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Author Topic: Amiga.Org users encouraged to help debate  (Read 4492 times)

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Offline a1200Topic starter

Amiga.Org users encouraged to help debate
« on: February 26, 2006, 02:45:21 AM »
Guys, I have a thread on Amigaworld that needs more brains than ever before:

Here is the thread. It asks the question:

"A plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor 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?"

Please comment on here and Amigaworld if you have any insight to this. It might be harder than answering "if the universe is expanding, what's beyond the universe?" question! Well maybe not but it is driving me potty. What is the answer?!?!

When you have made a decision, there is a poll here.

Edit: Title had spelling mistake  :roll:
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Offline Cymric

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Re: Amiga.Org users encoraged to help debate
« Reply #1 on: February 26, 2006, 09:00:30 AM »
Since the determinig factor for lift-off is the speed of the plane versus that of the surrounding air, the plane cannot take off, as it is standing still with respect to said air. Question solved.
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Re: Amiga.Org users encoraged to help debate
« Reply #2 on: February 26, 2006, 09:31:00 AM »
I'm not welcome at the Local site.
 

Offline Vincent

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Re: Amiga.Org users encoraged to help debate
« Reply #3 on: February 26, 2006, 02:25:33 PM »
I think Cymric is right.  The plane needs uplift from the surrounding air as the air passes past the wings.

If the air isn't passing the wings (just staying still) there's no updraft.
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Offline uncharted

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Re: Amiga.Org users encoraged to help debate
« Reply #4 on: February 26, 2006, 03:51:26 PM »
The plane can't take off under those circumstances.

Not going to rejoin AW for a poll though.
 

Offline blobrana

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Re: Amiga.Org users encoraged to help debate
« Reply #5 on: February 26, 2006, 03:57:19 PM »
Hum,
AOrg users are too smart....

Offline Boot_WB

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Re: Amiga.Org users encoraged to help debate
« Reply #6 on: February 26, 2006, 04:25:18 PM »
The problem is worded perfectly adequately, if one assumes a frame of reference of a point which is static at v=0:

The problem states that the runway moves with the same speed as the plane - if the plane is stationary the runway is also stationary.

The problem DOES NOT state that the runway applies an equal opposing force to the plane as that exerted by the plane engines on the surrounding air.

In the absence of a paint program and the use of algebraic symbols I'm not gonna start drawing free body diagrams, scanning them in, and writing the force-balance equations, but the upshot is:

There is an imbalance of forces - the plane accelerates, the plane takes off.

Step-by-step:

1) The plane begins to move - let's assume it moves at v=30m/s

2) The ground moves in an opposite direction relative to the plane at v=-30m/s.

3) The wheels are free to rotate about their bearings.

4) The wheels rotate as if the plane were moving at 60m/s, ie the planes velocity relative to the moving runway.

5) The plane doesn't give a crap how fast either the ground is moving or the wheels are rotating (Neglecting friction at the bearings - which, because they are bearings is negligible) - the only thing which concerns the plane is its own velocity relative to the air around it which (in simplified terms) creates lift.

6) The only thing the motion of the ground will accomplish is making the wheels rotate faster, and the bearings produce twice as much heat as normal due to friction.

7) The plane will take off.

Phewww! 4 years of studying mechanical engineering, and it seems some of it sticks after all.

Any arguement and I get the algebra out......
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Offline Cymric

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Re: Amiga.Org users encoraged to help debate
« Reply #7 on: February 26, 2006, 04:41:58 PM »
Quote
Boot_WB wrote:
5) The only thing the motion of the ground will accomplish is making the wheels rotate faster.

6) The plane will take off.

I trust you meant to insert a 'not' in there somewhere ;-).
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Offline Boot_WB

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Re: Amiga.Org users encoraged to help debate
« Reply #8 on: February 26, 2006, 04:49:55 PM »
@Cymric

No, but I edited my post, as I realised I couldn't count to 7 after writing the first draft :-D

The problem states the ground moves at the same speed as the plane - if the plane is still, the ground is still - if the plane moves a 5m/s the ground moves at -5m/s - the interface between the ground and plane (ie the wheels) measures their RELATIVE velocity, but the absolute velocity of the plane (in this case assumed relative to the surrounding air) remains unchanged by the ground motion.

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Offline Cymric

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Re: Amiga.Org users encoraged to help debate
« Reply #9 on: February 26, 2006, 05:08:42 PM »
Unfortunately for you, your reasoning is correct up until point 7. The plane does not take off, because there is no force imbalance. There is simply no net force in the vertical direction (a.k.a. lift), so the plane does not move upwards. That is the job of wings, alerons, flaps and slats: transform the horizontal motion of air passing over them into a vertical force which balances gravity, or exceeds it slightly (in which case Newton's Second Law stipulates an accelaration in the upward direction), or is slightly smaller (in which case there is accelaration in the downward direction).

That is also why loss of lift results in such catastrophic accidents. The plane still has lots of horizontal velocity relative to the air, but the areas which are responsible for lift don't do their job any more. The plane is then literally a brick due to impact on the ground within (I think) at most two or three minutes.

Later edit: Ack, ick, phooey. I never got the hang of the differences in torques in freely rotating bodies, or those which were powered to do so, when both are presented with a solid surface. Of course I am completely and utterly WRONG: The plane WILL take off. It's the same as when an airplane would be taking off from a completely iced-over runway. The wheels would just slip over the ice (and in fact, nearly stand still) so you'd lack the ability to steer unless you throttle the engines separately. But take off you will. The morale of the story: don't put a soapbox race car with JAT engine on those rolls used to test car performance in the lab, because you'll create one heck of a hole in the wall...
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Offline Doobrey

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Re: Amiga.Org users encoraged to help debate
« Reply #10 on: February 26, 2006, 05:43:45 PM »
Quote

Cymric wrote:
Since the determinig factor for lift-off is the speed of the plane versus that of the surrounding air, the plane cannot take off, as it is standing still with respect to said air. Question solved.


But unless the conveyor belt can produce an equal but opposite force to the plane ,it won't be standing still with respect to the surrounding air.
 The only interface between the belt and the airframe are the wheels, which aren't powered and are free to rotate at any speed they like (well, upto breaking point), so I fail to see how such an opposing force can be exerted.

Anyway, time for BootWB to break out the algebra, and I'll put some popcorn on.
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Offline uncharted

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Re: Amiga.Org users encoraged to help debate
« Reply #11 on: February 26, 2006, 06:44:04 PM »
In which case it would be moving relative to the belt, which seems to negate the original problem.
 

Offline jkirk

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Re: Amiga.Org users encoraged to help debate
« Reply #12 on: February 26, 2006, 08:37:02 PM »
the answer av web.net
The only stupid question is a question not asked.  


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Offline whabang

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Re: Amiga.Org users encoraged to help debate
« Reply #13 on: February 26, 2006, 10:08:26 PM »
Planes are not propelled through their wheels, so the speed of the conveyor belt is irrelevant. The plane will take off.
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Offline blobrana

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Re: Amiga.Org users encoraged to help debate
« Reply #14 on: February 26, 2006, 10:37:34 PM »
Hum,
If we assume that there is air and that the aircraft uses jets  (or the propellers do not push air past the wings) the airspeed relative to the aircraft is zero.

The plane cannot takeoff.... (the answer in  av web.net is wrong!)

You canna change the laws of physics.