We're having this debate on another forum, and it's pretty intersting, although it appears to be giving a lot of headaches...figured I'd post it on here and talk about it here as well

quote A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction). The question is: Will the plane take off or not?

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Nick www.naskie18.com AIM: Naskie182010

The plane will not fly. It needs air going over the wings to create lift. If it isn't moving thorugh the air it will not fly.

well this huge conveyor is going to be kicking up some air..

if you can imagine air to be something more viscous like honey, you can see anything moving through it will pull a wake behind it.... but it won't be enough to lift the plane, that's for sure.

This is simple. Unless the conveyor can create a 50kt headwind the plane will not fly, that is a general speed that even the smallest actual planes need to fly. Another thing is that a plane floating and a plane flying are two different things. If the conveyor does kick up enough air to get the plane off the ground, it will just be sitting in ground effect, in a sense sitting on a cushion of air, but it will not have the ability to actually fly on its own.

So final answer. There is no way that it will fly minus the idea that it is pointed into a 50 or 60kt headwind. And if that is the case you don't even need the conveyor belt, it will just take off and be pushed backwards and then drop back as it going backward will decrease in airspeed and fall back down.

So basically it can't be done.

no it will not fly. like driving your car on a dyno.. wheels turn car stays.. no drag or lift created.. need both for plane.

Alright, well you guys are all boring and agree with me.

The majority of the people on the other forum seem to be trying to tell me that the plane will fly....which is why the debate is fun....its boring if you all just agree with me

quote Originally posted by naskie18: Alright, well you guys are all boring and agree with me. The majority of the people on the other forum seem to be trying to tell me that the plane will fly....which is why the debate is fun....its boring if you all just agree with me

Well comon man, why WOULD it fly?!

Of course it will fly. wheels will just turn faster as it takes off.
Aircraft are not axle driven.

The plane will fly. What I mean is, the wheels aren't what's making the plane move, so their speed is irrelevent. The prop is what's pulling the plane forward, so the conveyor can't actually prevent the plane from moving forward, since it's not having any effect on the actual force of movement. I think the conveyor would probably go haywire trying to figure out what direction to go, since the conveyor itself is what would be moving the tires. Look at it this way, the wheels on the plane are unnecessary, they put pontoons on them. The wheels only give the plane sometehing to ride on until it reaches sufficient speed to become ariborne.

Wolfhound is correct. The prop or jet engines will still push the aircraft but as the conveyor tries to compensate, the wheels will simply be spinning twice as fast at takeoff, which has nothing to do with whether the plane will fly or not. Now if you relied on the WHEELS to make the plane go forward, then it would never fly, but you don't, they have nothing to do with it.

John Stricker

quote Originally posted by Wolfhound: Of course it will fly. wheels will just turn faster as it takes off. Aircraft are not axle driven.

quote Originally posted by jstricker: Wolfhound is correct. The prop or jet engines will still push the aircraft but as the conveyor tries to compensate, the wheels will simply be spinning twice as fast at takeoff, which has nothing to do with whether the plane will fly or not.

Push the aircraft where? The aircraft is only in motion relative to the conveyor belt. Relative to the air which surrounds it, it is motionless.

Unless you have one hell of a head wind, a motionless aircraft is not going to have a sufficient mass of air flowing over the wings to generate lift, and subsequently allow it to fly.

I thought we were assuming the aircraft is 'motionless' relative ground?

Hmm. Never had this problem arise when doing takeoff calculations when I was in Aerospace Engineering school.

However, after thinking about it a bit. I actually believe the plane would take off faster into the air, than if there were no conveyor belt. Why? The conveyor will drag along a boundary layer, increasing the relative dynamic pressure under the wings, in addition to the relative incident airflow due to thrust and resulting wing lift.

Ground effects are a real thing, as any pilot will tell you. The rolling friction increase due to the wheels spinning faster, would be small relative to the amount of thrust given up for take-off acceleration and velocity.

As was stated in a couple of previous posts, the wheels are simply there for the plane to roll around on, and prevent crashes during landings and things.

The wheels have nothing to do with providing power for take-off airspeed velocity.

[This message has been edited by zardoz (edited 12-02-2005).]

quote Originally posted by JohnnyK: I thought we were assuming the aircraft is 'motionless' relative ground?

me too. is the engine on?

Simply put, no.

You get in your car, start it, floor it, but you're on a dyno.
Yes, you're "moving" fast, but you'll never get down the block that way.

