@Frags

After some more thought I`ve concluded that the wheels would rotate twice as fast as during a normal takeoff.

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Fraggle

- insert profound text here -

@minator

Have you read the link http://www.straightdope.com/columns/060203.html ? What's you're take on that one ?

And the http://www.pprune.org/ forum links has some rather good points, in both it will fly and wil not fly "camps".

Good discusion on that forum. As the original question is flawed allready , how fast the conveyor belt and the wheels are moving (a=a+5 ??).

After reading pprune I think it will fly but still have my doubts..


/edit
After looking at that movie link, that one is flawed as well I think, isn't the paper pulled in the wrong direction ?

/edit 2 No it isn't.... Right ?

Last edited by Seer on 24-Feb-2006 at 09:44 PM.
Last edited by Seer on 24-Feb-2006 at 09:43 PM.

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Everything you say will be misquoted and used against you..
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@A1200

Haven't read rest of thread ..but.

The reason a plane trundles down a runway is to get enough wind passing over the wings to create lift.

If the conveyor nullifies the ground speed then not enough lift will be generated over wings.

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I dont suffer from Insanity - I enjoy it

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@The_Editor

If the conveyor nullifies the ground speed then not enough lift will be generated over wings.

That's just it I guess, according to the "it will fly" the conveyor in real world physics wouldn't nullify (sp?). I was completely on the it will not fly side, but reading the arguments it does sound plausible that it will.

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Everything you say will be misquoted and used against you..
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@Seer

Quote:

Quote: If the conveyor nullifies the ground speed then not enough lift will be generated over wings. That's just it I guess, according to the "it will fly" the conveyor in real world physics wouldn't nullify (sp?). I was completely on the it will not fly side, but reading the arguments it does sound plausible that it will.

It'll nullify the ground speed if the conveyer generates sufficient friction.
I think the link you gave pretty much answers it by saying the question is bad, the conveyer has to have huge power to generate that much friction, massively more than the plane's engines will generate.

In reality it may not be possible to build such a conveyer but physics questions like this don't generally concern themselves with boring things like reality!

I wonder though...

If you took a model aeroplane with worn bearings and placed it on a conveyer powered by a drag racer engine - what would win?

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Whyzzat?

Haven't read any of the links, but been an entirely THEORETICAL question,
Quote:

.....This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be EXACTLY the SAME (but in the opposite direction)....

The plane would NOT MOVE relative to the runway and therefor no air flow or lift.
I agree with "minator", the plane will NOT fly.

The question is not broken, as the conveyer will have to instantaneously match the speed (of the wheels) and thus offer an equal and opposite force (via friction on wheels to conveyer and bearings, plus air friction on rotating wheels) to the force generated by the jet engine.

The only way the plane will take off is if the conveyer CAN'T maintain the instantaneous "speed" of the plane. This will be a physical failing in the experiment, but has nothing to do with the THEORETICAL question.

Cheers,
Tim

@T_Power


You ARE totally WRONG!



The plane WILL FLY, the runway will not make any real difference to the plane.

The Aircraft WILL push its way through the air until sufficiant air speed
is obtained and it Will then take off (exactly the same as a static runway).


The wheels will be turning twice as fast, but so what; who cares..


See Newtons laws: For every action there ia an Equal and opposit reaction.

IE: The plane pushes exaust gases backwards, the exaust pushes the plane forwards.



End of arument

Now where is my plane ticket



Wol.

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It is my conviction that killing under the cloak of war is nothing but an act of murder.~Albert Einstein

@Wol

Wrong.

Exhaust gasses are not thrust.

Thrust is NOT lift. Air (wind) needs to pass over the wings to generate LIFT.

Put a car on a rolling road.. Rev it flat out in top gear... Will it take off ?

No.

No wings.... but the wings will be irrelevant if no lift is being generated by wind passing over the wings.

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@Thread

If the conveyor`s speed is maintained the same as the `plane`s speed relative to the ground but in the opposite direction, then I think it`s obvious that the `plane will take off assuming that it`s engine can overcome the slight extra friction caused by the wheels rotating twice as fast as normal. Discussing it this far (and I`m out on a limb here) is just what happens when computer geeks talk physics ;o)

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Fraggle

- insert profound text here -

@The_Editor



A car propells itself forward by turning its wheels and pushing the road backwards,
an airplane does not!


An airplane (JET) propells itself forward by pushing hot exaust
gases out of the rear of its motors at high speed.


An airplane (Propeller) propells itself forward by pushing the
air backwards with a propeller.


The Airplane will still move forward (Newtons Laws) through the air and the wings will generate lift same as before.


Because an airplane is NOT propelled by its wheels, it will not be affected
much by the conveyor belt runway; say only extra friction from wheels turning
faster.


You need to go and think some more.....



Wol.

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It is my conviction that killing under the cloak of war is nothing but an act of murder.~Albert Einstein

@all

There are factors here being ignorred.

