Plane on a treadmill .. to be on Mythbusters

[Sqrl]

drkohler said:

Sqrl said:

omgwtfbbq said:
..

..At that point the wheels would be rolling along the belt at 200 mph, but the plane relative to the earth would still be moving a 0mph and the same would be true of the airspeed over the wings. Thus no lift here.

Now since the plane's motive thrust is limited by the relative airspeed , which is currently at 0, it is capable of continuing to speed up and taking off on the treadmill if it should choose to do so.

The planes 'motive thrust' (whatever you mean by that) is limited by the power of its engine(s) only and only that. Did you actually read and try to understand what I wrote in #40 ? I guess you did neither.

So here is the situation of the myth once again:

A conveyor belt moves the belt at any given speed -x to the right.

A plane on the conveyor belt rolls to the left (firing up its engines enough) to reach the speed +x.

Now we have a plane that is not moving to an observer standing at the belt. Forget all the friction gibberish, this is our starting problem which any reasonable pilot on a plane can achieve without any tricks or problems - but needs a really stable belt system otherwise its killing time...

The question now is: Is there a minimum belt speed at which the stationary plane will lift off?

As I answered in #40, yes it is theoretically possible if the engine(s) driving the plane generate an airflow encompassing its wings (as the belt speed increases, the airflow produced by the engine(s) increases, therefore increases upward momentum).

This experiment is very difficult to actually perform due to several problems:

1. The engine has to produce forward thrust only. It may generate an additional downward thrust if the engine is not horizontal during the experiment (and the plane may start to 'hop' which is a very dangerous situation for the pilot).

2. The engine(s) must be mounted near the wings to create air flow on the wings. A rocket mounted to the wheels won't do, probably neither will jet engines mounted on the wings.

3. The required take-off speed is much greater than the conventional take-off speeds of planes, as in this case the air flow on the wings is only generated by the area of of air pushed by the engine(s). In a normal situation, the plane is not stationary and the total mass of the air around the wings contribute to vertical momentum. The engine(s) (and belt) may not be strong enough to achieve the required speed.

4. There are some additional factors that are probably too small to contribute to this problem.

This discussion is going nowhere. Once again, the propeller plane can and will take off if this experiment is performed correctly.

 

Come to think of this while typing, I actually remember a mythbuster thing where they demonstrated that a propeller plane can start and land on a truck roof moving at take-off speed. This is the same problem, only in a different frame of reference...

 

First of all, I have read every post in this thread, so whatever need you are finding to become short with me you can drop.  I won't tolerate you talking down to me or anyone else in this thread, so please either maintain a civil and respectful tone with everyone or you can find somewhere else to post. This thread is for light hearted debate not for someone to come in and proclaim they are omnicient in all things mythbusters and declare everyone else wrong.

Now, the issue at hand: The term "Motive Thrust" is a way of saying "however it is being pushed forward".  And in this case is stated with the assumption of standard aircraft thrust which is airspeed based regardless of whether you are using a prop or a jet.

There is a level of detail you are skipping over right now.  The power of the engine limits the acceleration of the craft relative to the airspeed..  Wind resistance is what sets the limit for an aircraft's top speed.  When the decelleration caused by wind resistance is greater than the accelleration provided by the motive thrust of the aircraft you have reached top speed.  This is why a rocket can accelerate in space without a top speed. Granted it is also worth noting that in atmosphere wind resistance isn't the only limiting factor as heat can become a concern at extreme speeds, and while it isn't directly limiting the speed it is a concern.

Now, in the case of the aircraft sitting on a conveyer belt moving to the left at a speed of x (or -x if you prefer) the windspeed relative to the plane is moving to the right at a speed of x.  Now when the plane turns on its propeller or jet it will start to move forward cancelling out the speed of the conveyer belt and also the airspeed relative to the plane will appear to slow down.  Once it reaches a speed of 0 relative to the earth the airspeed would also be 0, and the wheels of the plane would be rolling at x.  Now in this scenario if there was no friction between the plane and the belt it could turn off its engine and maintain its position indefinitely.  But due to the wheels in contact with the belt it cannot.

A great example of this was posted in the first post of the thread.  If you stand on a treadmill with rollerblades on and tie a rope to the front of the treadmill can you not pull forward?