This is first time I saw this kind of topic. My answer is "YES". The plane can fly on a moving conveyor which travel at the same speed in the opposite direction.

I think we need to read the statement again. Can a plane take off on a conveyor belt moving at the speed in opposite direction?. It didn't mention that the plane will be stationary. Reason why there is a large number of people in the "no camp" because they misinterpret that the airplane will be stationary when the conveyor is moving in the opposite direction. Allow me explain a bit.

Cheesenium has post an extensive lift principle, so I'll skip that. For my explaination, you need to know at least the definition of newton's third law.

In newton's third law, every action has a reaction in the opposite direction. On a moving conveyor, an object will remain stationary (to a third eye) if there is a force in the opposite direction reacting on the object. For example, a car. The reaction force will be driving the car at the same speed of the conveyor, in the opposite direction. Because the conveyor is moving, the car appear stationary, like a dyno machine. Agree?

Now, in airplane, the reaction force comes from the thrust of the engine. When we run the engine, the engine move air from the suction to the exit. Basically it doesn't move air on top and below the wing, but across and within the engine. So there is plenty of thrust generated, but without air moving across the wings, it will never fly.

Next, imagine we mount the engine on a trolley, or cart (just the engine, without the wing). The thrust generated will transfer the force to the trolley, and hence pushing the trolley forward. The same effect can also be achieved with fan on a trolley. As the trolley travel forward, air travel across it. Although trolley cuts through the air, it don't fly because there is no wing. Now, we exchange the trolley with a hull and wings, so we get an airplane.

WHEN a plane travel forward (on the runway), only will, the air move across the wing, and create lift. (Hope everyone can keep up with this.)

Now, we upgrade the experiment a bit. We put a conveyor at the bottom. By running the airplane on a conveyor, the engine actually create thrust onto the hull. Despite whatever the ground speed maybe, the thrust generate force onto the stationary air. If the ground travel at light speed yet the air above it remain stationary, the thrust will generate force onto the air, unlike car which generate thrust onto the ground. So, the airplane will just keep moving forward.

When it gain enough speed, sufficient air movement across the wing, the airplane will take off. In fact, it will still require the same runway length disregard of whatever speed the conveyor maybe. Mythbuster is right.

This post has been edited by Aurora: Jun 17 2009, 01:01 AM

Ok ok, when you mentioned regardless of what the conveyor belt is doing. I think you still mistunderstand the concept of the conveyor belt here.

The conveyor belt is moving at a constant speed to match the forward speed of the aircraft so the aircraft is effectively stationary. The conveyor belt accelerate at the same rate as the engine would accelerate the aircraft. Effectively canceling it's forward rolling speed.

Elliminating the conveyor belt, and let's say use an alternative method to keep the aircraft stationary at the same spot- wheel brakes.

Will the aircraft lift off?

There's another clip that support the 'no' camp argument. By, guess who? The one that busted the myth themselves.



Problem with this argument is, not all the aeronautics experts are in the same camp. Asking several engineer from Airbus/ Boeing / Bombardier/ Embraer may yield different answer. So far the debate carried on because experts in this field are yet to come together to give a definite answer.

This is basic answer.

The conveyor is just a distraction, and having nothing or contributor to the airspeed on the wing.

If it can, carrier already build a conveyor on its on board runaway so that fighter jet can fly without the aid the steam pressure jack (I don't what they called it) under the jet wheel to assist the take-off on a short run way.

Ok I've edited first post to reflect the purpose of the conveyor belt and clarity of the quesiton. There are other better method to keep a plane stationary I guess

eh, who's talking about the wheel la?

btw the engine just moves the plane forward. it only creates the thrust, not the lift.

the lift happens when the air passes above and under the wing, creating the pressure that generates the lift.

the idea behind the conveyor belt going backwards is for it to counter the force generated by the propeller / engine.

That's a difference in magnitude, not in principle. Look at this wind tunnel where model planes are tested (the planes are towed, like kites) -


They would fly up momentarily if you tug on the string (as this creates airflow against the kite, and subsequently lift), but unless you continue running backwards it will fall back down as it loses airflow/lift.

Try flying a small and heavy kite on a windless day. It's frustrating and your legs will get a good workout

If it generates thrust towards the hull, doesn't that create drag and result in force going in the backwards direction, working against the airplane?

And besides, the point of the conveyor is to match the airplane's forward speed but in the opposite direction, so "keep moving forward" won't happen unless the wheels loose frictional contact with the belt.

Seagates: The way you've formulated the question will only lead to the single answer that the plane doesn't take off, because the plane's wings do not move relative to the air [at least the simplified version - which you seem to have incorporated your own assumptions into, thus solving the problem].

Let me ask the following question then:

Assume ideal conditions (perfectly horizontal belt, perfectly round wheel). Take a plane, with the brakes on the wheel off. Put it on a conveyor belt. Rotate that conveyor belt. What do you expect would happen?

The (perhaps unexpected) answer is that the plane would not move at all, even with friction between the belt and the plane's wheels, since this friction only serves to rotate the wheel, and not drag it behind along with it.

So, now the question is: If we turned on the engines, would the plane move forward? Yes it would, because when the engines were switched off and the belt running, it did not move, and also because it's the engine propels the plane forward by pushing the air backwards, and not through the wheels of the plane. The plane would then take off as usual.

