Who's right?

Jotun · 20 Jan 2007 at 02:18

tim_enchanter said:
Sorry to quote myself but read this again

I think Athanor should try this with a shopping trolley and a treadmill tbh

Stand at the side and push the trolley along. Irrespective of how fast the treadmill is going he will make progress forwards.

Shock · 20 Jan 2007 at 02:18

Will the penny ever drop?

"andy" · 20 Jan 2007 at 02:19

the plane wont be able to move forwards so how can it take off ?

if the treadmill is equal to the thrust from engine then it would be like trying to take off with the brakes locked on

Scuzi · 20 Jan 2007 at 02:19

tim_enchanter said:
Sorry to quote myself but read this again

And to use your example of a trolley. Imagine you are in an airport and have a trolley. You come to the moving walkway and decide to walk on the normal surface but reach across and push your trolley along the moving walkway. If you are walking against the flow of the moving walkway, you can still push the trolley forward, regardless of what speed you or the walkway are moving at because the force you are applying is against a plane other than the walkway.

The trolley is an aeroplane.
The moving walkway is the conveyor runway.
You are the aircraft engines.

clv101 · 20 Jan 2007 at 02:20

jhmaeng said:
...I fear I'm wasting precious sleeping time, so I'll leave it to some other hapless soul to keep trying.

Indeed, he'll have worked it out in the morning.

paddy · 20 Jan 2007 at 02:20

Jotun said:
The key though is that the wheels can spin freely, this means there is no resistance on the plane from the treadmill. The speed of the wheels and treadmill are irrelevant, as there is no force on the plane as a result of them.

That I think is the key to it all....

imagine the belt spins at 100mph, so long as the plane overcomes the backwards FORCE generated by the friction in the wheels you'll sit still... as soon as the pilot adds any more power...... your go forward and generate lift, the converyour belt could be going at 500mph, the only thing that would happen is for the wheels to spin really really fast....

The Plane Flies... it lives dammit... it lives!

Jotun · 20 Jan 2007 at 02:21

andy8271 said:
the plane wont be able to move forwards so how can it take off ?

if the treadmill is equal to the thrust from engine then it would be like trying to take off with the brakes locked on

The treadmill speed is equal and opposite to the speed of the wheels, not the thrust. They are completely different.

As I said before:

Jotun said:
There is very little force on the plane from the wheels. The wheels rotate pretty much freely (with almost negligible resistance), and when you have the engines providing a huge force going forwards the plane moves forwards.

jhmaeng · 20 Jan 2007 at 02:21

andy8271 said:
if the treadmill is equal to the thrust from engine then it would be like trying to take off with the brakes locked on

The plane will eventually take off... it would just ruin the brakes (and possibly start a fire) in the process. Do you honestly believe that the tinny little brakes on the wheels stand a chance against the vast thrust provided by the engines?

Macro · 20 Jan 2007 at 02:22

clv101 said:
No, could you explane to me the difference between the wheels driving it (like a car) and the thrust from the engines driving it (like a plane)?

Ok....

If it was an UNPOWERED treadmill:

A plane uses it's engines for power and would roll happily along the treadmill and take off because there is no opposite force on the treadmill to counteract the forward motion of the plane. The treadmill would stay still.

On a powered treadmill instead of the plane moving forward it stays still as the forward force is being matched by the force of the treadmill being powered backwards by equaly powerfull engine. Without the planes engines trying to power it forward it would fall off the back of the treadmill. The same applies in reverse as to why it can't get forward on the treadmill.

The only reason a car on a rolling road doesn't overcome the "treadmill" is because it's sunk into rollers so it would have to match their backward movement AND provide additional power to overcome getting out of the hole (rolling roads tend to have rollers in a dip like this)

\o/ \o/

"andy" · 20 Jan 2007 at 02:22

how though ?

a plane isnt a rocket it needs to be going into air so that the air can go under the wings and produce lift

i know what you mean but a normal plane is too heavy for the engines alone to make it take off , it needs to be moving forwards too

Shock · 20 Jan 2007 at 02:24

Athanor said:
Ok....

If it was an UNPOWERED treadmill:

What has 'powered' got to do with anything? The treadmill will only move as fast as the wheels...

jhmaeng · 20 Jan 2007 at 02:24

Athanor said:
Without the planes engines trying to power it forward it would fall off the back of the treadmill.

Only if the brakes were on, and the engines didn't provide enough power to overcome the brakes.

Otherwise, as I said above, it will initially move backwards because of the initial friction of the wheels, but once that friction is overcome, the treadmill will stop moving the plane, only the wheels.

paddy · 20 Jan 2007 at 02:25

Can I spin this problem on its head... it might help people who are getting confused with the car/plane etc...

Imagine a wind tunnel and a Ferrari, the Ferrari must drive along the tunnel, but the wind tunnel will match in head wind speed the speed of the ferrari.

Will the Ferrari make it to the end of the tunnel ?

Of course.... because the 2 areas of power application (wheels on ground VS wind) are unrelated....

Does that help?

clv101 · 20 Jan 2007 at 02:25

Athanor said:
If it was an UNPOWERED treadmill:

A plane uses it's engines for power and would roll happily along the treadmill and take off because there is no opposite force on the treadmill to counteract the forward motion of the plane. The treadmill would stay still.

On a powered treadmill instead of the plane moving forward it stays still as the forward force is being matched by the force of the treadmill being powered backwards by equaly powerfull engine.

No, the plane would roll happily along the treadmill and take off with the wheels spinning backwards with the treadmill.

Jotun · 20 Jan 2007 at 02:26

andy8271 said:
how though ?

a plane isnt a rocket it needs to be going into air so that the air can go under the wings and produce lift

i know what you mean but a normal plane is too heavy for the engines alone to make it take off , it needs to be moving forwards too

The engines draw in air and push it out the back. This causes the plane to move forwards, hence air flows over the wings and we get lift.

divine` · 20 Jan 2007 at 02:26

Oh this is painful

VIRII · 20 Jan 2007 at 02:26

clv101 said:
No, the plane would roll happily along the treadmill and take off with the wheels spinning backwards with the treadmill.

As the plane is stationary and no air flows over the wings where does the lift come from?

jhmaeng · 20 Jan 2007 at 02:27

divine_madness said:
Oh this is painful

Tell me about it

The dodgy physics on the thread makes baby Jesus weep.

VIRII · 20 Jan 2007 at 02:27

Jotun said:
The engines draw in air and push it out the back. This causes the plane to move forwards, hence air flows over the wings and we get lift.

Then it would not remain on the treadmill. Or is this treadmill runway length..

divine` · 20 Jan 2007 at 02:28

VIRII said:
As the plane is stationary and no air flows over the wings where does the lift come from?

Thing is, the plane is not stationary.