Would the plane take off?

[SlartyBartFast]

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.

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.

[Army]

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]

Lohman, there are so many errors in your post....it's scary.

Yeh... there always is.

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.

[Army]

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]

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]

READ THE ORIGINAL QUESTION! The 747 IS moving forward, while the belt is moving backward!

I'm an idiot??

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.

[Army]

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]

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]

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



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]

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.

[Army]

I'm an idiot??

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


The treadmill does NOTHING in relation to the engine thrust. The engines themselves will attain take-off speed...the plane will naturally follow:)

[Miscue]

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



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]

You do understand, by your mind is denying it


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]

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 :)

[beam]

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.