Question

iflylow74 · 2005 November 22nd, 14:04

I posted a question on another forum and am having a hard time explaining the answer in laymens terms. Could you help?

Here it is:

A jet airliner is setting on a conveyor the length of a runway as it is trying to take off. The conveyor exactly matches the aircrafts movement BUT is going in the opposite direction of the aircraft. Will the aircraft be able to get airborne?

Thanks in advance.

**Fernanda** · 2005 November 22nd, 15:12

no because there is no air moving underneath and above the wing to cause the plane to lift. Besides, one other simple test is: let's you didn't know anything about lift, what happens as soon as the wheels lift off the conveyor?

What makes a plane lift is this:

The top of the wing has a longer path for air to travel than the lower wing. Thus, this air travels faster and faster moving air is lower in pressure than slow moving air. This difference causes the slower moving air to push up the plane from beneath the wing thus creating lift.

Here is a nice pic:

iflylow74 · 2005 November 22nd, 15:35

Yes I understand how the airfoil works but wouldnt the thrust of the airplane overcome the conveyor due to the wheels of the plane simply free rolling?

**Fernanda** · 2005 November 22nd, 16:16

but there is still no air MOVING around the wings

**Fernanda** · 2005 November 22nd, 16:20

ok, think of it this way. Its the difference between you running holding on the a kite, or using a treadmill while holding the kite.

Xerxes314 · 2005 November 22nd, 19:22

A key difference is that the airplane need not propel itself by pushing on the ground. It is perfectly capable of using its turbines to push itself with respect to the air.

Xerxes

2005 November 22nd, 20:30

Do the wheels rely on external force or just the jet engines?

If no -> then no moving air -> no take off ; since opposite forces will be equal and the plane would be just sitting there wrt the air.

if yes -> then no also -> I assume that the external motors ordinarily take the plane up to such a speed such that the jet engines work efficiently; but the wheels wouldnt make the air move, would it?

The original question demonstrates simple genius.

editor · 2005 November 22nd, 21:14

No it doesn't, it shows a surprising lack of common sense.

Also, how could the wheels move the air ?? They are just free rolling, unless the brakes are on. There's no external motors.

**Fernanda** · 2005 November 22nd, 22:39

if

Quote:

The conveyor exactly matches the aircrafts movement

if this is the condition, how can the plane EVER take off.

_mark_ · 2005 November 23rd, 06:56

If you have a huge fan infront of the conveyor???? :)

TortoiseShell · 2005 November 23rd, 10:12

Quote:

Originally Posted by _mark_

If you have a huge fan infront of the conveyor???? :)

If you have a huge fan, you don't even need the engines to be on, the plane will lift off the ground regardless.

**Fernanda** · 2005 November 23rd, 12:17

Quote:

Originally Posted by _mark_

If you have a huge fan infront of the conveyor???? :)

maybe in a wind tunnel...a very big wind tunnel

_mark_ · 2005 November 23rd, 14:29

(it was meant in jest :))

editor · 2005 November 23rd, 15:33

OK so physics is an experimental science. I borrowed a paraglider and strapped it on. Then I got on my daughter's treadmill and ran until I reached 30kph. There was no lift-off.
next time I'll try it outside the house.

kublai · 2005 November 23rd, 16:16

gotta have wheels, editor. it's all in the wheels.