Plane on a conveyor belt

Handsome_Chris

Wow! This is like Project Manager vs Engineer. PM plays the "this is what is written down" against the Engineer's " this is the reality of the situation" card.

I remember how confused people were by one of the Fast and Furious films where cars were driven into a cargo plane. 

A simple way of looking at it is that if a jet car was put on a rolling road and was pushed, or pulled, by an external force then would it move forward.

The answer is yes.

Maynehead

Handsome_Chris said:

A simple way of looking at it is that if a jet car was put on a rolling road and was pushed, or pulled, by an external force then would it move forward.

The answer is yes.

Not necessarily. If there was enough resistance in the wheel bearings (imagine they haven't been oiled for years), it could be possible to increase the speed of the rolling road enough for the friction in the wheels to counteract the pushing/pulling force, so that no matter how hard you pushed it, the rolling road would compensate by increasing its speed to match your force.

Ravenous

Maynehead said:

In practice the plane would take off, due to the physical limitations of designing a conveyor belt that precisely satisfied the requirements.

I disagree - as a few people have indicated, once the belt reached a certain limit (probably a few hundred miles per hour) the tyres on the undercarriage would shred anyway, because they have a speed limit. But, your main point is correct.

Handsome_Chris

Maynehead said:

Handsome_Chris said:

A simple way of looking at it is that if a jet car was put on a rolling road and was pushed, or pulled, by an external force then would it move forward.

The answer is yes.

Not necessarily. If there was enough resistance in the wheel bearings (imagine they haven't been oiled for years), it could be possible to increase the speed of the rolling road enough for the friction in the wheels to counteract the pushing/pulling force, so that no matter how hard you pushed it, the rolling road would compensate by increasing its speed to match your force.

I'll meet your theoretical conveyer belt with magic frictionless wheel bearings and indestructible tyres.

CabbageCat

viz said:

It's as though the aircraft were on incredibly slippery ice - no matter how hard the engines burn the wheels just spin, the aircraft is not propelled forwards, and never gains lift. 

No it's not. If it were on ice it would take off because the jets would propel it forward. The conveyor actively moves with the wheels, it doesn't just slip.

stratman3142

Stripping it down to it's bare essentials and attacking the problem in stages:
What is the source of the force (or forces) in the updwards direction that overcomes the downward force of mass times gravity?

If the only source of force is the jet engine, and that force starts in the horizontal, then there is no component in the vertical (orthogonal) direction because obviously cos(90deg) = 0. Hence, there would be no lift.
If the jet engine is tilted with respect to the horizontal, such that it has a vertical component then there will be some lift, but it needs to be enough to overcome mass times gravity.

If there is an airflow across the wings that is enough to create a upwards force that overcomes mass times gravity then the aircraft will lift. The 'airflow over the wings force' needs to be enough to lift the nose then the upward force from the tilted jet engine will also combine with the lift from the airflow over the wings. But that implies forward motion or a bloody strong wind.

Maynehead

Ravenous said:

Maynehead said:

In practice the plane would take off, due to the physical limitations of designing a conveyor belt that precisely satisfied the requirements.

I disagree - as a few people have indicated, once the belt reached a certain limit (probably a few hundred miles per hour) the tyres on the undercarriage would shred anyway, because they have a speed limit. But, your main point is correct.

Yes, I actually indicated that same conclusion in my very first post.

However, due to the relatively low friction in the wheel bearings and a well inflated tyre against the belt, the conveyor would have to accelerate at a practically impossible rate to keep up with the accelerating plane, so in practice I would have thought the conveyor mechanism would fail before the undercarriage.

However, your point is a very much valid one and should such a conveyor system be feasible (e.g. we could have 3 separate smaller units, one under each wheel, thus reducing the amount of stress forces that would be experienced in a single, large system), it could well result in the failure of the undercarriage/tyres before the belt itself.

CabbageCat

Handsome_Chris said:

Maynehead said:

Handsome_Chris said:

A simple way of looking at it is that if a jet car was put on a rolling road and was pushed, or pulled, by an external force then would it move forward.

The answer is yes.

Not necessarily. If there was enough resistance in the wheel bearings (imagine they haven't been oiled for years), it could be possible to increase the speed of the rolling road enough for the friction in the wheels to counteract the pushing/pulling force, so that no matter how hard you pushed it, the rolling road would compensate by increasing its speed to match your force.

I'll meet your theoretical conveyer belt with magic frictionless wheel bearings and indestructible tyres.

I think that the spirit off the question has indestructible, infinite-speed-capable, infinitely powered aeroplanes and conveyors. Which is why the plane would just stand still while the universe is annihilated by the jets.

Drew_TNBD

CabbageCat said:

Handsome_Chris said:

Maynehead said:

Handsome_Chris said:

A simple way of looking at it is that if a jet car was put on a rolling road and was pushed, or pulled, by an external force then would it move forward.

The answer is yes.

Not necessarily. If there was enough resistance in the wheel bearings (imagine they haven't been oiled for years), it could be possible to increase the speed of the rolling road enough for the friction in the wheels to counteract the pushing/pulling force, so that no matter how hard you pushed it, the rolling road would compensate by increasing its speed to match your force.

