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A question of physics.....

a simple little question, or is it?

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Gary Manuel12/10/2016 21:40:57
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The WHEEL (rather than the tire), will be moving at the same speed as the rest of the plane.

The conveyor will move backwards, matching the speed of the wheels, which will cause the wheels to rotate, by virtue of the contact between the conveyor / TYRE.

Q.E.D.

Martyn K12/10/2016 21:44:15
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If the conveyor belt is running at the same speed as the wheels. The top of the wheel is moving at the same speed as the bottom of the wheel but in the opposite direction so therefore the net speed of the wheel is zero.

The conveyor belt is therefore not moving..

The aircraft will then accelerate normally and take off in the normal take off distance.

Seemples.

Pete B - Moderator12/10/2016 21:48:06
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Leaving aside the pedantry (and the imprecise nature of communication prevalent on t'internetsmile), I think you have to assume the 747 is in full working order - and you have to ignore the picture and read the script carefully.

Note that the conveyor belt is described as being as wide and long as a runway - not the foreshortened running machine in the pic which is a bit of an optical diversion, IMHO.

If the engines are run up, the reaction to the thrust will move the 747 forwards, regardless of the conveyor belt. Once it reaches take-off speed, it will fly.....

There you go - I've put myself on offer...teeth 2

Pete

Flyer12/10/2016 21:52:22
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The question was though ' can it fly'........and the assumption has to be yes, as it's a 747, regardless of all of the distractions.

...........now back to my sanding............

WolstonFlyer12/10/2016 21:54:12
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Posted by Flyer on 12/10/2016 21:52:22:

The question was though ' can it fly'........

The question says 'can it take off' , not can it fly.

Flyer12/10/2016 21:56:25
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Posted by WolstonFlyer on 12/10/2016 21:54:12:
Posted by Flyer on 12/10/2016 21:52:22:

The question was though ' can it fly'........

The question says 'can it take off' , not can it fly.

I apologise Wolston, it's the merlot talking ! But same applies, aircarft CAN take off. That answers the question.

Now where did I leave those thinners..............

WolstonFlyer12/10/2016 21:59:23
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Posted by Pete B - Moderator on 12/10/2016 21:48:06:

Once it reaches take-off speed, it will fly.....

But will it reach take-off speed?

Please be aware that this is not my question, I am just being naughty posting it here for you good people to have a go at.

Flyer12/10/2016 22:02:35
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You are answering a different question BEB; the question was can it take off, not will it

Now awaits the wrath of the moderator............

smile p

WolstonFlyer12/10/2016 22:04:51
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You need some more wine Flyer, cocktail

Flyer12/10/2016 22:13:52
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Posted by WolstonFlyer on 12/10/2016 22:04:51:

You need some more wine Flyer, cocktail

Always . It's just the glass that gets smaller....... now maybe that's another thread..........

Harry Twist12/10/2016 22:38:02
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If the wheel rotational speed always matches the conveyor speed but in the opposite direction- then - whatever the thrust from the engines, the aircraft will remain totally stationary. If it is stationary the wings cannot generate any lift at all - so no it cant take off and wouldnt even move forwards or backwards.

My analogy would be - imagine being on a running machine and as you constantly increase your pace you balance that by constantly speeding up the conveyor that you are running on - you will stay in the same place on the machine.

I reckon.......!!


Peter Beeney12/10/2016 22:43:19
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I’d have also thought no, simply by the fact that if the engines were not running but the belt was going backwards the plane would move backwards. If the engines then were then started and the thrust resulting in the wheels turning forward to exactly match the belt under all conditions the plane would simply stand still. Thus to me it’s quite difficult to see how it could take off. How far does a person on a running machine actually move forward? All the forward movement is transformed into rearward movement, resulting in the runner staying in one place.

PB

Edit. Pipped at the post, Harry…  Great minds and all that!

Edited By Peter Beeney on 12/10/2016 22:45:41

David perry 112/10/2016 23:00:04
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If theres air flowing over the wings, yes. If not, absolutely no.
From the information in this question it seems to be suggested rhat the aircraft is stationary with respect to the base of the conveyor belt, ie the wings arent going forwards. In such a case tjat bird aint gonna fly.
David perry 112/10/2016 23:02:29
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A trickeir conundrum is rhis, imagine youre driving a car with the windows open and your passenger starts flying his quad inaide the car, holding it stationary. He then slides it sideways rhrough the window. Wjat bhappens?

Conversely, he flies the quad outside, pacing your car. Then slides it sideways in thriugh a window...what happens?

D
WolstonFlyer12/10/2016 23:06:17
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Tricky question David yes

Biggles' Elder Brother - Moderator12/10/2016 23:09:32
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It will take off.

It's not a car - it isn't driven through its wheels!! The wheels are therefore irrelevant. They are just casters!

The drive (thrust) is being supplied by the jet engines, which require no contact with the ground.

If you want to visualise what would happen imagine the exact equivalent sitution (physically) of four ropes tied to the engine nacelles and the aircraft being pulled along by them via some mechanism. If you pulled hard enough and fast enough (which the 747's engine would in practice of course) it will take off.

BEB

WolstonFlyer12/10/2016 23:16:37
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Another vote for Yes.... are you sure? The belt exactly matches the speed of the wheels rotating but the opposite direction, so does the plane ever move forward?

Biggles' Elder Brother - Moderator12/10/2016 23:37:17
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yes - I'm sure! smile

Try a free-body-diagram on it and you'll be sure too! wink 2

BEB

john stones 112/10/2016 23:44:49
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Hands up if you googled "free body diagrams" yes

Interesting stuff

Martin Harris12/10/2016 23:48:25
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My immediate thought was as BEB but I was assuming this was in the real world. I somehow suspect that such a conveyer belt could not be built with any materials/technology available today but assuming the conditions stated could be matched, it's impossible for the aircraft to be moving forward through the air if the conveyer belt is moving backwards at the same rate that the aircraft's wheels are rotating.

However, if the brakes could stop the wheels rotating and sufficient lubricant was applied to the belt (stupid, but just as likely as building such a conveyer - or teflon pads were strapped to the tyres?) within the conditions specified, there's no reason why the aircraft couldn't then move forward relative to the belt, so I will stick with my original judgement that it could take off!

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