Mythbusters will finally test...

[BlackArcher101]

Originally Posted by GreatWhiteEbola

Waiting...

Working... not on the FBD, I'm actually at work. So you'll be waiting longer.

[photon]

Originally Posted by GreatWhiteEbola

We have to get to the same page here. Would you agree that there is a relationship between the runway and the wheels where gravity is concerned? Nothing to do with drag coefficients or friction.

Not really agree, I don't see what relationship you are trying to establish. Gravity holds the plane on the runway so that it doesn't fly, but beyond that gravity plays only a indirect role in the scenario.

When the plane is at rest, gravity isn't what makes the plane resist moving. Inertia plays its part, and friction plays the other part (friction is influenced by gravity, the more gravity the more friction).

Let's remove flight from the equation entirely. If the airplane just sits on the treadmill, the thrust from the engine will be sufficient to move the airplane down the runway. Or change it to a hovercraft. The only difference between an airplane and a hovercraft is the amount of friction between the treadmill surface and the vehicle.

Now the reason an airplane can break free from gravity is because it has a stable base <the runway> of which it can propagate itself forward.

This is incorrect. An airplane does not push against the runway to propel itself forward, it uses Newton's 3rd law, it throws something backwards (air) and there's an equal and opposite reaction (the plane moves forward).

If you correct the concept that the plane is using the runway to help it fly then you'll be ok I think.

EDIT: Fixed some wording

[photon]

Originally Posted by AnarChrist

If the plane remains stationary on the conveyor belt, there is no airflow over the wing. Even if the engine is at max power, of the plane does not move forward to generate any airflow the plane stands no chance of getting off the ground.

What counteracts the force generated by the engines to cause the airplane to remain stationary?

[GreatWhiteEbola]

/

Originally Posted by BlackArcher101

Working... not on the FBD, I'm actually at work. So you'll be waiting longer.

The only way it will work is with the thrust force overcoming the counteracting force of the treadmill, be it friction, gravity, whatever. In that event the plane will move forward and take off.

What is the only difference with this take off than any other? The treadmill. If you use factors to negate the treadmill why is it even debated? Of course the plane will take off if the optimum airspeed is achieved, I think the Wright Brothers discovered that over a century ago.

[photon]

Originally Posted by GreatWhiteEbola

The only way it will work is with the thrust force overcoming the counteracting force of the treadmill, be it friction, gravity, whatever. In that event the plane will move forward and take off.

Exactly!

The plane is on wheels, so that counteracting force is minimal. If the treadmill is running at the plane's speed in the opposite direction, then the plane only has to overcome twice the rolling resistance it normally does on a normal takeoff (because the wheels are spinning twice as fast).

The amount of energy it takes to plow through the air at any speed is orders of magnitude greater than the resistance from the wheels on the ground.

[VanCity Cowboy]

If the wheels were perfect, completely without friction, the plane would stay in the same position while the treadmill moved below it, and the wheels spinning like crazy.

think of someone with roller blades on a treadmill; it would take minimal effort to counter act the friction of the wheel and keep you in the same spot.

The plane is not transferring power to the ground like a car, it is using the prop/jet to push air backwards, enough so that it gets it's opposite reaction, moving it forward.

I'll bet, the airplane has to over compensate for the 'minimal' amount of friction caused by the wheels, resulting in a slower acceleration and needing a longer 'treadmill-runway' than it would a regular runway.

[FlamesAddiction]

I think it can work. It might take a little extra thrust if the wheels have any resistance, but if they are lubricated really well so they can spin absolutely free, then I can't see why the plane wouldn't take off.

[AnarChrist]

Originally Posted by photon

What counteracts the force generated by the engines to cause the airplane to remain stationary?

I would say the conveyor. There is still going to be substantial pressure on the conveyor caused by the mass of the plane, so any thrust in one direction is going to be countered by movement in the opposite on the conveyor. The plane may gain some forward movement, but not near enough to develop enough airflow to generate lift.

[Burninator]

It will fly.

The only way I can see the plane not taking off is if the treadmill can pull the plane backwards off the runway before the trust of the engine can overcome the treadmill. Now I do not know a lot about planes but I assume the plane will easily overcome the force of the treadmill and take off with ease, and with a slightly longer take off distance.

[GreatWhiteEbola]

Plausible. I mean that I don't think they could recreate the myth to my liking.

[Hemi-Cuda]

so does this episode air on the canadian discovery channel on Wednesday as well?

[AnarChrist]

I don't see why the plane would fly. Say the plane weighs 10 tonnes. Gravity still acts on that 10 tonne object, pulling it straight down. You fire up the engines and produce some thrust, the plane tries to move forward, but gravity still holds the 10 tonnes where it is. The plane does not get any lighter and gravity does not get any weaker just because the engine uses thrust. The only thing that could allow the plane to develop any forward momentum is if the rollers on the conveyor were not well lubricated and the plane was allowed to gain traction. This would be the same for any sort of propulsion system.

