Topic: Can this airplane lift off?

[Duke McDukiedook]

You folks seem mechanically-minded...

There is an airplane on a runway. The runway is like a conveyor belt, and can detect the speed of the wheels of the airplane. When the runway senses the wheels moving, the runway will match the speed of the airplane's wheels, but in the opposite direction.

Can this airplane take off?

Discuss.

This is a theoretical thought problem:

Ok, I'll try and explain this as simple as possible. He says the runway will match the speed of the tires so that means the plane will have zero forward movement. The plane needs forward movement (air moving by to create pressure on the wings) to generate lift. If the plane is essentially standing still it will not create lift and fly.

If this was the case (plane taking off on a conveyor belt), then why don't they just put big conveyor belts on aircraft carriers?
You could make them a hell of a lot shorter if that were the case.

Newtons 3rd law- Two equal, opposing forces (Propeller and belt) would equal to zero, no flight.

[bunghole]

You folks seem mechanically-minded...

There is an airplane on a runway. The runway is like a conveyor belt, and can detect the speed of the wheels of the airplane. When the runway senses the wheels moving, the runway will match the speed of the airplane's wheels, but in the opposite direction.

Can this airplane take off?

Discuss.

This is a theoretical thought problem:

Ok, I'll try and explain this as simple as possible. He says the runway will match the speed of the tires so that means the plane will have zero forward movement. The plane needs forward movement (air moving by to create pressure on the wings) to generate lift. If the plane is essentially standing still it will not create lift and fly.

If this was the case (plane taking off on a conveyor belt), then why don't they just put big conveyor belts on aircraft carriers?
You could make them a hell of a lot shorter if that were the case.

Newtons 3rd law- Two equal, opposing forces (Propeller and belt) would equal to zero, no flight.

What you're missing is that the plane actually does move forward through the air (and forward relative to the earth) even though the wheels are spinning.

[MCRider]

Sorry DukieDook. I was with you back when I first heard this on MythBusters quite some time ago. And they had many people including the pilot of the plane (watch the video) who thought it wouldn't take off.

But its a trick question, the movement of the conveyor belt does not keep the plane stationary. In the vid there is a car/truck pulling a cloth in the opposite direction of the plane. Its as others have said, the rotation of the wheels has nothing to do with the forward motion of the plane.

If the wheels were geared to somehting, like on a car, then we might have a case. But they are not.

We lose.

[Gordon]

Ok, I'll try and explain this as simple as possible. He says the runway will match the speed of the tires so that means the plane will have zero forward movement.

And that's where you're going wrong in your reasoning, right from the beginning.  The conveyor belt will only match the speed of the wheels, not the plane.  In fact, the conveyor belt will determine the speed of the wheels.  The wheels can and will spin freely without regard for the forward airspeed of the airplane itself.  

The propeller, which is not in any way affected by the movement of the runway, will still pull the plane forward through the air, and moving forward through the air is what causes a plane to fly, not the speed that the wheels are spinning on the ground.  

Keep typing things like "I'll try and explain this as simple as possible", and the crow you'll be eating will just keep getting bigger.

[mattcb350f]

Holy crap, I need to sign up for whatever physics class you guys took in college.
What class was that, magical physics?

Aerodynamics actually  98% Class of 2008.

One of several courses taken to get my aviation diploma.

 Matt.

[Caaveman82]

I am not going to pretend like I know things but....


Dookie you said that the plane would not have lift with zero forward momentum...

though... if the jet engine was running... and it is a fact that the wheels are just over priced casters, they do what their told, even if the plane was not physically gaining ground the thrust is what actually lifts the plane any way, right?

So if that is true... the point of the run way is really just to allow the plane to build up the proper ammount of speed/thrust/what have you to take off

So I don't see how it would make a difference if the plane was physically moving forward or not, as long as the proper ammount of "speed" was obtained...

[MCRider]

I am not going to pretend like I know things but....


Dookie you said that the plane would not have lift with zero forward momentum...

though... if the jet engine was running... and it is a fact that the wheels are just over priced casters, they do what their told, even if the plane was not physically gaining ground the thrust is what actually lifts the plane any way, right?

So if that is true... the point of the run way is really just to allow the plane to build up the proper ammount of speed/thrust/what have you to take off

So I don't see how it would make a difference if the plane was physically moving forward or not, as long as the proper ammount of "speed" was obtained...

The plane must be physically moving forward, lift is generated by air over the wings.

But nothing in the set up of the experiment will keep the plane from moving forward. The plane is not geared to the wheels. Forward motion comes from the "lift" of the propellor, low pressure in front, high pressure behind, or thrust of a jet.

But yes it makes a difference that the plane is moving.

[Inigo Montoya]

dokie would it have been better if I put "" around "bites"? Even though what I said is still true. The plane uses the friction created by the propeller (or fan(turbine) in the case of a jet) against the air to move. So in a way, you CAN say it "bites" the air. The amount of pitch determines how much air is moved on each turn of the blade. As different conditions require different pitches, props are able to feather to different pitches. 

[Caaveman82]

I am not going to pretend like I know things but....


Dookie you said that the plane would not have lift with zero forward momentum...

though... if the jet engine was running... and it is a fact that the wheels are just over priced casters, they do what their told, even if the plane was not physically gaining ground the thrust is what actually lifts the plane any way, right?

So if that is true... the point of the run way is really just to allow the plane to build up the proper ammount of speed/thrust/what have you to take off

So I don't see how it would make a difference if the plane was physically moving forward or not, as long as the proper ammount of "speed" was obtained...

The plane must be physically moving forward, lift is generated by air over the wings.