Now this isnt quite the same as a jet, but the idea is the same.
Plane requires air over and under the wings. Lots of it. To get this it moves ittself forward at extreme speeds. Without this speed differance between the plane and the air around it, you get no lift.
Since the plane is remaining stationary, the only differance in air speed is from wind and turbulance from that belt. If that were enough to make a plane take off, we'd see our luggage shooting about airports near the baggage claims and parked cars going for a spin from the slightest breeze.

It could fly if it was a Harrier jump jet.

It's very simple. Relative wind makes an aircraft fly. Period. If the aircraft is moving into the wind fast enough, it will fly. A moving conveyor will not stop the aircraft from moving forward, it will just make the wheels spin faster since they don't provide the motive force. Having a conveyor move beneath and aircraft that is driven by a prop, jet, or rocket will not stop it from moving forward because the wheels don't provide the force.

John Stricker

quote Originally posted by jstricker: It's very simple. Relative wind makes an aircraft fly. Period. If the aircraft is moving into the wind fast enough, it will fly. A moving conveyor will not stop the aircraft from moving forward, it will just make the wheels spin faster since they don't provide the motive force. Having a conveyor move beneath and aircraft that is driven by a prop, jet, or rocket will not stop it from moving forward because the wheels don't provide the force. John Stricker

Hit the nail on the head.

Push the aircraft in a direction opposite the thrust applied. Spin that belt 200 knots for all I care, the aircraft is still going to go forward. Since the Earth IS a conveyor spinning a a little over 1,000 mph, aircraft take off on a "moving runway" all the time.

Extra point question, for a normal landing pattern on Runway 18, I fly a downwind at a heading of 360 degrees (north), a base at 270 degrees (west), and a final approach at 180 degrees, the same as the runway heading I'm landing on. Because I'm a moron I'm landing downwind and since it's KS, it's a strong wind FROM THE NORTH at 80 knots. My final approach speed is 80 knots. When I turn base to final, does my airplane fall from the sky (even if my wheels are spinning)? Second extra point question, what's my groundspeed when I touch down? What's my airspeed indicator read?

John Stricker

quote Originally posted by dguy: Push the aircraft where? The aircraft is only in motion relative to the conveyor belt. Relative to the air which surrounds it, it is motionless. Unless you have one hell of a head wind, a motionless aircraft is not going to have a sufficient mass of air flowing over the wings to generate lift, and subsequently allow it to fly.

No.
The conveyer belt idea is just shortening the runway in theory. Heck, put it on jack stands till it takes off.

[This message has been edited by cliffw (edited 12-02-2005).]

quote Originally posted by jstricker: It's very simple. Relative wind makes an aircraft fly. Period. If the aircraft is moving into the wind fast enough, it will fly.

John, you have to remember that planes fly by moving their fuselage (and wings). Having a plane on a conveyor would be like tying the rear end of the plane down so it is stationary. I don't think the prop *alone* can influence enough surface area of the wings to actually cause it to lift off the ground. It's not a helicopter.

No, you're assuming that a moving conveyor under a free wheel will keep something from moving forward, it will not. Now if you want to add conditions like the airplane has to be DRIVEN by the wheels or that the brakes have to be on, then that's another topic, but having landed, and taken off, on near glare Ice I can assure you that what the wheels are doing under there has little to do with keeping the aircraft from moving forward.

John Stricker

quote Originally posted by ryan.hess: John, you have to remember that planes fly by moving their fuselage (and wings). Having a plane on a conveyor would be like tying the rear end of the plane down so it is stationary. I don't think the prop *alone* can influence enough surface area of the wings to actually cause it to lift off the ground. It's not a helicopter.

I think you must all re-read the description !! It will not take off!! The conveyor belt is controlled to provide the same rearward motion of the conveyorbelt, as the forward motion of the plane.That means, if the plane's motive power would move the plane forward at say 120 mph, the conveyor belt would compensate by running at 120mph rearwards, thereby maintaining the plane in the same position...motionless!! The wheels would be spinning at 120 mph, the conveyor belt would be running in the other direction at 120 mph. There would be no air passing over the wings, because the plane is motionless with regard to ground/air.The best equivalent I can give, is running on a training mill. Except you run at a speed to match that of the treadmill. In this case, it is the treadmill running to match the speed of the aeroplane's attempted forward motion.Now, if a prevailing wind should happen to blow continuously at 120 mph, it would lift the plane, and the plane would hover, until its motive force pushed out enough power to move it forward at more than the 120 mph windspeed it was flying into.
Nick

[This message has been edited by fierofetish (edited 12-02-2005).]

