View Full Version : Would the plane take off?
This debate seems to be making it's way on every message boards so I tought I'd try it here.
So here it goes.
A plane (747 passenger jet) is sitting on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the planes speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction).
The question is:
Will the plane (747 passenger jet) take off or not?
Jeffy-CanCon
12-06-2005, 03:33 PM
No. Take-off is dependant on the lift produced by air moving over the wings at high speed. the speed at which the wheels are moving is irrelevant.
*edit again - Boy was I wrong. This is a pretty good debate.
rkjunior303
12-06-2005, 03:39 PM
No. Take-off is dependant on the lift produced by air moving over the wings at high speed. the speed at which the wheels are moving is irrelevant.
exactly.
if the plane is stationary but yet the wheels are moving, the wings will not produce lift and the plane will not leave the ground. thats like rudimentary physics..
rkjunior303
12-06-2005, 03:55 PM
This debate seems to be making it's way on every message boards so I tought I'd try it here.
So here it goes.
A plane (747 passenger jet) is sitting on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the planes speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction).
The question is:
Will the plane (747 passenger jet) take off or not?
http://cheerdollz.com/catalog/images/300300withstupid.jpg
magman007
12-06-2005, 04:43 PM
This debate seems to be making it's way on every message boards so I tought I'd try it here.
So here it goes.
A plane (747 passenger jet) is sitting on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the planes speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction).
The question is:
Will the plane (747 passenger jet) take off or not?
absolutely not! there is no lift, no drag, no foreward momentum to create lift. Read about bernouli's law and see if you can understand how the pressure differrntial created by a leading edge creates lift. the plane couldnt just take off on a vertical plane, unless it is an aircraft capable of VTOL(Hellicopter, harrier, JSF, or an osprey)
the plane would be sitting still, it would be like riding a bike on a tread mill, absolutely useless.
_magman007 your local pilot
the only thing that would ever result from this conveyor idea would be if the conveyor went in the same difrection only faster than the jet is actually propelling its self and making its own sort of catapult system Ala aircraft carrier, which would be invariably too much stress on a 747's airframe and gear system.
Also, how would you suggest landing the beast of all of these runways were in motion?
SCpoloRicker
12-06-2005, 04:46 PM
.
http://i18.photobucket.com/albums/b128/hiphopgamer26/HHG/rly.jpg
magman007
12-06-2005, 05:15 PM
also, please link to what other forums are debating this
maxama10
12-06-2005, 05:31 PM
http://i18.photobucket.com/albums/b128/hiphopgamer26/HHG/rly.jpg
thats great.... :p :bounce:
maxama10
12-06-2005, 05:49 PM
yup http://img105.imageshack.us/img105/3637/dropbearcopy4lp.jpg (http://imageshack.us)
neppo1345
12-06-2005, 06:00 PM
YES...the plane would take off
You people forget the the plane's motion is derived from the interaction of the jet exhaust with the air, and not from the planes landing gear with the tarmac.
If it was a car it would remain stationary, but the plane would have nothing to negate it's thrust and would therefore still accelarate.
Since there is nothing to counteract the thrust of the engines, the plane would still move forward on the conveyor and reach speed to take off.
The wheels would simply spin twice as fast (the groundspeed would be twice the airspeed until it got off the ground) and it might take more thrust to counteract the friction produced in the wheel bearings coeffecient of friction between the pavement...etc etc.
I am not going to school to be an aerospace engineer...I don't know what I'm talking about...
EDIT: Oh wait...I am...and I do...
nippinout
12-06-2005, 06:01 PM
Please, for the love of Jebus, don't make me draw a free body diagram.
Let's assume a plane needs to hit 100mph to take off.
That plane is travelling at 100mph down the conveyor belt, and the conveyor belt moves in reverse at 100mph.
The plane is travelling at 100mph relative to the ground, and 200mph relative to the belt. The plane takes off.
The conveyor belt and its speed is irrelevant. Thrust is produced by the engines, and not at the wheels.
Let's take the same problem with a car with wings. The car is designed to fly if it hits 100mph. Hit the gas until the speedometer hits 100mph. The belt also moves at 100mph in reverse. The car's speed relative to the ground is 0mph. Car does not take off. The plane does.
If this thread hits more than 3 pages, anyone that answers incorrectly after that point should be re-educated. With my fist!
wad04
12-06-2005, 06:13 PM
even if it could, once it leaves the belt wouldn't it flop anyway?
nippinout
12-06-2005, 06:14 PM
even if it could, once it leaves the belt wouldn't it flop anyway?
The car on a normal runway would eventually lose speed once it gained altitude. A skilled car pilot could hop along. :D
neppo1345
12-06-2005, 06:18 PM
YAAAAY...another smart person... :clap:
tropical_fishy
12-06-2005, 06:20 PM
My physics is a bit fuzzy (I had a moron for an AP Physics teacher who said things like "we don't call Fred 'Joe'. Thus, we do not call omega 'w'." But that's another story) but I would think that if the airplane isn't moving, then no lift can be generated. The conveyor belt is acting like friction, only friction... that acts in perfect opposition to the movement, rather than a fraction of it. I guess my point is that the engine can provide thrust, but the plane can only move forward as fast as the wheels are moving, and if the ground under the wheels is meant to move in perfect opposition, the plane will go nowhere.
But like I said, my physics are fuzzy.
Tobe2be
12-06-2005, 06:22 PM
i would have voted PIE but i guess you forgot to place it in the poll
For anyone who thinks the plane is going to take off drink some antifreeze, it's yummy.
I don't know how much more basic this can get.
How do planes fly? Lift...
How is lift generated? Air moving over the surfaces of the wings.
Can an object that is stationary relative to the surrounding air have any air passing over its surfaces? No
If their is no air passing over the surface of the wings can a plane have lift? No
Can a normal plane fly without lift? No
Someone disagree with me and get smacked.
nippinout
12-06-2005, 06:27 PM
My physics is a bit fuzzy (I had a moron for an AP Physics teacher who said things like "we don't call Fred 'Joe'. Thus, we do not call omega 'w'." But that's another story) but I would think that if the airplane isn't moving, then no lift can be generated. The conveyor belt is acting like friction, only friction... that acts in perfect opposition to the movement, rather than a fraction of it. I guess my point is that the engine can provide thrust, but the plane can only move forward as fast as the wheels are moving, and if the ground under the wheels is meant to move in perfect opposition, the plane will go nowhere.
But like I said, my physics are fuzzy.
Let's do an experiment. Take a sheet of paper. That will be your runway. Now use a toy airplane, or a Matchbox car if you have one. Let's pretend it's a 747 on a runway. That toy will have thrust- so move it along the paper, but the paper will move back at the same speed as the toy is moving forward. No matter how fast or how slow the toy moves, the toy is still able to move forward relative to the ground underneath the paper.
nippinout
12-06-2005, 06:29 PM
For anyone who thinks the plane is going to take off drink some antifreeze, it's yummy.
I don't know how much more basic this can get.
How do planes fly? Lift...
How is lift generated? Air moving over the surfaces of the wings.
Can an object that is stationary relative to the surrounding air have any air passing over its surfaces? No
If their is no air passing over the surface of the wings can a plane have lift? No
Can a normal plane fly without lift? No
Someone disagree with me and get smacked.
The plane is moving! Thrust from the engines moves air backwards, to propel the plane forwards. Equal and opposite... M1V1=M2V2
What happens at the wheels is not important! The engines produce forward thrust no matter what is going on with the wheels.
tropical_fishy
12-06-2005, 06:31 PM
Let's do an experiment. Take a sheet of paper. That will be your runway. Now use a toy airplane, or a Matchbox car if you have one. Let's pretend it's a 747 on a runway. That toy will have thrust- so move it along the paper, but the paper will move back at the same speed as the toy is moving forward. No matter how fast or how slow the toy moves, the toy is still able to move forward relative to the ground underneath the paper.
There's no way you can move them at the same rate though.
....draw me a diagram, then I'll believe it.. What can I say, I'm a visual person.
nippinout
12-06-2005, 06:32 PM
New experiment.
Don't actually do this, you'll hurt yourself.
You need:
Rollerskates, helmet, treadmill, yourself, and a friend.
Strap on the skates and helmet and get on the treadmill. Turn on the treadmill.
You'll remain pretty much stationary. But you have thrust. Have your friend push you. You are moving forward! Your skate wheels and the treadmill don't factor into the speed at which your friend can push you!
The wheels and treadmill are irrelevant! Your friend can push you no matter how fast things are moving.
The plane is moving! Thrust from the engines moves air backwards, to propel the plane forwards. Equal and opposite... M1V1=M2V2
What happens at the wheels is not important! The engines produce forward thrust no matter what is going on with the wheels.
You sir deserve a smack.
The plane is remaining on one spot on the belt. This means you can stand right next to the belt and the plane will not move an inch relative to you correct? That is the situation he is describing in the question.
How in the hell can their be air moving over the surface of the wings if the plane is not moving relative to the air around it. Stupid people suck :(
neppo1345
12-06-2005, 06:38 PM
For anyone who thinks the plane is going to take off drink some antifreeze, it's yummy.
I don't know how much more basic this can get.
How do planes fly? Lift...
How is lift generated? Air moving over the surfaces of the wings.
Can an object that is stationary relative to the surrounding air have any air passing over its surfaces? No
If their is no air passing over the surface of the wings can a plane have lift? No
Can a normal plane fly without lift? No
Someone disagree with me and get smacked.
Smack me...come on...do it...judging by your response...thats about all you'll be doing for the rest of your life...hope you're a big guy...
What we have here my "friend" are TWO systems...
System 1. The 747's four Prat and Whitney PW4056 Turbofan Engines interacting with the Air
System 2. The 747's 18 tires and the Tarmac
System 2 is virtually frictionless meaning that with a little thrust forward the plane could stand still with the conveyor moving at 1.2 million miles per hour in the opposite direction. Once System 1 overcomes the little friction created by system two the resulting motion will be caused strictly by system 1, will be in a forward direction, and will eventually result in takeoff...
nippinout
12-06-2005, 06:39 PM
You sir deserve a smack.
The plane is remaining on one spot on the belt. This means you can stand right next to the belt and the plane will not move an inch relative to you correct? That is the situation he is describing in the question.
How in the hell can their be air moving over the surface of the wings if the plane is not moving relative to the air around it. Stupid people suck :(
So we're looking at a word problem wrapped in semantic tomfoolery? 'Plane is sitting on a runway' and 'plane moves in one direction, while the conveyor moves in the opposite direction' are both in the question. Looks like a physics problem to me.
Tobe2be
12-06-2005, 06:41 PM
if a cat chokes on a dead mouse who killed who?
Miscue
12-06-2005, 06:45 PM
You don't have to have lift from an airfoil for flight. Look at a rocket for instance. Thrust alone will get it into the air.
With sufficient thrust, you can get something that looks and works nothing like an airplane to fly around.
But if this airplane is able to take off without lift... is it still an airplane?
I guess it has to do with what you think the requirements are for something to be considered an airplane.
Personally, I think that something that can take off from a standstill is more of a rocket than an airplane. What I consider to be a pure airplane would not be able to take off in this conveyor belt situation.
nippinout
12-06-2005, 06:48 PM
You don't have to have lift from an airfoil for flight. Look at a rocket for instance. Thrust alone will get it into the air.
With sufficient thrust, you can get something that looks and works nothing like an airplane to fly around.
But if this airplane is able to take off without lift from the wings... is it still an airplane?
I guess it has to do with what you think the requirements are for something to be considered an airplane.
Personally, I think that something that can take off from a standstill is more of a rocket than an airplane. What I consider to be a pure airplane would not be able to take off in this conveyor belt situation.
Dear god, not you Miscue! Get on that treadmill now!
UTDragun
12-06-2005, 06:48 PM
the answer is no, you wanna know why? cuz you didnt specify what kind of conveyor runway, have you seen pearl harbor? they had trouble with a medium bomber, now way a 747 can take off a carrier :p
as far as I can see, If your thrust > friction of wheels then it shouldnt make a difference whether its on a conveyor or not
Miscue
12-06-2005, 06:50 PM
YES...the plane would take off
You people forget the the plane's motion is derived from the interaction of the jet exhaust with the air, and not from the planes landing gear with the tarmac.
If it was a car it would remain stationary, but the plane would have nothing to negate it's thrust and would therefore still accelarate.
Since there is nothing to counteract the thrust of the engines, the plane would still move forward on the conveyor and reach speed to take off.
The wheels would simply spin twice as fast (the groundspeed would be twice the airspeed until it got off the ground) and it might take more thrust to counteract the friction produced in the wheel bearings coeffecient of friction between the pavement...etc etc.
I am not going to school to be an aerospace engineer...I don't know what I'm talking about...
EDIT: Oh wait...I am...and I do...
Pretentiousness... yay! Let us know when you decide that, "It's not what I really wanted to do," and end up enrolling in a liberal arts program.
nippinout
12-06-2005, 06:51 PM
In the probleme's set-up, the belt moves at the speed of the plane in the opposite direction. Plane goes 100mph, the belt moves at 100mph opposite to it. Read that again. The plane is going 100mph! The wheels are travelling at 200mph, but the plane is travelling at 100mph!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
neppo1345
12-06-2005, 06:53 PM
PWNED...ALL OF YOU (http://www.avweb.com/news/columns/191034-1.html)
UTDragun
12-06-2005, 06:57 PM
PWNED...ALL OF YOU (http://videos.streetfire.net/player.aspx?fileid=35E964D9-38DB-4EFD-BE8D-D6BA1A43A06B)
this link doesnt work
and here... (http://www.avweb.com/news/columns/191034-1.html)
this one does
nippinout
12-06-2005, 06:58 PM
I was under the impression that the conveyor belt exactly matched the plane's speed at all times... so it would be at a standstill although the wheels spin.
Try visualizing the treadmil example.
The wheels are not creating the forward movement of the plane, the engines pushing air is. The wheels are just spinning happily along due to the conveyor belt, while the engine is pushing the plane forward. Idealize the wheels as frictionless bearings. (For the love of god people, do not go into analyzing friction. Any engineer in the real world idealizes everything.)
neppo1345
12-06-2005, 06:58 PM
works fine for me....
nippinout
12-06-2005, 07:01 PM
I'm waiting for Mythbusters to test this and blow up a 747 in the process.
SlartyBartFast
12-06-2005, 07:05 PM
But can the wheels support the extra speed? Or do they explode = plane doesn't take off?
Here's my 2 cents.
The plane will take off. As long as the propulsion system is independent of the interaction with the ground, the ground has no effect.
Case in point: I can take off of an icy runway just as easily as a dry one. Why? My wheels don't "slip" because it is the prop that is pulling me forward. Now...stopping is another issue. My breaks are based at the wheels, stopping on ice is tough. Do you see?
nippinout
12-06-2005, 07:07 PM
But can the wheels support the extra speed? Or do they explode = plane doesn't take off?
The question isn't whether or not the wheels/bearings/hydraulic fluid/brake fluid/rubber/tire pressure can handle the situation. Let's just assume nothing will break.
