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The great plane on the moving conveyor debate

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Can the plane take off?
Yes
47%
 47%  [ 34 ]
No
52%
 52%  [ 38 ]
Total Votes : 72

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Lord Percy
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PostPosted: 10:35 - 13 Oct 2016    Post subject: The great plane on the moving conveyor debate Reply with quote
Apparently this is an old and divisive topic, but I only came across it for the first time yesterday. Did a BCF search and found it discussed 10 years ago and again 8 years ago, but that's a pretty long time so thought I might as well see whose feathers can be ruffled this time around Very Happy.

Here it is:

https://scontent-lhr3-1.xx.fbcdn.net/v/t1.0-9/14670683_561345320741333_8632519001810725959_n.jpg?oh=6ed0660317c119ad0d25fba52960779e&oe=589ABD73

Last edited by Lord Percy on 10:40 - 13 Oct 2016; edited 1 time in total
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Lord Percy
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PostPosted: 10:37 - 13 Oct 2016    Post subject: Reply with quote
To me it's pretty obvious that the plane will take off because the forces on the body of the plane resolve like so:

https://scontent-lhr3-1.xx.fbcdn.net/v/t1.0-9/14705623_10155283557098496_5859441039462131173_n.jpg?oh=d97ad4a2f2dc0251edc09ff8063d2d20&oe=58618755

Conveyor simply increases rotation of the wheels, while the engines are the only linear force acting on the body of the plane so the result is forward motion.

To me this is blindingly obvious but over on facebook where I found it it's been quite controversial. Apparently the issue is down to how people interpret the description.
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Loui5D
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PostPosted: 10:40 - 13 Oct 2016    Post subject: Reply with quote
Lord Percy wrote:
To me it's pretty obvious that the plane will take off because the forces on the body of the plane resolve like so:

https://scontent-lhr3-1.xx.fbcdn.net/v/t1.0-9/14705623_10155283557098496_5859441039462131173_n.jpg?oh=d97ad4a2f2dc0251edc09ff8063d2d20&oe=58618755

Conveyor simply increases rotation of the wheels, while the engines are the only linear force acting on the body of the plane so the result is forward motion.

To me this is blindingly obvious but over on facebook where I found it it's been quite controversial. Apparently the issue is down to how people interpret the description.


The plane will only take off due to lift, this requires a moving air current to form a pressure difference underneath the wing.

No movement of air = no lift.
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The Shaggy D.A.
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PostPosted: 10:41 - 13 Oct 2016    Post subject: Reply with quote
The plane won't take off. Any forward thrust will be counteracted by the conveyor, so there's no airflow over the wings to generate any lift.
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Lord Percy
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PostPosted: 10:41 - 13 Oct 2016    Post subject: Reply with quote
Loui5D wrote:


The plane will only take off due to lift, this requires a moving air current to form a pressure difference underneath the wing.

No movement of air = no lift


if the plane has no engines.
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Lord Percy
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PostPosted: 10:43 - 13 Oct 2016    Post subject: Reply with quote
The Shaggy D.A. wrote:
The plane won't take off. Any forward thrust will be counteracted by the conveyor, so there's no airflow over the wings to generate any lift.


The division begins! Laughing

The conveyor just makes the wheels spin at some arbitrary speed, totally unrelated to the force that drives the plane forward and lets it create lift.
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The Shaggy D.A.
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PostPosted: 10:46 - 13 Oct 2016    Post subject: Reply with quote
Lord Percy wrote:
[The conveyor just makes the wheels spin at some arbitrary speed, totally unrelated to the force that drives the plane forward and lets it create lift.


No, it doesn't :-

You wrote:
The conveyor belt is designed to exactly match the speed of the wheels, moving in the opposite direction.


The plane will remain stationary.
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Loui5D
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PostPosted: 10:50 - 13 Oct 2016    Post subject: Reply with quote
You need lift for flight, no lift = drop like a stone, hence why gliders have to be towed to generate enough airflow to lift. They then use thermal's to rise.


