Airplane on a conveyor belt - will it fly?

[Duster929]

Actually, I was almost convinced it would fly, but I don't think it does.

What happens is that the plane picks up speed, but the conveyor spins faster and faster, so the wheels spin much faster than they would under normal conditions at that airspeed. The heat in the tires exceeds their design parameters and at least one of the tires blows. Then the plane nosedives into the conveyor or slides sideways off it. Or maybe ingests some fragments of the tire into and engine, which bursts into flames. Either way, people are going to die.

--- D

[suprPHREAK]

Actually, I was almost convinced it would fly, but I don't think it does.

What happens is that the plane picks up speed, but the conveyor spins faster and faster, so the wheels spin much faster than they would under normal conditions at that airspeed. The heat in the tires exceeds their design parameters and at least one of the tires blows. Then the plane nosedives into the conveyor or slides sideways off it. Or maybe ingests some fragments of the tire into and engine, which bursts into flames. Either way, people are going to die.

--- D

You win

[bear22099]

Actually, I was almost convinced it would fly, but I don't think it does.

What happens is that the plane picks up speed, but the conveyor spins faster and faster, so the wheels spin much faster than they would under normal conditions at that airspeed. The heat in the tires exceeds their design parameters and at least one of the tires blows. Then the plane nosedives into the conveyor or slides sideways off it. Or maybe ingests some fragments of the tire into and engine, which bursts into flames. Either way, people are going to die.

--- D

the wheels really have little to do with it. They spin freely on the plane, does not matter what the ground does actually, it could even run double speed in the same direction as the plane, and other than the plane needing to pay attention to direction, it would not make it run faster, it would just spin the wheels faster, and they are not driving to the aircraft, just following along.. in other words, the plane would fly.

[nobbie48]

Imagine you were riding a skateboard on a treadmill and keeping your position by holding onto a rope. You would be stationary. If someone pulled on the rope you would move forward. The computer controlling the speed would do a melt down because it couldn't ever get the speed to correct to zero because it is linked to the belt speed not the person pulling the rope. Either that or the belt / wheels would fry. If you were pulled fast enough and you had wings (Fat chance on this site) you would fly.

[shil]

It never says “speed to zero”; it says “same as plane”. If your pal is pulling you forward at 10 kph, then the belt’s running at -10 kph. You’ll get pulled off the treadmill at 10 kph, with your wheels turning 20 kph.

The plan takes off, with its wheels tuning twice as fast as normal.

[kraigar]

Clipped from another site...

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).
Can the plane take off and why?

The plane flies. Everyone seems to be working off of one of two assumptions.

1) the conveyor will travel at the speed that the wheels move.
or
2) the somehow magically the plane starts to howver because of thrust over the wings.

Here is the key as I see it. The "conveyor has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same"

So.. If the plane is moving, air is moving over the wings and generating lift, who cares how fast the conveyor moving as it tracks the flight speed.

If the plane is not moving because the conveyor is going in the opposite direction as the thrust., it still doesn't matter because in that case the conveyor is also not moving.

Remember the conveyor tracks the plane speed, not the wheel speed. The point is that in this example the plane IS moving, and therefore it WILL fly.

Now where's my free VFR as a prize?

[farmerjoe]

I love it - even the aerospace engineers are all over this one...

Here's a good reference for a credible explanation:

http://www.straightdope.com/columns/060203.html

[Gaggu P]

+1 i agree

The answer has always been, and always will be, YES the plane will fly.

Unlike a car, it does not derive its speed from pushing off the ground. Its speed comes from thrust to (pushing off of) the air.

So, if the ground is moving at -100kmh, as long as the thrust from the engine is sufficient to overcome the friction of the wheels on the ground (which we know it is, since planes fly every day) then the plane will continue to accelerate forward, relative to the air around it, and the ground speed becomes irrelavant.

Why is this true? Because once a plane leaves contact with the ground, it continues to accelerate. If it was dependant on contact to the ground, then it would never ever fly, even if the ground was stationary.

ANYONE WHO KNOWS HOW PLANES FLY WILL AGREE.

[Riceburner]

the plane takes off.....it leaves the wheels behind....cause they melted into the conveyor belt..

[donut]

the wording of the riddle is rather cryptic, am i correct in thinking it means that the conveyor will adjust speed automatically to keep the position of the plane wrt to the ground next to the conveyor the same? think of a car on a dyno or something like that.

if that's the case, then i dont believe the plane will take off. lift has very little to do with thrust, it relies on the wing passing thru the air at speed. if the speed of the wing relative to the surrounding air is zero, then there is no lift. even the air being sucked into the jet engines will have little or no lift effect.

for the people who say it will fly, consider this. if the plane did take off, immediately after leaving the conveyor it would be some distance off the ground with zero air speed at full throttle, it might move a few feet forward before crashing back down onto the conveyor and once again, eveyrone dies a horrible horrible death.