Another thought to show the minimal impact of the moving runway: If the bearings in the wheels were perfectly frictionless [hey we're talking about a conveyor runway!], and the plane's engines were off, the plane still wouldn't move, the wheels would just spin. Now add the thrust of the engines an there you go. Acceleration and lift.
nippinout
12-06-2005, 07:18 PM
Another thought to show the minimal impact of the moving runway: If the bearings in the wheels were perfectly frictionless [hey we're talking about a conveyor runway!], and the plane's engines were off, the plane still wouldn't move, the wheels would just spin. Now add the thrust of the engines an there you go. Acceleration and lift.
HIGH FIVE!!!
http://www.greasyelbow.com/korea/waryong/dogs05.jpg
nippinout
12-06-2005, 07:39 PM
Miscue, the conveyor belt isn't keeping the plane from moving. It is simply moving in a reverse direction from the plane. The wheels are spinning freely, sort of like isolating the plane from a moving or stationary runway.
maxama10
12-06-2005, 07:39 PM
The plane is moving! Thrust from the engines moves air backwards, to propel the plane forwards. Equal and opposite... M1V1=M2V2
What happens at the wheels is not important! The engines produce forward thrust no matter what is going on with the wheels.
actually yeah now that i think about it the m1v1=m2v2 is right and made it clear for me thanks!
slade
12-06-2005, 07:56 PM
My physics is a bit fuzzy (I had a moron for an AP Physics teacher who said things like "we don't call Fred 'Joe'. Thus, we do not call omega 'w'." But that's another story) but I would think that if the airplane isn't moving, then no lift can be generated. The conveyor belt is acting like friction, only friction... that acts in perfect opposition to the movement, rather than a fraction of it. I guess my point is that the engine can provide thrust, but the plane can only move forward as fast as the wheels are moving, and if the ground under the wheels is meant to move in perfect opposition, the plane will go nowhere.
But like I said, my physics are fuzzy.
at first i thought the guy was an idiot, because the way he words it it sounds like hes asking what would happen if the plane does not move, but the engines are running. of course it wouldnt take off, it needs to have air flowing past it.
the thing is, cars are powered through their wheels, they use contact with the ground to push themselves forward. thus a car on a conveyor belt would not move relative to the ground.
however, a plane is powered by a jet engine/propeller which both use the air. the wheels are only to support the plane while it is on the ground, they rotate freely. when the plane starts the engines pull it forward through the air. that is what powers the plane. the wheels just rotate to keep up with the plane as the engines power it forward. the belt would cause the wheels to rotate faster which would add friction and reduce the speed, but most likely a neglidgeable amount.
tropical_fishy
12-06-2005, 08:06 PM
Miscue, the conveyor belt isn't keeping the plane from moving. It is simply moving in a reverse direction from the plane. The wheels are spinning freely, sort of like isolating the plane from a moving or stationary runway.
Except the problem says that no matter what, the conveyor belt's speed is exactly equal and opposite of the plane's.
tropical_fishy
12-06-2005, 08:16 PM
New experiment.
Don't actually do this, you'll hurt yourself.
You need:
Rollerskates, helmet, treadmill, yourself, and a friend.
Strap on the skates and helmet and get on the treadmill. Turn on the treadmill.
You'll remain pretty much stationary. But you have thrust. Have your friend push you. You are moving forward! Your skate wheels and the treadmill don't factor into the speed at which your friend can push you!
The wheels and treadmill are irrelevant! Your friend can push you no matter how fast things are moving.
This analogy only works if as my friend pushes me and I pick up speed, the treadmill speeds up to match my new speed.
Please play again.
magman007
12-06-2005, 08:38 PM
I see where i was wrong, and when flying today, i considered the problem, and retract my statement of it not being able to take off.
Consider this, the plane is still able to run down the runway, whether the wheels are turning or not, that doesnt matter. Its like spin in paintball, it doesnt do a damn thing. LEts put it this way, i was flying backwards today, how you may ask?
I was doing slowflight in a dirty configuration (for non pilots thats pitching up to just above stall speed in landing configuration) the wind was string enough to effectively put me in a hover/ -2 or 3 kts ground speed. I was effectively flying, because there was still wind going over the wings, just i was not traveling forewards.
Now, that conveyor belt can run as fast as it wants, in a perfect world, the jet would not have to over come its own weight on the wheels, if it could do that in a real world, then you could pull a 747 around on a string. but you cant.
so assuming the wheels are turning 100% effortlessly, the wheels will just rotate faster and faster as the jet pushes its way forewards, allowing air to flow over the wings effectively generating lift.
is there any sort of advantage to this conveyor style? NO. Because your TO distance is still the same. you still need to achieve your airspeeds before you pitch back for your climb out speed.
get it q?
BD_Paintball
12-06-2005, 08:38 PM
My friend who is a senior at Purdue majoring in Aeronautical/Astronautical Engineering said it will not take off, and I believe him. he said it will not gain any air speed and it wont have any lift at all. he also game me this equation "L=.5*cl*rho*U^2*s. U is wind velocity. If U = 0, L=0, L is lift force, cl is lift coeficient, rho is air density, U is freestream velocity, and s is wing span"
i win
tropical_fishy
12-06-2005, 08:43 PM
I see where i was wrong, and when flying today, i considered the problem, and retract my statement of it not being able to take off.
Consider this, the plane is still able to run down the runway, whether the wheels are turning or not, that doesnt matter. Its like spin in paintball, it doesnt do a damn thing. LEts put it this way, i was flying backwards today, how you may ask?
I was doing slowflight in a dirty configuration (for non pilots thats pitching up to just above stall speed in landing configuration) the wind was string enough to effectively put me in a hover/ -2 or 3 kts ground speed. I was effectively flying, because there was still wind going over the wings, just i was not traveling forewards.
Now, that conveyor belt can run as fast as it wants, in a perfect world, the jet would not have to over come its own weight on the wheels, if it could do that in a real world, then you could pull a 747 around on a string. but you cant.
so assuming the wheels are turning 100% effortlessly, the wheels will just rotate faster and faster as the jet pushes its way forewards, allowing air to flow over the wings effectively generating lift.
is there any sort of advantage to this conveyor style? NO. Because your TO distance is still the same. you still need to achieve your airspeeds before you pitch back for your climb out speed.
get it q?
The only thing you haven't explained is exactly how you're going to overcome physics. The problem says that no matter HOW FAST your wheels are spinning-- freewheeling-- the conveyor is going toexactly match that speed. I'm not convinced I'm right-- but no one has explained how exactly this plane is going to overcome 0 net movement.
neppo1345
12-06-2005, 08:52 PM
My friend who is a senior at Purdue majoring in Aeronautical/Astronautical Engineering said it will not take off, and I believe him. he said it will not gain any air speed and it wont have any lift at all. he also game me this equation "L=.5*cl*rho*U^2*s. U is wind velocity. If U = 0, L=0, L is lift force, cl is lift coeficient, rho is air density, U is freestream velocity, and s is wing span"
i win
You don't win...its common sense that if an airplane has no airspeed there is no lift...this has nothing to do with the problem at hand...
The wheels and the runway have no effect on the system as a whole because they are virtually frictionless...
neppo1345
12-06-2005, 08:55 PM
The only thing you haven't explained is exactly how you're going to overcome physics. The problem says that no matter HOW FAST your wheels are spinning-- freewheeling-- the conveyor is going toexactly match that speed. I'm not convinced I'm right-- but no one has explained how exactly this plane is going to overcome 0 net movement.
Yes, the wheels will be spinning at exactly the same speed as the conveyor...there is very little friction here...
Once the massive jet engines overcome this tiny amount of friction, the rest of the thrust will go towards accelerating the jet towards takeoff velocity...
BD_Paintball
12-06-2005, 08:59 PM
You don't win...its common sense that if an airplane has no airspeed there is no lift...this has nothing to do with the problem at hand...
The wheels and the runway have no effect on the system as a whole because they are virtually frictionless...
no i win b/c i gave you scientific proof that it will not take off!! :cheers: :cheers:
neppo1345
12-06-2005, 09:08 PM
no i win b/c i gave you scientific proof that it will not take off!! :cheers: :cheers:
You gave me scientific proof that it will not fly as long as it's not moving forward...
I'm telling you it's going to move forward despite the conveyor belt...so your fact only proves that it will fly...
BD_Paintball
12-06-2005, 09:12 PM
You gave me scientific proof that it will not fly as long as it's not moving forward...
I'm telling you it's going to move forward despite the conveyor belt...so your fact only proves that it will fly...
how is it going to move forward if the conveyor belt is moving the other way the same speed as the plane is going? if the conveyor belt is matching the speed of the plane but in the opposite direction it will just sit there.
magman007
12-06-2005, 09:13 PM
no i win b/c i gave you scientific proof that it will not take off!! :cheers: :cheers:
but you didnt
BD_Paintball
12-06-2005, 09:15 PM
but you didnt
L=.5*cl*rho*U^2*s. U is wind velocity. If U = 0, L=0, L is lift force, cl is lift coeficient, rho is air density, U is freestream velocity, and s is wing span"
The plane flies.
It has forward movement regardless of the belt. The belt will match the forward speed. Regardless of how fast the belt moves, the plane will continue to accelerate to flight speed.
There will be lots of rubber tire chunks all over...but it will be flying!
Think this way: the plane is already flying with wheels up, just very low and against the travel direction of the belt. It does not matter the belt speed, the plane flies. Now put the gear down. Does the plane instantly stop? No, it is still at flight speed. The plane does NOT get it's forward thrust/movement from any drive system to the wheels. Forward movement is independant of the belt.
Picture a car on a Dynomometer, the wheel thrust powers the drums the wheels are sitting on. The faster you spin the car wheels, the faster the Dyno turns, but the car can go nowhere since its "road" rolls as fast as it does. Put that same car back on the Dyno with the motor off but now the DYNO making the car wheels roll....and light the RATO pack now strapped to the roof. Where is the car going to go? Yup, airborne.
Independant thrust negates the speed or direction of the belt.
I am right. You are wrong. Worship me.
UTDragun
12-06-2005, 09:17 PM
Well I just hope he does what Miscue says and turns to major in liberal arts
Im a mechanical engineer and I still got it right.
neppo1345
12-06-2005, 09:23 PM
Yeah...considering that I'm an aerospace major as well...that equation looks sorta familiar, even in it's butchered form...i can spit out some more "science" if you want, but it's not going to make any difference.
It's one of those things you just have to get...if you don't have a decent math/physics/flight background you just wont get it...
I sure hope I never end up "flying" in anything he designs...
Miscue
12-06-2005, 09:26 PM
Well I just hope he does what Miscue says and turns to major in liberal arts
Im a mechanical engineer and I still got it right.
Graduate of Howard R. Hughes College of Engineering right here... although who has what education is irrelevant in solving this problem. Just means you have less of an excuse to go about it the wrong way.
tropical_fishy
12-06-2005, 09:28 PM
I hate calculus. Can I announce that in this thread? It's semi-math related. I. hate. calculus. Right now. I hate sequences and series, I hate that I have calc at 8:30 in the morning and I rarely go, I hate that I don't remember how to do conditional covergence for the alternating series test, and I don't know how to find sums of p-series. Oh, and magman007 explained the plane thing and I'm all better now.
Back to your regularly scheduled program.
BD_Paintball
12-06-2005, 09:32 PM
how is it going to fly when it has no air speed. no lift = not flying
Miscue
12-06-2005, 09:36 PM
I hate calculus. Can I announce that in this thread? It's semi-math related. I. hate. calculus. Right now. I hate sequences and series, I hate that I have calc at 8:30 in the morning and I rarely go, I hate that I don't remember how to do conditional covergence for the alternating series test, and I don't know how to find sums of p-series. Oh, and magman007 explained the plane thing and I'm all better now.
Back to your regularly scheduled program.
Study more and it becomes simple! Take some harder math classes and you'll realize how easy Calculus really was.
tropical_fishy
12-06-2005, 09:57 PM
Study more and it becomes simple! Take some harder math classes and you'll realize how easy Calculus really was.
I really do like math. I really do like calculus. I just hate tests. And I hate mornings. So when I have tests and mornings, that is extra-super bad.
Just think of it as if you were looking at it from the side, one part at a time. There is a conveyor belt moving 100 mph to the right, and then imagine a 747 moving 100 mph to the left (in the air). Now lower the 747 till it touches the conveyor belt. It will still be moving at 100 mph to the left and the wheels will be spinning at 200 mph. I'm an engineer (not aero or mech) and when I first read it I just didn't think about how a plane actually made itself move and I thought it would stand still. The way the question is worded tricks you into thinking like it's a car.
I really do like math. I really do like calculus. I just hate tests. And I hate mornings. So when I have tests and mornings, that is extra-super bad.
I felt the same way. Graduated from RPI with an EE and CSYS degree. Now I'm working as a nuclear engineer on submarines. I also used to think 8:30 was early, now I'm happy when I get to sleep in till 5. yay!!
Miscue
12-06-2005, 10:36 PM
I felt the same way. Graduated from RPI with an EE and CSYS degree. Now I'm working as a nuclear engineer on submarines. I also used to think 8:30 was early, now I'm happy when I get to sleep in till 5. yay!!
Congrats! Working with the Navy?
Just think of it as if you were looking at it from the side, one part at a time. There is a conveyor belt moving 100 mph to the right, and then imagine a 747 moving 100 mph to the left (in the air). Now lower the 747 till it touches the conveyor belt. It will still be moving at 100 mph to the left and the wheels will be spinning at 200 mph. I'm an engineer (not aero or mech) and when I first read it I just didn't think about how a plane actually made itself move and I thought it would stand still. The way the question is worded tricks you into thinking like it's a car.
I answered that.
The 747 does not use its wheels to generate forward motion.
Miscue
12-06-2005, 11:00 PM
Ok, these are my last thoughts on this:
It is not the case that a conveyor belt can prevent an airplane from moving forward no matter how fast it goes (ideally). The plane does not have to be in a state of flight for this to apply. The median used to propel the plane forward (air) is not moving and that's the important idea here... the moving conveyor belt becomes irrelevant. It is relevant that power is not being applied to the wheels of the plane, I initially didn't think so until I thought about it more. The condition in which the plane is stationary and net velocity is 0... does not occur.
So yes... it can take off because it will generate airspeed and go forward regardless. It still needs enough runway.
Same thing Army and others said basically... now I understand it.
billybob_81067
12-06-2005, 11:08 PM
Think this way: the plane is already flying with wheels up, just very low and against the travel direction of the belt. It does not matter the belt speed, the plane flies. Now put the gear down. Does the plane instantly stop? No, it is still at flight speed. The plane does NOT get it's forward thrust/movement from any drive system to the wheels. Forward movement is independant of the belt.
That sir is the best explanation yet! I understood what the people saying "Yes it will fly" were getting at, yet could not think of a real good way to word a description, but you hit it right on the head! :hail:
wanna-b-ballin'
12-06-2005, 11:24 PM
it wont take off. how can a plane get lift if it is not moving forward?
it wont take off. how can a plane get lift if it is not moving forward?
