Plane on a conveyor revisited Options

[james]

Dragging up the past a bit - TV show Mythbusters ran it's much anticipated episode with a practical experiment version of this question on it a couple of nights ago over in the US.

I don't know when it will come on over here but for the curious Kottke liveblogged it and also links to another good explanation from a guy with a Sc.D. in Nuclear Science and Engineering (whatever that is).

And for the hard of attention and understanding - yes of course it takes off.

[camsmith]

You could just have used his graphic...

[jamie]

I just want to publicly state at this point


I still don't thin it'd take off

[Jeffers]

Brando, Brando, Brando......

If you strapped a rocket to my arse when I was running on a treadmill and fired it up, it wouldn't matter if I were Linford Christie on Speed, I would be propelled forwards and out through the walls of the gym. If you nailed some wings to my shoulders, then I would probably be able to make some sort of attempt at flight.

There is no way the treadmill can stop the plane moving forward and gaining lift as the wheels are free spinning and reduce the opposing force needed to act against the thrust of the engines. The force supposedly generated by the treadmill.

If you took the wheels off the plane and just sat it on the treadmill and the friction generated by the treadmill was equal and opposite to the thrust generated by the engines, then the plane would just sit there burning jet fuel.


It would take off but not as some people imagine.
i.e... Just stationary and then magically lifting of the ground.


Jeff

This post has been edited by Jeffers: Feb 25 2008, 10:21 AM

[paul]

If you strapped a rocket to my arse when I was running on a treadmill and fired it up, it wouldn't matter if I were Linford Christie on Speed, I would be propelled forwards and out through the walls of the gym. If you nailed some wings to my shoulders, then I would probably be able to make some sort of attempt at flight.
Jeff

You would also be a rich man cos I'd pay to see it

[james]

It would take off but not as some people imagine.
i.e... Just stationary and then magically lifting of the ground.

Reading that lit up a bulb for me - I never considered that people might have been thinking like that - I always assumed that everyone saw the plane would move forward along the runway/conveyor until it reached takeoff speed and then up, up and away! If people have been thinking we meant it would just suddenly lift off relative the the earth without any forward momentum, that would explain how come people aren't getting it!

[jamie]

no, I can see how you are thinking.. Its just if the planes engines were at low power taxying speed say the tread mill would counter the speed, thus leaving the plane stationary. Regardless of whether the power was coming from the engines or the wheels.

As long as the treadmill could keep up, the plane could never jump to the stage where its forward force we greater than the opposing force by the tread mill. The only way it could is by taking its downward gravitational force out of the equation which it could only do by moving forward and getting some airlift...

Anybody else see what I'm saying?!! Anybody?

[Jeffers]

Your wrong

no, I can see how you are thinking.. Its just if the planes engines were at low power taxying speed say the tread mill would counter the speed, thus leaving the plane stationary. Regardless of whether the power was coming from the engines or the wheels.

As long as the treadmill could keep up, the plane could never jump to the stage where its forward force we greater than the opposing force by the tread mill. The only way it could is by taking its downward gravitational force out of the equation which it could only do by moving forward and getting some airlift...

Anybody else see what I'm saying?!! Anybody?

[paul]

no, I can see how you are thinking.. Its just if the planes engines were at low power taxying speed say the tread mill would counter the speed, thus leaving the plane stationary. Regardless of whether the power was coming from the engines or the wheels.

As long as the treadmill could keep up, the plane could never jump to the stage where its forward force we greater than the opposing force by the tread mill. The only way it could is by taking its downward gravitational force out of the equation which it could only do by moving forward and getting some airlift...

Anybody else see what I'm saying?!! Anybody?

Completely agree mate!



A plane needs forward motion to create airflow over the wings which in turn creats up lift.



Of course I know nothing about it so could be as wrong as you! Especially as Jeff disagrees

[james]

Ok - as long as you didn't think we meant it suddenly leapt into the air from a stationary (relative to a static observer) start!

I've read back over your reply and this is the key bit:

... the tread mill would counter the speed, thus leaving the plane stationary

That would be true if the forward propulsion of the plane was through the wheels, but it isn't!

[jamie]

that only makes sense if the plane were suspended above the treadmill wheels touching the runway but no force (gravity) pushing down and causing drag.

Which it is not.

[Emily]

It took my a while to get this but there was one example that just explain it for me, because I could imagine the situation from real experiences.

In this example your not trying to take off, just get up to speed (as the plain would need to do to take off - getting air flowing over the wings etc)

here goes...

your on a treadmill (a long one), you've got skates on, imagine you have a rope attached to a wall a good distance in front of you and your treadmill reaches all the way to the wall.
the tread mill starts up and you have to hang onto the rope to stop yourself from being propelled backward. Your bored now and so you begin to pull yourself alone the rope expelling the used rope behind you - will you stay on the treadmill or will you be able to pull yourself alone it?

[jamie]

dont get me wrong, that makes sense. But there is no outside force pulling the plane. It is propelling itself.

[Emily]

the engine is pulling the plane through the air - air goes in one end out the other, its not the wheels doing the work its the engine pushing through the air, like with the rope the wheels are just letting the plane move more easily while the engines move it
this is as far as my explanation goes, i can't think of anything else

[Jeffers]

Emily is correct.

the engine is pulling the plane through the air - air goes in one end out the other, its not the wheels doing the work its the engine pushing through the air, like with the rope the wheels are just letting the plane move more easily while the engines move it
this is as far as my explanation goes, i can't think of anything else