[photon]

Originally Posted by AnarChrist

I would say the conveyor. There is still going to be substantial pressure on the conveyor caused by the mass of the plane, so any thrust in one direction is going to be countered by movement in the opposite on the conveyor. The plane may gain some forward movement, but not near enough to develop enough airflow to generate lift.

If a plane takes off without the conveyor moving, then there's x amount of force holding the plane back due to the wheels and landing gear and such right? Then if the conveyor is activated, then only 2x that force is there.

I can push a car if the brake isn't on, the amount of energy to overcome the friction of rolling on the runway is very small compared to the amount of energy to push the plane through the air (it's a cube power relationship).

[photon]

Originally Posted by AnarChrist

I don't see why the plane would fly. Say the plane weighs 10 tonnes. Gravity still acts on that 10 tonne object, pulling it straight down. You fire up the engines and produce some thrust, the plane tries to move forward, but gravity still holds the 10 tonnes where it is.

This is true treadmill or no treadmill; by this argument no airplane should fly.

Gravity doesn't hold the 10 tonnes where it is, inertia and friction do.

The only thing that could allow the plane to develop any forward momentum is if the rollers on the conveyor were not well lubricated and the plane was allowed to gain traction. This would be the same for any sort of propulsion system.

The airplane doesn't use traction on the conveyor to propel itself forward. Float planes don't use traction with the pontoons and the water, and ski planes don't use traction with snow. The plane throws air backwards to propel itself forward.

[kermitology]

Well we know that air moving over the wings is what provides lift. Are some people suggesting that the engines are what generate the airflow?

They certainly would cause air to flow over the wings, but I don't think that the engines alone (say on the Learjet 36 I fly on almost daily) would generate the 150+ knots required for takeoff.

I think it's busted. I'm going to ask my pilots tomorrow..

[GreatWhiteEbola]

Originally Posted by Hemi-Cuda

so does this episode air on the canadian discovery channel on Wednesday as well?

According to the channel guide, nope, although they have been known to be wrong. Torrent it on Thursday if need be.

[GreatWhiteEbola]

Originally Posted by kermitology

Well we know that air moving over the wings is what provides lift. Are some people suggesting that the engines are what generate the airflow?

They certainly would cause air to flow over the wings, but I don't think that the engines alone (say on the Learjet 36 I fly on almost daily) would generate the 150+ knots required for takeoff.

I think it's busted. I'm going to ask my pilots tomorrow..

Yeah there's no way that the engines provide the airflow for liftoff, the question is if the plane can break the forces that hold it back on the tread mill.

There is a point where the fictional forces will be overcome by the thrust, thusly giving the plane forward momentum. Here is where I don't get the debate. Why is there a question? If the plane can be free to move forward on the treadmill, the treadmill is neglected and it becomes like a normal takeoff. Of course it will take more force but it can happen.


This is accounting for the fictional force being less than the thrust provided by the prop/turbine.

[photon]

Originally Posted by GreatWhiteEbola

Why is there a question?

Heh, well of course any question becomes moot once the answer is clear.. the point of this one is most people are thinking about airflow over the wings and the plane not moving, so they forget that there's no reason the plane won't move forward as it normally would.

Look at it this way, as you say the question is can the plane overcome the forces that hold it back on the treadmill.

A Learjet 36 has two engines, each of which can generate 3500lbs of thrust. The max speed is 540 miles per hour, or 47520 feet per minute. 7000lbs over 47520 feet in one minute is 332.6 million lb-ft per minute. 1 horsepower is 33,000 lb-ft per minute, so the Learjet 36 takes 10,000 equivalent horsepower to move it through the air at max speed.

How much thrust is required to overcome the friction of the landing gear? Far far far less than 7000lbs of thrust worth (which is how much would be required to stop the plane from accelerating).

[McG]

Originally Posted by FlamesAllTheWay

EDIT: Hmmm, the myth is a little different than what I thought, rendering my prediction useless.

Originally I thought the treadmill was going forward and the plane was sitting stationary on the treadmill moving forward as fast as the treadmill would take it. I would guess the plane would take off just fine as long as the treadmill was fast enough. Sorta like on an aircraft carrier.

For what the myth actually is (plane moves forward as treadmill moves backwards), I'm not so sure. The plane itself would still be stationary regardless of how fast the treadmill was moving backwards. I'm no scientist but I say this one gets busted... would be cool if it actually worked though.

exactly my thoughts. no lift on the wings means it doesn't fly...

[kermitology]

Originally Posted by photon

A Learjet 36 has two engines, each of which can generate 3500lbs of thrust. The max speed is 540 miles per hour, or 47520 feet per minute. 7000lbs over 47520 feet in one minute is 332.6 million lb-ft per minute. 1 horsepower is 33,000 lb-ft per minute, so the Learjet 36 takes 10,000 equivalent horsepower to move it through the air at max speed.

Funny you should mention a Learjet 36..

This is the plane that I'm working with (currently in Little Rock)