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.

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Originally Posted by Bell

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.

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(Thread drift)
BTW: I saw an ineresting sight when I was coming home from San Antonio. As I was driving with the top down, a red truck past by with a turbine engine lit up.

I last saw one when one of the engineers at Ayres in Albany, GA had one attached to his Ford Econoline Pickup back in 1995.

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Originally Posted by Majik

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.

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OK... I see it now.... The treadmill is irrelevant... The thrust the engine generates is going to move the plane forward, and lift will be generated. It doesn't matter if the treadmill moves forward with the plane or if the plane drives off the treadmill from thrust. As long as the wheels spin freely, the treadmill can't prevent the forward motion of the plane. It will take off. Can I change my vote now? ::Angel::

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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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You are thinking that the plane is driven by it's wheels...

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Originally Posted by Bell

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.

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Same with the plane... wheels or no wheels, ground or no ground, hit the thrust and it will move.

But the point is, it's on it's wheels, on a treadmill moving, presumably at the same speed in the opposite direction, thereby the wing itself is not moving through the air. It is, for intents and purposes, stationary.

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Originally Posted by channelmaniac

You are thinking that the plane is driven by it's wheels...

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Really? I'd have to be an idiot to believe that... c'mon man... :rolleyes:

Also, the plane relies both on forward thrust from the engine and lift from the wings to get up.

The rocket is a brutal machine relying only on strong upward thrust to get airborne. If you put it on it's side, put wheels on it, put some imaginary treadmill that could match it's speed in the opposite direction thereby negating any forward motion, put wings on it, it still would not fly.

ok once and for all, what is the air speed in the original question? my guess was no, since if there was no airspeed, and no (relative-to the air) movement, then no, it wont fly, but if the air moved as fast as the treadmill, or faster, then of course itll fly....

When I was 16, my Father was in a Cesna 150 with me and we were flying against the wind. The plane was in a climb with the airspeed at 50 mph and the road below us was moving forward. Of course, he had trimed the engine speed way back on purpose and pulled back on the yoke.

I saw a Aerocoupe doing the same thing trying to put down on the runway and the wind was too strong for him to be moving forward. It looked very funny from a spectators point, though the pilot in command didn't look like he was having all that much fun. :rolleyes:

This is an interesting question. However, my opinion as an engineer and a pilot, is that the plane will move forward, generate lift, and sooner or later reach a velocity that allows flight.

Here's why: Let's draw what engineers and physicists commonly call a "free body diagram" around the plane. From rest, you have three forces acting on the plane - gravity, the "normal" force acting through the contact with the runway that counteracts gravity, and the thrust of the engines. There is no force that opposes the thrust of the engines, so by simple physics a force acting unopposed on an object with mass will cause that mass to accelerate. Once the mass in question reaches the right velocity, the lift generated by the wings will be enough to overcome gravity, and the airplane will start to rise.

The moving runway does not offer a counteracting force, because the wheels are free to rotate. The thrust of the engines does not act against the runway, it acts against the air mass surrounding the plane. Now, if you were able to provide a wind tunnel environment, where both the ground and the airmass could be moved at the same velocity to counteract the forward thrust of the aircraft, you would see no relative forward motion form an outside point of reference... but you'd still have enough relative air velocity over the wings to produce lift, so the aircraft could still take off, just not fly forward.

Case in point:I've actually "hovered" a Cessna 152 before. With a stall speed in the high 30mph range in landing trim, all you need is a stiff enough wind... and a stupid enough pilot. :mrgreen:

the airplane does not move with its tires but with either a turbine or the propeller so the threadmill will not keep up/stop the plane from propelling through the air...