I'll meet your theoretical conveyer belt with magic frictionless wheel bearings and indestructible tyres.

I think that the spirit off the question has indestructible, infinite-speed-capable, infinitely powered aeroplanes and conveyors. Which is why the plane would just stand still while the universe is annihilated by the jets.

They said the same thing about dragons, but where are they now huh!?!

viz

CabbageCat said:

viz said:

It's as though the aircraft were on incredibly slippery ice - no matter how hard the engines burn the wheels just spin, the aircraft is not propelled forwards, and never gains lift. 

No it's not. If it were on ice it would take off because the jets would propel it forward. The conveyor actively moves with the wheels, it doesn't just slip.

Forgot to mention - my ice is special backwards-facing ice. 

scrumhalf

How does the plane get onto the conveyor belt?

Reverse that process and then let the plane take off normally from the runway. Problem solved.

Sheesh, it takes an acountant to work out a practical answer.

paul_c2

The issue is whether you "take it as read" that this infinitely-capable conveyor belt can, or does exist. If it does, then its only fair that the plane can also possess infinite capabilities, such as wheels which can rotate at infinity and jet engines which can overcome infinite drag by generating infinite thrust.

If we accept the infinite, theoretical stuff then the answer is undefined since 1) infinity - infinity (the horizontal force balance) is undefined. 2) its theoretical anyway.

If we don't accept this conveyor belt can exist, but look to actual real-world components, then the plane can take off for the fairly simple reason that the conveyor belt can't match the wheel speed because it can't know the wheel speed because aeroplanes don't have wires dangling from them, and don't have a radio link which is constantly providing the data about the wheel speed. (And even if they did, ie a remote engineering/monitoring/data acquisition system, the data is encrypted).

DiscoStu

The plane on a conveyor belt is the same as a car on a rolling road. You could get a car up to top speed on a rolling road but it is still stationary. The plane is the same, the thrust from the engines moves the plane forward slightly and in turn makes the wheels turn which makes the conveyor move and the plane is left effectively stationary with not enough speed to generate lift. If all it took was engine thrust to get a plane off the ground then we wouldn't need runways, all planes would be able to rev their engines then hop into the air but they can't.

Ravenous

The question clearly indicate "a 747" which is presumably a real plane. It says nothing about the conveyor performance. So it's up to you if the conveyor breaks before the 747's tyres do.

stratman3142

Did anyone else see the cuddly toy? I think I saw a coffee maker as well.

ESBlonde

With no (appreaciable) movement of air over the wing surfaces there would be no lift and just a noisy static lump of metal.
OK so the jet engines would cause some AIR 'suck' over the wings and the wheels would have friction blah blah, but without the air splitting above and below the wing airfoil section there is no lift. 'THE CONVEYOR IS DESIGNED TO EXACTLY MATCH THE SPEED OF THE WHEELS'

menamestom

DiscoStu said:

The plane on a conveyor belt is the same as a car on a rolling road.

No it's not, because the car is driven via the wheels.  it's more like a bike on a treadmill with somebody on the bike but not pedalling.  You hold them upright as the treadmill starts by pushing them on their back.  The only thing stopping you then pushing them forward (because even at a relatively high speed of treadmill it would be easy) is the limitation of the question.  If you pushed them very hard, the treadmill would have to go incredibly fast to stop forward movement, which in essence the paradoxical limitation of the question.

My issue with the question is it is not at all real world, thrust would easily overcome drag, in practise. The treadmill would have to increase exponentially with respect to the thrust, putting the speed of the treadmill at full thrust beyond the bounds of possibility. 

I'll put it another way, it's a paradox, not a physics question.

stickyfiddle

The analogy to a car on a rolling road would be a plane in a wind tunnel...

Myranda

The posited question doesn't suggest the bearings should be ignored, but lets assume they are... 

the conveyor would in this magical situation be capable of sustained rolling of 700+ miles per hour... without the tyres or belt melting... 

but air is being pushed at 700 miles per hour in that situation - that air has to go past the wings and would create lift - the plane would at first lift vertically and then go forwards... the mechanical lift generated occurs not because of the movement of the plane, but movement of the air over the stationary plane. Yes the greatest movement of air would be through the engine, but it would force the air around those 4 small horizontal columns of air to move too 

In a real situation of as described the belt/tyres would not be capable of sustaining the speed of movement long enough to work... and so would allow the thrust of the engines to overcome the drag of the wheels... but I think in these hypothetical situations it's fair to assume we're ignoring lots of factors. which probably means we're supposed to ignore the drag the air will have on the surrounding air... 

CabbageCat

This is obviously not a real world situation, but if it were to be I think that the question would be thus:

If a plane applied thrust smoothly as it normally would to get up to 200mph over 60 seconds and the conveyor belt that it was on accelerated smoothly up to 200mph in the opposite direction over 60 seconds would the plane take off?

I think that the answer is "no" for the reasons Maynehead has explained.