A puzzle for all.

[stunned_monkey]

Compltely off the wall, this one, but bear with me.

This appeared on the Skyline Owners forum 3 days ago and in that time has produced 232 replies across 15 pages and has had nearly 2000 hits. It's absolutely incredible the amount of arguing this has generated (and a bit of googling shows it's not limited to us - even "airliners.net" has had a good fight).

I'm tempted to post my reply, but I think I can predict, in advance, that Nick T, and Rich H will offer the correct answer.

But try convincing those who disagree with you.............

-------------------------------------

" A plane is on a runway that is made of a large conveyor belt. The plane starts up its engines and starts to move forward, but as it moves forward, the conveyor belt senses the speed of the plane's wheels and moves at exactly the same speed in the opposite direction. Can the plane take off???"

[Guinney1971]

ok..........just told Phill that one.........

He said....."what if the plane was a VTOL (Like a Harrier Jump Jet?) then it could take off. "

A conventional aeroplane surely wouldnt be able to take off without lift? (but I was crap at Physics at school and only just scraped a GCE O'Level pass in 1987)

Or of course, the aircraft could be a helicopter - like a VTOL, they can also move along the runway if they want.

[stunned_monkey]

Don't try and read lots into it, it's not a trick question. It's just a boggo standard aeroplane at the end of a very long imaginary conveyor belt.

No, it's not a harrier.

(Skyline forum now up to 16 pages and still going stong, with figures in excess fo £10k being bet on the result of an as-yet-to-be-decided experiment)

[Emagine]

Ok..... my reply would be this.....

A planes speed is measured in relation to the air. A plane relies upon air rushing across the wing to create a low pressure under the wing, effectivly meaning the wing (and plane) are sucked into the air. As the converyor belt is spinning at the same speed as the as the plane is trying to move forward the plane if effectivly stationary and so there would be no movement of air across the wing meaning it would not take off.

Thats my basic understanding of it all.

I like it though.... made me think

[Outatime]

This is a very good question! I kind of made my mind that it won't take off in one sense... but right until I had this example in mind inspired by a friend of mine:

Imagine this:

1.You wear roller skating shoes and go onto a running machine with a conveyor belt thing.

2. Then turn on the running machine and make that belt go as fast it can go (With your hands grabbing the handles of course!)

3. Then try to pull yourself foward! (and the answer is that you can still pull yourself forward!)

Since the plane applies its forward movement by jet propulsion (not by friction on the wheels as a car would do) it won't be stationary! The plane can still take off because the power is not applied to the ground by means of friction but to the air to gain actual airspeed.

The plane will require more power than standard runway, but still possible to take off.

[NickT]

I'd best upgrade the forum database

NickT.

[NickT]

I know the answer to this but I won't spoil it 8)

Think about these points:

The conveyor is giving closed loop feedback to the aeroplanes wheels so it is always adjusting the conveyor to them.

The wheels are not driven as in a car. There is only small frictional forces at the wheels bearings.

Enjoy.

NickT.

[Outatime]

NickT Wrote:I know the answer to this but I won't spoil it 8)

Think about these points:

The conveyor is giving closed loop feedback to the aeroplanes wheels so it is always adjusting the conveyor to them.

The wheels are not driven as in a car. There is only small frictional forces at the wheels bearings.

Enjoy.

NickT.

Gosh Nick, you words of "closed loop feedback" brings back the horror feeling I just had not long ago.... my machine intelligence and control exam paper!

[stunned_monkey]

I deliberately didn't post my answer because I've got enough of a fight on my hands over on the Skyline forum. That said, you've now provided both ends of the typical response

John and Jan are correct (if you believe me)

Stu has answered with the typical wrong answer (in my opinion)

For anyone who agree with Jan, John and me, the answer is obvious. But apparently it's also obvious to everyone in the "No" camp.

THis is probably a better psychological question than one of physics!

[NickT]

Because the wheels are not driven by the aeroplane newtons 3rd law is applied to the thrust and the mass of the plane. This is shown if you draw a force diagram.

It would be generally assumed by the average person that the wheels would have friction or linked to the drivetrain like a car or think that it would be like trying to run up a downward moving escalator.

The conveyor feedback even assuming it is a lagless feedback function would not apply as the wheel rotation is independant of the forces of the engine.

The plane would take off, the theoretical speed of the planes wheels would be approx twice then speed as a normal take off, assumiing perfect friction of tyre, no loss due to rolling resistance etc, the bearings were frictionless and the feedback function of the conveyors control system had zero lag.

NickT.

[stunned_monkey]

But I guarantee you most people think the thrust from the engines can be counteracted by the conveyor and no amount of examples will convnce otherwise.

Maybe DeLorean owners are more intelligent than airline pilots and skyline owners (the latter isn't hard)

[NickT]

It's the airline pilots that worry me !

[stunned_monkey]

http://www.airliners.net/discussions/te ... in/136068/

Have a read for yourself (387 replies!!!!!)

[Rich H]

Absolutely not.

Where's the lift coming from?

[bozzzydmc]

Rich beat me too it ....

If plane it not in motion there is no lift......