[Biggsy]

Refactor the problem.

 ->-           PLANE
[_-_-_-]       BELT

Imagine we take the convey belt and unravel it down the runway. The plane runs down the runway as per usual. Only there a tail wind exactly the same as the speed of the plane - so effectivly the plane is sitting in still air.

@@@                           WIND >>
       ->-                    PLANE
--------------------------    BELT UNRAVELED

Could it fly in that? I'd say so.

What the wheels are up to isn't really material (especially once flying)

[donut]

dammit... after reading superphreak's post a few times thinking he's an idiot... i realized it was me that was confused.

there is no "driving" connection between the plane and the wheels so long as the brakes arent on. so, no matter what the conveyor does, it is incapable of keeping the plane in the same spot, the wheels will be turning at very high speed but that has nothing to do with lift or thrust or blah blah blah..

fack.

[donut]

Refactor the problem.

 ->-           PLANE
[_-_-_-]       BELT

Imagine we take the convey belt and unravel it down the runway. The plane runs down the runway as per usual. Only there a tail wind exactly the same as the speed of the plane - so effectivly the plane is sitting in still air.

@@@                           WIND >>
       ->-                    PLANE
--------------------------    BELT UNRAVELED

Could it fly in that? I'd say so.

What the wheels are up to isn't really material (especially once flying)

no, it wouldnt fly. that's why planes take off into the wind.. it would end up in the westbound 401 collectors and kill everybody.

[Mikey-D]

The air is cycled through the engines, not over the wings, which is what counts. No airflow over the wings; no lift; plane stays still.

That is true if the plane's engine are ONLY producing enough thrust to JUST overcome the friction for the wheels and bearings on the ground at that speed. Since a plane generates much MUCH more then the plane will acclerate beyond that.

**************

No im surprised no one here has spotted the way this riddle is worded. It says the plane is moving forward. If the plane is moving forward, there is air over the wings, which means the plane will fly.

Again, a planes forward motion is not derived from its wheels, but by the engines on the air.

Again, I'm not buying it. The plane is standing (which was also stated) and knowing that, it would need to accelerate to move. Not only that, your idea of the friction being overcome is false. Let's use a Boeing 747-100 as an example.

Max Takeoff weight = 735 000 lbs
Thrust = 50 100 x 4 = 200 400 lbs
Coefficient of friction between rubber and tarmac = between 1.00 and 2.00

(If you remember your high school physics, and specifically how to do an FBD, it'd help here)

Friction Force = Coeff. of Friction x Normal Force (which in this case is the weight)

Thus, Friction > Thrust.

If you'd like a discussion about aerodynamics, propulsion theory or for me to solve this problem using the Navier-Stokes equations, I can do that too; however, Newton's Second Law is all that's really needed to disprove it.

assuming your numbers are correct, you forgot one thing...
while the coefficient of static friction between rubber and dry concrete is about 1.0 (so assume a bit higher for asphalt), that is the coefficient of friction for a block of rubber. The wheels spin on their axles, they dont slide along the ground. so the coefficient of friction that you would need is between the wheels and the axle (which is mostly overcome with bearings, i would assume), not the wheels and the tarmac (that friction would be negligible)


by using your assumptions/numbers, the plane would NEVER be able to take off.

[donut]

by using your assumptions/numbers, the plane would NEVER be able to take off.

his logic is still wrong, but i initially thought the same thing.

you cant slow down the plane by moving the conveyor backwards, it's simply impossible as there is no drive link between the plane and wheels.

the engines "push off" against the surrounding air, not the conveyor, the only affect the conveyor will have is making the wheels turn really really fast.

[Martin]

It won't fly. If I understand the question correctly the planes displacement is ZERO over the time the conveyors belt is turning. The only way a plane can lift off is air needs to pass over the wings to create lift. If the plane is stationary the wheels are turning and the conveyor belt is holding it in place then how the hell can there be lift created if no air is passing over the wings? ??? ? ? ??

THE PLANE WOULD HOWEVER FLY if there was a gigantic fan placed in front of it to create air speed over the plane to match the planes tire speed. Then theoretically it would take off once it hit the correct speed.

Without air passing over the planes wings it will not fly.

Here's a little demonstration. Go jump on your treadmill and pretend you are the plane, crank the thing up as high as it goes and run as fast as you can. You don't feel much against your face do you in terms of air speed. Now go outisde and run down the street as fast as possible and notice how the air hitting you. That is what the plane needs to fly.

