Mythbusters will finally test...

[octothorp]

Originally Posted by burn_baby_burn

How big of treadmill are we talking about?

If you look at the photo in the link at the beginning of the article, it looks like it's a few hundred feet long. This isn't a treadmill that the plane is intended to stay stationary on.

[ken0042]

Originally Posted by Ducay

Guys, the plane takes off.

I guess my thinking is that if the plane needs to hit 100 knots to take off, and the treadmill is going at 100 knots, the plane will now require a ground speed of 200 knots to take off.

And looking at the plane in the picture, I'm not sure it is capable of reaching a speed of double its takeoff velocity; be it ground speed or air speed.

[Bobblehead]

Originally Posted by burn_baby_burn

I was assuming that the plane would be stationary on the treadmill similar to a person running on a treadmill. How would air move over the wings?

Why would it stay stationary? The prop or jet engine isn't pushing against the treadmill, it is pushing against the air.

[GreatWhiteEbola]

I don't think it will fly, I think the forward thrust is meant to counter act gravity until the plane achieves liftoff. Now with the treadmill counter acting the effects of gravity in relation to the wheels and the runway, I don't think there will be enough forward motion to create lift. I am still trying to wrap my head around the whole idea.

[photon]

Originally Posted by burn_baby_burn

I was assuming that the plane would be stationary on the treadmill similar to a person running on a treadmill. How would air move over the wings?

Why would you assume that? What would cause the airplane to remain stationary? Like Bobblehead says, the engine causes the plane to move by throwing air backwards, not by acting on the treadmill somehow.

[photon]

Originally Posted by ken0042

I guess my thinking is that if the plane needs to hit 100 knots to take off, and the treadmill is going at 100 knots, the plane will now require a ground speed of 200 knots to take off.

And looking at the plane in the picture, I'm not sure it is capable of reaching a speed of double its takeoff velocity; be it ground speed or air speed.

Ground speed means nothing to an airplane, airplanes can fly with zero ground speed. The only thing that matters with respect to being able to fly is airspeed.

In a way you are right, if the plane needs 100 knots airspeed to take off, and the treadmill is going 100 knots, the plane WILL require a "treadmill speed" of 200 knots to take off, but it will still only need 100 knots of air speed.

And the amount of energy required to reach 100 knots won't change by much, it isn't going twice as fast relative to what matters; the surrounding air. It will take a little more energy to overcome the friction of the wheels spinning twice as fast though.

[photon]

Originally Posted by GreatWhiteEbola

I don't think it will fly, I think the forward thrust is meant to counter act gravity until the plane achieves liftoff.

Forward thrust goes forward, gravity goes down, the two act at right angles to each other, one can't counteract the other.

Forward thrust moves the plane forward, as air moves over the wings lift is generated which counteracts gravity.

[GreatWhiteEbola]

The thrust doesn't create lift until it counteracts gravity, the relationship between the runway and wheels is key. Gravity will prevent liftoff as there will not be adequate high/ low pressure differentiation on the airfoil.

[photon]

Originally Posted by GreatWhiteEbola

The thrust doesn't create lift until it counteracts gravity

Thrust doesn't create lift, it moves the plane forward. Lift is created by the wings, with more lift being generated the faster the plane goes forward (well that and wing geometry and angle of attack and stuff, but keeping it simple for argument's sake).

the relationship between the runway and wheels is key. Gravity will prevent liftoff as there will not be adequate high/ low pressure differentiation on the airfoil.

The only relationship between the treadmill surface and the wheels is that one will cause the other to turn.

Yes gravity will prevent liftoff if there is not enough lift, but what would cause there to not be enough lift? Once the airplane reaches the appropriate airspeed it will take off, so for the plane to not be able to take off, something has to stop it from moving forward. What's stopping it from moving forward?

[fredr123]

In case it hasn't already been posted, here's one explanation: http://www.straightdope.com/columns/060203.html

[burn_baby_burn]

Originally Posted by photon

Why would you assume that? What would cause the airplane to remain stationary? Like Bobblehead says, the engine causes the plane to move by throwing air backwards, not by acting on the treadmill somehow.