Although it may have a mostly flat surface, that conveyor belt would begin to produce an induced frictional flow in the air, close to it, in its direction of movement.

The physics of how aircraft wings produce lift is incorrectly understood by most:

HOW DO AIRPLANE WINGS *REALLY* WORK?

More correctly, during the takeoff run, and immediately after (BEFORE an aircraft gains altitude), an aircraft is experiencing the higher lift from Ground Effect.

The rearward induced flow produced by the conveyor belt will act to LESSEN the usual gain from ground effect IF the aircraft were to actually accellerate towards normal lif-off speed (i.e. with landing gear wheels spinning TWICE as fast) . . . thus the USUAL aircraft ground effect will tend to be replaced by racing car type ground effect which acts to multiply the apparent WEIGHT of the aircraft!

So, even if the aircraft is moving forward relative to its GPS co-ords it MAY be TOO heavy to achieve lift before it runs into the airport boundary!

(more on ground effect
and still more, while this linked page is on Car Ground Effect.
Another name for induced flow is Boundary Layer Flow. Just as wind flowing OVER a flat surface will produce a boundary layer - so, a moving surface (relative to still air) will produce a boundary layer! )

Last edited by vortexau on 25-Feb-2006 at 02:53 PM.

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-vortexau, who's A1 XE-G4 remains at half-RAM !
A2000HD (from 1991) 060 64Mb PicassoII with OS3.5 . . . still working.

@Wol

Lets see....what does a plane need to fly?

For a plane to lift off the runway it needs to move forwards through the air, the shape of the wing is designed so that the air passing over the wing has a slightly longer path which make the air pressure lower on the top of the wing. This in turn makes the air on the underneath of the wing generate "lift" and given the large surface area of the wing there is enough "lift" generated to move the plane "upwards".

However, since the jet/propellors only act on the air to push the plane along the runway, and maintain forward speed for the flight, if the plane is on the conveyor belt which matches the "speed" of the planes travel in the opposite direction then the wheels will turn on the belt but plane will never "physically" move forward and thusly there will be no air passing along the planes wings to generate the lift needed.

It won't fly.

Damn it...this is annoying to figure out!

'm flying to Bristol on wednesday...I'll see if we can test some of these theories!

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@vortexau

LOL!

Well that makes my explanation a waste of time!

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@many here

There seems to be an imperfect understanding of aircraft thrust.

Thrust explanation

Propeller - jet - rocket, . . all produce a reaction, via their mounting bolts, to the airframe from moving a swift mass of gasses rearwards. There is no real "push" to the air. Recall that the rocket engine still works in a partial vacuum (space is NOT fully empty).

Have you ever watched moving pictures showing a fireman trying to hold still while holding a high-pressure fire hose? The high velocity mass rushing from the hose-nozzel forces the nozzel, and hose, backwards in a reaction to the exiting water-flow.

Recoil of a discharged gun is another example.

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A2000HD (from 1991) 060 64Mb PicassoII with OS3.5 . . . still working.

@vortexau

Note:

The reaward induced airflow will also add to the airflow over
the wings producing a little more lift, this may cancel any loss of
ground effect lost and extra frictoin of the wheels.

But I can't be ars@d to work it out to that detail.



Wol.

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It is my conviction that killing under the cloak of war is nothing but an act of murder.~Albert Einstein

@Thread

Having thought about this for a while I have to ignore my instant thought of the plane will not take of and agree with Wol. It doesn't matter how fast the convayor runs in the opposite direction of the plane it will still move forward because it is not propelled by its wheels they just move freely, so providing the tyres don't give out under the increased stress the plane will still take off.

Quote:

L8-X wrote: @Wol I'm flying to Bristol on wednesday...I'll see if we can test some of these theories!

You'll need to find a conveyor-belt runway. I wouldn't be surprised if Gates has considered investing in that, he can afford it!

If the "conveyor runway" came to a sudden halt, would the plane go TWANG through the air?

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SEX SUX

Sex doesn't prove to me that sex doesn't suck.

@RoqueFort

I can take the plane down to the nearest MOT centre and put it on the rolling road, should be the same effect.

Still don't think the plane would fly since there would be no forward motion and therefore no lift on the wings......whatever bloody method they use! (@vortexau)

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Quote:

L8-X wrote: @RoqueFort I can take the plane down to the nearest MOT centre and put it on the rolling road, should be the same effect. Still don't think the plane would fly since there would be no forward motion and therefore no lift on the wings......whatever bloody method they use! (@vortexau)

Yes, it doesn't seem possible to me, but Wol has another idea.

The thread title is right, it's driven me mad!

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SEX SUX

Sex doesn't prove to me that sex doesn't suck.

@RoqueFort

RoqueFort wrote:
Quote:

Yes, it doesn't seem possible to me, but Wol has another idea. The thread title is right, it's driven me mad!

Yes, it is a cracking puzzle!...had me thinking about it when I woke at 7.00am.

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