You clearly don't understand how flying works.
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Hetzer
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PostPosted: 10:54 - 13 Oct 2016    Post subject: Reply with quote
The Shaggy D.A. wrote:
The plane won't take off. Any forward thrust will be counteracted by the conveyor, so there's no airflow over the wings to generate any lift.


Yes it will. All the conveyor does is make the wheels spin faster.
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Lord Percy
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PostPosted: 10:56 - 13 Oct 2016    Post subject: Reply with quote
The Shaggy D.A. wrote:
Lord Percy wrote:
[The conveyor just makes the wheels spin at some arbitrary speed, totally unrelated to the force that drives the plane forward and lets it create lift.


No, it doesn't :-

You wrote:
The conveyor belt is designed to exactly match the speed of the wheels, moving in the opposite direction.


The plane will remain stationary.


What linear force does the conveyor exert on the plane to counteract the linear force of the thrust created by the engines?

Loui5D wrote:
You need lift for flight, no lift = drop like a stone, hence why gliders have to be towed to generate enough airflow to lift. They then use thermal's to rise.


You clearly don't understand how flying works.


Ah ok so the consecutive Advanced Mechanics and Advanced Fluid Mechanics classes I've just walked out of have taught me naff all then Very Happy

You clearly forget that an aeroplane's engines don't drive the wheels which are free spinning.
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UnknownStuntm...
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PostPosted: 10:56 - 13 Oct 2016    Post subject: Reply with quote
On a normal aircraft there's no limitation on the wheels rotation during takeoff. The conveyor would be acting as a brake, so the plane remains stationary while the engines do their best to overcome this. They can't so no lift is generated by the wings and so on until someone's head is bleeding with the thought of this.

And that conveyor would need to be driven by some fuckoff jet engines, with bearings made of supercooled magnets that would most likely still assplode after a few seconds real work.

TL;DR: stop reposting nonsense. Laughing
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Hetzer
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PostPosted: 10:59 - 13 Oct 2016    Post subject: Reply with quote
The Shaggy D.A. wrote:
Lord Percy wrote:
[The conveyor just makes the wheels spin at some arbitrary speed, totally unrelated to the force that drives the plane forward and lets it create lift.


No, it doesn't :-

You wrote:
The conveyor belt is designed to exactly match the speed of the wheels, moving in the opposite direction.


The plane will remain stationary.


Imagine instead a rope attached to the front of the plane, pulling it. Do you think the plane won't be pullable? The wheels don't provide a static force, they are independent of any force moving the plane (other than the tiny amount of friction from the bearings).

Or imagine instead the plane with engines off. If you start the conveyor very gently you'll get the plane to move backwards on the conveyor, but start it fast enough to overcome the initial friction from the wheels and the plane will barely move backwards, it's wheels will spin instead. Now start the engines and go full-thrust, the plane will move forwards minus that tiny amount of friction from the wheels.
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Last edited by Hetzer on 10:59 - 13 Oct 2016; edited 1 time in total
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Lord Percy
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PostPosted: 10:59 - 13 Oct 2016    Post subject: Reply with quote
UnknownStuntman wrote:
The conveyor would be acting as a brake


How so? The conveyor just makes the wheels spin completely arbitrarily. The free wheels which have zero connection to the engine which is what gives the forward motion.
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angryjonny
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PostPosted: 11:00 - 13 Oct 2016    Post subject: Reply with quote
The Shaggy D.A. wrote:
You wrote:
The conveyor belt is designed to exactly match the speed of the wheels, moving in the opposite direction.


The plane will remain stationary.

I would argue the following:

The wheels do not drive the plane. The engine thrust does. The wheels are there simply because its better than the plane scraping its belly along the ground. The plane can move forward just as easily (some minor friction ignored) on the moving conveyor as it could on a regular runway.