But of course, we don't live in ideal conditions, and we then bring in a new concept: rolling resistance. This resistance leads to the wheel slowing down and ending up being dragged along by the belt. But let's now consider the (linear approximation) form of the equation, which we can get off Wikipedia [http://en.wikipedia.org/wiki/Rolling_resistance#Physical_formula]



where F is this rolling resistance that opposes the motion of this force
Crr a coefficient that relates the rolling resistance with the normal force of the aircraft on the wheel and the wheel's weight down onto the ground
Nf is the normal force of the aircraft on the wheel and the wheel's weight down onto the ground

So what do we get here? We have that the resistive force on the wheels (in a sense, 'pulling it backwards') is proportional to the force applied on the ground by the aircraft and the wheels, and not proportional to the velocity at which the airplane moves relative to the belt, i.e. the rolling resistance force is constant, assuming away fuel being burnt out and stuff factoring into that normal force and also assuming away any lift generated by the plane's wings when it moves.

It then becomes a question of: Can the engines then create enough forward thrust, or in essence, a force pushing the plane forwards such that it will overcome this rolling resistance being applied on the wheels which is 'pulling the plane back'? If yes, the plane will move forward, which does not comply with your question at all. If not, the plane won't move forward, and either stay still, or move backward if the thrust isn't large enough to overcome the rolling resistance.

And there's the answer: If the forward thrust is large enough as to overcome this added rolling resistance (added to the other dragging forces on a plane), it will still move forward on this conveyor belt, no matter how fast the conveyor belt spins, because the rolling resistance force remains constant, and the speed of the conveyor belt will only lead to the wheel spinning faster and faster, and the wheel will still go forward, because the added thrust of the plane will cause it to spin even faster than the conveyor belt ever can. And if this thrust can overcome the usual drag forces present when the plane's in motion wrt the air, plus this rolling resistance, it'll achieve the velocity needed to take off and it will take off.

Note: We only consider a linear approximation, it might be that the force is indeed dependent on the relative velocities at higher orders, and this argument will fail, but this semi-ideal model should give you an idea.

Note 2: The wings, etc, etc, don't matter at all, because the question that matters is: Will the plane move at all relative to a stationary observer not on either the belt or the plane, but say standing at the airport watching, and we also assume that the air remains still in the inertial frame of that observer (or to simplify further, will the wheels move forward). If it does move forward, it will also move forward wrt the air, and thus you have lift, and takeoff soon after if the thrust is also large enough

Note 3: The key word here is that we're talking about wheels, not cuboids on the belt or something. Friction on a cuboid will drag it backwards, but friction on a wheel will cause it to only rotate and nothing else [assuming again ideal conditions with no rolling resistance].

This post has been edited by bgeh: Jun 17 2009, 09:30 AM

I don't follow the discussion.

but as long as there's sufficient airflow across the wings, the plane will take flight.

It's the airspeed we should be looking at, not the ground speed. ie: in strong headwinds winds, a light aircraft can appear to be "hovering" above the ground.

In my opinion this is not done cause it will be wasting too much energy.

I'm struggling to follow the discussion as well.
But I agree with as long as there is sufficient airflow across the wings it will take off.

The plane's relative velocity with respect to the conveyor belt will be twice the speed then if it is actually moving on the ground.

google mythbuster, they busted the myth long time ago....

and yes, the plane can take off on a conveyer belt

This post has been edited by styrwr91: Jun 18 2009, 01:10 AM

Assume that instead of a conveyor belt, that we have 1000000 idiots pushing the plane because they have nothing better to do. Also assume that the surface area of the aircraft is so high that and it's weight so low that take off speed becomes largely irrelevant. Since there is motion and thus airflow, the plane will take off. As long as the brakes on the wheels are on and the coupling of energy complete, it should fly.

The problem with the conveyor belt ideal is actually a transfer of power at the wheels, because it's well known that you can throw aircraft off other devices that deliver power, like the steam catapult on US Navy carriers. When you turn the conveyor belt, the wheels would turn in the opposite direction, but this assume the wheels are perfect and have no loss, meaning they turn the forward energy of the conveyor belt perfectly into rearward energy in the wheel, thus you have an equalibrium.

The great big question is, due to losses, it would move, but in what coupled direction?

Yet the video clearly shows the ultralight moving with respect background behind it, thus generating air speed and lift.
While the thrust is indeed relative to the air (and not the ground), it is the air speed that generates the wings' lift.
So I'm not clear what the Mythbusters guys supposedly busted.

If the aircraft is on the conveyor belt . Parking brake on , the aircraft will not be going anywhere then

i think every physics question need diagram so that everybody wont misinterpret. because some people understand sentence differently

so yeah, a diagram would help

it very simple actually.if u have a miniature plane made of foam,tied one end of a string to the front of it and another to a rigid stationary object.switch on a table fan in front of it.u will see the plane flying rite?more or less.what happens is the and pushes the air through the wings of the plane to create lift.imagine the fan is swapped with propellers attached to the plane.it will do the same thing.so yes plane could take off on conveyor belt but only for very light propeller plane.air pushed by the propeller must be strong enough to create lift on the wings.i think u can visualize this, while u're at it try jet planes being put in similar condition,it wont take off

sometimes mythbusters do overlook some variables in an experiment.maybe we should call them missbusters?

now im confuse, mind explaining more on the jet aircraft?

Jet aircraft would be the worst type of aircraft to demonstrate this problem.

Somehow I realize this debate would not exist if somebody say 'Hey! Conveyor belt is the worse method of keeping a plane stationary'

waaa.. where's the diagram..
coz my lecturer told me, that he think in picture. so,
diagram please.
coz i could imagine 2 possibilities of the situation here..

Cherroy is right. The belt is distraction.

Imagine this:
A person running on a threadmill, then he gets a push from the back (which is the forward momentum from propeller/jet engine).
The guy either falls down face first, or he moves forward and run in front of the threadmill.

Same thing to the plane:
The conveyor belt will not be effective in keeping the plane stationary.
The plane will move forward DESPITE the conveyer belt.

OR,
there might be a possibility that the plane will just nose dive into the ground before it even moves anywhere (if the force applied is off centre)

The only way to get a plane to fly stationary is in the wind tunnel.