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.

 

/crap deleted in hindsight

 

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. 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...

 Whoah, there killer.  The truck example has nothing in common... The truck is MOVING and therefore there is air movement across the wings of the plance hence providing lift.

Also note that it isn't going to take but a small amount of thrust to overcome the rolling resistance of the plane on the conveyer belt and that level of throttle won't be anywhere near enough to actually get a plane airbore if it was on a concrete runway.  It might be enough to taxi, but not enough to actually generate any real lift... 

drkohler said: omgwtfbbq said: you're all getting it wrong! you're all saying "if the plane has no airspeed it won't fly" which is of course true, but you're missing the point that they're trying to slow down a plane with a treadmill. It simply won't work! The plane is pushing against the air. If it were a car it would be stationary. Imagine this scenario then: You have a car driving at 100 MPH. It's a front wheel drive. The two back wheels are on a treadmill (ok, you'll have to imagine a mobile treadmill here so that the back wheels are on it and the front wheels aren't) and this treadmill is going backwards at 100 MPH. Will the car stay stationary? Of course not! The front wheels are providing the force, the back wheels are simply there for the ride. It doesn't matter how fast those back wheels are spinning, the front wheels will keep the car moving at 100 MPH. This is the exact same scenario. The plane will move forward because no treadmill in the world can stop a plane from moving. Oh boy... nobody is trying to slow down anything in that experiment. Now in your car case, put the belt where it belongs, under the FRONT wheels that DEFINE the momentary axis of rotation (it does not make any logic sense to place the belt under the wheels of the free running axle). Of course the plane can and will stay stationary on the conveyor belt if its forward speed (generated by whatever means) exactly matches that of the belt. Now you can increase both speeds slowly to 'lift-off' speed, If the pilot can balance the plane long enough. At some (probably very high) speed, a propeller plane will lift off although being stationary to an observer standing at the belt. However, in the actual experiment a normal propeller engine might not be able to reach that speed (which is higher than normal 'lift-off' speeds) but the plane lifts off anways because it is probably not running horizontally but with engines slightly inclined giving the accelerated air a downward momentum.   Ever watched those fitness freaks running on a treadmill? They do exactly the same, they run just as fast forward as the belt runs backwards to stay in place...

Exactly my point! The Front wheels are providing the movement, so putting the treadmill on the back wheels isn't going to slow down anything.

With the plane, what provides the movement? The propellors. By your own admission then, placing a treadmill under the wheels isn't going to slow down anything!

EDIT: to clarify, you said "it does not make any logic sense to place the belt under the wheels of the free running axle". This sums up my position entirely. The wheels on a plane ARE the free running axle.

Gnizmo said: So long as the wheels are part of the body themselves, there is a treadmill that could stop the plane from taking off. A theoretical one at the very least. Since it is part of the body, a portion of the force used to push the wheels back will transfer to the plane as a whole. This makes sense since the plane cannot move forward and just leave a solid part of it behind. The problem would be one of getting a treadmill that could exert enough force on the wheels to completely counter the forward thrust of the engine without just breaking the wheels. It would work similar to this. If I tied a rope around your arm and pulled back on it, could I keep you from running forwards? The power is all in your legs, but the effect on the body from an external source is still a factor. I think everyone here is assuming the treadmill will be using only the wheels of the plane for its speed, but I can't imagine why anyone would make that assumption. All modern treadmills I have seen can move on their own opening up the possibility to increase the force on the wheels independant of their actual motion.

Those scenarios are totally different. If you pulled back on a rope on my arm, then of course it would stop me. Same as if you tied the plane to something, it wouldn't be able to take off. Because the force provided by the rope always exactly matches the force provided by the plane/my legs. Therefore I won't move until either the rope breaks or whatever is holding it breaks.

This scenario is completely different. Essentially the treadmill is being attached to free running wheels. No matter how fast the treadmill is going, all it's succeeding in doing is spinning the wheels faster.

 

