Plane on a conveyor belt

monquixote

DiscoStu said:

'the wheels don't allow the plane to move forward, the engines do' is a common phrase in this thread.

So what if the brakes were on? It would still move forward exactly the same on the conveyor belt? No.
What if the undercarriage was not down and the plane was sitting on its belly on the conveyor belt? What then?

The wheels are what allows the plane to move forwards, but as they can't turn faster than the 'ground' they are sitting on there cannot be forward motion and that is what you need to get lift.

If you didn't need wheels to move a plane forward and get it airborne they wouldn't have wheels and planes wouldn't need runways.

If you fire up the engines of a 747 with the brakes on it will definitely move.

It will simply slide on it's tires.

monquixote

CabbageCat said:

monquixote said:

DiscoStu said:

You need air flowing OVER a wing to create lift. The engines on this plane are mounted underneath and no matter how much thrust they create they do not force any air OVER the wing.
With zero windspeed, the plane needs to be moving forward through the air to force air over the wing at a high enough speed to lift it off the ground but as the 'ground' the plane is sitting on is moving in an opposite direction to the wheels (which allow the plane to move forward) any forward movement is negated and the airspeed is still zero, which means NO LIFT.
The video of the seaplane on the trailer us a completely different scenario to the question posed as the car is pulling the plane forward through the air therefore air is passing over its wings allowing it to take off. 

The wheels don't allow the plane to move forward the engines do. 

Imagine a plane on skis on a conveyor belt, it amounts to the same thing.

Does your position rely on the fact that the plane is effectively skidding on tyres at 180mph pushing wheel bearings beyond the point where their friction is equal to that of rubber on road? Do you think a real-world plane would tolerate that? If not then indestructible tyres can also be infinitely tractable (if that is the word) or indestructible bearings can be frictionless.

If that's the case then it's simply a question of pick the set of bogus assumptions you desire and get the result you want. 

You can use the same logic to prove that perpetual motion machines work. 

monquixote

CabbageCat said:

monquixote said:

DiscoStu said:

You need air flowing OVER a wing to create lift. The engines on this plane are mounted underneath and no matter how much thrust they create they do not force any air OVER the wing.
With zero windspeed, the plane needs to be moving forward through the air to force air over the wing at a high enough speed to lift it off the ground but as the 'ground' the plane is sitting on is moving in an opposite direction to the wheels (which allow the plane to move forward) any forward movement is negated and the airspeed is still zero, which means NO LIFT.
The video of the seaplane on the trailer us a completely different scenario to the question posed as the car is pulling the plane forward through the air therefore air is passing over its wings allowing it to take off. 

The wheels don't allow the plane to move forward the engines do. 

Imagine a plane on skis on a conveyor belt, it amounts to the same thing.

Does your position rely on the fact that the plane is effectively skidding on tyres at 180mph pushing wheel bearings beyond the point where their friction is equal to that of rubber on road? Do you think a real-world plane would tolerate that? If not then indestructible tyres can also be infinitely tractable (if that is the word) or indestructible bearings can be frictionless.

You have to assume an indestructible plane or the answer is "the plane is almost immediately destroyed"

snakemanStoo

Is anyone else sick of these motherfucking planes on these motherfucking conveyors?

http://www.eonline.com/resize/600/600//eol_images/Entire_Site/201073/293.jackson.snakes.plane.lc.080410.jpg

CabbageCat

monquixote said:

CabbageCat said:

monquixote said:

DiscoStu said:

You need air flowing OVER a wing to create lift. The engines on this plane are mounted underneath and no matter how much thrust they create they do not force any air OVER the wing.
With zero windspeed, the plane needs to be moving forward through the air to force air over the wing at a high enough speed to lift it off the ground but as the 'ground' the plane is sitting on is moving in an opposite direction to the wheels (which allow the plane to move forward) any forward movement is negated and the airspeed is still zero, which means NO LIFT.
The video of the seaplane on the trailer us a completely different scenario to the question posed as the car is pulling the plane forward through the air therefore air is passing over its wings allowing it to take off. 

