Who's right?

"andy" · 20 Jan 2007 at 02:52

yeah you missed my post where i admitted wrongness

this un grr

andy8271 said:
oooops lol i see what ive done wrong now , having had a few drinks i failed to apprehend that they suck air into the front

was trying to work it out as though they where just mad thrusters pushing "summit" out the back

oops , so yea it will definetly take off

VIRII · 20 Jan 2007 at 02:54

andy8271 said:
how can they be both right they both think different lol

although virii is slowly swaying towards the right answer

Because there are several things being discussed here and confused by a number of people.

First one is can the treadmill stop the plane from moving forward in space.
The answer is no it can't.

Second one is if the plane is stationary can it still take off when the engines are at full thrust (someone suggested the sucking and blowing of the jet engine caused the lift).
The answer is no, the lift is created by the wings, the thrust by the engine.

The third thing i wanted clarified (only came into this thread a few minutes ago) is the length of the runway because technically for a plane to take off whilst "on" the treadmill it would have to be a big treadmill in length.

tim_enchanter · 20 Jan 2007 at 02:54

The plane would need to only apply a small amount of thrust to hold stationary against any friction in the bearings. Once it has that level of thrust against the air around it the treadmill/wheels can rotate at any speed they like the plane will hold still relative to solid ground.
Increase the amount of thrust against the air and it will overcome the friction caused by the wheels and begin to move forward relative to the surroundings.
The more thrust it applys the faster it will go forward until it takes off.
Yes the wheels may go so fast the catch fire or whatever but thats not really the point of it all.

also I think for the purpose of the argument

debate the treadmill should be whatever length is required for takeoff if it is possible otherwise what would be the point of the question in the first place

blighter · 20 Jan 2007 at 02:55

VIRII said:
The third thing i wanted clarified (only came into this thread a few minutes ago) is the length of the runway because technically for a plane to take off whilst "on" the treadmill it would have to be a big treadmill in length.

or a small plane

/runs to nearest exit

VIRII · 20 Jan 2007 at 02:57

blighter said:
or a small plane

/runs to nearest exit

LOL

"andy" · 20 Jan 2007 at 02:57

VIRII said:
Second one is if the plane is stationary can it still take off when the engines are at full thrust (someone suggested the sucking and blowing of the jet engine caused the lift).
The answer is no, the lift is created by the wings, the thrust by the engine.

the air displacement from front to back of the "engine" will push the plane forwards (its what i didnt think about when i posted at first)

once its going forwards it will get the lift

as someone said its why seaplanes work

blighter · 20 Jan 2007 at 02:58

VIRII said:
LOL

sorry im just really bored and can't think of anything to do apart from sleep... and im not tired as have just drunk a litre of red rooster :eek:

EDIT: VERY good point about seaplanes... i think you could have solved the argument... they have a bigger surface area compared to planes wheels, so therefore my theory about the treadmill breaking is less accurate then i thought...

divine` · 20 Jan 2007 at 02:59

andy8271 said:
the air displacement from front to back of the "engine" will push the plane forwards (its what i didnt think about when i posted at first)

once its going forwards it will get the lift

as someone said its why seaplanes work

I think VIRIIs point is if you assume a plane is stationary, regardless of whether the treadmill would cause this.

Jotun · 20 Jan 2007 at 02:59

VIRII said:
The third thing i wanted clarified (only came into this thread a few minutes ago) is the length of the runway because technically for a plane to take off whilst "on" the treadmill it would have to be a big treadmill in length.

Actualy, if you read the original question (which I hadn't properly till now either) it says:

Hypothetical Question

A plane is standing on a runway that can move like a giant conveyor belt. The plane applys full forward power and attempts to take off. This conveyer has a control system that tracks the plane's wheel speed and tunes the speed of the conveyer to be exactly the same but in the opposite direction, similar to a treadmill.

The question is:

Will the plane take off or not?

So our conveyor belt is runway length

VIRII · 20 Jan 2007 at 03:00

divine_madness said:
I think VIRIIs point is if you assume a plane is stationary, regardless of whether the treadmill would cause this.

Yup, I wanted to make the distinction between thrust and lift.

"andy" · 20 Jan 2007 at 03:00

it was probly me suggesting it caused "lift" to be honest but i meant it would move it forwards !

drinking and physics are bad mmkay

VIRII · 20 Jan 2007 at 03:01

Jotun said:
Actualy, if you read the original question (which I hadn't properly till now either) it says:

So our conveyor belt is runway length

Heheh I confess to only reading the last 2 pages.

VIRII · 20 Jan 2007 at 03:01

andy8271 said:
it was probly me suggesting it caused "lift" to be honest but i meant it would move it forwards !

drinking and physics are bad mmkay

Butof course you did

tim_enchanter · 20 Jan 2007 at 03:01

Jotun said:
Actualy, if you read the original question

/snip
our conveyor belt is runway length

Well spotted

Jotun · 20 Jan 2007 at 03:02

VIRII said:
Heheh I confess to only reading the last 2 pages.

To be fair I only quickly glanced at the original question and stopped reading when I realized it was "that treadmill question" again

tim_enchanter · 20 Jan 2007 at 03:05

I have to agree with VIRII when he says that people were confusing thrust and lift.
The plane must use thrust to gain forward speed to create lift to enable takeoff same as any normal takeoff.
The point is the treadmill cannot stop the plane from gaining airspeed by thrusting against the air around it.

blighter · 20 Jan 2007 at 03:06

tim_enchanter said:
thrusting against the air around it.

am i the only one that LOL'd?

i got an image of a stickman making thrusting movements now

it think i need sleep...

tim_enchanter · 20 Jan 2007 at 03:08

Yeah maybe I could have worded that slightly better but hell it's gone 3am

"andy" · 20 Jan 2007 at 03:08

speaking of flying red bull gives you wings

it also stops you been able to go to sleep even at 3 am :/

VIRII · 20 Jan 2007 at 03:11

tim_enchanter said:
I have to agree with VIRII when he says that people were confusing thrust and lift.
The plane must use thrust to gain forward speed to create lift to enable takeoff same as any normal takeoff.
The point is the treadmill cannot stop the plane from gaining airspeed by thrusting against the air around it.

What is needed is a good explanation as to why the wheels and treadmill have no or little effect on moving the plane forward or stopping it moving forward.
A car on a rolling road has direct drive to the wheels and that drive is a rotational force not a forward thrust hence the treadmill or rollers are turned.
Strap a batman style rocket on the car and it's going to overcome the rolling road.

I can't adequately verbalise why the wheels (which are not direct driven) on the plane have little effect on a treadmill but the forces involved are very different in size and direction. I guess there comes a point where the downwards force (mavity) keeping the wheels in contact with the treadmill is simply insignificant compared to the forward thrust so the wheels will simply skid forward / lose grip on the treadmill even if still turning.