Airplane on a Treadmill

[stebo0728]

Ok ... this appeals to the Etc. portion of this forum, not the Politics. Thought Id try and lighten the debate abit.

Question: Would an airplane placed on a large scale treadmill be able to take off?

This has been the subject of some pretty heated nerd debates that I have seen. Any takers?

[heliochrome85]

my initial guess, no. planes unlike cars, require air for lift, and to further power their turbine engines to produce thrust. if they were like cars, where the wheels were powered, maybe, but as it stands, the wheels are only for movement on the ground, with any momentum provided during taxiing, by the turbines.

[C-Kwik]

Your question needs clarification. I'll pose a few scenarios though in an attempt to answer what you might be asking.

If the plane is on the treadmill but its velocity with respect to the ground (not the treadmill) is zero, then it will not fly as there will be no air across the wing to provide lift.

If the plane is to use its own thrust and has to try and reach the velocity it needs while on the treadmill, then the plane will fly. Since the thrust from the engines is acting on the air, the movement of the ground beneath it will have a negligible effect. Basically, the only resistance the treadmill will cause is friction at the bearings of the wheel. There would likely be some rotational inertia as well that might act back against the treadmill, but it would likely be really small considering a typical plane will weigh much more than its wheels and the thrust from the engines is likely to be quite strong and should have little problem overcoming those forces. This scenario was actually tested by Mythbusters.

Last scenario is probably not at all what you are asking, but if you faced a plane on a treadmill in the direction of the belt, it will take off if the speed is high enough without its own power. But it will not stay up very long without its own power as the friction from the air will decrease the velocity and lift will reduce to the point it can no longer keep the plane up.

[wingFeather]

Yes. Harrier Jump Jet

[IBCoupe]

Yes. Harrier Jump Jet

+10 points for creativity.

[stebo0728]

Your question needs clarification. I'll pose a few scenarios though in an attempt to answer what you might be asking.

True, thats sort of what the discussion usually leads to, but the original question is always as stated.

If the plane is on the treadmill but its velocity with respect to the ground (not the treadmill) is zero, then it will not fly as there will be no air across the wing to provide lift.

Quite true. The question becomes less about the plane and more about the treadmill and the planes wheels. The wheels are passive, they only turn when other forces acting on the plane cause the pane to move, and the wheels lessen the friction. If the treadmill is powered, and moving along, the question moves to whether or not the treadmill would be able to move the plane through space. If the weight of the plane causes the force of friction to be great enough, then the treadmill will turn the wheels of the plane as it goes, but the plane itself will never overcome inertia, hence it will not actually move. Then it will not fly.

If the plane is to use its own thrust and has to try and reach the velocity it needs while on the treadmill, then the plane will fly. Since the thrust from the engines is acting on the air, the movement of the ground beneath it will have a negligible effect. Basically, the only resistance the treadmill will cause is friction at the bearings of the wheel. There would likely be some rotational inertia as well that might act back against the treadmill, but it would likely be really small considering a typical plane will weigh much more than its wheels and the thrust from the engines is likely to be quite strong and should have little problem overcoming those forces. This scenario was actually tested by Mythbusters.

If the plane is using its engines to move, then regardless of what the treadmill is doing, the plane will move forward, and flight will be achieved. The treadmill action will have effects on the wheels of the plane, and how they move, but the plane is getting its equal and opposite reaction from the air.

Last scenario is probably not at all what you are asking, but if you faced a plane on a treadmill in the direction of the belt, it will take off if the speed is high enough without its own power. But it will not stay up very long without its own power as the friction from the air will decrease the velocity and lift will reduce to the point it can no longer keep the plane up.

This is somewhat related to the part above as to whether the treadmill/wheel interaction will be such that the force of inertia of the plane can be overcome.

[stebo0728]

Another fun one is imagine your car sitting on a hill, with its brakes on. In what direction is the force of friction acting? This one question occupied an entire class session in my college Physics class.

[IBCoupe]

Friction between the ground and the tire or the friction between the brake components?

[audtatious]

The plane one was shown on Mythbusters already.

[stebo0728]

Friction between the ground and the tire or the friction between the brake components?

Actually I said that one wrong, it would be a car driving up the hill, not stopped. And Im asking about friction between the tires and the road.

[stebo0728]

The plane one was shown on Mythbusters already.

I saw that episode ... it was a joke.

[IBCoupe]

Wouldn't the direction of the friction from the ground against the tire be pointing up the hill? The tire's trying to pull the asphalt down the hill, and the asphalt is staying put. Friction would be up.

But I barely paid attention to physics in high school, and that was a number of years ago anyways.

[stebo0728]

Yes in fact you are correct, and that was my position on the class, but ohhhhhhh i remember the vehement folks insisting the contrary, and a professor that couldnt tell for sure which was right ... needless to say once everyone's little light bulbs came one i had a good ROFLMAO moment ...