Go back, and sloooowly read the original question.
wanna-b-ballin'
12-07-2005, 12:03 AM
if its already in motion, as in flying, when it comes over the conveyer belt, then sure, it can take off because it will have speed, and lift.
whereas i think the question is stating that the plane and conveyor belt are both at a stop, and both start and accelerate at the same time, thus not moving it anywhere.
i dont see how you guys think it can lift off, unless i'm missing something major.
Case in point: I can take off of an icy runway just as easily as a dry one. Why? My wheels don't "slip" because it is the prop that is pulling me forward. Now...stopping is another issue. My breaks are based at the wheels, stopping on ice is tough. Do you see?
This sounds like the best way to explain that the runway surface friction/direction doesn't really do a damn thing in the take off of the plane.
The plane flies.
wanna-b-ballin'
12-07-2005, 12:06 AM
with a propeller driven plane, i could see it taking off, as the propeller is directly pushing air over the wings.
where as on a jet, it is sucking air in under the wings, and pushing it out really fast behind the plane. i dont think it gives it lift.
i dont see how you guys think it can lift off, unless i'm missing something major.
Yeah, you're missing that a plane doesn't use the ground to accelerate.
To cite an example above, an icy runway doesn't effect the takeoff distance.
if its already in motion, as in flying, when it comes over the conveyer belt, then sure, it can take off because it will have speed, and lift.
whereas i think the question is stating that the plane and conveyor belt are both at a stop, and both start and accelerate at the same time, thus not moving it anywhere.
i dont see how you guys think it can lift off, unless i'm missing something major.
The 747 does NOT rely on its wheels for forwrd motion---they are unpowered. The jet's engines push it forward, unheeded by the conveyor or wheel speed. When the 747 reaches flight speed, it will, regardless of the speed or direction of the belt and wheels.
with a propeller driven plane, i could see it taking off, as the propeller is directly pushing air over the wings.
where as on a jet, it is sucking air in under the wings, and pushing it out really fast behind the plane. i dont think it gives it lift.
Propellers pull, jets push. The wind they generate is only for forward motion, NOT supplying airflow over or under the wings. Forward motion will provide that.
wanna-b-ballin'
12-07-2005, 12:10 AM
This sounds like the best way to explain that the runway surface friction/direction doesn't really do a damn thing in the take off of the plane.
The plane flies.
uugh. for the plane to get lift it has to be moving forward. the wheels do not make it go forward, they just let the plane roll with the force created by the jet. on ice, the jet will roll forward.
on the conveyer, it will not because the "land" it is on is going the opposite direction, thus preventing the plane from being able to move forward.
i really can't explain it more.
Bear_Claw
12-07-2005, 12:17 AM
Yes the plane would accelerate but the tires would blow out due too excessive speed and the plane would crash their for not taking off.
Nope i aint no enginer either im a mehcanic LOL
wanna-b-ballin'
12-07-2005, 12:19 AM
damnit, now i have to go try this crap. i'm starting to understand armys point, but i just can't bring myself to completely understand how the jet would go forward.
i dont have a treadmill, but my friend does.
i have a remote controll airplane, propeller driven.
i think it will simulate it well enough if i put the two together. but i think his parents would be a little pissed if it did take off, as they have too much expensive fragile crap in that room.
i'll ask his dad that question though, he is a pilot, and is very good at comprehending that sort of difficult/should-be-easy sort of stuff.
billybob_81067
12-07-2005, 12:25 AM
uugh. for the plane to get lift it has to be moving forward. the wheels do not make it go forward, they just let the plane roll with the force created by the jet. on ice, the jet will roll forward.
on the conveyer, it will not because the "land" it is on is going the opposite direction, thus preventing the plane from being able to move forward.
i really can't explain it more.
The thrust pushing the plane forward pushes off of the AIR BEHIND THE JET ENGINES, NOT THE GROUND/CONVEYOR BELT... Therefore unless the conveyor belt could move the air above it the jet will still take off...
Hmmm... even if the conveyor could move the air, that air movement would then make lift due to it passing over the wings. :eek:
wanna-b-ballin'
12-07-2005, 12:32 AM
why are people thinking i'm saying the wheels are powered. i know they arent. they just allow the plane to move easier than if it were just dragging its *** across the ground. they help with the friction.
the plane has to be moving forward at a high enough speed to create lift with its wings. with the conveyor moving the opposite direction, the plane cannot move forward. yes, it will be accelerating, and the jet pushing harder and harder, yet so will the conveyer belt. so the plane will not be able to take off.
someone mentioned the rocket thing. so how is the plane going to all of a sudden have enough thrust to take off like a rocket?
billybob_81067
12-07-2005, 12:44 AM
why are people thinking i'm saying the wheels are powered. i know they arent. they just allow the plane to move easier than if it were just dragging its *** across the ground. they help with the friction.
the plane has to be moving forward at a high enough speed to create lift with its wings. with the conveyor moving the opposite direction, the plane cannot move forward. yes, it will be accelerating, and the jet pushing harder and harder, yet so will the conveyer belt. so the plane will not be able to take off.
someone mentioned the rocket thing. so how is the plane going to all of a sudden have enough thrust to take off like a rocket?
But the air that the jet engines push against with their thrust is not moving backwards with the conveyor belt...
The wheels would spin like crazy, yet the jet would still push forward.
Think of it this way. Say you're running on a threadmill at 15 mph, and someone turns a fan on behind you. You can feel that air right? That's cause the conveyor/treadmill has NO effect on the air.
Or say you've got a skateboard on one of those conveyor walkways at the airport. if you just stand there on it then the backwards motion of the conveyor cancells out the forward motion of the skateboard. But now run a rope tied to a pole at the end of the conveyor and you can pull yourself through the conveyor walkway no matter if it's going 5 or 100 mph, because the conveyor has no effect on the rope.
bleachit
12-07-2005, 01:04 AM
"So far the J-3 has not moved, nor has the conveyor. At idle power, there's not enough thrust to move the J-3 forward on a level surface, so Manfred starts to bring up the power, intending to take off. The propeller rpm increases and the prop shoves air aft, as it does on every takeoff, causing the airplane to move forward through the air, and as a consequence, forward with regard to the ground. Simultaneously the conveyor creaks to life, moving east, under the tires of the J-3. As the J-3 thrusts its way through the air, driven by its propeller, the airspeed indicator comes off the peg at about 10 mph. At that moment the conveyor is moving at 10 mph to the east and the tires are whirling around at 20 mph because the prop has pulled it to an airspeed, and groundspeed, of 10 mph, westbound. The airplane is moving relative to the still air and the ground at 10 mph, but with regard to the conveyor, which is going the other way at 10 mph, the relative speed is 20 mph.
Manfred relaxes a bit because the conveyor cannot stop him from moving forward. There is nothing on the airplane that pushes against the ground or the conveyor in order for it to accelerate; as Karen -- one of our techies here at the Lounge -- put it, the airplane freewheels. In technical terms, there is some bearing drag on the wheels, but it's under 40 pounds, and the engine has overcome that for years; plus the drag doesn't increase significantly as the wheel speed increases. Unless Manfred applies the brakes, the conveyor cannot affect the rate at which the airplane accelerates.
A few moments later, the roaring Continental, spinning that wooden Sensenich prop, has accelerated the J-3 and Manfred to 25 mph indicated airspeed. He and the airplane are cruising past the cheering spectators at 25 mph, while the conveyor has accelerated to 25 mph eastbound, yet it still has no way of stopping the airplane's movement through the air. The wheels are spinning at 50 mph, so the noise level is a little high, but otherwise, the J-3 is making a normal, calm-wind takeoff.
As the indicated airspeed passes 45 mph, groundspeed -- you know, relative to where all those spectators are standing beside the conveyor belt -- is also 45 mph. (At least that's what it says on Manfred's GPS. Being brought back to life seemed to create an insatiable desire for electronic stuff.) The conveyor is also at 45 mph, and the wheels are whizzing around at 90 -- the groundspeed plus the speed of the conveyor in the opposite direction.
Manfred breaks ground, climbs a few hundred feet, then makes a low pass to see if he can terrify the spectators because they are Americans, descendants of those who defeated his countrymen back in 1918. "
thats from the link a while ago, as there still seems to be some who dispute the ability to fly. I think that says it rather clearly.
anomoly40
12-07-2005, 05:38 AM
Just watch this.
http://media.putfile.com/Plane-runway-conveyor
If this helps.
Imagine someone sitting on a skateboard. Now imagine that skateboard is on a tread mill. Now imagine that the person is holding a rope that is tied to a wall. Now no matter how fast that treadmill spins the person isn't using the wheels to hold theirself in place, they're using the rope. Now if they pull the rope they move foward no matter how fast the treadmill is going.
The person on a skateboard is the Jet.
The treadmill is the conveyor.
The rope is the surrounding air.
The person pulling on the rope is showing how the jet pulls the air to move.
Pacifist_Farmer
12-07-2005, 08:33 AM
^
He Wins, it flies, thats that
Lohman446
12-07-2005, 09:30 AM
If the jets are creating forward movement the wheels of 100MPH and the treadmill is making that remain stationary to the non-mobile ground around it the plane will not take off.
Regardless of the amount of thrust from the engines it is not the engine thrust over the wings on most aircraft that create lift. It is the movement of air over the wings. Because the plane is stationary to the stable ground and air around it there is not the required movement of air over and under the wings to create lift.
"But the thrust of the jet" - nonsense. Planes glide without engine power, in fact there are some that are designed to not have an engine. Take a 747 and disable the engines while in flight. It will still "fly" though not gain altitude once it falls below a certain speed.
Air speed causes lift, not some manufactured ground speed. If all one needed to lift off was the thrust of the jet engine then we would not use wheels for take off, the plane would simply sit, locked to the ground (wheels chocked for instance) and lift off without a runway.
rkjunior303
12-07-2005, 09:33 AM
The only thing you haven't explained is exactly how you're going to overcome physics. The problem says that no matter HOW FAST your wheels are spinning-- freewheeling-- the conveyor is going toexactly match that speed. I'm not convinced I'm right-- but no one has explained how exactly this plane is going to overcome 0 net movement.
I think this problem is dependent on the fact that these wheels would be completely frictionless, which would then allow them to spin freely at any speed.
tropical_fishy
12-07-2005, 09:56 AM
I think this problem is dependent on the fact that these wheels would be completely frictionless, which would then allow them to spin freely at any speed.
Yeah, I got it, bill explained it to me last night.
SlartyBartFast
12-07-2005, 11:52 AM
If the jets are creating forward movement the wheels of 100MPH and the treadmill is making that remain stationary to the non-mobile ground around it the plane will not take off.
But, you CANNOT keep a jet stationary on a treadmill.
If you were stanfing on skateboard, on a treadmill, holding onto a rope, would you be able to pull yourself along? YESWould the treadmill be able to keep you stationary (fight your pulling of the rope)?NO
Regardless of the amount of thrust from the engines it is not the engine thrust over the wings on most aircraft that create lift.
But, engine thrust creates the movement. Engine thrust either overcomes inertia to accelerate to take off speed or overcomes air resistance to maintain airspeed.
It is the movement of air over the wings. Because the plane is stationary to the stable ground and air around it there is not the required movement of air over and under the wings to create lift.
But the plane wouldn't be stationary. A treadmill can only keep something motionless if the "something" depends on traction for forward motion. Planes do not need traction. Nor do skateboarders holding a rope.
"But the thrust of the jet" - nonsense. Planes glide without engine power, in fact there are some that are designed to not have an engine. Take a 747 and disable the engines while in flight. It will still "fly" though not gain altitude once it falls below a certain speed.
That is true. Except for the opening "nonsence" bit.
Air speed causes lift, not some manufactured ground speed. If all one needed to lift off was the thrust of the jet engine then we would not use wheels for take off, the plane would simply sit, locked to the ground (wheels chocked for instance) and lift off without a runway.
True. I've seen biplanes take of vertically at an airshow.
THe pilots were really not very amused by the fact the CF-18 doing a 360 in front of them decided to "gun" the engines for the turn. :rofl:
But, it is VERY difficuly under normal circumstances to get airspeed without groundspeed. And to reach ground speed, best bet is wheels.
In the situation of a plane on a moving treadmill.
Assuming no headwind and guessing 100mph airspeed for take-off:
Final gound speed: 100mph
final speed on treadmill: 100mph + whatever speed the treadmill is moving at.
Lohman, there are so many errors in your post....it's scary.
Please tell us how a large aircraft, such as a 747, gets to take off speed at the start of the runway...from a completely stopped position?
Yes, that's right, the thrust of the engines overwhelming its inertia. There is NO POWER to the wheels, NO THRUST is generated at the wheels, there is NO RESISTANCE at the wheels. That the belt is also moving, will only make the wheels roll faster, NOTHING ELSE.
The 747 relies on forward movement being created by the push of its engines....which the belt would have absolutely NO INFLUENCE on.
The skateboard/rope/treadmill is a perfect analogy. Make the rope shorter, and where are you in relation to the treadmill as compared to a long rope? Obviously, you will have moved forward REGARDLESS of the treadmills belt speed.
The 747 is simply making its rope shorter.
Lohman446
12-07-2005, 01:25 PM
Lohman, there are so many errors in your post....it's scary.
Yeh... there always is. :D
But the base - a plane requires air flow over the wings to fly. If the wings are not moving in relation to the air, the plane is not going to have the lift to fly. If the plane moves in relation to the air around it then it will fly, if it does not it will not.
Regardless of the propulsion used if the treadmill allows the plane to remove stationary in regards to the nonmoving ground (and more importantly air) around it it will not take off. There is likely an issue here, I cannot seem to grasp for certain if I beleive the jet would move in regards to the ground around the treadmill assuming the treadmill could keep up with it. The difference between traction based movement and the jet propulsion... yeh well my mind is having a hard time with that.
I'm thinking if we could use a quick moving treadmill type thing to get the planes off the ground without forward movement (in relation to the rest of the ground) they would already exist on military vessels. I don't see them.
READ THE ORIGINAL QUESTION! The 747 IS moving forward, while the belt is moving backward!
The difference between traction based movement and the jet propulsion... yeh well my mind is having a hard time with that.
Jet propulsion does NOT RELY on traction, which makes the belt/wheel speed moot.
octane2079
12-07-2005, 01:39 PM
The jet will move forward. The wheels are not used for thrust in any way shape or form their only purpose is to reduce friction between the two surfaces. The thrust from the engines is not affected by the treadmill so it will move forward, just the wheels will be spinning twice as fast because of the treadmill.
Its a trick question. The question is phrased in such a way as to make it seem the conveyors function is to keep the plane in one position. However this is not the case. The thrust of the engine will move the plane regardless of the runway being a conveyor.
Consider a car on frictionless ice. If a standard motor is used to turn the wheels then the car will definitely not move because there is no friction to cause wheel roll. But if we now strap a jet engine on top of the car, the car will move regardless of the wheels rolling or just sliding. The wheels on a plane serve the same function.
In this case, the plane will move and eventually take off. Assuming the conveyor is the same length as a real runway. And assuming the bearings on the wheels can handle it.
Lohman446
12-07-2005, 01:40 PM
READ THE ORIGINAL QUESTION! The 747 IS moving forward, while the belt is moving backward!
