Plane on a treadmill (Debate topic) 2

[NonAeroNauticalGuy]

Hello all,

I would like your help in an ongoing debate on another forum. Here is the concept:

A jet engine plane is placed on a hypothetical treadmill that turns freely, matching the speed of the plane's wheels. Let's theoretically assume the treadmill can match this speed, not matter how fast the wheels turn. The engines are started and turned up to takeoff speed.

What happens?

To a lot of us on the other forum, it seems like "common sense" that when the engines start to drive the plane forward on it's wheels, and the treadmill will begin turning the other way, cancelling out all forward motion. Without forward motion, there will be no wind moving over the wings, and there will be no lift.

Others have said that since the wheels have no power themselves, and the plane is driven forward by the engines only, that the treadmill cannot prevent the plane from moving forward. That doesn't make sense to me, but perhaps someone could break it down for me.

Feel free to use extremely technical language, as we have plenty of posters on that forum who will be able to understand it..

Thanks,

Russ

 

[berkshire]

The tread mill can only stop the aircraft from moving forward if it can move fast enough to create enough rolling resistance to prevent the aircraft from moving.
Highly unlikely.
B.E.

 

[40818]

Can you tell me where this other forum is please?? I wish to have a look?

 

[NonAeroNauticalGuy]

http://forums.azbilliards.com/forumdisplay.php?f=13

 

[tbuelna]

The treadmill will only move if the aircraft wheels are driven, which they aren't. You remember the whole thing about equal and opposite reactions? The jet is propelled forward due to the momentum of the exhaust gas mass being expelled out its rear and the delta P from inlet to outlet, not due to its wheel's contact with runway surface.

Does a concrete runway move backwards when a plane takes off from it?

Of course, if you want to prevent the aircraft from moving in the direction of thrust, you can lock its wheels/brakes. Then it won't move until the thrust overcomes the fiction produced at the tire/runway interface.

 

[40818]

I just had a look on the web and the other forum the OP is talking about, and it seems the topic is debated quite vigorously on both sides.

Unfortunately half the people are wrong, and the correct answer is as Tbuelna replied.

For the laymans approach:
The fwd motion is only relative to the thrust and drag, where drag contains innumerous components, such as friction, inefficiencies etc, so simply by allowing a hypothetical frictionless surface on the "wheel interface" will not hinder or reduce the amount of thrust produced. Imagine a missile dropped from a stationary balloon, the engine kicks in and it moves forwards. Why, because the thrust was greater than the drag. Now back to the treadmill, all it will do is to give zero drag/friction on its wheel interface.

And before anyone quotes about the aircraft has to generate lift to fly, and that lift has a velcity squared term, please dont, as they fly equally well through just thrust.

 

[IRstuff]

This subject was already discussed to death somewhere in Eng-tips with the same result and the same reason that rockets can fly in a vaccuum.

TTFN

FAQ731-376

 

[rb1957]

i think tbuelna's has it right. consider a ski racer, they'd love a zero friction launcher 'cause they are driven by an external force (gravity). i think the jet is the same, it's going to generate it's thrust (without needing forward speed) ... Newton rules (F = ma)

 

[swearingen]

Looking only at friction, and assuming the aircraft could take off if unhindered the cases are as follows:

Case 1: If the treadmill is frictionless and the wheel bearings have any amount of friction, the treadmill will move (in the direction of flight), the wheels will not turn, and the plane will lift off.

Case 2: If the treadmill has any friction and the wheel bearings do not, the treadmill will not turn, the wheels will turn, and the plane will lift off as if on a conventional runway.

Case 3: If the treadmill and wheel bearings have infinite friction, the aircraft will sit there under maximum power and not move, unless maximum power overcomes the friction between the wheel and the treadmill, then all hell breaks loose.

Case 4: If the treadmill and wheel bearings both have some friction, but not enough to stop the plane from reaching takeoff velocity, either the treadmill or wheels will turn, or both the treadmill and wheels will turn, and the plane will take off (albeit with a longer takeoff roll than in Cases 1 & 2)

Case 4 is what actually happens and has a few subcases to it:

Case 4a: There is a threshold of treadmill friction where, if the wheel bearings have a much smaller friction, the wheels will turn and the treadmill will not.

Case 4b: Conversely, it's possible for the wheel bearings to have a relatively high friction compared to the treadmill where the treadmill turns and the wheels do not.

Case 4c: It is also possible for combinations of treadmill/wheel bearing friction to have both the treadmill turning AND the wheels turning during the takeoff roll.

However, in ALL cases, if the treadmill moves, it will ALWAYS be in the direction of flight.




If you "heard" it on the internet, it's guilty until proven innocent. - DCS

 

[Unionmason]

I never been so frighten by an Internet post before in my life. Please contact your respective supervisors with a copy of this post. Just kidding a little. Have you every seen a plane with skis instead of wheels? Or a sea plane? (I'd love to see what coefficient of friction a 777 wheel bearings could increase to before they melted during a take off.) Didn't Firestone or Goodyear develop a tracked landing system for grass runways during The cold war? How fast is top of the landing gear tire moving when the plane takes off at 140mph?(on an old fashion fixed runway) By the way under the conditions of the original post the plane would accelerate slightly faster w/o landing gear tire rotational inertia.

 

[swearingen]

My guess is the inertia of a treadmill that could hold a 777 would far outstrip that of the wheels and tires...



If you "heard" it on the internet, it's guilty until proven innocent. - DCS

 

[rb1957]

maybe it's a model of a 777 ...