Author Topic: Something to fry your brains?  (Read 1869 times)

Offline DC

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Something to fry your brains?
« on: July 24, 2008, 12:24:52 AM »
Hi Peeps,

I came across this question on another site I use:

A plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction). Can the plane take off? (Harriers excluded! Now come on Suzie, get thinking... ;D ;D)

Anybody care to comment? I know my thoughts on the matter, I'll post them next Friday (1st August)

Dave.
« Last Edit: July 24, 2008, 02:53:49 AM by DC »
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StrayDog

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Re: Something to fry your brains?
« Reply #1 on: July 24, 2008, 12:32:47 AM »
A Boeing treadmill, interesting.

I think it can. I could go into a long discourse on why, but that would be boring.
This'll be a, "here's my answer, sod how I worked it out," type of exam question.

Offline DC

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Re: Something to fry your brains?
« Reply #2 on: July 24, 2008, 01:30:05 AM »
A Boeing treadmill, interesting.

I think it can. I could go into a long discourse on why, but that would be boring.


Nah, come on Ollie, that's the idea of the question...
Life should NOT be a journey to the grave with the intention of
arriving safely in an attractive and well preserved body, but rather to
skid in sideways - Chardonnay in one hand - open throttle in the other -
body thoroughly used up, totally worn out and screaming : "Woo Hoo, what
a ride!"

Offline Akeith (Gray)

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Re: Something to fry your brains?
« Reply #3 on: July 24, 2008, 01:30:41 AM »
If I understand correctly, the plane is held stationary by the belt underneath it moving in the opposite direction. Therefore, no air is moving over the wings and consequently, no lift would be generated over the wings in turn. It is the flowing air over the wings, not the speed of the wheels, that generate the lift. Uh, right?

But then, if it were a prop plane, with the propellers were in front of the wings, it might lift off anyway.

With a jet engine, I say it would not.

Gray

(...of course now I won't be able to fall asleep tonight until I figure this out for sure.  :-\  Curse you.  ;D )

Edit: I finally fell asleep from exhaustion but...the prop plane wouldn't lift either. 
« Last Edit: July 24, 2008, 02:10:06 PM by GrayAyes »

StrayDog

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Re: Something to fry your brains?
« Reply #4 on: July 24, 2008, 01:43:53 AM »
Quote
Nah, come on Ollie, that's the idea of the question...

I'll give it a shot.

=

Just because the plane isn't going anywhere when it's on the treadmill, it still has a velocity.
It's not so much that the treadmill counteracts the plane, more that it becomes an endless runway.

That's all I can stand to write about this.

SuzieHarris

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Re: Something to fry your brains?
« Reply #5 on: July 24, 2008, 02:15:14 AM »
It could if it were a Harrier Jump Jet as they can take off vertically.

S x

Offline Mark H

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Re: Something to fry your brains?
« Reply #6 on: July 24, 2008, 04:20:45 AM »
No. [Edit: that is not a no to Suzie  :) it is a no to the original question.]

Neither jet nor prop will take off. The prop works like a vertical wing and has no effect other than moving the plane forwards (like the jet), but as the conveyor is moving it backwards at the same rate the wing is static in relation to the body of air. No movement of air over the wing means no lift.

Mark

[EDIT (again) for clarity - the props would have a secondary effect of generating airflow over the wing but it would not be sufficient to generate lift.]
« Last Edit: July 24, 2008, 04:32:00 AM by Citabria »
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Offline Amie

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Re: Something to fry your brains?
« Reply #7 on: July 24, 2008, 05:18:44 AM »
I agree with Cita and Gray. If there's no airflow relative to the wing you won't get lift and so the plane won't take off. If this turns out to be incorrect, we'll have to seek out and fire my first year fluid dynamics lecturer ;)
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PaulW

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Re: Something to fry your brains?
« Reply #8 on: July 24, 2008, 10:10:52 AM »
I agree - no air around the wings means no pressure differential = no lift. Unless the prevailing wind speed was sufficient it wouldn't be going anywhere. However - if the conveyer snapped, flinging the plane into the air, that could be a totally different matter. As described though, all it's going to do is wear the tyres out.

Here's the maths if anyone is interested.

http://en.wikipedia.org/wiki/Bernoulli's_principle


Offline Akeith (Gray)

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Re: Something to fry your brains?
« Reply #9 on: July 24, 2008, 02:08:24 PM »
We could always use our anti-gravity belts for a lift. I hear Apple has come out with a shoestring model.

Gray


Offline Spell Chick

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Re: Something to fry your brains?
« Reply #10 on: July 24, 2008, 03:35:46 PM »
Fixed wing aircraft might not be able to lift off, but rotary wing aircraft could. The other name for those are helicopters    ;D
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Offline Gyppo

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Re: Something to fry your brains?
« Reply #11 on: July 24, 2008, 03:50:40 PM »
Without needing - or fully understanding - the maths all my instincts, from several years of flying models and some time flying gliders, say no.  As others have said, you need airflow over the wing in order to get lift, and the bigger and heavier the aircraft the more speed it normally needs to fly.

Some forty+ years ago at an airshow I saw an old WW1 pilot almost 'throw' a Tiger Moth biplane into the air - in a stiff wind- with almost no take-off run.  He had made a bet with some of the WW2 polots that it could be done.  But it wasn't sustained flight and he flared it back down from about twenty feet with quite a crunch.

The Harrier or similar is surely a special case, a 'brute force lift'.  Similar to a prop aircraft making a truly vertical climb hanging from the prop. It's not 'flight' in the conventional sense of the word.

Just my thoughts on the subject.

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