I'm an idiot?? :confused:
What I should have said - the speed of the plane in relation to the air around it (the stationary ground, etc.) is all that is important. The treadmill will not reduce or increase the air speed the plane needs to achieve to take flight.
OK! Best analogy ever!
We've all been on an escalator...and most of us have tried to walk "up the down", right? OK, imagine the stairs matching your speed, keeping you in one position, right? No matter how fast you run up those stairs, the downward travel will keep you "right there".
Now have a friend reach out and grab your arm as HE rides the UP stairs....guess which way you are going, regardless of the down moving stairs? The external and independant force (your friend/jet engines) will make the movement of your feet and the down stairs irrelevant, as the independant force will move you up.
Run as fast as you want, you are no longer slaved to the steps, but to your friend.
Miscue
12-07-2005, 01:49 PM
If the jets are creating forward movement the wheels of 100MPH and the treadmill is making that remain stationary to the non-mobile ground around it the plane will not take off.
Regardless of the amount of thrust from the engines it is not the engine thrust over the wings on most aircraft that create lift. It is the movement of air over the wings. Because the plane is stationary to the stable ground and air around it there is not the required movement of air over and under the wings to create lift.
"But the thrust of the jet" - nonsense. Planes glide without engine power, in fact there are some that are designed to not have an engine. Take a 747 and disable the engines while in flight. It will still "fly" though not gain altitude once it falls below a certain speed.
Air speed causes lift, not some manufactured ground speed. If all one needed to lift off was the thrust of the jet engine then we would not use wheels for take off, the plane would simply sit, locked to the ground (wheels chocked for instance) and lift off without a runway.
I was thinking the exact same thing. Then I thought about it this way:
Lets say a guy is standing on a skateboard on a conveyor belt. If he pushes himself forward with his feet, he will never go forward. However, let's say that there is a rope. He holds on to this rope. Even if the conveyor belt goes really fast backwards, he never goes backwards. He starts pulling on the rope... he goes forward regardless of how fast the conveyor belt is going. And he pulls himself forward just as fast as if the conveyor belt was not there (ignoring friction).
But the airplane is not hooked to a rope... yes yes. Well... the idea here is that there is a second medium used for propulsion. When you have thrust, it works the same way. Take a medium that is relatively stationary (rope or air), and apply force to it.
Lets say the conveyor belt is going 100 mph backwards. To stay stationary, the plane does not need to use the same amount of thrust it would to normally get 100mph of ground speed. It just needs enough thrust to defeat the wheel's mechanical friction... and then the wheels start to freewheel. All of the conveyor belt's efforts get spent on spinning the wheels, instead of moving the plane backwards.
Lohman446
12-07-2005, 01:51 PM
I was thinking the exact same thing. Then I thought about it this way:
Lets say a guy is standing on a skateboard on a conveyor belt. If he pushes himself forward with his feet, he will never go forward. However, let's say that there is a rope. He holds on to this rope. Even if the conveyor belt goes really fast backwards, he never goes backwards. He starts pulling on the rope... he goes forward regardless of how fast the conveyor belt is going. And he pulls himself forward just as fast as if the conveyor belt was not there (ignoring friction).
But the airplane is not hooked to a rope... yes yes. Well... the idea here is that there is a second medium used for propulsion. When you have thrust, it works the same way. Take a medium that is relatively stationary (rope or air), and apply force to it.
Lets say the conveyor belt is going 100 mph backwards. To stay stationary, the plane does not need to use the same amount of thrust it would to normally get 100mph of ground speed. It just needs enough thrust to defeat the wheel's mechanical friction... and then the wheels start to freewheel. All of the conveyor belt's efforts get spent on spinning the wheels, instead of moving the plane backwards.
Yeh, I've been proven an idiot, I thought the question was too elementary and the answer too easy. I should have known to stay away from this thread :D
:cheers:
Thanks for the examples though, great ones because I finally understand it fully :) . One would have thought I would have been able to get it from the skateboard example etc but the escalator one, well aside from laughing when I considered it, crystalized the concept for me.
Miscue
12-07-2005, 01:53 PM
OK! Best analogy ever!
We've all been on an escalator...and most of us have tried to walk "up the down", right? OK, imagine the stairs matching your speed, keeping you in one position, right? No matter how fast you run up those stairs, the downward travel will keep you "right there".
Now have a friend reach out and grab your arm as HE rides the UP stairs....guess which way you are going, regardless of the down moving stairs? The external and independant force (your friend/jet engines) will make the movement of your feet and the down stairs irrelevant, as the independant force will move you up.
Run as fast as you want, you are no longer slaved to the steps, but to your friend.
Hehe. Well... how about a people-mover going 50mph (faster than you can skate) and a guy on roller blades... while his friend that's not on the people-mover pulls him.
I'm an idiot?? :confused:
What I should have said - the speed of the plane in relation to the air around it (the stationary ground, etc.) is all that is important. The treadmill will not reduce or increase the air speed the plane needs to achieve to take flight.
You do understand, by your mind is denying it :D
The treadmill does NOTHING in relation to the engine thrust. The engines themselves will attain take-off speed...the plane will naturally follow:)
Miscue
12-07-2005, 01:56 PM
Yeh, I've been proven an idiot, I thought the question was too elementary and the answer too easy. I should have known to stay away from this thread :D
:cheers:
Thanks for the examples though, great ones because I finally understand it fully :) . One would have thought I would have been able to get it from the skateboard example etc but the escalator one, well aside from laughing when I considered it, crystalized the concept for me.
I didn't get it either at first... I don't think the explanations were clear. And part of the problem is how the question is worded.
Lohman446
12-07-2005, 02:00 PM
You do understand, by your mind is denying it :D
The treadmill does NOTHING in relation to the engine thrust. The engines themselves will attain take-off speed...the plane will naturally follow:)
The part of the engines is what threw me. I guess I did not see (it was probably said) or comprehend at first that the treadmill would nto keep the plane from moving forward. Because everyone wanted to talk about the engine thrust my mind latched onto that. The engine thrust forces movement but not flight. I kept thinking people were arguing that the thrust of the engines created flight which we know is not true - the thrust of the engine creates movement (of the air over the wings) which creates flight. My mind wanted to deny that the plane would be mvoing because of the treadmill - which was obviously wrong.
billybob_81067
12-07-2005, 02:09 PM
The part of the engines is what threw me. I guess I did not see (it was probably said) or comprehend at first that the treadmill would nto keep the plane from moving forward. Because everyone wanted to talk about the engine thrust my mind latched onto that. The engine thrust forces movement but not flight. I kept thinking people were arguing that the thrust of the engines created flight which we know is not true - the thrust of the engine creates movement (of the air over the wings) which creates flight. My mind wanted to deny that the plane would be mvoing because of the treadmill - which was obviously wrong.
You have been converted!
Haha, reminds me of that Simpsons episode when Bart was playing that Christian video game called "Bible Blaster" with Rod and Tod and they had to "convert the heathens"
LOL
What a great show :)
My friend who is a senior at Purdue majoring in Aeronautical/Astronautical Engineering said it will not take off, and I believe him. he said it will not gain any air speed and it wont have any lift at all. he also game me this equation "L=.5*cl*rho*U^2*s. U is wind velocity. If U = 0, L=0, L is lift force, cl is lift coeficient, rho is air density, U is freestream velocity, and s is wing span"
i win
Haha good for him. I'm a '98 BS Aeronautical Studies from the best flight school in the world University of North Dakota. I hold a Commercial, CFI, CFII certificate SEL. The example of taking off on an icy runway was a real-world example that I have done in my life.
I think this issue has already been answered enough. The airplane will fly.
ScatterPlot
12-07-2005, 07:16 PM
Wow I felt like an idiot after the first about four posts. I thought no at first, then I was like, duh, yeah it will. For any people still needing an explanation, here's my go at it:
The wheels cannot produce any force other than straight up. Simple statics concept, if you try to push the ground under a cart then the cart won't move (assume frictionless wheels). The treadmill, therefore, cannot exert any force upon the airplane. The airplane in turn pushes against the air. Since there is a force pushing the plane to the left (say it's facing left) and nothing on the right (since the wheels can only push upwards, any other direction and they rotate) there is a net force to the left. What do imbalanced forces do? Induce motion. Hence, the plane will take off because it will start moving and behave EXACTLY like normal. There is a groundspeed, the plane does move sideways before taking off. The belt is irrelevant.
Example...
Say the belt is already moving at 100,000 miles an hour. With NO friction, will the plane move? No, so why would it not move when the plane is going 2 miles an hour in the opposite direction? 10 miles an hour? 300 miles an hour?
Nother example...
Say there was a hovercraft with wings. Put the hovercraft on a runway. Can it still move? Yes.
Cow hunter
12-07-2005, 07:44 PM
looked it up/...... doesnt sound right but sayz the 747 has a takoff speed of 500mph...... maybe...... or not.......
neppo1345
12-07-2005, 10:00 PM
Haha good for him. I'm a '98 BS Aeronautical Studies from the best flight school in the world University of North Dakota. I hold a Commercial, CFI, CFII certificate SEL. The example of taking off on an icy runway was a real-world example that I have done in my life.
I think this issue has already been answered enough. The airplane will fly.
Whats scary is that one day...you may be flying something that he him or myself have designed :)
Except I got the question right...
benzy2
12-08-2005, 01:10 AM
At very first i was thinking that the thrust was dependent on wheel speed. Once the first guy said it would fly it made sence that it would move forward no matter how fast the ground under it was moving backwards. As long as the treadmill doesnt go so fast to bust something or cause so rediculously great amount of friction that the engines cant over come it the plane, as most have agreed on, will fly. The jets thrust air having nothing to do with the ground or its speed.
anomoly40
12-08-2005, 02:08 AM
Haha good for him. I'm a '98 BS Aeronautical Studies from the best flight school in the world University of North Dakota.
Well he's just BS'in his way through it.
That joke never gets old. :rofl:
bjjb99
12-08-2005, 08:52 AM
Interesting puzzle.
From a purely physics standpoint, the wheels decouple the 747 from the conveyor belt runway, and the plane just takes off as it normally would. The wheels would just spin at twice their normal rate as the 747 rolls down the conveyor belt during takeoff.
Cow Hunter posted that the takeoff speed of a 747 was around 500 miles per hour. This seemed high to me, so I did a bit of digging. According to the following link:
http://www.airnewzealand.co.nz/aboutus/fleet/aircraft_statistics.htm
the takeoff speed is around 250 kilometers per hour, or about 155 miles per hour. This link:
http://www.aerospaceweb.org/question/performance/q0088.shtml
lists the takeoff speed to be 180 miles per hour. I think we can safely say that the takeoff speed for a 747 is at least 150 miles per hour, which will be important later in this post.
I also came across something interesting from Boeing's website. They have some PDF files listing various performance characteristics of aircraft for the purposes of airport planning. The section on a 747-400, the most common variant these days, lists a tire speed limit of 235 miles per hour.
If the plane is moving in one direction at some speed "V" and the conveyor belt is moving in the opposite direction at that same speed "V", the wheel speed will be 2V. Since 235 miles per hour is the tire speed limit, 2V = 235 mph. V = 117.5 mph. The 747 will exceed its tire speed limit by anywhere from 27 to 53 percent before it reaches its takeoff speed (of 150 or 180 mph). As to whether this will result in exploding tires or not is up for speculation, but if the tires do blow then the plane is no longer decoupled from the conveyor belt.
I'm not so sure the plane can take off from a conveyor belt runway under "real world" conditions, given the above tidbits of information... shame I already cast my vote in the poll.
BJJB
bleachit
12-08-2005, 09:01 AM
if the tire speed limit is 235 mph, then my guess is after prolonged speeds of 235 you will see a failure. This failure will occur more rapidly as the speed increases. However, you may be able to get it up to 260 270 for short periods of time before seeing a failure, possible causing permanent tire damage making it unsafe to land.
well thats my speculation. safety is usually built into those numbers.
If you want to analyze it to that degree, you would have to take in consideration that the amount of friction on the wheels is changing as the 747 is gaining speed. Once those wings start creating lift, which is long before takeoff, the amount of friction would start to decrease.
benzy2
12-08-2005, 12:15 PM
If you want to analyze it to that degree, you would have to take in consideration that the amount of friction on the wheels is changing as the 747 is gaining speed. Once those wings start creating lift, which is long before takeoff, the amount of friction would start to decrease.
Also you would have to account for which direction the wind was blowing. You would also need to take into consideration altitude of the takeoff and many other things. If we really go by real world situations you would need to take into consideration all real workd factors and not just the capability of the tires.
epterry
12-08-2005, 12:23 PM
Interesting puzzle.
From a purely physics standpoint, the wheels decouple the 747 from the conveyor belt runway, and the plane just takes off as it normally would. The wheels would just spin at twice their normal rate as the 747 rolls down the conveyor belt during takeoff.
Cow Hunter posted that the takeoff speed of a 747 was around 500 miles per hour. This seemed high to me, so I did a bit of digging. According to the following link:
http://www.airnewzealand.co.nz/aboutus/fleet/aircraft_statistics.htm
the takeoff speed is around 250 kilometers per hour, or about 155 miles per hour. This link:
http://www.aerospaceweb.org/question/performance/q0088.shtml
lists the takeoff speed to be 180 miles per hour. I think we can safely say that the takeoff speed for a 747 is at least 150 miles per hour, which will be important later in this post.
I also came across something interesting from Boeing's website. They have some PDF files listing various performance characteristics of aircraft for the purposes of airport planning. The section on a 747-400, the most common variant these days, lists a tire speed limit of 235 miles per hour.
If the plane is moving in one direction at some speed "V" and the conveyor belt is moving in the opposite direction at that same speed "V", the wheel speed will be 2V. Since 235 miles per hour is the tire speed limit, 2V = 235 mph. V = 117.5 mph. The 747 will exceed its tire speed limit by anywhere from 27 to 53 percent before it reaches its takeoff speed (of 150 or 180 mph). As to whether this will result in exploding tires or not is up for speculation, but if the tires do blow then the plane is no longer decoupled from the conveyor belt.
I'm not so sure the plane can take off from a conveyor belt runway under "real world" conditions, given the above tidbits of information... shame I already cast my vote in the poll.
BJJB
I was about to say something like that. I would say that the answer could be yes or no and it would depend on several things. This is a very short list and even then it is just about the tires:
1 At what speed/load combination do the tires blow?
2 If the tires do not go then at what speed do the bearings go?
3 Is the conveyer moving in a -1 to 1 ratio or does it just speed up as the planes actual ground speed (not the conveyer) increases? In this case the wheels would definitely go.
4 Would it take the jets more time to spool up?
5 Would that mean that it spent more time on the ground, changing tire rating?
6 Would the conveyer cause a different tire rating then concrete?
7 would the tires and the conveyer create increased rotational friction?
8 What about temp(if it was really really really cold say -100C then what of the tires)?
9 What elevation (by increasing takeoff speed then the tires may go)
That is as many as I can think of off the top of my head, but that is all I need to say that this question needs a lot more details. Oh and if it had the necessary details, I would not be qualified to answer anyway.