I didn't see the picture. How would it remain stationary? The same way a person running on a treadmill stays stationary. Allthough your feet are moving. Your body stays in one place.

[GreatWhiteEbola]

Originally Posted by photon

Thrust doesn't create lift, it moves the plane forward. Lift is created by the wings, with more lift being generated the faster the plane goes forward (well that and wing geometry and angle of attack and stuff, but keeping it simple for argument's sake).



The only relationship between the treadmill surface and the wheels is that one will cause the other to turn.

Yes gravity will prevent liftoff if there is not enough lift, but what would cause there to not be enough lift? Once the airplane reaches the appropriate airspeed it will take off, so for the plane to not be able to take off, something has to stop it from moving forward. What's stopping it from moving forward?

The runway/ treadmill. Gravity is sticking the plane to the runway. If there is no thrust the plane will move in the same direction as the treadmill. Opposite of what is required for an adequate airspeed for lift off. Now, I'm assuming that the treadmill with be matching the forward thrust of the plane or the experiment is debating a moot point as we all know how a plane works.

[photon]

Originally Posted by burn_baby_burn

I didn't see the picture. How would it remain stationary? The same way a person running on a treadmill stays stationary. Allthough your feet are moving. Your body stays in one place.

Your body doesn't remain stationary though, your body is moving constantly with relation to what you are pushing against; your feet push against the treadmill to move you forward. The treadmill surface itself is moving backwards though so you remain stationary, but only in relation to the walls and floor of your house.

In this case the plane is moving air to generate thrust, so the plane will move in relation to the surrounding air; the treadmill doesn't have anything to do with that, there's no energy being directed into the treadmill from the airplane.

[GreatWhiteEbola]

The interaction between the runway and the wheels is what provides forward motivation to get the appropriate airspeed for liftoff.

[photon]

Originally Posted by GreatWhiteEbola

The runway/ treadmill. Gravity is sticking the plane to the runway. If there is no thrust the plane will move in the same direction as the treadmill. Opposite of what is required for an adequate airspeed for lift off.

Right, but the force of that drag is no where close to the force of the thrust. The plane is on wheels which are designed to roll with minimal friction.

Now, I'm assuming that the treadmill with be matching the forward thrust of the plane or the experiment is debating a moot point as we all know how a plane works.

That's the assumption of the scenario.. now if you could spin the treadmill fast enough that the friction of the wheels generated enough drag to counteract the thrust perfectly, then yes the plane would stand still and nothing would happen. Though more likely the plane would flip over if that much drag was generated at the landing gear.

[photon]

Originally Posted by GreatWhiteEbola

The interaction between the runway and the wheels is what provides forward motivation to get the appropriate airspeed for liftoff.

Not in any airplane I've seen, the wheels on an airplane aren't driven, they just spin free (and have brakes).

[BlackArcher101]

Originally Posted by GreatWhiteEbola

The interaction between the runway and the wheels is what provides forward motivation to get the appropriate airspeed for liftoff.

You are really looking at this the wrong way. Like Photon said, this isn't true for any existing aircraft. Planes are driven/propelled through the wheels like a car.

I should just do a free body diagram right now and just put this thing to rest.

[GreatWhiteEbola]

Originally Posted by photon

Not in any airplane I've seen, the wheels on an airplane aren't driven, they just spin free (and have brakes).

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.

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. Now introducing a moving counteracting surface will cause the free wheeling land gear of the plane to no longer allow forward movement.

All forces being equal

[AnarChrist]

What makes airfoils function? Airflow over the wing. The plane will not generate enough airflow over the wing to generate the lift required to achieve flight. 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.

Does anyone know how planes take off on aircraft carriers? They use a steam powered catapult system that accelerates the plane to a speed where enough air flows over the foil to lift the plane. To make this function better, the ship turns into the wind to maximize the lift the plane gets from the slingshot. The plane is airbourne before the engines have a chance to pull/push the plane forward through the air. This is busted before it begins.

[GreatWhiteEbola]

Originally Posted by BlackArcher101

I should just do a free body diagram right now and just put this thing to rest.

Waiting...