lol.. we got some example which plane no need wheel to fly like amphbia plane and ski plane...
The plane need moving air at it wings to produce lift... now thats engine job to provide the force...

i know..
i meant i read through the pages, it seemed that some people got the idea that the wheel that provide forward thrust to the plane.
which is not.

yup, the wheel is free to move, but it can be stopped by the brake of course

dont you all think that plane on a conveyor is same like plane with parking brake engaged or plane with engine running but stay stationary?

when a plane remain stationary, how did it air travel above the wing and below the wing, which means there's no low pressure above the wing and high pressure below the wing, also can say as no lift, so how can a plane fly without lift?

let's take boeing 747 or other passenger aircraft for example, the engine produce thrust and push the plane forward, the forward motion of the plane causes the air to around the wing and create low pressure above the wing and high pressure below the wing, thus create lift so the plane takeoff

unless you are putting the plane's engine in vertical position (similar to VTOL), otherwise you can artificially create airspeed at the wings of a plane to create lift, yes, i mean like those wind tunnel test where the model plane are in stationary position.

No it doesn't. It basically cancels out all the friction that is supposed to pull the plane backwards, well, because it has the ability to roll. That's the gist of it. Spin the conveyor belt and the wheel will simply roll, either forwards or backwards. It doesn't provide any thrust at all.

Or put it this way: The plane isn't advantaged in any way if you got the conveyor belt to spin forwards either.

Heck let me put in the force analysis:
Suppose the conveyor belt's already moving backwards, with the plane also moving backwards. Make some simplifying assumptions:

1) The wings do not generate any 'negative lift' if the plane move backwards
2) Fuel isn't lost

These 2 assumptions will be needed to ensure the weight of the plane on whatever object it rests upon stays constant.

Right, from A level/SAM (I think)/F6 Physics, we model friction by F = mu*N, mu the coefficient of friction, N the normal force on the belt

So if the thrust is less than this frictional coefficient (we assume for simplicity's sake that the coefficient of sliding friction = coefficient of static friction = mu. Analysis still holds because usually coefficient of sliding friction < coefficient of static friction)

Suppose the thrust is larger than the coefficient of friction. The plane will accelerate relative to this belt, and the key word is, it will continue accelerating relative to the belt no matter how fast the belt is moving, it only depends on the initial velocity the belt was travelling at when this thrust was applied (remember, I assumed that plane travels backwards with belt). Plane takes off nonetheless because it'll continue accelerating and at some point its velocity to some observer in the airport will turn positive, and take off occurs.

Of course we never see this, because the landing gear would probably shatter into pieces if we attempted it, and even that's assuming we have those mystical super powerful engines that can provide that much thrust.

This post has been edited by bgeh: Jun 22 2009, 03:28 AM

So, since many of us here agree that the plane can take-off even when it is on the conveyor belt, can we use this to decrease the length of the runway?

*Let's assume that we can manufacture a conveyor belt that is strong enough

i don't think the plane can take off...

plane need wind to lift off. thats why they need runway.
if stationary, no wind to lift their wings? then how to fly?

i suggest u to include big fan infront of the plane (to blow wind) + conveyor belt under it's wheels.

then we can start to discuss.

if i dont get you wrong, what you are trying to say is, the plane is moving forward eventhough the conveyor belt is moving backward, because the thrust generate by the plane is more than the speed of the conveyor belt, so the plane accelerate instead of stationary, if that's the case, when there is sufficient pressure build up on the wing then the plane will take off.

then i ask you how can you generate enough pressure to lift the plane??

No, the thrust of the plane has absolutely nothing to do with the speed of the conveyor belt. The thrust will only determine the acceleration of the plane relative to the conveyor belt supposing the force by the engines is larger than the frictional force. So the question to ask is whether the plane will accelerate relative to the belt. The speed of the conveyor belt becomes irrelevant in that context, and that's the point.


In the case of the 'free spinning wheel', basically even without thrust, and in the presence of a moving conveyor belt, ideally the plane will not move, even though there is friction between the tyre and the belt; this is because the conveyor belt simply causes the wheel to roll, and doesn't move the plane at all. The wheel will continue rolling and rolling matching the speed of that conveyor belt, even with 0 thrust. A simple experiment (I used thumbscrews - which are far from ideal, if possible get as round a cylinder as possible with moderate mass (basically not too light)) is to take a cylinder and put it under say a plastic ruler or a flat surface with enough friction. Pull that ruler and you'll see that the object will just roll and stay at the same place. Why? Conservation of momentum of that object. It'll also seem to roll slowly in the direction that you pulled the ruler. Why? Rolling resistance that I discussed earlier. But in the ideal case, the rolling resistance doesn't exist, and thus the plane will stay in the same place no matter how quickly you move the belt. The semi ideal case can be modelled by adding an extra 'dragging force' term, the rolling resistance. If the thrust can handle that, we're done, and plane will take off. That's it.

so you are in no camp, the plane will not take off unless there is enough pressure generate and lift the wing right?