Heh I just thought of a good way to demonstrate a little thing about tires. Put a hot-weels car on a grinder... heh
epterry oh sorry about the nasty grammar I am a little sick and therefore not all here today.
billybob_81067
12-08-2005, 03:24 PM
I think I'd worry more about a ginormous conveyor runway blowing up at over 100mph than the tires on the plane blowing up at 235 mph.
That's just me though.
;)
Heh I just thought of a good way to demonstrate a little thing about tires. Put a hot-weels car on a grinder... heh
epterry oh sorry about the nasty grammar I am a little sick and therefore not all here today.
Yes, very good way!
But notice that you must hold the car onto the grinder (your hand/jet engines). Now, simply roll the car around the grinder...easy, right? Nothing stopping you from doing that, as the wheels easily match whatever speed the grinder is doing, AND what YOU induce. Now, here's the tricky part....
....take the car off the grinder while making the "PTPTPTPTPTPTPT" noise to simulate a take off using the propulsion from your hand/jet engine. Hey look, the car is flying!
Guys, it was a trick question that had NOTHING to do with physics, calculus, wind shear, Toby Kieth, or godzilla spit. The belt and wheels mean NOTHING.
I think eveyone is overengineering this.
the statement only says that the plane goes with the conveyor belt. it mentions nothing of the planes jet engines actually engaging. so, as far as the plane taking off, I dont believe it would take off in the sense it would fly. It would probably eventually get out of control and crash off the conveyor belt if it is not secured down say for example a car on a dyno.
Amazing....
J, the plane IS moving (re-read the original post). The ONLY way the plane can possibly move, is by its engines. The ONLY way the plane can actually fly, is for the engines to push it to take off speed.
There is NO way the wheels or belt will have anything to do with that.
Forward motion is NOT dependant on the belt or wheels. Heck, you could have snow-skis on each strut, the plane will still fly.
THIS JUST IN!!!!
I have no idea why it didn't dawn on me before!!!
Float planes, lacking wheels, quite often land on MOVING RIVERS. The plane...WITHOUT WHEELS AND AGAINST THE CURRENT....still flies.
THIS JUST IN!!!!
I have no idea why it didn't dawn on me before!!!
Float planes, lacking wheels, quite often land on MOVING RIVERS. The plane...WITHOUT WHEELS AND AGAINST THE CURRENT....still flies.
I kid you not...I came on here to post that very thing!!
Good call Army.
slateman
12-08-2005, 11:10 PM
http://i18.photobucket.com/albums/b128/hiphopgamer26/HHG/rly.jpg
yup http://img105.imageshack.us/img105/3637/dropbearcopy4lp.jpg (http://imageshack.us)
http://img431.imageshack.us/img431/8098/boomyarly4jp.jpg
Oh and I asked my uncle, who is a pilot in the Air Force and has a Masters degree in aerodynamics, and he said
No. The plane will not take off. Only the wheels are moving. No airflow (other than the wind) is going over the wings to generate lift. Without airflow over the wings, lift cannot be generated and thusly the plane cannot take off. If I understand the question correctly, the wheels of the 747 will rotate as the treadmilling runway moves underneath the jet but now airflow is going over the wings.
Target Practice
12-08-2005, 11:20 PM
Oh and I asked my uncle, who is a pilot in the Air Force and has a Masters degree in aerodynamics, and he said
You are not explaining the quesiton correctly.
I'll have to draw a picture.
Warewolf50
12-09-2005, 01:00 AM
did anyone actually read the link on like the 2nd page a guy put a toy plane on a treadmill and it took off wiht the treadmill running ad full spead.
IT'S A TRICK QUESTION! IT WAS MEANT TO CONFUSE YOU ABOUT THE WHEELS AND CONVEYOR. THEY HAVE NOTHING TO DO WITH THE FORWARD MOTION OF AN EXTERNALLY POWERED AIRCRAFT. THE 747 HAS NO CHOICE BUT TO ACCELERATE TO FLIGHT SPEED, REGARDLESS OF HOW FAST THE CONVEYOR OR WHEELS ARE TURNING!!!
Target Practice
12-09-2005, 01:08 AM
IT'S A TRICK QUESTION! IT WAS MEANT TO CONFUSE YOU ABOUT THE WHEELS AND CONVEYOR. THEY HAVE NOTHING TO DO WITH THE FORWARD MOTION OF AN EXTERNALLY POWERED AIRCRAFT. THE 747 HAS NO CHOICE BUT TO ACCELERATE TO FLIGHT SPEED, REGARDLESS OF HOW FAST THE CONVEYOR OR WHEELS ARE TURNING!!!
quotin' dis
Oops...did I shout that?
:D
SuiciDal Sn Y p ER
12-09-2005, 01:53 AM
does anyone feel somewhat smarter by understanding the physics of this? or am i just learning useless information?
http://img431.imageshack.us/img431/8098/boomyarly4jp.jpg
Oh and I asked my uncle, who is a pilot in the Air Force and has a Masters degree in aerodynamics, and he said
http://img360.imageshack.us/img360/2031/owlorry8eq.jpg
billybob_81067
12-09-2005, 02:28 AM
I'll have to draw a picture.
Will this one suffice? :)
http://i9.photobucket.com/albums/a87/billybob_81067/plane.jpg
Oh and I asked my uncle, who is a pilot in the Air Force and has a Masters degree in aerodynamics, and he said
Well, my best friend happens to also be a pilot in the Air Force (F-16s, C-130s) and I work on C-130 simulators, and we both disagree with your uncle. It is obvious that he did not understand the question.
If you've never been on an airplane, it's difficult to imagine the amount of thrust that they put out. Even for a turbo-prop like a C-130.
Instead of a 747 in the example, picture a C-5 Galaxy. If you've ever seen one of those bad boys take off, I guarantee that you were amazed at how in the hell it even made it off the ground.
zaqwert6
12-09-2005, 10:42 AM
Hmmm...
5 pages and the complete explaination and obvious, unneccessary (so it would seem) answer was given in the link on Page 2.
And AO is supposed to be the 'smarter' Forum. :rolleyes:
Why does anyone think that free spinning wheels, with no mechanical connection to an airplane will slow down it's forward excelleration? The plane simply fires the engines and moves forward at normal take off speed...assuming the brakes aren't on of course. :D The conveyor has no impact on it's forward speed what so ever.
Lohman446
12-09-2005, 11:18 AM
Hmmm...
5 pages and the complete explaination and obvious, unneccessary (so it would seem) answer was given in the link on Page 2.
And AO is supposed to be the 'smarter' Forum. :rolleyes:
Why does anyone think that free spinning wheels, with no mechanical connection to an airplane will slow down it's forward excelleration? The plane simply fires the engines and moves forward at normal take off speed...assuming the brakes aren't on of course. :D The conveyor has no impact on it's forward speed what so ever.
It was a pretty hard answer to grasp, even when explained and it took several examples for me to fully "get" it.
/Then again, I could be slow.
army,
I re-read after I posted it and debated it a bit with rob. I understand the explanation that you and miscue both gave, but I guess I am having a hard time seeing it in my head. I guess the only reasonable explanation is to call the myth busters and let them hash it out.
I would love to see a giant conveyor belt with the 747 on it. the potential for disaster just makes me tingle inside.
bleachit
12-09-2005, 11:28 AM
It was a pretty hard answer to grasp, even when explained and it took several examples for me to fully "get" it.
be that as it may, there was a link 3 pages ago as to why it would fly, and folk still argued that it wouldnt. I believe that was the of zaqwert6
slateman
12-09-2005, 11:35 AM
Well, my best friend happens to also be a pilot in the Air Force (F-16s, C-130s) and I work on C-130 simulators, and we both disagree with your uncle. It is obvious that he did not understand the question.
If you've never been on an airplane, it's difficult to imagine the amount of thrust that they put out. Even for a turbo-prop like a C-130.
Instead of a 747 in the example, picture a C-5 Galaxy. If you've ever seen one of those bad boys take off, I guarantee that you were amazed at how in the hell it even made it off the ground.
This is exactly what I wrote him:
A plane (747 passenger jet) is sitting on a runway
that can move (some sort of band conveyor or a giant
treadmill). The plane moves in one direction, while
the conveyor moves in the opposite direction. This
conveyor has a control system that tracks the planes
speed and tunes the speed of the conveyor to be
exactly the same (but in the opposite direction).
The question is:
Will the plane (747 passenger jet) take off or not?
He sent me the answer I previously stated. BTW - He DOES fly C-5s out of Dover. And I've seen them. The reason they take of is because the earth isn't moving as fast as the plane. The 4 giant engines create enough thrust to propel the plane forward. Enough speed = enough lift because of the air going under and over the wings. On a treadmill, you don't get the wind rushing by because you're not actually moving through the air.
SlartyBartFast
12-09-2005, 11:42 AM
He sent me the answer I previously stated. BTW - He DOES fly C-5s out of Dover. And I've seen them. The reason they take of is because the earth isn't moving as fast as the plane. The 4 giant engines create enough thrust to propel the plane forward. Enough speed = enough lift because of the air going under and over the wings. On a treadmill, you don't get the wind rushing by because you're not actually moving through the air.
Sorry, but that just proves that even the ignorant can fly planes... :rolleyes:
What the heck does the earth's rotation have to do with aircraft take-off? :confused:
neppo1345
12-09-2005, 11:43 AM
PWNED...ALL OF YOU (http://www.avweb.com/news/columns/191034-1.html)
"Because the conveyor cannot stop [the plane] from moving forward. There is nothing on the airplane that pushes against the ground or the conveyor in order for it to accelerate; as Karen -- one of our techies here at the Lounge -- put it, the airplane freewheels. In technical terms, there is some bearing drag on the wheels, but it's under 40 pounds, and the engine has overcome that for years; plus the drag doesn't increase significantly as the wheel speed increases. Unless Manfred applies the brakes, the conveyor cannot affect the rate at which the airplane accelerates. "
Glickman
12-09-2005, 11:46 AM
He sent me the answer I previously stated. BTW - He DOES fly C-5s out of Dover. And I've seen them. The reason they take of is because the earth isn't moving as fast as the plane. The 4 giant engines create enough thrust to propel the plane forward. Enough speed = enough lift because of the air going under and over the wings. On a treadmill, you don't get the wind rushing by because you're not actually moving through the air.
yes the engines create thrust to create speed to create lift.
but going back to miscue...
that doesnt mean that you cant use thrust alone as propellant. i could do the calculation to find out how much it would need but that would involve not being lazy :D
all you people with "family within the airforce":
Yea, ur friends are right, but thats only considering that the only possible way to get lift off is with lift, which is not physically true. Most effecient yes, but not the only way
bleachit
12-09-2005, 12:53 PM
all you people with "family within the airforce":
I wonder how many of them took a physics or engineering course while learning to fly?
very few if any. knowing how to fly and knowing why you fly are two different things.
Lohman446
12-09-2005, 12:57 PM
I wonder how many of them took a physics or engineering course while learning to fly?
very few if any. knowing how to fly and knowing why you fly are two different things.
My dad's a private pilot, and I have seen the course work to get licensing, most pilots know why, or at least passed a "ground test" on it.
This is a little off-topic, but since we've already answered the original question, here is another one. Most pilots should be able to answer this in around 4 words:
What makes an airplane turn?
**Edit - the answer is not: The pilot**
bleachit
12-09-2005, 01:10 PM
My dad's a private pilot, and I have seen the course work to get licensing, most pilots know why, or at least passed a "ground test" on it.
they may know the basics, as they are taught basics. something more advanced, such as this, is not something they would normally know.
they may know the basics, as they are taught basics. something more advanced, such as this, is not something they would normally know.
Actually, as Army has stated, bush pilots take off of moving rivers all the time. They know what the impact is of taking off of a "moving runway"
Physics, schmisics. Why does an airboat drive equally well on ice and water? The surface medium has nothing to do with it's movement. Why? It's propulsion system is not tied to that medium. It operates in a different medium...air. The air is not moving.
Here is a scenario that would not allow the 747 to fly. Add a tailwind that is equal to the plane's velocity. The plane WILL NOT FLY.
Do you see it now?
SlartyBartFast
12-09-2005, 01:18 PM
Add a tailwind that is equal to the plane's velocity. The plane WILL NOT FLY.
And when that happens while you're in the air, it called "dead" air. Been there, 767, done that, not pleasant.
ScatterPlot
12-09-2005, 01:41 PM
This is a little off-topic, but since we've already answered the original question, here is another one. Most pilots should be able to answer this in around 4 words:
What makes an airplane turn?
**Edit - the answer is not: The pilot**
I'm not pilot here, and unless this is a trick question, could it be "aelerons, rudder, and elevators"?
Cause from what I know of planes and stuff, there's a bunch of ways it could "turn." If you mean like the yaw, then just the rudder could do it, or you could pitch to one side and use the elevators to do it, or I guess if you had engines on each side of the plane controlled separately (like not a Cessna) then that could do it, or maybe the answer is "the wind" or something... Am I overanalyzing this? :D
Anywho, back to you who don't get the question and whoever that was with the uncle and whatnot. You seem to think the plane will not move. The plane will not take off if it is not moving, but the issue here is that the conveyor belt will not impede movement. Think of it this way- with frictionless wheels and a belt going a hundred thousand miles an hour, will the plane be moving? No, because the wheels are frictionless. Now add a teensie bit of thrust to that, the plane will move a teensie bit. Add a lot of thrust, and the plane will take off.
Your uncle might be a pilot, but the question here is not whether or not there can be lift at zero speed. Obviously no. Tell him some of the responses seen here, he will probably hit himself in the forehead with a resounding "duh" like I did when I figured it out.
slade
12-09-2005, 05:33 PM
after looking over the question, i realized its not worded that clearly and there are multiple ways of looking at it. we dont know much about the "tracking system". To hopefully clear everything up, i decided to post this.
method 1: it tracks the wheel RPM, and uses the wheel diameter to calculate the MPH of the wheels, and sets the belt to rotate at this speed. if this is the method used, when the plane starts and moves forward, its wheels will begin to rotate, and the belt will begin to move. the belt will speed up the wheels, and will soon far exceed their mas RPM, break, and the plane will fall down and crash. since the belt speed matches the wheel RPM, the plane CANNOT move forward. the belt just keeps increasing rapidly until the wheels break.
method 2: the potential speed of the plane is tracked. the speed the plane would go if on a normal track is recorded and the belt moves at the same speed (relative to time) in the opposite direction in the actual trial. the wheels just spin at twice the RPM and the plane takes off. this is how most people are looking at the question.
*forget method 3. for all practical purposes its almost the same as 2*
SlartyBartFast
12-09-2005, 06:18 PM
To hopefully clear everything up, i decided to post this.
method 1: ...
method 2: ...
method 3: ...
Clear things up?!?
You realise that each of those methods is exactly the same, right?
RPM of wheels at take-off speed, speed compared to a normal take-off, or actual speed from a radar gun.
ALL result in the wheels doing twice the RPMs that they would without the moving surface.
slade
12-09-2005, 06:31 PM
Clear things up?!?