However, the key point is "The conveyor belt is designed to exactly match the speed of the wheels".

This changes everything. Because:

Assume the plane starts doing 1mph.
(1) When the plane does 1mph, the wheels turn at the equivalent of 1mph
(2) The conveyor, then, by design, does 1mph. At this point the plane itself is still doing 1mph but the wheels are now doing the equivalent of 2mph to counter the moving conveyor.
(3) So the conveyor, as designed, then speeds up to 2mph, to match the wheels. As the plane does 1mph the wheels are now doing 3mph. So the conveyor speeds up again. The wheels are always trying to do 1mph more than the conveyor. The conveyor is always accelerating to catch up.
(4) This process continues. The conveyor does 100 mph so the wheels do 101. The conveyor does 1000mph so the wheels do 1001. Eventually something gives up, probably the conveyor. It disintegrates, parts fly out and damage the plane. Now it can't take off.

TL;DR - as soon as the plane starts moving the conveyor goes faster and faster until it implodes.

Last edited by angryjonny on 11:04 - 13 Oct 2016; edited 2 times in total
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Aceslock
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PostPosted: 11:02 - 13 Oct 2016    Post subject: Reply with quote
It wont take off because there is no wind resistance to lift the wings. It will probably nose dive when the conveyor blows up Laughing
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Lord Percy
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PostPosted: 11:03 - 13 Oct 2016    Post subject: Reply with quote
angryjonny wrote:

(1) When the plane does 1mph, the wheels turn at the equivalent of 1mph
(2) So the conveyor, by design, does 1mph. At this point the plane is still doing 1mph but the wheels are now doing the equivalent of 2mph to counter the moving conveyor.
(3) The conveyor, as designed, then speeds up to 2mph. So as the plane does 1mph the wheels are doing 3mph. So the conveyor speeds up.
(4) This process continues. The conveyor does 100 mph so the wheels do 101. The conveyor does 1000mph so the wheels do 1001. Eventually something gives up, probably the conveyor. It disintegrates, parts fly out and damage the plane. Now it can't take off.

TL;DR - as soon as the plane starts moving the conveyor goes faster and faster until it implodes.


Nope. I thought about this too and it seemed to give some kind of infinity where the wheels and conveyor are constantly trying to catch up with each other.

But this isn't true.

It's basically two connected rotational bodies where the plane engine controls their shared rotational speed.
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Loui5D
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PostPosted: 11:06 - 13 Oct 2016    Post subject: Reply with quote
wheels & engines do not generate lift, wings do.
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Hetzer
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PostPosted: 11:06 - 13 Oct 2016    Post subject: Reply with quote
angryjonny wrote:

I would argue the following:

The wheels do not drive the plane. The wheels are there simply because its better than the plane scraping its belly along the ground. The plane can move forward just as easily (some minor friction ignored) on the moving conveyor as it could on a regular runway.

However, the key point is "The conveyor belt is designed to exactly match the speed of the wheels".

This changes everything. Because:

(1) When the plane does 1mph, the wheels turn at the equivalent of 1mph
(2) So the conveyor, by design, does 1mph. At this point the plane is still doing 1mph but the wheels are now doing the equivalent of 2mph to counter the moving conveyor.
(3) The conveyor, as designed, then speeds up to 2mph. So as the plane does 1mph the wheels are doing 3mph. So the conveyor speeds up.
(4) This process continues. The conveyor does 100 mph so the wheels do 101. The conveyor does 1000mph so the wheels do 1001. Eventually something gives up, probably the conveyor. It disintegrates, parts fly out and damage the plane. Now it can't take off.

TL;DR - as soon as the plane starts moving the conveyor goes faster and faster until it implodes.