I remember it like it was yesterday ... "BUT THE CAR IS DRIVING UP THE HILL, HOW CAN FRICTION BE FACING UP THE HILL!!"

[audtatious]

The plane one was shown on Mythbusters already.

I saw that episode ... it was a joke.

How so? Putting a plane on a treadmill does not induce lift to the wings.

or did you mean you asking the question was a joke.....

[stebo0728]

No, the episode was a joke, first of all, I applaud them for trying, but there IS NO airplane sized treadmill in existence. They instead used a long sheet type something, I forget what, and pulled it behind a vehicle. Thats not a very good correlation to the problem being explored. All that is required, as helio pointed out, is for lift, and lift occurs as air is forced across the wings, a differential air pressure occurs, and the air pressure under the wing is larger, thus bringing the plane up. You can do this 2 ways. Move the plane fast enough through a section of air, or, move a section of air fast enough across the wings (wind tunnels). If the plane in face moves forward on the treadmill fast enough, then lift occurs.

The confusion comes in imaging a car on a dynamo, or person walking on a treadmill, where the motive force comes from the implement contacting the treadmill. For a plane, the motive force does not come from such implement, it comes from the engines, turbine or prop, pushing against air and moving the plane forward. If the plane was driven by the tires and not the engines, you would be correct.

But then thats where the question is a bit ambiguous. If in fact you placed the plane on a treadmill, and it was the treadmill's obligation to move the plane, then as soon as the first ftlb of lift was achieved, contact with the treadmill would cease, and the motive force of the plane would be gone, and gravity would take back over.

[wingFeather]

How so? Putting a plane on a treadmill does not induce lift to the wings.

or did you mean you asking the question was a joke.....

I looked that video up. They had the plane moving forward, creating lift on the wings. That is cheating!!! The idea here is to keep the plane stationary....

[stebo0728]

How so? Putting a plane on a treadmill does not induce lift to the wings.

or did you mean you asking the question was a joke.....

I looked that video up. They had the plane moving forward, creating lift on the wings. That is cheating!!! The idea here is to keep the plane stationary....

That would never work without dilithium crystals.

[wingFeather]

That would never work without dilithium crystals.

Or enough NOS to blow himself up period!

[bigbadberry3]

Aw man, I'm late to the physics stuff.

[C-Kwik]

True, thats sort of what the discussion usually leads to, but the original question is always as stated.

Perhaps I wasn't clear. You asked a question with too many variables f which, the answer would start with "It depends". Hence the reason for pointing out a few different scenarios. If it was intended to be

Quite true. The question becomes less about the plane and more about the treadmill and the planes wheels. The wheels are passive, they only turn when other forces acting on the plane cause the pane to move, and the wheels lessen the friction. If the treadmill is powered, and moving along, the question moves to whether or not the treadmill would be able to move the plane through space. If the weight of the plane causes the force of friction to be great enough, then the treadmill will turn the wheels of the plane as it goes, but the plane itself will never overcome inertia, hence it will not actually move. Then it will not fly.

This is somewhat related to the part above as to whether the treadmill/wheel interaction will be such that the force of inertia of the plane can be overcome.

Actually it should have no problem overcoming the inertia as long as the force remains. The weight you speak of will also cause a greater friction load at the bearings of the wheels, which will transfer some of the force into moving the plane. Not to mention the wheel's moment of inertia will also resist the rotational movement allowing some of the friction force between the tire and the ground to translate the plane's motion. They will have limitations on how much force can be applied but so long as force is still transferred, the plane will accelerate. Force = mass x acceleration. What it will probably have a problem with is the aerodynamic resistance. But remove the atmosphere from this and the plane will eventually match the speed of the treadmill.

Another fun one is imagine your car sitting on a hill, with its brakes on. In what direction is the force of friction acting? This one question occupied an entire class session in my college Physics class.

Its acting in both directions actually. It depends on what you define as the object. If the car is the object, then typically one would draw the external force of friction as being up the hill. But if you consider the Earth to be the object, then the force of friction acting on Earth by the car would be down the hill.

No, the episode was a joke, first of all, I applaud them for trying, but there IS NO airplane sized treadmill in existence. They instead used a long sheet type something, I forget what, and pulled it behind a vehicle. Thats not a very good correlation to the problem being explored. All that is required, as helio pointed out, is for lift, and lift occurs as air is forced across the wings, a differential air pressure occurs, and the air pressure under the wing is larger, thus bringing the plane up. You can do this 2 ways. Move the plane fast enough through a section of air, or, move a section of air fast enough across the wings (wind tunnels). If the plane in face moves forward on the treadmill fast enough, then lift occurs.

It was actually a pretty good experiment for the problem THEY were seeking an answer to. As I stated initially and reiterated, your question is too broad. I thought of three scenarios your question can apply to. There may be more. You need to state the conditions more clearly. If your scenario is different, then their experiment would not apply to YOUR scenario. But they did a pretty good job finding a reasonable answer to the question for THEIR scenario.