You realise that each of those methods is exactly the same, right?
RPM of wheels at take-off speed, speed compared to a normal take-off, or actual speed from a radar gun.
ALL result in the wheels doing twice the RPMs that they would without the moving surface.
re-read my post slowly...
the first method is the speed of the wheels. the speed of the wheels will be increased by the belt rotating. if the linear speed of the wheels exactly matches that of the belt, the wheels CANNOT move forward and thus the plane cannot move forward. note i did NOT say RPM of the wheels at takeoff speed. i said the actual linear speed of the wheels in the experiment.
the second method is the belt spins at the speed of the plane in a normal takeoff. this results in the wheels rotating at about twice their normal rate.
the third method is the belt speed adjusts to match the actual forward speed of the plane, as compared to the ground. the friction from the belt will slow the plane down somewhat, although not much, so it wont be exactly the same. it will be close to method 2... ehh, maybe i should have just not mentioned it. although it is a different interpretation of the question, they all are valid interpretations.
ntn4502
12-09-2005, 06:33 PM
I be pilot...
its kind of tricky, but this is what would happen:
as the jets increase their thrust the plane would speed further down the conveyor belt until it reaches its Vr speed, or speed in which the plane will take off
/the belt has no effect on creating the lift
//tires would be doubling in speed...possible failure
/// would need a conveyor belt as long as the normal runway
////picture a kid wearing a jet pack and roller blades on a treadmill
zaqwert6
12-09-2005, 06:43 PM
It was a pretty hard answer to grasp, even when explained and it took several examples for me to fully "get" it.
/Then again, I could be slow.
Well there are 2 issues here and I don't mean any real disrespect but....
1) Inbility to grasp the fact that airplanes don't use thier wheels to accelerate/go forward...should be obvious to anyone who has seen a plane fly without it's wheels touching hte ground. :)
B) Inability to 'read' a link posted 3/4 pages ago containing the complete thorey, example and practical explaination.
I won't pretend to know why either of , or apparently both of those two would be. :D
SlartyBartFast
12-09-2005, 06:47 PM
re-read my post slowly...
Rethink your position slowly.... :rofl:
the first method is the speed of the wheels. the speed of the wheels will be increased by the belt rotating.
But it's one to one. THe belt MATCHES the wheel rotation.
if the linear speed of the wheels exactly matches that of the belt, the wheels CANNOT move forward and thus the plane cannot move forward.
Hold a hot wheels car on a belt sander. Rapidly increase the speed of the belt. At what point does the belt stop you from being able to mover the car forward? NEVER.
note i did NOT say RPM of the wheels at takeoff speed. i said the actual linear speed of the wheels in the experiment.
Your missing the very basic concept. Belt speed is irrelevant. No matter what the belt does, the plane will take off at normal take off speed, in the normal length of runway. Jus the wheels will be doing twice the RPM.
The belt cannot slow or stop the plane.
the second method is the belt spins at the speed of the plane in a normal takeoff. this results in the wheels rotating at about twice their normal rate.
Which is the same as the first. Please tell me how it can't be.
the third method is the belt speed adjusts to match the actual forward speed of the plane, as compared to the ground. the friction from the belt will slow the plane down somewhat, although not much, so it wont be exactly the same. it will be close to method 2... ehh, maybe i should have just not mentioned it. although it is a different interpretation of the question, they all are valid interpretations.
WHAT THE HOOBY is different between the "actual" forward speed, linear speed calculated through RPM, and speed at normal take off?
slade
12-09-2005, 07:21 PM
Rethink your position slowly.... :rofl:
But it's one to one. THe belt MATCHES the wheel rotation.
Hold a hot wheels car on a belt sander. Rapidly increase the speed of the belt. At what point does the belt stop you from being able to mover the car forward? NEVER.
Your missing the very basic concept. Belt speed is irrelevant. No matter what the belt does, the plane will take off at normal take off speed, in the normal length of runway. Jus the wheels will be doing twice the RPM.
The belt cannot slow or stop the plane.
Which is the same as the first. Please tell me how it can't be.
WHAT THE %&^*$ is different between the "actual" forward speed, linear speed calculated through RPM, and speed at normal take off?
*slaps forehead*
gah!
re-read my post again. you obviously missed something. your hotwheels and belt sander idea is completely irrelevant to what i said. the belt sander runs at a constant RPM. its speed is NOT ADJUSTABLE.
first lets state that if the belt starts moving while the plane is off, the plane will certainly move backward due to friction through the wheels.
let me rephrase this. you know what linear speed is, right? its the speed of the outer edge of a wheel. its a function of the RPM and diameter of the wheel. on a belt, the linear speed is the speed of any point on the belt. now, in this way of interpreting the question, the linear speed of the belt matches that of the wheel, as long as the plane is on the ground and not airborne. think of them as interlocked gears. if they match speed one cannot move faster than the other. because of the incredible force when the engines of the jet start, the plane will be pulled forward... but the sensors will pick up the rotation of the wheels, and attempt to match the belt speed to it, so the belt will begin turning... and that turning will cause the wheels to turn faster. the sensors will pick up that higher RPM and speed up the belt... which will cause the wheels to rotate faster. the plane will be slowed down because of the friction from the wheels, but not substantially, so the belt speed will essentially increase rapidly, to the point where the wheels break, and the plane will crash. the system will attempt to keep the plane from moving forward by negating the force of the jet through the RPM of the wheels, but since the friction is neglidgeable compared to the force of the engines, the belt will rapidly increase in speed. This is of course assuming the belt is powered in a manner that can support a plane, but still turn at an incredibly high RPM... im not sure which id hate to fund more, the engineers or the power bill.
i realize that post is sort of confusing, but my logic is not flawed.
think back to your belt sander theory. if you hold the car on the belt sander, the wheels spin at the same speed as the belt. but if you move the car forward, the speed of the wheels will increase slightly compared to the speed of the belt, and what im saying is the belt will speed up attempting to match that speed... but in speeding up, the wheels will speed up to match the belt PLUS the speed you are pushing it forward. the ultimate goal of the setup is to cancel the force you put on the car (or plane) through friction through the interface (wheels and belt). this of course cannot happen perfectly, but if you set it up this way with a very powerful belt system, it will result as i predicted.
as i explained, the second method is the speed in a normal takeoff and the third is the actual speed. i shouldnt have mentioned the third, since it is basically the second with the minimal difference that will come from friction through the belt/wheels. almost everyone is interpreting the question as method 1 or 2. ignore 3, forget i mentioned it.
Shives
12-12-2005, 11:48 AM
The only 2 facts everyone seems to agree on are...
1) Air needs to move across the wings to provide lift.
2) The jet engines don't provide lift or air flow over the wings.
People are pointing out that the tires and conveyor belt act like a frictionless surface. So, the engines should "slide" he plane along despite what the wheels are doing. Other people think the conveyor belt will hold the plane back.
The big problem I see in the discussion is that people want to leave some parts "real world" while other parts "ideal".
I looked at it sort of upside down. The treadmill starts moving. If the planes wheels were frictionless, they would start turning, and the plane thanks to inertia, would stay still. The wheels aren't frictionless, so the plane would start to move, in this case backwards. Now, say the pilot starts to feel this backwards movement and powers up the engines giving it a little forward thrust. The plane is not moving anywhere in relation to the air around the wings. If the conveyor belt speeds up, the pilot will also have to increase thrust to stay stationary. However, he will still be just keeping up w/ the backwards movement the conveyor belt is causing. If this keeps up until the plane is at full thrust, it won't take off. It would be a very fine balancing act, and the conveyor would probably have to move faster than it would to just move the plane at that speed. I would imagine the landing gear would also give out resulting in a huge mess:)
That's my thought. Am I right? I haven't a clue. What is the right answer? Until it can be done in real life, who knows.
Shives
Lohman446
12-12-2005, 12:12 PM
The only 2 facts everyone seems to agree on are...
1) Air needs to move across the wings to provide lift.
2) The jet engines don't provide lift or air flow over the wings.
People are pointing out that the tires and conveyor belt act like a frictionless surface. So, the engines should "slide" he plane along despite what the wheels are doing. Other people think the conveyor belt will hold the plane back.
The big problem I see in the discussion is that people want to leave some parts "real world" while other parts "ideal".
I looked at it sort of upside down. The treadmill starts moving. If the planes wheels were frictionless, they would start turning, and the plane thanks to inertia, would stay still. The wheels aren't frictionless, so the plane would start to move, in this case backwards. Now, say the pilot starts to feel this backwards movement and powers up the engines giving it a little forward thrust. The plane is not moving anywhere in relation to the air around the wings. If the conveyor belt speeds up, the pilot will also have to increase thrust to stay stationary. However, he will still be just keeping up w/ the backwards movement the conveyor belt is causing. If this keeps up until the plane is at full thrust, it won't take off. It would be a very fine balancing act, and the conveyor would probably have to move faster than it would to just move the plane at that speed. I would imagine the landing gear would also give out resulting in a huge mess:)
That's my thought. Am I right? I haven't a clue. What is the right answer? Until it can be done in real life, who knows.
Shives
While realistically the plane has some friction in the bearings the thrust will overcome that bit of friction and create movement when forward power is applied by the engined.
nippinout
12-12-2005, 09:22 PM
The only 2 facts everyone seems to agree on are...
1) Air needs to move across the wings to provide lift.
2) The jet engines don't provide lift or air flow over the wings.
People are pointing out that the tires and conveyor belt act like a frictionless surface. So, the engines should "slide" he plane along despite what the wheels are doing. Other people think the conveyor belt will hold the plane back.
The big problem I see in the discussion is that people want to leave some parts "real world" while other parts "ideal".
I looked at it sort of upside down. The treadmill starts moving. If the planes wheels were frictionless, they would start turning, and the plane thanks to inertia, would stay still. The wheels aren't frictionless, so the plane would start to move, in this case backwards. Now, say the pilot starts to feel this backwards movement and powers up the engines giving it a little forward thrust. The plane is not moving anywhere in relation to the air around the wings. If the conveyor belt speeds up, the pilot will also have to increase thrust to stay stationary. However, he will still be just keeping up w/ the backwards movement the conveyor belt is causing. If this keeps up until the plane is at full thrust, it won't take off. It would be a very fine balancing act, and the conveyor would probably have to move faster than it would to just move the plane at that speed. I would imagine the landing gear would also give out resulting in a huge mess:)
That's my thought. Am I right? I haven't a clue. What is the right answer? Until it can be done in real life, who knows.
Shives
The pilot doesn't get a license in matching thrust to forces acting against forward motion. When you have 200,000 pounds of thrust available, you don't need to worry about a dumb conveyor belt, river, wind, or Mr. T holding the plane back.
bleachit
12-12-2005, 09:33 PM
When you have 200,000 pounds of thrust available, you don't need to worry about a dumb conveyor belt, river, wind, or Mr. T holding the plane back.
sig worthy, if only sigs were around..
cant believe people are still arguing it.. click on the link in the second page.. I think it was the second page.
MantisMag
12-13-2005, 11:26 AM
opening this thread has made me dumber. :cry:
silentdeath55
12-13-2005, 11:42 PM
If I understand this corectly, the plane wont move at all, just as a person running on a tredmill wont move at all no matter how fast they run (if they keep increasing the speed of the belt). Furthermore, aircraft need lift as was mentioned before. If you look up Bernulies (sp?) Principle you will find that fast air creates low pressure, therefore the shape of the wing coupled with the movement of air around it creates low pressure allowing the aircraft to take off.
nippinout
12-13-2005, 11:45 PM
If I understand this corectly, the plane wont move at all, just as a person running on a tredmill wont move at all no matter how fast they run (if they keep increasing the speed of the belt). Furthermore, aircraft need lift as was mentioned before. If you look up Bernulies (sp?) Principle you will find that fast air creates low pressure, therefore the shape of the wing coupled with the movement of air around it creates low pressure allowing the aircraft to take off.
If you run on a treadmill, and a friend shoved your forward, do you stay in place? No, you move forward.
MantisMag
12-13-2005, 11:54 PM
no it would be more like if you were the coyote and you strapped on roller skates and a rocket pack. no matter how fast that treadmill goes do you really think it can stop you with your wheels and rocket pack?
THE PLANE WILL FLY! THE PLANE WILL FLY!! THE PLANE WILL FLY!!!
Anybody not get that yet? Do I need to repeat it? You, in the back? No?
THE PLANE WILL FLY! THE PLANE WILL FLY!! THE PLANE WILL FLY!!!
Any more questions? Anyone? Buehler? Buehler? Buehler?
anomoly40
12-14-2005, 02:16 AM
To the guy who says the earth isn't moving as fast as the plane isn't realizing the earth rotates 360* in 24 hours. Now the earths circumference is approx 25,000 miles. Divide that by 24 and you get about 1040 miles an hour. Now I've seen jets take off just as good east as they can west with the same wind.
nippinout
12-14-2005, 02:22 AM
What if I was on a treadmill, facing backwards, blindfolded, in a vacuum, and holding a baby wildebeast?
nippinout
12-14-2005, 02:24 AM
To the guy who says the earth isn't moving as fast as the plane isn't realizing the earth rotates 360* in 24 hours. Now the earths circumference is approx 25,000 miles. Divide that by 24 and you get about 1040 miles an hour. Now I've seen jets take off just as good east as they can west with the same wind.
Don't forget the Earth's orbit speed around the sun. Might as well factor in Einstein's frame-dragging while you're at it.
bleachit
12-14-2005, 02:35 AM
THE PLANE WILL FLY! THE PLANE WILL FLY!! THE PLANE WILL FLY!!!
Anybody not get that yet? Do I need to repeat it? You, in the back? No?
THE PLANE WILL FLY! THE PLANE WILL FLY!! THE PLANE WILL FLY!!!
Any more questions? Anyone? Buehler? Buehler? Buehler?
wait a minute... I disagree....
you say the plane will fly? but the mill.. or tread.... thermodynamics..... what if the treadmill got some grass on it from when they mowed the lawn?
maybe it COULD fly if it was one of those airplanes with the pointy noses... but the fat round ones... not a chance in hell.
DIE THREAD DIE!!
its German for "The thread, the"
LS1 WS6
12-14-2005, 05:13 AM
This thread has made me laugh so hard!! My first thought was that it would not fly. Then realizing ( as this thread has clearly pointed out) that the thrust and wheel motion is not related. The plane will fly. A car would go nowhere and a plane or something that is not wheel driven will move. :cheers:
Glickman
12-14-2005, 09:51 AM
NEITHER! you're all wrong!
the plane cannot take off cause it isnt moving , neither is the tredmill :p
:dance: :dance: :dance: :dance:
RRfireblade
12-17-2005, 12:57 PM
Mmm.....Candy !!
http://www.oldtimecandy.com/images/assortments/assortment.jpg
Will Wood
12-17-2005, 06:13 PM
Candy Buttons! Damnit.
That's what they were called.
I was so having an in depth discussion with my classmates the other week trying to remember what they were called.
:ninja:
No. Take-off is dependant on the lift produced by air moving over the wings at high speed. the speed at which the wheels are moving is irrelevant.
There is the correct answer.
