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A question of physics.....

a simple little question, or is it?

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Piers Bowlan13/10/2016 17:04:49
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No it doesn't! All the tyres overheat as their maximum tyre speeds are exceeded and they burst (or the fuse plugs melt) and the tyres deflate. Don't think it will take off with flat tyres!

kc13/10/2016 17:12:21
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Haven't you chaps got any aeromodelling to do?

Cliff Bastow13/10/2016 17:13:55
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No but I had better get off here before my boss gets mad!

Cliff Bastow13/10/2016 17:15:00
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Just as an aside, planes take off with burst tyres quite often!

Don Fry13/10/2016 17:21:56
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But it is a 747, a real machine. It has design parameters, and this scenario is orders of magnitude outside them. The wheels don't move, they therefore are spinning very very fast, think say 100,000 rpm. The belt does not care, it goes as fast as requires to stop the wheels, and therefore the wings, and therefore the engines are going anywhere.

As I said earlier, the belt is done by a very competent engineer, and has a starship, and warp drives. The 747 is mere the efforts of Boeing. The 747 does not move.

I do take the point, however, that a belt a runway wide, moving at very high speed will generate wind, fluid dynamics tells us that. I have no idea if it would lift a 747. All I say is,would the undercarriage last long enough?

That belt is a scary machine.

Don Fry13/10/2016 17:23:36
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And Cliff, this one has all its tyres gone, and it hasn't even moved yet.

john stones 113/10/2016 17:27:32
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Why don't you just google it, wink

Erfolg13/10/2016 17:36:23
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Come on, this is not a real scenario, when was the last time you saw an aircraft on a conveyor belt, never mind a 747.

All the consideration of real world issues , is at best amusing.

The question has all the hallmarks of the type of question asked in "A" level applied maths, without the merit of physics or maths.

Rich too13/10/2016 17:41:46
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Posted by ron evans on 13/10/2016 16:13:50:

OK my turn for some real physics.

1/ the 747 cares not a jot what the wheels do once the brakes are off.

2/ the engines go whooooosh.

3/ the wings lift

4/ the plane flies....Yipee

Yes, and confirms what Gary said. It took me ages, and I watched the mythbusters video, but eventually I got it. Since the drive pushes the air rather than the ground, it doesn't matter what the wheels are doing..took me a long time to get my head around it though!

john stones 113/10/2016 17:45:17
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Posted by Biggles' Elder Brother - Moderator on 13/10/2016 12:32:30:

Newton's third law!

Jet engines work by creating a reactive force against the air - not the ground!!

This resultant force means they will accelerate relative to the air. (F=ma)

The aeroplane is firmly attached to the engines so it will accelerate through the air as well.

The airspeed will increase over the wings, lift will result and the aircraft will take off!!!!!

Its all about the air - what the ground is doing is utterly immaterial! It could be still, it could be moving, it could be doing the bosa-nove - it matters not one jot!

BEB

PS Got to stop here - about to suffer a serious, frustration driven, sense of humour failure!!! smile o

The air

Martin Harris13/10/2016 17:54:24
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...but doesn't the machine sense that the tyres are rotating faster than the belt, as soon as the aircraft moves?

According to the conditions specified in the question, it has sensed the fact that the wheels are turning faster than the belt, the belt speeds up, but there is no mention of any lag in the system and it has to be assumed that there is no slip or hysteresis. The relative movement still increases so it speeds up again, and repeats until the speed is infinite. Since that can't happen, either the machine breaks, the energy source driving it becomes inadequate or the aircraft wheels explode!

Rich too13/10/2016 17:56:51
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Posted by Biggles' Elder Brother - Moderator on 13/10/2016 12:32:30:

Newton's third law!

Jet engines work by creating a reactive force against the air - not the ground!!

This resultant force means they will accelerate relative to the air. (F=ma)

The aeroplane is firmly attached to the engines so it will accelerate through the air as well.