[CageyB]

by using your assumptions/numbers, the plane would NEVER be able to take off.

his logic is still wrong, but i initially thought the same thing.

you cant slow down the plane by moving the conveyor backwards, it's simply impossible as there is no drive link between the plane and wheels.

the engines "push off" against the surrounding air, not the conveyor, the only affect the conveyor will have is making the wheels turn really really fast.

Yup. I thought about this some more last night and realized it. My assumption was that the conveyor moves to counteract the plane's movement, and not at the opposite speed to the plane, in which case it takes off.

Also, interesting to note that even if the conveyor were strong and fast enough to keep the plane still, it could still take off since the conveyor would be essentially acting like a fan due to air viscosity and boundary layer effects.

[donut]

It won't fly. If I understand the question correctly the planes displacement is ZERO over the time the conveyors belt is turning. The only way a plane can lift off is air needs to pass over the wings to create lift. If the plane is stationary the wheels are turning and the conveyor belt is holding it in place then how the hell can there be lift created if no air is passing over the wings? ??? ? ? ??

THE PLANE WOULD HOWEVER FLY if there was a gigantic fan placed in front of it to create air speed over the plane to match the planes tire speed. Then theoretically it would take off once it hit the correct speed.

Without air passing over the planes wings it will not fly.

Here's a little demonstration. Go jump on your treadmill and pretend you are the plane, crank the thing up as high as it goes and run as fast as you can. You don't feel much against your face do you in terms of air speed. Now go outisde and run down the street as fast as possible and notice how the air hitting you. That is what the plane needs to fly.

that demonstration is invalid... you are driven by your feet, sort of like wheels, a plane is not.

next time, put on some roller blades and get on the same treadmill with a rocket pack on your back.... no amount of treadmill speed will keep you from moving forward.

[mrcantrell]

It won't fly. If I understand the question correctly the planes displacement is ZERO over the time the conveyors belt is turning. The only way a plane can lift off is air needs to pass over the wings to create lift. If the plane is stationary the wheels are turning and the conveyor belt is holding it in place then how the hell can there be lift created if no air is passing over the wings? ??? ? ? ??

THE PLANE WOULD HOWEVER FLY if there was a gigantic fan placed in front of it to create air speed over the plane to match the planes tire speed. Then theoretically it would take off once it hit the correct speed.

Without air passing over the planes wings it will not fly.

Here's a little demonstration. Go jump on your treadmill and pretend you are the plane, crank the thing up as high as it goes and run as fast as you can. You don't feel much against your face do you in terms of air speed. Now go outisde and run down the street as fast as possible and notice how the air hitting you. That is what the plane needs to fly.

that demonstration is invalid... you are driven by your feet, sort of like wheels, a plane is not.

next time, put on some roller blades and get on the same treadmill with a rocket pack on your back.... no amount of treadmill speed will keep you from moving forward.

I'm not much of a scientist, but I believe you are correct. Because the engines on the wings are propelling the plane, not the wheels/tires the plane will progress down the runway regardless of what the wheels are doing... they have no drive.

[suprPHREAK]

rewritten for clarity. This is also sent to an aerospace engineer, so I will let you know his answer:


Any aviator will tell you that it is not ground speed that is required for lift-off, but airspeed. As I am sure you all agree, it is air over the wings that works to create lift, and thus flight.

A plane derives this airspeed by moving forward as a reaction of the thrust of the engine (be it jet or propeller) pushing against the surrounding air.
The wheels of an aircraft are not like a car, in that they are not powered in any way. They have no effect on the aircraft's ability to accelerate. They are just dollies to, for arguments sake, eliminate friction with the ground.

So, since the wheels have effectively no friction with the ground, the conveyor can move as fast as it wants, but it will have no effect on the plane's ability to accelerate through the air.

The result of this, is if the plane is moving at +100kmh airspeed (since the engines act upon the air, not the ground), the conveyor is now moving at -100kmh groundspeed. Typical math says that this cancels each other out to equal 0 effective ground speed. However since the wheels are not part of the propulsion of the plane, and not causing the plane to go +100kmh, it will in fact add up to +200kmh ground speed, while still maintaining +100kmh airspeed. As long as the speed reaches the point where lift exceeds gravity, then the plane will fly.

Summary: due to wheels, there is zero friction with the ground (arguments sake, lets say they are mythical perfect zero friction bearings. Heck, we are talking about an airplane sized superfast conveyor after all). Because of this, the conveyor is presenting zero opposing force to the plane to slow down its acceleration.