Alpha
12-18-2005, 08:21 AM
I be pilot...
its kind of tricky, but this is what would happen:
as the jets increase their thrust the plane would speed further down the conveyor belt until it reaches its Vr speed, or speed in which the plane will take off
/the belt has no effect on creating the lift
//tires would be doubling in speed...possible failure
/// would need a conveyor belt as long as the normal runway
////picture a kid wearing a jet pack and roller blades on a treadmill
Too bad the treadmill can put out the same speed as the jet pack can put out in thrust.
Its obvious that the plane could not take off. Relative to the AIR, there is no movement. period. The control system on the conveyor is responcible for this.
Do we all agree that the plane, relative to a fixed point in the air, is NOT moving.
Also, the plane, relative to the ground outside the conveyor runway is NOT moving.
There is NO air moving over the wing, considering there is no wind.
Lift is generated because air over the top of the wing is lower pressue then the air below the wing.
The air pressure is equal right now, because no air is movign over it.
Lift is not generated.
You guys are looking at the wheels as if the wheels create lift. It doesnt matter how fast they are going (unless of course you look at the speed at which they break).
Someone needs to make an animation.
Alpha
12-18-2005, 08:24 AM
This thread has made me laugh so hard!! My first thought was that it would not fly. Then realizing ( as this thread has clearly pointed out) that the thrust and wheel motion is not related. The plane will fly. A car would go nowhere and a plane or something that is not wheel driven will move. :cheers:
If that was true, then the plane would not lift off. I believe the idea of the problem is that the entire plane is not moving
What about if it had props :rofl:
Lohman446
12-18-2005, 08:37 AM
Do we all agree that the plane, relative to a fixed point in the air, is NOT moving.
Also, the plane, relative to the ground outside the conveyor runway is NOT moving.
No we do not agree on that or else we would not think the plane would fly. As has been illustrated in various examples because the plane does not depend on anything that touches the ground to for movement (traction) the treadmill will have little to no effect on it (some bearing resistance, but really not enough to matter once thrust is applied). The plane will move relative to the air around it, the stationary ground, etc.
bleachit
12-18-2005, 08:40 AM
http://www.avweb.com/news/columns/191034-1.html
good thing that link was posted 4 pages ago....
billybob_81067
12-18-2005, 11:35 AM
Why is this even being debated any more!!! Good lord people. Go back and read the past 5 pages of posts and then if you still think the plane won't fly smack/punch yourself in the: face, nose, nuts, or combination thereof.
:tard:
-=Squid=-
12-18-2005, 12:24 PM
Why is this even being debated any more!!! Good lord people. Go back and read the past 5 pages of posts and then if you still think the plane won't fly smack/punch yourself in the: face, nose, nuts, or combination thereof.
:tard:
Careful... This guy's from NASA.
...
You guys are looking at the wheels as if the wheels create lift. It doesnt matter how fast they are going.....
And YOU are looking at it, as if the wheels supply forward motion....THEY DON'T/CAN'T/WON'T!
The engines are going to accelerate to take off speed regardless of what the wheels are doing. The plane has no choice but to follow the engines into the air.
RRfireblade
12-18-2005, 09:33 PM
Too bad the treadmill can put out the same speed as the jet pack can put out in thrust.
Its obvious that the plane could not take off. Relative to the AIR, there is no movement. period. The control system on the conveyor is responcible for this.
Do we all agree that the plane, relative to a fixed point in the air, is NOT moving.
Also, the plane, relative to the ground outside the conveyor runway is NOT moving.
There is NO air moving over the wing, considering there is no wind.
Lift is generated because air over the top of the wing is lower pressue then the air below the wing.
The air pressure is equal right now, because no air is movign over it.
Lift is not generated.
You guys are looking at the wheels as if the wheels create lift. It doesnt matter how fast they are going (unless of course you look at the speed at which they break).
Someone needs to make an animation.
OMG !!
So much wrong there I don't even know where to start...........
RRfireblade
12-18-2005, 09:44 PM
http://www.brooklandsconcorde.com/diary/gear12.jpg
Anyone want to point out where in this picture is the Jet engines attached by drivetrain or transmission to the wheel? :D
Last attempt:
Matchbox car....
Treadmill going 50MPH...
You , holding the matchbox car in your hand, touch the wheels of the car to the moving tread mill....
Does the treadmill rip the car out of your and throw it backwards at 50 mph?
Umm.....no.
Why do you you think is?
Furthermore...
Your holding the car on the treadmill while the wheel are now spinning at teh speed of the tread mill....can you push the car forward on the treadmill?
Umm...yeah.
Why do you think this is?
Now:
Your Hand = Jet Engines abilty to push the car forward.
Anyone still lost on this subject needs to stop posting and go back to kindergarden and start over. :)
bjjb99
12-19-2005, 08:31 AM
Man, and here I thought this thread was bad... check out the 2400+ posts in this one.
http://forum.physorg.com/Plane-on-conveyer...-Will-it-ever-take-off_2417.html
BJJB
Glickman
12-19-2005, 09:38 AM
jebus...
Yes, the plane can take off, and no it can not.
ANY PLANE CAN TAKEOFF WITHOUT LIFT!
granted it needs alot of thrust (and an angle) , but its possible
Lohman446
12-19-2005, 09:41 AM
jebus...
Yes, the plane can take off, and no it can not.
ANY PLANE CAN TAKEOFF WITHOUT LIFT!
granted it needs alot of thrust (and an angle) , but its possible
Tha plane will have lift though....
bleachit
12-19-2005, 09:42 AM
Man, and here I thought this thread was bad... check out the 2400+ posts in this one.
http://forum.physorg.com/Plane-on-conveyer...-Will-it-ever-take-off_2417.html
BJJB
this is ao... if the thread dies, it will be resurected in 6 months and debated all over... in a never ending cycle...
or at least it would have on the older ao.
Glickman
12-19-2005, 09:45 AM
Tha plane will have lift though....
the point im trying to make is that lift is not required, its very helpful, but not required
Somebody please tell me what the SECOND sentence of the question says?
I'll help you...
"THE PLANE MOVES IN ONE DIRECTION...."
This means...in case you need comprehension help.....the plane can, and is moving on the belt.
With the plane moving independantly of the wheels, due to the thrust of the engines against the air------NOT THE BELT OR GROUND OR WHEEL ASSEMBLY----acceleration will happen. Acceleration to flight speed will happen.
Belt speeds, wheel speeds, shopping cart speeds, TSA Security speeds, the moons of Jupiter speeds mean NOTHING to the question.
Here's one last analogy for those who don't know airplanes:
Same conveyor belt, same runway: YOU are standing NEXT to it (not on it). Grasping firmly to a skateboard, you place the wheels onto the belt.
Are you with me so far? You are standing next to the belt, holding the wheels of a skateboard to it. Got it? Follow on then:
Now, still holding the wheels to the belt------walk forward. YES, the wheels roll. YES, the belt now speeds up. But are YOU being rooted to the spot because of this? NOOOOOOOO! You are OBVIOUSLY not, and CANNOT!!!
Why? Because YOUR forward motion is independant of the belt or skateboard. NOTHING is holding YOU back.
Heck, you can even RUN next to the belt, and lift the skateboard into the air!
WHY? Because YOUR forward thrust----JUST LIKE THE ENGINES----has NO bearing on what the wheels or belt are doing.
The damn plane flies....and you get arrested for trespassing.
teufelhunden
01-30-2008, 09:03 PM
Necropost!
Mythbusters is doing this now :)
teufelhunden
01-30-2008, 09:45 PM
They tried it out with an ultralight and a 2000' tarp and the plane was able to take off. I don't know if it took more distance to do so [or if there were other differences], as they didn't note it, but it took off.
VTLO910
01-30-2008, 10:43 PM
Necropost!
Mythbusters is doing this now :)
Keep us posted for when this will be aired please. I wanna see this one!
devildog
01-30-2008, 11:42 PM
I didnt read the whole thread, but here is what i am thinking.
i voted yes by the way
edit, i answered my own question
teufelhunden
01-31-2008, 09:08 AM
Keep us posted for when this will be aired please. I wanna see this one!
It aired last night at 9. I'm sure it'll be rerun 400 times in the next week... tvguide.com
Pneumagger
01-31-2008, 10:08 AM
Whithout reading the multitude of pages I'm going to have say yes it would take off. It's a trick question.
Sure the conveyor belt is moving the opposite direction... but that point is moot. The engines on the wings provide the thrust which drives the plane and produces the relative airspeed difference between the airfoil/air to produce lift forces. The relative speed difference between the plane and air is what is important... this is why aircraft carriers fly INTO the wind and INCREASE headwind by speeding up when planes are taking off. This is also why a "stationary" wing can produce lift in a wind tunnel.
Any opposite directional force like a conveyor belt cannot be realized on the body of the plane because the wheels of plane rotate freely upon takeoff. All that would happen is the wheels would spin twice as fast as the plane took off.
Now if there as a wicked tailwind on the runway...
Pneumagger
01-31-2008, 10:20 AM
New question...
What if the plane is a JSF X-35B (F-35 LightningII STOVL)?
Pneumagger
01-31-2008, 10:26 AM
Whats scary is that one day...you may be flying something that he him or myself have designed :)
Except I got the question right...
I love this thread... so much oportunity for shenanigans.
One day, something you designed will get pwned when something I designed shoots it down.
I win :clap:
txaggie08
01-31-2008, 11:25 PM
Sure the conveyor belt is moving the opposite direction... but that point is moot. The engines on the wings provide the thrust which drives the plane and produces the relative airspeed difference between the airfoil/air to produce lift forces. The relative speed difference between the plane and air is what is important... this is why aircraft carriers fly INTO the wind and INCREASE headwind by speeding up when planes are taking off. This is also why a "stationary" wing can produce lift in a wind tunnel.
they kinda did it on national tv last night dude....
What the ones who argued against it missed are the definitions of "speed"
In order for something to be "going" a certain speed implies forward motion.
SuiciDal Sn Y p ER
02-01-2008, 12:21 AM
can cold temperatures be harnessed to produce energy?
slade
02-01-2008, 01:14 AM
can cold temperatures be harnessed to produce energy?
yes.
and enthalpy can be combined with dark matter to form microfluidic pressure differentials.
slade
02-01-2008, 01:16 AM
(actually, temperature differences can be used to create electricity. I don't see how that's relevant to this thread though. this thread that should have died long, long ago.)
secretweaponevan
02-01-2008, 01:38 AM
No.
Airspeed creates lift.
We can't build a conveyor that can achieve unlimited speeds however, so any "real world" test will always have the plane generate airspeed by out-accelerating the conveyor and take off.
If a plane is flying at 200 mph and encounters a 200 mph tailwind, the plane will drop as its relative airspeed is now zero. Think air pockets. If a plane is on a theoretical treadmill that can keep it from moving forward by exact opposite movement, airspeed stays at zero and I never make it to LAX.
Edit:
If it was possible for a planes to take off from a conveyor belt runway, the U.S. would already be stacking more planes on our aircraft carriers as the current runway space takes up valuable fighter/bomber real estate.
Re-edit:
Yes. The plane will take off. A conveyor cannot keep a plane from gaining forward momentum and therefor airspeed.
If the question was, "If a plane is kept stationary in relation to the air around it, will it take off?" the answer would be no. It is in the interpretation of the question.
slade
02-01-2008, 01:46 AM
No.
Airspeed creates lift.
We can't build a conveyor that can achieve unlimited speeds however, so any "real world" test will always have the plane generate airspeed by out-accelerating the conveyor and take off.
If a plane is flying at 200 mph and encounters a 200 mph tailwind, the plane will drop as its relative airspeed is now zero. Think air pockets. If a plane is on a theoretical treadmill that can keep it from moving forward by exact opposite movement, airspeed stays at zero and I never make it to LAX.
LAX?
get away from me, stalker <.<
acropilot19
02-01-2008, 02:09 AM
Let's assume a plane needs to hit 100mph to take off.
That plane is travelling at 100mph down the conveyor belt, and the conveyor belt moves in reverse at 100mph.
The plane is travelling at 100mph relative to the ground, and 200mph relative to the belt. The plane takes off.
The conveyor belt and its speed is irrelevant. Thrust is produced by the engines, and not at the wheels.
The key to this problem IS: WHAT is causing the forward movement of the aircraft, I.E. thrust? It is NOT the interaction of the gear to the ground (this would actually be a form of drag). The engine(s) generate rearward thrust, which pushes against the atmosphere, and the ground. This WILL propell the aircraft forward to an eventual flying speed, irrespective of the interaction of the gear with the ground.
:)
The conveyor belt does not affect the thrust output of the engine(s). Newton's 3rd law is still in effect (for every action, there is an equal & opposite reaction).
:cool:
An example was given by Nippinout of an automobile with wings. If the forward thrust was geneated through the wheels, then the car would NOT take off, because the conveyor is counteracting (I.E. equal & opposite to) the force of the forward thrust.
Problem Solved.
;)
God, I LOVE this stuff!
slade
02-01-2008, 02:27 AM
...jesus, the thread is dead. the question has been answered repeatedly. there's nothing more to be said on the subject.
acropilot19
02-01-2008, 10:57 AM
...jesus, the thread is dead. the question has been answered repeatedly. there's nothing more to be said on the subject.
I know, I got suckered in before I realized how old it was, and how long. :tard:
dave p
02-01-2008, 11:54 AM
...jesus, the thread is dead. the question has been answered repeatedly. there's nothing more to be said on the subject.
the only thing left to be said is this: the thing that was really demonstrated here is that what happens when people of average intelligence at best, are lead to believe they are smart their whole life. you get know it alls who cite equations they only half understand. people who highly overestimate their intelligence, and then some plain old stupid people arguing over a point where the answer is clear by intuition and common sense alone. one guy was right from the start. all you future engineers and aerospace people argued him with such conviction and you were wrong. and some of you aspire to careers that will effect the live of thousands, if not millions of people. it is absolutely terrifying that people may die over a design flaw or miscalculation because some arrogant engineer refused to see the truth.
wow, just wow.
Thursday, January 31, 2008
Airplane on a treadmill
I created this blog specifically to make this post. It may be the only post I ever write, but since human ignorance is seemingly unbounded, perhaps it won't be.
I thought that today would be a monumental day for this topic. Today, the Mythbusters debuted their long-awaited "Airplane on a treadmill" episode. For years, physics teachers around the world have cringed in horror at heated internet debates concerning a ludicrous thought experiment. Sadly, half of them recoiled in disgust at the correct arguments. Forum posters signed their names with such epithets as "Ph.D. Aerospace Engineer" and "20-year pilot." Somewhat tellingly, these ego-boosters were most often employed by those delivering the wrong answers. Mythbusters finally attempted to end the insanity by performing the experiment themselves.
AND YET...
The debate rages on. Even after being shown seemingly conclusive evidence of the other side's argument, forum-goers from near and far continued to staunchly defend their own theories.
Here and now, the debate will end. I intend this long-winded article to be the definitive answer to the great AOAT conundrum. No further debate is necessary - simply direct the ignorant people to this page, tell them to read it, and let's all get on with answering more intriguing questions, like does P = NP?