That's pretty cockeyed. Laughing For the conveyor to impart enough friction to the wheels, such that the friction can overcome the thrust from the engines, it would have to be doing a billion RPM. If the 'conundrum' is to be regarded as a realistic conjectural event the conveyor has to be assumed to be operating within realistic parameters.

innit.
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angryjonny
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PostPosted: 11:06 - 13 Oct 2016    Post subject: Reply with quote
Lord Percy wrote:
Nope. I thought about this too and it seemed to give some kind of infinity where the wheels and conveyor are constantly trying to catch up with each other.

But this isn't true.

It's basically two connected rotational bodies where the plane engine somehow controls their shared rotational speed.

No. If the conveyor was designed to match the speed of the plane, this would be true. But the conveyor is designed to match the speed of the wheels, which are themselves affected by the turning of the conveyor itself.

Accept that the conveyor moving doesn't affect the plane itself moving because the jet engines thrust the plane forwards regardless of what is going on underneath it. Then:

Wheel speed = plane speed + conveyor speed
and
Conveyor speed = wheel speed
means
Conveyor speed = plane speed +conveyor speed.

Hence calamity.

Last edited by angryjonny on 11:12 - 13 Oct 2016; edited 1 time in total
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Hetzer
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PostPosted: 11:11 - 13 Oct 2016    Post subject: Reply with quote
Having just re-read the OP's picture, I've decided it's bollocks from the get-go, it makes no sense. 'Matching the speed of the wheels' is non-sensical and the author of the question should be designated a muppet.

Very Happy
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The Shaggy D.A.
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PostPosted: 11:13 - 13 Oct 2016    Post subject: Reply with quote
Hetzer wrote:
Imagine instead a rope attached to the front of the plane, pulling it. Do you think the plane won't be pullable?


Depends. If you yourself are not on the conveyor, then yes the plane will move, relative to the air it needs to generate lift. But, if you are on the conveyor too, then the distance between you and the plane will change, but it will remain stationary, relative to the air - no air flow, no lift.

Quote:
The wheels don't provide a static force, they are independent of any force moving the plane (other than the tiny amount of friction from the bearings).

Or imagine instead the plane with engines off. If you start the conveyor very gently you'll get the plane to move backwards on the conveyor, but start it fast enough to overcome the initial friction from the wheels and the plane will barely move backwards, it's wheels will spin instead. Now start the engines and go full-thrust, the plane will move forwards minus that tiny amount of friction from the wheels.


The plane will move forward in relation to the conveyor, but the conveyor is moving the plane backwards at the same speed. If the take-off speed is (say) 100mph but the conveyor is going in reverse at 100mph, where's the airflow for the lift coming from?
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STONEY!
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PostPosted: 11:13 - 13 Oct 2016    Post subject: Reply with quote
Imagine you are running on a conveyor belt with a paper plane in your hands, what happens when you throw it?

The paper plane will still fly as normal as the force propelling it is seperate to the motion of the conveyor, the same will happen for the plane.
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angryjonny
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PostPosted: 11:14 - 13 Oct 2016    Post subject: Reply with quote
Hetzer wrote:
Having just re-read the OP's picture, I've decided it's bollocks from the get-go, it makes no sense. 'Matching the speed of the wheels' is non-sensical and the author of the question should be designated a muppet.

Very Happy

Hence my careful use of the phrase "the wheels are now doing the equivalent of..."

It is, as you say, nonsensical.

Does "wheel speed" mean the rate at which the circumference of the wheel is passing over the conveyor, or the rate at which the hub is moving forward relative to the earth? Because that changes everything.
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Rogerborg
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PostPosted: 11:15 - 13 Oct 2016    Post subject: Reply with quote
Conservation of momentum will move the plane forwards relative to the ground and take off every 'ZIG'. The wheels will simply spin faster than they would if the treadmill wasn't there.

You need to add two pterosaurs for stability though.

https://i2.wp.com/imgs.xkcd.com/blag/sauropod.png
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Last edited by Rogerborg on 11:18 - 13 Oct 2016; edited 4 times in total
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