The wheels don't allow the plane to move forward the engines do. 

Imagine a plane on skis on a conveyor belt, it amounts to the same thing.

Does your position rely on the fact that the plane is effectively skidding on tyres at 180mph pushing wheel bearings beyond the point where their friction is equal to that of rubber on road? Do you think a real-world plane would tolerate that? If not then indestructible tyres can also be infinitely tractable (if that is the word) or indestructible bearings can be frictionless.

If that's the case then it's simply a question of pick the set of bogus assumptions you desire and get the result you want. 

You can use the same logic to prove that perpetual motion machines work. 

So don't make assumptions outside of the parameters of the question. Use a real-world plane. What would happen to a plane in that situation do you think? I confess that I don't know but parts of it would very definitely be having to deal with things a long way outside what they are expected to have to deal with.

Drew_TNBD

Christ this is probably the driest and most boring thread in the database. Digiscream, a purge!!

sinbaadi

Without wishing to get entrenched.

Imagine on your desk, you have a conveyor-belt which can perfectly match the speed of a wheel which you also have.  On the wheel is a frame which forms a handle and is fixed through the axle of the wheel.  You can sit and spin the wheel by directly driving the wheel with your hand all day and it won't move.

You can also hold the handle of the wheel.  

There are 2 outcomes from your interaction with the handle of the wheel and the belt it sits on:
1.  You do not move the wheel, the rules of the experiment are upheld, nothing rolls, no speed to match.
2.  You pull the wheel toward one end of the belt, and you break the experiment.  You make its speed exceed that of the belt.  By moving the axle of the wheel it has to, because no matter how much belt travels beneath the wheel, the wheel will roll that distance plus the distance you have pulled it.  

You cannot move the wheel, you cannot give it speed, unless you break the rule in the first place.

Maynehead

monquixote said:

Maynehead said:

If by that you mean it always maintains traction with the belt surface then yes, that is one of the many assumptions.

Ofcourse you can circumvent the problem completely by appying the brakes and powering through the locked up wheels with the sheer force of the engines and take off that way, with the wheels, and therefore the belt, never turning; but that would not be in the spirit of the question

Where does it say you have to make that assumption?

It does seem a bit silly.

So you are assuming that all of the retarding force on the plane is coming from the friction in the wheel bearings?

I agree that this is one of the more critical assumptions we need to make, because it directly affects outcome of the core question, which is: "Can a wheel translate in space if it is rolling on a surface that matched its velocity exactly and instantaneously?".

The answer to which could be: "Depends if the wheel can slide along the surface.".

So to make the question better and more concise we need to add the clause: "Given that there is perfect traction between the wheel and the surface.".

In which case the answer would be "No.".

ICBM

The whole point of thought experiments is that you can have solutions which tend to infinity if you need to.

Since I tend to agree with monquixote that those who want to interpret it as a not-real-world-physics question can simply choose whatever conditions they want in order to be right, I will answer this one instead:

DiscoStu said:

The video of the seaplane on the trailer is a completely different scenario to the question posed as the car is pulling the plane forward through the air therefore air is passing over its wings allowing it to take off. 

Not quite. The car isn't pulling the plane forward, or if it is it's unintentional - the plane would fall off the back of the trailer as it got close to flying speed and the friction between the floats and the trailer decreased if it was.

The plane is accelerating through the air under its own engine power. The plane and the car are accelerating at the same rate, or as close to it as the pilot and driver can manage.

The important point being that the plane reaches flying speed not due to the speed of the plane relative to whatever it's resting on, only relative to the air.

And that is really all I'm going to say because it's clearly become a nonsense if you don't just look at the actual physics of it.