The airspeed will increase over the wings, lift will result and the aircraft will take off!!!!!

Its all about the air - what the ground is doing is utterly immaterial! It could be still, it could be moving, it could be doing the bosa-nove - it matters not one jot!

BEB

PS Got to stop here - about to suffer a serious, frustration driven, sense of humour failure!!! smile o

yes

Don Fry13/10/2016 17:57:43
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Martin, in a nutshell, that belt is a scary machine.

John F13/10/2016 18:06:03
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Posted by Rich2 on 13/10/2016 17:41:46:
Posted by ron evans on 13/10/2016 16:13:50:

OK my turn for some real physics.

1/ the 747 cares not a jot what the wheels do once the brakes are off.

2/ the engines go whooooosh.

3/ the wings lift

4/ the plane flies....Yipee

Yes, and confirms what Gary said. It took me ages, and I watched the mythbusters video, but eventually I got it. Since the drive pushes the air rather than the ground, it doesn't matter what the wheels are doing..took me a long time to get my head around it though!

The problem is that the Mythbusters got it wrong!! The aircraft managed to outpace the car and travel forward.No forward speed means no lift being generated by the wings. That's why we have aircraft that stall - you need air travelling over the wing.That's also how rolling roads work. If the plane could get airborne from a geostationary position with no air flowing over the wing then the car would too!

Don Fry13/10/2016 18:10:29
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And Erf, we had a very fine discussion the other day about software control on machines, in this case an Airbus hitting the ocean. One of the factors was the software people did not envisage fools trying to do that to their machine. Failure to envisage, and of the operators to own their own machine.

And regarding myth busters, first on must address the question, did they go to school. Or bunk off, do weed, and eye crumpet from afar.

Andy4813/10/2016 18:25:31
1394 forum posts
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For those of you that think the plane stays still, consider this:

If the plane stays still, then somehow the conveyor belt must be applying an equal but opposite thrust to the plane's engines through the wheels. How can it do that when all it does is make the wheels revolve faster?

Martin Whybrow13/10/2016 19:05:51
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Posted by Andy48 on 13/10/2016 18:25:31

If the plane stays still, then somehow the conveyor belt must be applying an equal but opposite thrust to the plane's engines through the wheels. How can it do that when all it does is make the wheels revolve faster?

Spot on Andy.

The question is ridiculous anyway as it assumes the 747 obeys the laws of physics but the conveyor doesn't; at what point in the system does magic take over from physics?

Piers Bowlan13/10/2016 19:07:40
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Posted by Cliff Bastow on 13/10/2016 17:15:00:

Just as an aside, planes take off with burst tyres quite often!

Wot, 18 of them !!

Martin McIntosh13/10/2016 19:29:38
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Did you ever finish and fly your Tucano James?

Rich too13/10/2016 19:48:16
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Posted by John F on 13/10/2016 18:06:03:
Posted by Rich2 on 13/10/2016 17:41:46:
Posted by ron evans on 13/10/2016 16:13:50:

OK my turn for some real physics.

1/ the 747 cares not a jot what the wheels do once the brakes are off.

2/ the engines go whooooosh.

3/ the wings lift

4/ the plane flies....Yipee

Yes, and confirms what Gary said. It took me ages, and I watched the mythbusters video, but eventually I got it. Since the drive pushes the air rather than the ground, it doesn't matter what the wheels are doing..took me a long time to get my head around it though!

The problem is that the Mythbusters got it wrong!! The aircraft managed to outpace the car and travel forward.No forward speed means no lift being generated by the wings. That's why we have aircraft that stall - you need air travelling over the wing.That's also how rolling roads work. If the plane could get airborne from a geostationary position with no air flowing over the wing then the car would too!

Rubbish, the only fly in the ointment is friction. Of course the aircraft moved forward, that's the point!

Edited By Rich2 on 13/10/2016 19:49:44

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