Airplane can take off because of the wing lifting, which is generate through differentiation of pressure between upper and lower part.

Strip up the wing, no matter how fast the plane travel, it cannot fly.

The thrust on plane is to enable the plane to travel fast enough so that the airflow is fast enough between the wing to have enough pressure of lifting force.

So if the airflow is fast enough, you don't need to have thrust of engine to propel the plane forward to fly (in reality impossible as you won't have those kind of air speed in ordinary situation), just open up the wing will do, just like what they do in air-tunnel test or just like a bird, when you see there is enough wind, they just spread out the wing, then they are hanging up in the air already.

No, I am saying that there is a nuance. It depends on the magnitude of the thrust, that's it. I did not say whether it's a definite yes or no.

I'll state my position again: The thing that matters is to ask whether the plane can move forward with some positive velocity. If it can, then yes it will fly, if it has enough thrust. We've shown (somewhat) that the plane does move forward, so it indeed can fly. The wings and pressure difference etc, etc aren't really important, because all you'll need is some mystical 'more powerful engine' to solve that problem if the engines you already have can move the plane forward, but lack sufficient thrust to cause the plane to fly.

This post has been edited by bgeh: Jun 24 2009, 12:13 AM

discussing physics in words will lead to confusion.


the plane is on infinitely long conveyor belt.

so, if someone could set,
whether
-the force that moves the plane forward solely from the engine. or,
-the force that moves the plane forward solely from the wheel rotation,

whether
-the conveyor belt is freely moving, or
-the conveyor belt in the case of wheel rotation, move freely with the wheel,
-the conveyor belt moves at constant speed

It makes no difference in any above option, what the sole important factor that dictate the airplance can fly or not is the airflow between the wing.

If any method can, carrier already implemented the conveyor method to shortern the run away for fighter jet take off purposesly.

After reading for a while, I think I am switching camp

The explanation given is that, the conveyor only apply force to the wheel, and since the wheel is free rolling, the force doesn't transfer to the plane, so it won't be affecting the plane.

If conveyor belt is moving back at A speed, and plane have a forward trust of B speed. The only difference is that the wheel will be moving at A+B speed while the plane move forward at B speed.

Even if the conveyor belt is moving to match the forward trust of the plane. It simply means the wheel move at 2xB speed while the plane moves at B speed.

So on a infinitely long conveyor belt. The plane will still treat it as if it's a static tarmac and take off after gaining enough forward movement to achieve take off speed.

Reason wheel brakes can keep a plane on the ground is that the brake pads is the transfer medium for the force on the wheel to the aircraft.

Did I get it right?

Exactly, but with one caveat: Thrust is a force, not speed, i.e. it's proportional to acceleration. The only reason we equate the idea of thrust to speed is because the drag force imposed by the air is proportional to the square of the velocity [at higher velocities, it's linear for low velocities], so you need a higher velocity to get a drag force that will cancel out a larger forward thrust.

Yes, the brakes applied suddenly changes the 'free spinning wheel' into a cuboid like object, which requires a lot more thrust for the plane to move, if it manages to overcome the friction (see one of the above posts)

To those who actually support the 'YES' answer, what are you guys thinking?

I'm am not a scientist. In fact, I'm pretty ignorant when it comes to science. But even I know that most airplanes, in this case, a 747, need to have wind pass it's wings in order for it to fly.

If a plane is on a conveyor belt that is moving backwards as fast as the plane is moving forward, then the plane is actually just staying still. If the plane stays still, then there is no wind passing it's wings.

And if you think a plane, with enough propulsion, can take off while staying still, then you are right, but those planes are called VTOLs (Vertical Take Off and Landing), and they use very powerful thrusters to lift them UPWARDS when taking off.

Other, more traditional planes have thrusters (propeller, jet turbine, etc) that move the plane FORWARD so the wind will do the job of lifting the aircraft.

UPDATE: Okay, I've seen the Mythbuster episode, and I am now in doubt about my own answer. But that doesn't mean I fully accept the other camp, though. The plane flown was just a light, propeller based plane, not a huge jet turbined behemoth like a 747.

UPDATE: Okay, I changed my mind. Actually it is possible to take off, even for a 747.

This post has been edited by mylife4nerzhul: Jun 24 2009, 08:48 PM

I think no matter what it will fly.
Cause the wheel is going freewheel.
Is just the matter will it get enough speed.

Nope, it wouldn't help the plane to take off earlier. We will still need a conveyor as long as the runway.


Glad that you change your mind. The thrust is acting on the stationary air, not on the conveyor. If we have wind traveling backward that fast, the airplane will take off vertically. That is how scientist did the simulation in wind tunnel.

i thought myth buster already confirmed this that it can take off?