Time for an argument

I saw this on another board. Very compelling arguments in both directions, and I dont have enough experience with airplanes to be able to decide, what do you guys think.

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An airplane is standing on runway that can move (some sort of gigantic conveyer belt or treadmill). This gigantic runway/treadmill has some kind of system so that when the plane moves in one direction, the treadmill moves in the opposite direction at exactly the same speed.

The question is: Will the plane be able to take off?

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Nope... you'd need the forward motion of the plane on the tarmac in order to generate lift. The air has to move over the wings...

I am no engineer, but my guess is NO.

Im gonna change my position and say yes, since airplanes depend on airspeed not ground speed for lift.

Seems to me that the actual 'flying' part takes place when air is moving over/ under an airfoil- or wing. Whether the ground is moving under the plane means nothing. There is a difference between 'air speed' and 'ground speed'.
But I'm probably just talking nonsense. I'm sure there are some pilots out there.....;)

But you need the grounds friction against the wheels to move the aircraft forward to generate the needed lift. Sitting in the same spot, not moving forward, would never create lift no matter how much you gunned the motor.

Imagine your car on ice.

But the engines develop thrust by moving the air, not by turning the tires.

The question would still be are the jets being used to move the jet in a forward motion in aspect to the air around it. If the jets are just used to match the speed of the moving ground and not moving the plane forward in relation to surrounding air there is no lift. The air has to pass over the wing in order to generate the lift

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Originally Posted by Majik

But the engines develop thrust by moving the air, not by turning the tires.

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Interesting things might occur if the moving air comes from in front of the wings (like a prop plane), but I suspect the prop would have to be going pretty fast to develop enough air movement. I am not sure how the uneven force applied would effect things.

Now if you put the plane on the treadmill in a wind tunnel large enough, then yes, with the air moving over the wings, it would generate lift.

Hell, with the wind tunnel, you wouldn't need the treadmill...

The key is moving the wing through the air or the air over the wing enough to generate lift.

Nice quote, Titus!

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Originally Posted by Titus

Interesting things might occur if the moving air comes from in front of the wings (like a prop plane), but I suspect the prop would have to be going pretty fast to develop enough air movement. I am not sure how the uneven force applied would effect things.

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I meant that the engines develop the thrust for forward motion by moving air. I didnt mean they move the air over the wings directly. The force the engines develop moves the wings through the air that the wings need to create lift.

I'm with Majik.

The wheels don't move the plane. Nothing driving them at all.

A propeller pushes against the air as it turns. A jet engine expands air that pushes against air.

The plane is going to get it's wing moving across the air as long as the propeller/jet can create thrust against the air.

The wheels would essential be spinning twice as fast, but they're just spinning.

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Originally Posted by Treibenschnell

Nice quote, Titus!

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lol.

Good one Todd! :clap:

The funny thing is that it says that the treadmill "moves at the same speed" but in the opposite directon as the plane. So if takeoff speed is 130 knots then the treadmill will be moving at 130 knots in the opposite direction.

The plane will still take off but the wheel bearings probably won't handle the added heat/stress of spinning 2x what they should be spinning on takeoff.

Remember - this isn't a car. The thrust is given by turbines or engines moving lots of air, not by the wheels on the ground.

RJ

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Originally Posted by Treibenschnell

Nice quote, Titus!

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::Naughty:

The only way a wing generates lift is if it is moving through air... if the plane and the treadmill are moving at the same speed in opposite directions, then the plane isn't really moving forward - so no lift, no fly.

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Originally Posted by Titus

::Naughty:

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You have to admit, they were nice...

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Originally Posted by Treibenschnell

The only way a wing generates lift is if it is moving through air... if the plane and the treadmill are moving at the same speed in opposite directions, then the plane isn't really moving forward - so no lift, no fly.

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Let picture a rocket.

Lay it on it's side.

Launch it.

It's going to move.

Wheels, no wheels, ground, no ground the thing is going to move because of thrust.

Now attach wings to it and you'll have lift. Ground or no ground.

Having worked with and on airplanes as a career, the treadmill is a moot point. Planes fly all of the time without forward motion when a wind storm or tornado comes by. Lift is a cause of a high preasure and low preasure differential when the air goes through the airfoil. A hellicopter is turning a airfoil to create lift rather than pushing the aircraft through the air to create the pressure differential.