CabbageCat

monquixote said:

CabbageCat said:

monquixote said:

DiscoStu said:

You need air flowing OVER a wing to create lift. The engines on this plane are mounted underneath and no matter how much thrust they create they do not force any air OVER the wing.
With zero windspeed, the plane needs to be moving forward through the air to force air over the wing at a high enough speed to lift it off the ground but as the 'ground' the plane is sitting on is moving in an opposite direction to the wheels (which allow the plane to move forward) any forward movement is negated and the airspeed is still zero, which means NO LIFT.
The video of the seaplane on the trailer us a completely different scenario to the question posed as the car is pulling the plane forward through the air therefore air is passing over its wings allowing it to take off. 

The wheels don't allow the plane to move forward the engines do. 

Imagine a plane on skis on a conveyor belt, it amounts to the same thing.

Does your position rely on the fact that the plane is effectively skidding on tyres at 180mph pushing wheel bearings beyond the point where their friction is equal to that of rubber on road? Do you think a real-world plane would tolerate that? If not then indestructible tyres can also be infinitely tractable (if that is the word) or indestructible bearings can be frictionless.

You have to assume an indestructible plane or the answer is "the plane is almost immediately destroyed"

I agree. Which is why I'm in the "doesn't take off" camp. If you have the proverbial spherical-chicken-in-a-vacuum plane then it stays still without taking off. If you have the real life plane it explodes without taking off. If you have some chimera of assumptions then you can take off or not take off depending on which assumptions you like best.

menamestom

Sporky said:

In that sense it reminds me of a puzzle in my dear old mum's university alumni newsletter, which I'd read before. Basically your doctor gives you two lots of pills, which are identical in appearance. You must take one of each each day or you'll die. Don't take them? Die. Take more than one of either? Die. You go on holiday the next day to a really remote place, and on arriving (your transport having departed) you discover that the containers broke and the pills spilled out into your suitcase. They look identical and have the same mass, size, smell, blah blah blah. The cottage you're staying in has a pestle and mortar and some very accurate scales.You're not being picked up again for a fortnight and can't ontact the outside world. How do you make it out alive?

There's a classic answer, and there's the obviously correct answer

Weigh one pill.  Grind the lot, mix it all up.  Weigh an amount of powder which is twice the weight of the single pill.  Then consume that amount daily.  Hopefully the total amount is enough to last 2 weeks.  Then question the wisdom of going to a really remote place with a life threatening illness. 

monquixote

Maynehead said:

monquixote said:

Maynehead said:

If by that you mean it always maintains traction with the belt surface then yes, that is one of the many assumptions.

Ofcourse you can circumvent the problem completely by appying the brakes and powering through the locked up wheels with the sheer force of the engines and take off that way, with the wheels, and therefore the belt, never turning; but that would not be in the spirit of the question

Where does it say you have to make that assumption?

It does seem a bit silly.

So you are assuming that all of the retarding force on the plane is coming from the friction in the wheel bearings?

I agree that this is one of the more critical assumptions we need to make, because it directly affects outcome of the core question, which is: "Can a wheel translate in space if it is rolling on a surface that matched its velocity exactly and instantaneously?".

The answer to which could be: "Depends if the wheel can slide along the surface.".

So to make the question better and more concise we need to add the clause: "Given that there is perfect traction between the wheel and the surface.".

In which case the answer would be "No.".

You don't need to, you choose to. 

If you decide to go with that set of assumptions then you are asking what do equal and opposite forces do which is an obvious and boring question.

Paul_C

stratman3142

I think the safety hazards analysis wouldn't be able to reduce the risks to a sufficiently low level that it would be possible to get airworthiness approval from the CAA. So it's never going to fly.

CabbageCat

monquixote said:

Maynehead said:

monquixote said:

Maynehead said:

If by that you mean it always maintains traction with the belt surface then yes, that is one of the many assumptions.

Ofcourse you can circumvent the problem completely by appying the brakes and powering through the locked up wheels with the sheer force of the engines and take off that way, with the wheels, and therefore the belt, never turning; but that would not be in the spirit of the question

Where does it say you have to make that assumption?

It does seem a bit silly.

So you are assuming that all of the retarding force on the plane is coming from the friction in the wheel bearings?