Somebody just get an RC plane on a treadmill, please!!!! ::Clown::

I've got a good one: There is an airplane in flight. A bird is flying inside the airplane, If you were to weigh the airplane, would the birds weight appear on the scale? The air inside the airplane is supporting the bird, but the bird isnt touching anything. :D

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Originally Posted by blackzx3_13

Somebody just get an RC plane on a treadmill, please!!!! ::Clown::

I've got a good one: There is an airplane in flight. A bird is flying inside the airplane, If you were to weigh the airplane, would the birds weight appear on the scale? The air inside the airplane is supporting the bird, but the bird isnt touching anything. :D

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No. Not only would the bird not register on the scale while in flight inside the aircraft, but the aircraft in flight would register zero on the scale as well.

Free body diagram time again - assuming the bird and the plane were both in level flight (i.e., the net vertical acceleration of both is zero), then there can be no net force acting vertically, since force is defined as mass times acceleration. No acceleration, no force generated. And what we commonly call weight (as opposed to mass) is actually a measurement of force.

So, no net downward acceleration equals no net downward force equals no measurement of weight on the scale. :mrgreen:

Iain takes all the fun out! ;)

Iain - If you were travelling at the speed of light and turn on your headlights... would they work?

::Clown::

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Originally Posted by altiain

No. Not only would the bird not register on the scale while in flight inside the aircraft, but the aircraft in flight would register zero on the scale as well.

Free body diagram time again - assuming the bird and the plane were both in level flight (i.e., the net vertical acceleration of both is zero), then there can be no net force acting vertically, since force is defined as mass times acceleration. No acceleration, no force generated. And what we commonly call weight (as opposed to mass) is actually a measurement of force.

So, no net downward acceleration equals no net downward force equals no measurement of weight on the scale. :mrgreen:

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So the upward force on the wing is equal to the downward force of gravity on the plane, hence it not falling.

And the upward force on the bird's wing is equal to the downward force of gravity on the bird, hence the bird not falling.

And if the bird is flying in the plane it must be exerting a downward force on the air in the plane.

And that force againt the air must be pushing the air down on the floor of the plane.

And therefore the plane must create more lift or upward force to support the bird as insignificant as that may be.

I'm dizzy.

http://www.avweb.com/news/columns/191034-1.html

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Originally Posted by channelmaniac

Iain takes all the fun out! ;)

Iain - If you were travelling at the speed of light and turn on your headlights... would they work?

::Clown::

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Sure they'd work... but don't expect 'em to actually throw any light out ahead of the car. ::Bleh::

So if a tree falls in the woods and there's nobody around, does it make a sound?

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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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read the article...it will fly...the secret is in the wheels. they don't apply force to the ground. They freewheel. So 10 mph on the belt and 10 mph from the plane in the other direction means 20 mph for the wheels to turn. The plane can accelerate against the belt because there is nothing to stop the wheels from spinning, like a brake. Unless the pilot hits them of course.

Lets reverse the situation...what if a plane tried to LAND on a belt going the opposite way at the exact same speed? If the landing speed was 130 and the belt was spinning opposite at 130, the wheels would do that nice little "chirp" and start accelerating to a max speed of 260. The plane wouldn't change air speed until the pilot hit the brakes and let the force between the tires and the belt affect the plane. The pilot could even to a touch and go and take back to flight.

i like this one...i tried to convince someone that when a tire spins that the "contact patch" is essentally stationary or not moving. They didn't believe me. i used a tank tread as a comparison...still didn't work...:rolleyes:

While on the treadmill this morning, I started thinking about this. Spinning wheels are irrelevent. The spinning prop is not creating lift. It's creating thrust. Think about a pusher prop or a jet...... The only way to generate lift is with airspeed, real or simulated (wind tunnel). It will not fly. :rolleyes:

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Originally Posted by MadMerv

While on the treadmill this morning, I started thinking about this. Spinning wheels are irrelevent. The spinning prop is not creating lift. It's creating thrust. Think about a pusher prop or a jet...... The only way to generate lift is with airspeed, real or simulated (wind tunnel). It will not fly. :rolleyes:

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Yeah it is developing thrust that isnt going to be held back by the treadmill. A person on a treadmill isnt a good example because we develop our thrust through our feet. That thrust is going to push it forward. If you were standing on the ground next to the imaginary treadmill the plane would pass you.