Sqrl said: omgwtfbbq said: you're all getting it wrong! you're all saying "if the plane has no airspeed it won't fly" which is of course true, but you're missing the point that they're trying to slow down a plane with a treadmill. It simply won't work! The plane is pushing against the air. If it were a car it would be stationary. Imagine this scenario then: You have a car driving at 100 MPH. It's a front wheel drive. The two back wheels are on a treadmill (ok, you'll have to imagine a mobile treadmill here so that the back wheels are on it and the front wheels aren't) and this treadmill is going backwards at 100 MPH. Will the car stay stationary? Of course not! The front wheels are providing the force, the back wheels are simply there for the ride. It doesn't matter how fast those back wheels are spinning, the front wheels will keep the car moving at 100 MPH. This is the exact same scenario. The plane will move forward because no treadmill in the world can stop a plane from moving. I don't think anyone is arguing if the plane's own motive thrust can overcome the treadmill. What we are saying is that the myth seems to prescribe a plane that cannot move. By the rules of the myth it's only ever allowed to provide itself enough thrust to overcome the backwards thrust produced by it's wheels friction with the treadmill. And thus no airspeed is obtained and thus no lift. If you put a plane on an infinite treadmill going to the left at 200mph and let the plane get up to 200mph due to the friction on it's wheels, theorotically it would still be able to overcome this with its own motive thrust because its motive thrust is based on relative windspeed and is limited by such. Therefor it would be able to slowly speed up and overcome the leftward 200mph thrust until it reached a speed vector of 0. 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. But as I said before the myth is poorly worded and I don't think anyone here can really claim to have the absolutely correct interpretation of the myth. I really hope they do a better job of explaining the myth on the show, but in the meantime I have double checked my season pass and am looking forward to it =) @Bod, thats pretty much what I said in my first post also =)

you are correct. If they do this, the plane won't take off. But I'd be very impressed if they manage this, though. I suppose they could move the plane at minimum speed, and then slowly increased the speed of the treadmill until the friction in the wheels stopped the plane moving. But this is a ridiculous scenario, because the plane won't take off without the treadmill, of course putting a treadmill underneath it won't change anything.

This whole scenario is stupid. The treadmill quite smply will not make any difference at all to whether the plane will take off. If a plane uses its full thrust to accellerate, then no treadmill in the world will stop it. If it doesn't, then moving a treadmill BACKWARDS will certainly not help it take off. In the end the treadmill does absolutely nothing to stop or help a plane take off.

I suppose it would be more interesting to see whether a treadmill can actually accellerate a plane enough to get it to lift off

There will be some force transfered from the wheels to the body of the plane however. Look at it like this, if you put the plane on a tread mill and started it going would the plane move backwards? The answer is, of course, yes it would. So we have established that the ground can, infact, exert force on the entirty of the plane. As such, it is no longer a question of if you can counter the force of the engine by applying a force on the wheels, but what happens when you put that much force on the wheels themselves. And, if it is even possible to create a machine to exert that kind of force.

Id say in theory you could take off depending on the placement and type of engines used, a turbojet with an engine set over the leading of the wings, so the wash actually helps create lift could do it, there is a russian aircraft that was designed this way for short takeoffs and landing ... thing looks strange as Hll though. really its all about creating lift so it depends on how the aircraft does it volt aircraft, no problem, gyroplanes/rota planes, rocket powerd vehicals...(the tredmill could not keep up). really the bigest problem i see here is keeping a giant tread mills bearing cool durring this test

goddog said: Id say in theory you could take off depending on the placement and type of engines used, a turbojet with an engine set over the leading of the wings, so the wash actually helps create lift could do it, there is a russian aircraft that was designed this way for short takeoffs and landing ... thing looks strange as Hll though. really its all about creating lift so it depends on how the aircraft does it volt aircraft, no problem, gyroplanes/rota planes, rocket powerd vehicals...(the tredmill could not keep up). really the bigest problem i see here is keeping a giant tread mills bearing cool durring this test

Amen to that - at least somebody else got it right. In order to get rid of this 'wheels' discussion, let's mount a sled under the plane instead of wheels so there is no rotation anymore of anything (except your brain, maybe). Everything in the myth problem remains the same. (Now the engines have to be much stronger so that the plane can sort of scratch on the conveyor belt, overcoming initial adhesion and gliding friction. As long as the engine(s) can create high airflow on the wings, that plane is taking off at some belt speed as I have tried to explain before. (That thing with the truck was crap, I figured that one out after sending off the message).

 

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? 

omgwtfbbq said: epsilon72 said: They're doing this myth? I would've thought it would be a no-brainer... Maybe one of their short 5 minute ones or something...   edit: wow, some people here think it might be possible? No forward velocity--->no lift--->no takeoff. Now, if they were to blow a huge amount of air at a high velocity at the conveyor-belt-plane, then it would possibly work (and I can see mythbusters attempting this on a small scale, possibly) it's clearly not a no-brainer since you got it wrong =) no forward velocity won't happen, no treadmill's going to stop a plane taking off

Huh? If the conveyor belt is moving this way --->

and the plane is trying to move this way <---  (at the same speed - one of the original problem's stipulations) it'll be stationary.

If the atmosphere surrounding the plane does not move at the same speed+direction as the conveyor belt, the plane ain't takin' off.

What they're trying to do in essence is have a stationary plane lift off of the ground - which would only happen if you had something that could do vertical takeoff and landing...

 

@drkohler - a stationary 'normal' plane won't take off.