I agree that this is one of the more critical assumptions we need to make, because it directly affects outcome of the core question, which is: "Can a wheel translate in space if it is rolling on a surface that matched its velocity exactly and instantaneously?".

The answer to which could be: "Depends if the wheel can slide along the surface.".

So to make the question better and more concise we need to add the clause: "Given that there is perfect traction between the wheel and the surface.".

In which case the answer would be "No.".

You don't need to, you choose to. 

If you decide to go with that set of assumptions then you are asking what do equal and opposite forces do which is an obvious and boring question.

I dunno, I think it's quite an interesting question. Sorry everyone who doesn't like this thread. I think it's great.

Even with the super-assumption model there are some layers to it. If the force of the jets are what makes the plane move and the plane moving is what makes the wheels turn and the wheels turning are making the conveyor move and the conveyor moving is what keeps the plane still then there is no movement so the wheels wouldn't have turned in the first place but the jets are still firing and the plane is still stationary (ery?). I know I've repeated that a couple of times now but it amuses me to think of it. It's a lot more fun than irrepressible force vs immovable object. 

Maynehead

monquixote said:

You don't need to, you choose to. 

If you decide to go with that set of assumptions then you are asking what do equal and opposite forces do which is an obvious and boring question.

Quite true, and in the absence of the said clause there is definitely room for debate.

I must say though, this is actually the first well reasoned and logical counter argument I have seen in this thread for why the plane MAY take off. It remains within the confines of the question and does not stray from the limitations explicitly set out.

hywelg

Gassage said:

Point of order- on the belt, the wheels would be spinning at c. 480mph- twice their design speed. So they'd fall off.

Show me a conveyor that can travel over 100mph.......

DiscoStu

ICBM said:

The whole point of thought experiments is that you can have solutions which tend to infinity if you need to.

Since I tend to agree with monquixote that those who want to interpret it as a not-real-world-physics question can simply choose whatever conditions they want in order to be right, I will answer this one instead:

DiscoStu said:

The video of the seaplane on the trailer is a completely different scenario to the question posed as the car is pulling the plane forward through the air therefore air is passing over its wings allowing it to take off. 

Not quite. The car isn't pulling the plane forward, or if it is it's unintentional - the plane would fall off the back of the trailer as it got close to flying speed and the friction between the floats and the trailer decreased if it was.

The plane is accelerating through the air under its own engine power. The plane and the car are accelerating at the same rate, or as close to it as the pilot and driver can manage.

The important point being that the plane reaches flying speed not due to the speed of the plane relative to whatever it's resting on, only relative to the air.

And that is really all I'm going to say because it's clearly become a nonsense if you don't just look at the actual physics of it.

The seaplane is being held to the trailer by gravity and friction.
Its engines may well be helping it stay on the trailer but it is the car pulling the trailer which is making air pass over the plane's wings, creating the crucial lift it needs to take off.
Take away the car and the trailer and sit the seaplane on the ground and it won't take off. It might scrape along on its skis but I doubt at a high enough speed to cause lift.
Stick wheels on the bottom though and that's another matter! The wheels don't have drive but they do freewheel along a static ground and allow the plane to move forward properly.

The wheels on the plane in the question are still vital to the puzzle and as they are rotating on a moving surface which has an opposite rotation to the wheels the plane can't move forward.

Here's a 747 at full thrust. Why isn't it moving? Why hasn't it taken off?


Same here, the engines are running but the blocks stop the wheels from turning so it doesn't go anywhere.

Emp_Fab

It appears that only a few of you have read my explanation ;-)

Emp_Fab said:

You know what..... @ICBM is right....  the plane would take off !!!   The humble pie is ready for us all to eat !!!

The little drawing in the original post is misleading - it implies a little conveyor belt (just bear this in mind for the moment).

The reason we're all saying the plane wouldn't fly is because we're all assuming the plane remains stationary relative to the ground (and the surrounding air).  The plane doesn't remain stationary - it moves along the (very long) conveyor belt and takes off exactly the same as if it were on a tarmac runway.