For those of you just joining us, "Airplane On A Treadmill" is a thought experiment in physics. Some consider it a litmus test for assessing one's knowledge of airplane physics. In its most basic form, the experiment is worded thusly:
A plane is standing on a large treadmill or conveyor belt. The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction). Can the plane take off?
The question suffers from many rewordings that muddle much of the debate about the thought experiment. The basic idea is that there's a plane, on a treadmill, and we're going to run the treadmill backwards in an attempt to stop the plane from taking off. And here, at the very beginning of this explanation, is the definitive answer. There are in fact two correct answers to this question:
-No, the plane can't take off.
-Yes, the plane can take off.
Fooled you! But that's just the point. The experiment is meaningless, and the passionate internet debates more so, if we cannot agree on what is truly meant by the question. But don't worry, I won't pull a Lost on you - I do intend to give a truly airtight answer later on. For now though, we need to debate semantics.
Really, we do.
You see, the AOAT confusion all arises from misses - misconceptions, misinterpretations, and misunderstandings. Consider three rewordings of the question:
1) An airplane is sitting at rest on a very powerful treadmill. You are at the controls of the treadmill, while I am at the controls of the airplane. On some signal, I begin to attempt to take flight in the plane, and you attempt to match my speed to try to keep me stationary. Will the plane take off?
2) An airplane is sitting at rest on a very powerful treadmill. You are at the controls of the treadmill, while I am at the controls of the airplane. On some signal, I throttle up the airplane and you turn on the treadmill, and we conspire by our joint effort to try to keep the plane stationary relative to the ground. Will the plane take off?
3) An airplane is sitting at rest on a very powerful treadmill. You are at the controls of the treadmill, while I am at the controls of the airplane. On some signal, I attempt to take flight in the plane, but you match my speed with the treadmill and keep me stationary relative to the ground. Will the plane take off?
Here are the absolute, 100%, bet-your-life-on-it answers to these rewordings:
Yes.
No.
Whoever asked this question is an idiot.
And that's about all this debate comes down to, folks. If we could all agree on one set of rules for the thought experiment, then we ought to be able to make the explanation of the answer clear. As it stands, normally one side has interpretation (1) in mind, and argues vehemently with someone else who has interpretation (2) in mind, and the whole thing blows up into a senseless squabble.
Here are the three core facts that are rock-solid:
A) If the plane remains stationary relative to the ground, it will not take off.
B) If the plane moves relative to the ground, it will take off.
C) The person operating the conveyor belt cannot by himself make the plane remain stationary relative to the ground.
(EDIT: Really, you should substitute the word "air" for ground in the above facts. I use "ground" throughout this post because of a consistent mistake made by "no-flys" in their assumption that the plane remains stationary. It doesn't remain stationary, relative to the ground orthe air. The important point is that it moves relative to the air, not the ground, but I'm assuming throughout this post that there is no significant tailwind or headwind. I discuss the implications of this briefly in the section about windtunnels.)
That's about all you need to know to argue whichever interpretation is appropriate. I'll discuss why these facts are true in a moment. In the meantime, look back at the three re-wordings of the question above.
In (1), the key phrase is "you attempt to match my speed to try to keep me stationary." Since we know from fact (C) that you cannot keep me stationary, it follows from (B) that I will take off successfully.
In (2), we conspire together to keep the plane stationary. This is possible, albeit stupid. We know from fact (A) that I will not take off.
In (3) - and this is the important part - the actions being described are impossible. We know from (C) that the conveyor operator cannot keep the plane stationary. The most powerful conveyor belt the world couldn't do it. David Copperfield couldn't do it. It can't be done. Only
if the pilot "plays along" can the plane be made to remain stationary.
Unfortunately, most of the "no-flys" - the label given to those who argue that the plane won't take off - are assuming that interpretation (3) is what is being asked. They accept that the plane remains stationary, and say it won't take off. The "will-flys" know that the plane can't remain stationary, and say it will take off. Add to the mix a few people who see that in one way, the plane could be forced to be stationary by some pilot-conveyor cooperation, and you've got a deadly internet forum explosion cocktail.
Let's examine the physics behind the three key facts, so that there will be no doubt as to their validity. The first two are pretty easy to follow. Airplanes create lift by causing air to flow over their wings. This airflow is caused by the motion of the wings relative to the air - that can happen in two ways. The first way is to move the plane forward through the air. The second is to blow air against the plane and over the wings. As far as the plane is concerned, these two scenarios are equivalent. So you could put a plane in a very powerful wind tunnel, blow air over its wings, and have it fly stationary relative to the ground. But that's another question.
In our treadmill scenario, the air is stationary relative to the ground, so the plane has to move relative to the ground in order to gain flight. If it doesn't move, it simply won't fly. There will be no airflow over the wings, and there will be no lift. A lot of people get confused here, and think that the original thought experiment is some sort of trick question, and that the propeller of the airplane, or possibly the jet engines, will be blowing air backwards over the wings, which will create lift. While there will be a certain amount of airflow created by the propeller or engines, it is not enough to create flight. I promise you, that's not what the question is asking.
Really, I promise. Please, please stop talking about airflow created by the prop. It isn't part of the question.
So we have facts (A) and (B) well taken care of. If the plane moves, it flies. If it doesn't move, it doesn't fly. The real question is, will it move? Again, the answer is unambiguous - if the pilot doesn't try to make the plane stay still, it won't. If he does, it will. This is always, always the part that confuses people, so stick with me for a few more paragraphs.
When a plane is sitting on a runway, it moves by using its engines. It does not move by any sort of motorized wheel. The propellers or jets create thrust that pushes against the surrounding air and causes the plane to move forward. A plane wouldn't move at all in a vacuum chamber. Compare this to a car, which moves by applying torque to the wheels. A car would drive just fine in a vacuum chamber - at least, as long as the driver could survive (and technically, it would need some sort of air reservoir to provide something to mix with the fuel. An electric car wouldn't have this problem.) However, a car could not drive on a frictionless surface - imagine, for example, that you had your car on a super slippery frozen lake. As you hit the gas, the wheels would simply spin and spin in place, and the car wouldn't move forward. You may even have firsthand experience with this situation if you've ever gotten stuck in a snow bank. In contrast, a plane would have no trouble moving on a frictionless surface. The jet engines or propeller would still push against the air, and the plane would still move forward. If it were on a truly frictionless surface, then you would see the wheels sliding along the ground, not rotating.
I hope those two scenarios clearly illustrate the difference in motive force between cars and planes. Cars create their forward movement from torque applied to the wheels, which push against the ground and create forward motion from friction. Planes create their forward movement from thrust applied to the air, which pushes the plane forward regardless of the surface it is on.
Imagine a plane without wheels. The fuselage would sit on the runway, and as you fired up the engines, it would skid spectacularly along the runway, possibly spewing sparks in its wake and doing quite a number on the body of the aircraft. No matter how fast it was going, the frictional force against the airplane would be constant; friction does not depend on speed! If the engines were strong enough to get the plane up to the critical take-off speed, then it would still take off. The only reason planes have wheels is to reduce this sliding friction. The wheels roll along the runway instead of sliding, and the only friction that the plane feels is in the bearings of the wheels. This is substantially less than the friction that a sliding fuselage would create, and it's a much smoother ride for the passengers as well.
(Edit: Technically, there are some factors that would make the friction change with speed. The classic idealized model called "coulomb friction" doesn't really apply to bearings. As the bearings spun faster and faster, they would generate heat, which would increase the friction slightly on the wheels. However, it would never be enough force to prevent take-off. The only time this would prevent take-off is if the wheels locked up or broke off, and then we'd have a much bigger problem and catastrophic failure.)
So what does this all have to do with treadmills? Well, now let's place our plane on that treadmill and see what happens. If the wheels were perfect - that is, there is no friction in the bearings (and no deformation of the wheels as they spin) - then something interesting happens. When we turn on the treadmill, the plane stays stationary on its own. The wheels simply spin along the track, and impart no force to the plane. If you had a car with frictionless axles, and you disconnected the whole drive train, the same thing would happen to your car.
The only reason that a plane or a car moves backwards on a treadmill is that the wheels are somehow partially locked to the axles. In a plane, this is because of minor friction in the bearings. In a car, it's because of the drive train. If you want the car to stay still, you have to turn the drive train at the proper speed. If you want the plane to stay still, you have to overcome the minor bearing friction. And again, since friction does not change with speed, you don't have to exert any more force at higher speeds. If you run the treadmill at 5mph and turn on the plane's engines just slightly, they will provide enough thrust, pushing against the air, to keep the plane still. If you then increase the treadmill speed to 500 mph, you won't need to adjust the throttle on the airplane - it will remain stationary. That's because it's seeing the same frictional force that it was at 5mph. Thus, it doesn't matter how fast the treadmill is moving - if the pilot does not want to remain stationary, then he won't. It only uses the very first bit of power from the engines to keep the plane stationary. As the throttle is increased from that point, it moves forward just as it would on any other runway. It's pushing against the stationary air!
If you don't believe me, imagine this (or even try it at home): you're standing on a skateboard on a treadmill. You hold onto the handrails of the treadmill and turn it on. Of course, you'll remain stationary (relative to the ground). In fact, you only need to use a very light touch to stay stationary - perhaps a few fingers pressed against the handrails. Crank up the treadmill speed as high as you like. You'll still only need the same light touch to remain stationary. At any time you like, you can move forward - closer to the treadmill console - by simple pulling on the handrails. If you had a jet engine, or super-strong hairdryer, you could use this to propel yourself forward instead of holding onto the handrails. In fact, if you're really careful, you might be able to do this at home with a skateboard and a leafbower, but I doubt you'll have a sensitive enough control of your leafblower thrust to get yourself to remain stationary.
So you see (oh please tell me you see), the conveyor operator cannot force the plane to remain stationary. And if the plane isn't stationary, it can take off.
And yes, if we interpret the question in a different way, and assume that for some reason the pilot is colluding with the conveyor operator and keeping the plane stationary, then the plane can't take off.
But what is the question really getting at, anyway? There are really two "spirits" of the question. In the first, we're asking "can a plane take off with no runway, if I replace the runway with a treadmill?" The answer, as we know now, is no. The plane must move relative to the ground in order to take off. In another deep-meaning of the question, we're asking "is it possible to prevent a plane from taking off, by moving the runway backwards under it?" The answer again is no, you can't prevent it from taking off.
The interesting thing about all this is that in both scenarios, you'd wind up with a plane moving relative to the ground. In the first scenario, you might think you're being clever by allowing a plane to take off from a very small field, by using a treadmill runway. If you actually tried it, you'd be attempting to take off, so the plane would move, and would likely crash into something, or fall off a cliff, or suffer some other catastrophe that you were trying to avoid with questionable physics. In the second scenario, you'd give the plane plenty of room and safety to take off, but attempt to play a practical joke on the pilot by moving the runway backwards, and you'd wind up with a plane in flight, much to your chagrin.
When the "no-flys" saw the Mythbusters episode, they all complained that it wasn't done properly, because the plane didn't remain stationary. But think about it for a moment. No, really think about it, don't just spout about Bernoulli's principle and airflow and all that. In what possible scenario would the plane actually stay still? The only way this can happen is if the pilot is trying to stay still, and this only happens if he just barely applies the throttle, making no attempt to take off. This makes no sense. Either you're trying to prevent him from taking off with your clever and misinformed use of a conveyor belt, or he's trying to defy physics by taking off in a too-small area. There is no scenario in which the plane would realistically stay still. We know what would happen if it did - it would sit on the runway, not taking off, and we'd all stare at each other in an all-too-short silence punctuated by loud exclamations of "I told you so!". But that's not really what the thought experiment is getting at, no matter how you reasonably interpret it. Luckily for all of us, if we agree on the interpretation, reasonable or not, we should all agree on the answer. So let's get back to the next great internet debate, shall we?
This is not mine, I found it for a guy at work that is arguing with me that it will not take off, luke. :spit_take
devildog
02-01-2008, 03:49 PM
what happened in the mythbusters episode just wondering?
Pneumagger
02-01-2008, 05:36 PM
Air is a fluid of some known viscosity. When the conveyer belt moves it causes the air over it move with it at some speed inversely proportional to the distance from the belt. The further away from the belt the air is, the slower it is moved by the belt. This called a boundary layer and develops because the air in contact the actual moving belt is travelling the same speed as the belt itself and viscosity causes the air to (more or less) impart drag on itself.
Theoretically, if the conveyor belt moves fast enough it could creat a large enough high speed boundary layer which generates enough lift at the plane's wings to cause takeoff without the plane moving relative to the ground. Sort of an open tube wind tunnel. Practically this is impossible... but it would work if you gat a conveyor belt moving like a zillion mph and the planes wheels had zero friction.
Stick that in your pipes and smoke it. :dance:
bornl33t
02-02-2008, 04:16 AM
the only thing left to be said is this: the thing that was really demonstrated here is that what happens when people of average intelligence at best, are lead to believe they are smart their whole life. you get know it alls who cite equations they only half understand. people who highly overestimate their intelligence, and then some plain old stupid people arguing over a point where the answer is clear by intuition and common sense alone. one guy was right from the start. all you future engineers and aerospace people argued him with such conviction and you were wrong. and some of you aspire to careers that will effect the live of thousands, if not millions of people. it is absolutely terrifying that people may die over a design flaw or miscalculation because some arrogant engineer refused to see the truth.
wow, just wow.
And you're hanging out with us, that doesn't exactly speak highly of you.
There we're all equal again....
dave p
02-04-2008, 10:09 AM
And you're hanging out with us, that doesn't exactly speak highly of you.
There we're all equal again....
true true, but i dont claim to know anything :) .
Hilltop Customs
02-08-2008, 03:25 AM
This debate seems to be making it's way on every message boards so I tought I'd try it here.
So here it goes.
A plane (747 passenger jet) is sitting on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the planes speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction).
The question is:
Will the plane (747 passenger jet) take off or not?
Wow this question changes everytime I see it. But its all relative to where you measure the motion of the jet from. There are 3 distinct objects to measure the speed from: the conveyor, the air, and the ground off of the conveyor(basically the same as air assuming theres no wind).
If its airspeed the jet will easily take off, the speed measured at the wheels will be 2x the actual speed.
If the groundspeed is considered to be a stable object(the ground) off of the conveyor then the plane will always take off. Once again with the wheels spinning at 2x the rate of a normal takeoff, just like with airspeed.
Now if its groundspeed(assuming that the conveyor is considered the ground) the jet will never take off becasue it is motionless compared to the surrounding air and there will be no lift generated by the wings. The groundspeed in this case would quickly accelerate to infinity and the landing gear would disintegrate. This is because the aircraft is still applying force to the air and trying to accelerate....which is constrained, by the question, not to(because speed of jet - conveyor = 0). The only way to apply a force to negate the forward thrust is friction created by the landing gear, thus the extreme acceleration of the conveyor to the point of failure.
Its a horribly ambiguous question....no wonder everyone doesnt agree on an answer. Good questions have single correct solutions....this on the other hand is a perfect example of a badly worded question that has amazingly wasted a lot of peoples time :rofl:
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