Plane on a conveyor belt

littlegreenman

Sporky said:

It's a bit like if you asked "A mouse, who can speak only Esperanto, wishes to book a hotel room in Moscow - how does he go about it?", and all the biologists go off about how mice don't have vocal cords

How very dare you Sir? If they didn't then how do you explain this?

CabbageCat

Myranda said:

monquixote said:

Myranda said:

Sporky said:

stratman3142 said:

All this theory is making my brain hurt. I vote for an empirical approach. Let's construct an experiment. Then we can come up with a theory that fits what's observed in practice.

I don't think you could build the conveyor.

It's a bit like trying to solve the "irresistible force meets immovable object" paradox - no universe can support both.

We could create a scale model using a running machine and a remote control plane... a handful of sensors... 

rig up an arduino to control speeds exactly ... could even add weights etc to ensure a proper scale experiment

At any scale the conveyor needs to instantaneously spin up to infinite speed.

Pretty sure a 747 doesn't achieve FTL speeds... I think we could use much lower speeds ... how about 5 times the take off speed for the model plane? that would be a much greater difference than between 747 take off speed and maximum speed (160-180 take off and 675 maximum) ... you'd need a VERY well made conveyor but it should still be possible

It might not need to. Since there is no air resistance with the plane not moving the only limits are the speed that the wheels can spin and the speed of the conveyor. 

Sporky

littlegreenman said:

Sporky said:

It's a bit like if you asked "A mouse, who can speak only Esperanto, wishes to book a hotel room in Moscow - how does he go about it?", and all the biologists go off about how mice don't have vocal cords

How very dare you Sir? If they didn't then how do you explain this?

Quite simply, my dear fellow.

We know from another episode (The Mouse Mill) that the mice are confidence tricksters. I posit that the Mouse Organ is a mechanical contrivance that produces their "voices" and that the mce simply mime in time. That task made easier, of course, by the shoddy animation techniques of the age.

monquixote

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?

Drew_TNBD

https://i.imgur.com/5C7Xi6e.mp4

CabbageCat

monquixote said:

Maynehead said:

sinbaadi said:

I thought this was stupid until I read the OP properly.

Can the aircraft take off whilst in a stationary position (assuming no headwind)?  No.

The question is whether you would be able to move the aircraft at all given the rules imposed by the treadmill.

Of course nothing happening at the wheels is stopping a Eurofighter at full thrust, so a rolling wheel isn't go to either, and that thing just grabs a massive wodge of air and pulls itself through it.  The aircraft is going to build airspeed and accelerate.

But if a wheel rolls at 1 mph over a surface that rolls in the opposite direction at 1mph, that wheel is not going anywhere.  End of.  Add as many zeroes as you like.

I don't think the aircraft can take off without breaking the rules of the thought experiment.  The wheel has to turn at a speed greater than that of the conveyor for the aircraft to have the first iota of its forward momentum, but both have infinitely high possible speeds.

OK, time to hit youtube.

And we have another winner!

Btw the only value the YouTube "proof" videos offer is entertainment.

So do we have to assume that the wheel is infinitely tractable? 

I don't think it would need to be. Just more tractable than the bearings, as I said. When driving a car a wheel has a choice of skid or turn. I don't see why the plane's wheels wouldn't make the decision to turn since the plane would be wanting them to turn in the direction that they are already turning.

Sporky

monquixote said:

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

The plane is locked into position by the premise, not by physics.

It's not (and can't be) a real plane or conveyor.

monquixote

Sporky said:

monquixote said:

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

The plane is locked into position by the premise, not by physics.

It's not (and can't be) a real plane or conveyor.

Hang on, if the premise locks it into place then there is no discussion to have. 

Surely the question hinges on if the plane moves?

I think this is rapidly becoming "make up a set of assumptions to fit any outcome you choose"

Mkjackary

Still don't buy the not moving argument.

We have now established in the real world the aircraft would take off.

Please someone explain the physical constraints to me because I just can't get it.