Why does it move ?  Because the forward thrust from a plane is generated by its engines against the surrounding air.  The surrounding air has no 'knowledge' of the existence of the conveyor belt.  If the forward thrust was applied through the plane's wheels instead, then yes, the plane would remain stationary and wouldn't take off.  However, a plane with powered wheels is useless - it's just a car with wings.  The reason any plane moves is because of the force it applies against the surrounding air.  What is going on beneath the wheels is irrelevant (unless the brakes are applied).  Discounting the minor effect of friction in the wheel bearings, the wheels have no role to play in any of this other than to keep the plane from hitting the ground.  The ground / conveyor belt could be moving sideways for all it matters (presuming the wheels were on castors !).  The only force that has any relevance here is the thrust of the engines against the surrounding air - which is what makes the plane move forward, regardless of the ground beneath it.

Of course, in the OP's post, it's a tiny conveyor belt, in which case, the plane would just fall off the end.

Sporky said:

The plane cannot achieve any forward motion, because the premise is that the conveyor moves at the same speed as the wheels but in the opposite direction. It doesn't matter that the plane isn't propelled by its wheels.

@Sporky The plane will achieve forward motion because the thrust is against the air, not the conveyor.  The plane will push against the air and move forward.  What the conveyor is doing underneath the wheels is irrelevant - the only thing the conveyor movement has any effect on is the rotational speed of the wheels.  You have to assume that friction in the wheel bearings plays no part in this.  Picture a huge mass sitting on frictionless wheels that is sitting on a conveyor belt.  When the belt moves, the huge mass is going to stay put, because there is no force being applied to it.  Do you agree so far ?  Now, if you accept that moving the ground underneath a huge mass with frictionless wheels doesn't move the mass, then you have to accept that if you apply a force to the mass, such as from jet engines bolted onto it, the mass will move - because it now has a force applied to it.  The plane is the huge mass and its engines are the force applied.  The rest of it is a mental distraction.

The error in the thinking of the 'non-flyers' (as I was previously), is believing that the plane will move backwards as the conveyor moves (which is then 'counteracted' by the forward thrust of the engines).  In order for this to be true, there has to be a rearwards force applied to the plane - and the only path that this force could possibly take would be via the friction in the plane's wheel bearings.  There is a range in values of friction from 'total' - i.e. wheels locked, in which case the plane will move rearwards in unison with the conveyor, to 'none', in which case no force can be applied to the plane and, as no force is imparted, no movement is possible - and the plane will just stay still as the conveyor moves underneath it.  However, as the planes engines are bolted to it, directly applying force, the plane will move - and it will move forward along the conveyor that is happily spinning away beneath it.
Even allowing for wheel bearing friction, within the range of 'total' friction (rearward movement) to 'none' (plane stays stationary) there will be a point, very early, at which the force of the engines overcomes the miniscule friction of the wheels and the plane starts accelerating forward.

Maynehead

menamestom said:

Sporky said:

In that sense it reminds me of a puzzle in my dear old mum's university alumni newsletter, which I'd read before. Basically your doctor gives you two lots of pills, which are identical in appearance. You must take one of each each day or you'll die. Don't take them? Die. Take more than one of either? Die. You go on holiday the next day to a really remote place, and on arriving (your transport having departed) you discover that the containers broke and the pills spilled out into your suitcase. They look identical and have the same mass, size, smell, blah blah blah. The cottage you're staying in has a pestle and mortar and some very accurate scales.You're not being picked up again for a fortnight and can't ontact the outside world. How do you make it out alive?

There's a classic answer, and there's the obviously correct answer

Weigh one pill.  Grind the lot, mix it all up.  Weigh an amount of powder which is twice the weight of the single pill.  Then consume that amount daily.  Hopefully the total amount is enough to last 2 weeks.  Then question the wisdom of going to a really remote place with a life threatening illness. 

This heavily relies on the assumption that each bottle contained an equal amount of pills.