But nothing in the set up of the experiment will keep the plane from moving forward. The plane is not geared to the wheels. Forward motion comes from the "lift" of the propellor, low pressure in front, high pressure behind, or thrust of a jet.

But yes it makes a difference that the plane is moving.

I think that is what I was trying to get at...

It may not be "moving" but the lift or thrust is really the important thing...

I guess when I said moving I ment as in you can see it move from point a to point b...

if the propellor/turbine is spinning, the wheels are turning, even if it does not "move" from point a to point b, i.e. if it was on a belt,  it is still moving, right?

[markb]

I don't the the Mythbusters proved anything.  The reverse speed of the "conveyor" didn't match the forward speed of the wheels on the airplane otherwise the airplane would have been stationary and therefore wouldn't be able to take off.  Think what would happen if the conveyor was going faster.  The plane would be going backwards.  Could it still take off?  OK soichiro, what's the answer?

[Duke McDukiedook]

The propellers on the plane or a jet engine do not create lift, they create thrust.
The differential in air flow caused by the shape in the wings with air running past them cause lift, not the engine or props. If engines created lift why would you need wings on an airplane?

I still say I am right in that if the conveyor belt goes fast enough to prevent forward movement of the plane and can match the thrust force of the airplane, you will not fly. There will be no air running over the wings to create lift. And I didn't say zero forward momentum.

Please tell me where that logic is wrong. Two forces acting in opposite vector directions will always equal zero. Newton says so and so do I and you are not going to get me to change me opinion. I guess we can agree to disagree on this one.

 

[Caaveman82]

Not attacking you Dook... I am just askin questions.

If the wheels are not geared then really the engines are making the forward momentum, so it would not be at zero even on the belt. No?

[Bob Wessner]

It will take off. I have spoken.

Really, it will.. provided the conveyor belt is at least as long as the shortest take-off run and the plane has some pretty stout rubber on the wheels.

[shacolaid]

I watched the video a few times. The weight of the plane creates friction on the tarmac allowing the plane to move forward with the propulsion of the prop.

EVery pilot prior to takeoff performs an engine check taking the engine to full throttle while applying brakes. The plane does not take off. Would it not be same thing if the wheels and the conveyor are moving at the same speed in opposite directions? There has to be air passing over the wings. For the Bernolli (sp) effect to occur there has to be enough air flow over and under the wings(airfoil) to provide lift.

Again, if the conveyor belt speed and the airplane were truly matched in opposite directions, there would be no air flow created and the airplane would be standing still.

[Caaveman82]

It will take off. I have spoken.

Really, it will.

I believe in Bob.

[Duke McDukiedook]

And the theoretical belt would match the thrust force created by the engine, hence zero relative movement. The plane could go 100 mph and the belt would match it according to the problem.
I did say this was a theoretical problem from the start didn't I?

No offence taken.

[Borkunit]

I still say I am right in that if the conveyor belt goes fast enough to prevent forward movement of the plane and can match the thrust force of the airplane, you will not fly. There will be no air running over the wings to create lift. And I didn't say zero forward momentum.

Please tell me where that logic is wrong. Two forces acting in opposite vector directions will always equal zero. Newton says so and so do I and you are not going to get me to change me opinion. I guess we can agree to disagree on this one.

The problem with your logic is that the treadmill does not act as a force on the plane that is equal and opposite to the thrust produced by the propellers.  Fundamentally all the treadmill is doing is spinning the wheels.  There will be some small amount force applied to the plane in the rearward direction due to friction in the bearings (and probably other places) but this is not *necessarily* (nor is it even *incidentally*) the equal and opposite to the force created by the props.

Make sense?

[Bob Wessner]

This thread my challenge some others for length.

[Duke McDukiedook]

So translation besides rotation of the wheels plays no part in this?

[Duke McDukiedook]

Again, if the conveyor belt speed and the airplane were truly matched in opposite directions, there would be no air flow created and the airplane would be standing still.

I dunno man, clearly some are not fazed by scientific fact.

[tramp]

wheeles smeeles
the plane must have moving air under the wings for lift
an ultra light has a lot of lift due to it's light weight
a plane does not have to move forward to fly
it must have air moving under the wings for lift
if you were to lock that ultralight in place so movement is only up and down the plane will not lift off
i don't think the air from the propeller is enough to lift the plane up
what if it was a rear prop job

[Borkunit]

Look at it this way - the purpose of the free-spinning wheels on a plane is to NEGATE the influence of forces of the tarmac on the plane.  If the wheels were fixed (didn't rotate) the plane would never take off, unless it generated some massive amount of thrust and was able to drag itself over the asphalt.  The wheels are basically a lubricant that allows the plane to slide over the runway as freely as possible.  

[HavocTurbo]

Look at it this way - the purpose of the free-spinning wheels on a plane is to NEGATE the influence of forces of the tarmac on the plane.  If the wheels were fixed (didn't rotate) the plane would never take off, unless it generated some massive amount of thrust and was able to drag itself over the asphalt.  The wheels are basically a lubricant that allows the plane to slide over the runway as freely as possible.  

wow.... 4 pages of this??

already?

Don't forget the brakes man..... the brakes!!!!

[Duke McDukiedook]

The wheels allow forward movement of the plane to get airflow over the wings which create lift.
No forward movement, no fly. The theoretical conveyor belt prevents forward movement and matches the thrust of the engine, pound per pound.
The bearings push on the axle, which in turn moves the plane in the rearward direction, they don't just rotate.

[Steve F]

This whole thread about an airplane not being able to take off on a conveyor belt...
That's like saying you can't take off on any east-bound directon on the face of the earth because you're going with the earth's rotation and can't possibly fly that fast.