The engines push air backwards causing thrust, what is that counteracted by? It can't be drag, the only thing it can be is friction.
Everyone agreed on that?

DiscoStu

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 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. 

monquixote

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.

Maynehead

Mkjackary said:

Still don't buy the not moving argument.

We have now established in the real world the aircraft would take off.

Please someone explain the physical constraints to me because I just can't get it.

The engines push air backwards causing thrust, what is that counteracted by? It can't be drag, the only thing it can be is friction.
Everyone agreed on that?

Yes, the only thing that stops the belt having to turn at infinitely high speeds as soon as the wheels move 1 micron is friction. The belt needs to turn fast enough (however fast it needs to be) for the friction to counteract the thrust. If the belt cannot turn fast enough and the plane starts to move then the belt has not done the job it's specified to do, which is to match the speed of the wheels EXACTLY.

monquixote

Maynehead said:

Mkjackary said:

Still don't buy the not moving argument.

We have now established in the real world the aircraft would take off.

Please someone explain the physical constraints to me because I just can't get it.

The engines push air backwards causing thrust, what is that counteracted by? It can't be drag, the only thing it can be is friction.
Everyone agreed on that?

Yes, the only thing that stops the belt having to turn at infinitely high speeds as soon as the wheels move 1 micron is friction. The belt needs to turn fast enough (however fast it needs to be) for the friction to counteract the thrust. If the belt cannot turn fast enough and the plane starts to move then the belt has not done the job it's specified to do, which is to match the speed of the wheels EXACTLY.

The belt can match the speed of the wheels and the plane can move as soon as the wheels are delivering the maximum amount of frictional force they can deliver because they will slip.

CabbageCat

Mkjackary said:

Still don't buy the not moving argument.

We have now established in the real world the aircraft would take off.

Please someone explain the physical constraints to me because I just can't get it.

The engines push air backwards causing thrust, what is that counteracted by? It can't be drag, the only thing it can be is friction.
Everyone agreed on that?

Friction. yeah, but it's friction on a surface that is actively pulling against the plane. If a stationary plane is sitting on a moving conveyor then friction is a pretty big deal.

CabbageCat

Sporky said:

monquixote said:

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

The plane is locked into position by the premise, not by physics.

It's not (and can't be) a real plane or conveyor.

I think it can be a real plane. It can't be a real conveyor.

Sporky

monquixote said:

Sporky said:

monquixote said:

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

The plane is locked into position by the premise, not by physics.

It's not (and can't be) a real plane or conveyor.

Hang on, if the premise locks it into place then there is no discussion to have. 

Surely the question hinges on if the plane moves?

I think this is rapidly becoming "make up a set of assumptions to fit any outcome you choose"

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.

Effectively the premise can only be true (with real world physics) if the plane's engines don't work. In the real world the conveyor couldn't function as described if the engines work, so the only logical conclusion is that either all real world physics is out of the window, or the engines don't work.

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.

DiscoStu

'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.

Sporky

CabbageCat said:

Sporky said:

monquixote said:

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

The plane is locked into position by the premise, not by physics.

It's not (and can't be) a real plane or conveyor.

I think it can be a real plane. It can't be a real conveyor.

Actually I think you might be right there.

monquixote

Sporky said:

monquixote said:

Sporky said:

monquixote said:

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

The plane is locked into position by the premise, not by physics.

It's not (and can't be) a real plane or conveyor.

Hang on, if the premise locks it into place then there is no discussion to have. 

Surely the question hinges on if the plane moves?

I think this is rapidly becoming "make up a set of assumptions to fit any outcome you choose"

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.

Effectively the premise can only be true (with real world physics) if the plane's engines don't work. In the real world the conveyor couldn't function as described if the engines work, so the only logical conclusion is that either all real world physics is out of the window, or the engines don't work.

No, this isn't correct. 

At some point the wheels will be delivering the maximum amount of frictional force that they are capable of delivering and at that point the plane will begin to move forward. The wheels and the conveyor will still be moving at the same speed, but the plane will be effectively sliding forward. 

CabbageCat

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.