Extra point ground speed Zero. apparent hover. Of couse someone would have to tie it down before you could kill the throttle.
Art Scholes once flew a J-3 down the runway backwards using the fact the wind speed was greater than the stall speed.

But, by having a conveyor propelling the plane backwards, in theory, you are decreasing air speed.

quote Originally posted by jstricker: No, you're assuming that a moving conveyor under a free wheel will keep something from moving forward, it will not. Now if you want to add conditions like the airplane has to be DRIVEN by the wheels or that the brakes have to be on, then that's another topic, but having landed, and taken off, on near glare Ice I can assure you that what the wheels are doing under there has little to do with keeping the aircraft from moving forward. John Stricker

No, we're assuming that the planes ground speed is 0, but that the converyor belt is moving, therefore so are the planes wheels. So the answer is no, the plane is stationary. Everyone is interpreting the question differently. If the plane was rolling for takeoff, then yes, it would take off.

yes because the plane is moving the air w/ the prop... its not turning its wheels... so all the belt is doing is making the wheels spin faster... i think it would actually take off faster because of the wind being produced by this big belt.

Read it again, Wolfhound. I'm landing DOWNWIND.

I have landed a Piper Tomahawk with a forward ground speed of less than 10 knots. I was interesting, to land on the numbers and be stopped before passing the end of the numbers.

John Stricker

Edited to add, I may have been unintentionally misleading when I said I was flying my "downwind" leg at 360 degrees. While in a normal pattern, that would be a fact because in THIS example I specified my landing stupidity, that would really be flying UPWIND.

quote Originally posted by Wolfhound: Extra point ground speed Zero. apparent hover. Of couse someone would have to tie it down before you could kill the throttle. Art Scholes once flew a J-3 down the runway backwards using the fact the wind speed was greater than the stall speed.

[This message has been edited by jstricker (edited 12-02-2005).]

The problem states clearly that the plane is moving.

You're assuming it's going backwards. To have that happen other conditions will also have to be in place. The brakes will have to be causing more drag than the engines thrust, for instance. Or the engine would have to be so anemic that it couldn't overcome the simple rolling drag of the free-wheeling tires. But those weren't part of the stated conditions.

Now if you want to add conditions that the aircraft remain MOTIONLESS RELATIVE TO THE WIND then the aircraft will not fly, because relative wind makes an aircraft fly, nothing else. FWIW, it's quite possible (and not unusual in the case of an ultralight) for there to be enough wind to make them fly with no power or motion whatsoever.

John Stricker

quote Originally posted by cliffw: But, by having a conveyor propelling the plane backwards, in theory, you are decreasing air speed.

It would fly, if it is prop. Because the faster the plane pushes forward, the prop wash might give you just enough lift over the wings to pop that bad boy a little off the ground. With full flaps and a high angle of attack, you will fly off with ease.

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[This message has been edited by jstricker (edited 12-02-2005).]

Strickter, If your landing downwind in an 80 knot wind I,m not flying with you.

No you won't. If you could, you could take off by doing a full power runup and you can't even come close.

John Stricker

quote Originally posted by Wichita: It would fly, if it is prop. Because the faster the plane pushes forward, the prop wash might give you just enough lift over the wings to pop that bad boy a little off the ground. With full flaps and a high angle of attack, you will fly off with ease.

quote Originally posted by jstricker: And exactly HOW are you going to make it's groundspeed zero if the wheels supply no motive force? John Stricker

Because it's a THEORETICAL physics question. The question is implying that the planes ground speed is 0, at least thats what I gather he wanted the question to be since he specified it 'tracks to the speed of the airplane'. I think he wanted to know, if the wheels were rolling fast, but the plane was not moving, would it take off, and the answer is no. Plane, stationary, runway, flying by.

Yeah, those who think the plane won't fly are stuck on the idea that the wheels actually have anything to do with the forward motivation of the plane. The conveyor can NOT prevent the plane from moving forward because they have nothing to do with the plane's propulsion. If this were a car with wings, that relied on the drive wheels to gain forward momentum, then yeah, you'd probably not get off the ground.

If you put racing slicks on an airplane will it take off faster? And why?

[This message has been edited by Wolfhound (edited 12-02-2005).]