Plane on a treadmill...

[badass]

Right, I've got one word to solve all of this.


Helicopters.



Go away, find out how they fly (not how you THINK they fly but actually how they really do fly, ie, go a research and read), then come back and admit that either the plane will take off or helicopters cannot fly.

Go on, stop reading this post, go and Google.

I know how they really fly (as in the blades constantly shange pitch creating a kind of rowing motion) but dont see how this is relavant?

[Nick]

Wrong.

Go and read.

[badass]

Its quite easy, the plane isn't moving and neither is the treadmill... the belt of the treadmill is moving and so are the wheeels of the plane.

Giant hand situation, nope still wouldn't take off, sorry bud. to take your analogy a step futher, if you imagine a giant hand on the back of the plane as the engines, imagine an equally giant hand on the front of the plane pushing back (this is the treadmill) with exactly the same force - the plane doesn't move.

Its all a matter of forces.

NONONONONO NO!
that bold bit is wrong!
How can the force be the same?
Is there some magical system not mentioned that means that whatever force the engines are exerting, the treadmill accelerates till it reaches the point?
The wheels freewheel. If you hold the plane still and move the conveyor back and forth, there will be very little force required to keep the plane still.
If you accelerate the plane on any normal runway to say 100 MPH, then completely switch off the engines what happens?
The plane will slow down, but very slowly
The friction from the wheels is miniscule compared to the power output of the engines.
How do I know this.
Get on a plane. During take off and for the first few mins, the engines are at full throttle.
Whilst at full throttle the plane accelerates down the runway. When the wheels leave the goroun, is there a surge in acceleration?
NO
However the whole time the plane is accelerating HARD, both on the runway and immediately after takeoff.
That proves the engines have enough power to completely ignore the friction of the wheels at take off speed.
However the conveyor effectively doubles that speed.
So the friction will be increased as a result of the conveyor, but unless the wheel friction increases hugley between takeoff speed and double takeoff speed, then it will make no noticable difference.

I will also ask this:
If the friction is high enough to overcome the engines power, all of that energy has to go somewhere, usually as heat. I believe Jet engines are in the 20MW plus range of power.
Thats a lot of heat to dissapate for some wheels/bearings etc. Some might argue that's impossible

[autopilot]

Right, I've got one word to solve all of this.


Helicopters.

Nice one Nick. May i go one step further and suggest that a jet basically (ignoring the afterburner etc) like a helicopter on it's side.

redsky009 - When you have your eureka moment and realise you have got in so very wrong (that's the brilliant thing about this - even bright people get in a muddle and are totally sure they are right. The more they explain it to themselves, the more convinced they get) will you admit it?

[TeePee]

Is this thread really going to devolve into a discussion of retreating blade stalls?

[Nick]

Ooh how did I bloody know YOU were gonna come up with one?

(Which to some extent have been reduced by a 20 degree sweep on the wing tip to counter sonic compression and blade stall depending on the point of rotation... iirc... )

[MSIC]

If you think this discussion divides people, you should try getting money out of my sister.
You lend her money, she'll find a way to explain why you still owe her a tenner.

[TeePee]

Yeah, the aerodynamics of retreating blades are largely countered in the design.

I don't fly helicopters (I checked the book, and I'd need 50 hours to get a commercial rating. Maybe one day!)

I don't think they fly at all, just beat the air into submission.

[j.o.s.h.1408]

imo the plane wont take off because the air is not flowing to the wings for it to take off.

interesting discussion which seems to not lead to a definete solution

[Skie]

I actually think that rather than the plane taking off, it would crash

If the treadmill could keep up with the thrust of the engines and keep the plane at a standstill (and therefore having no lift), eventually the wheels would reach their limit and break. Then the plane would land on the runway/treadmill with something that wouldn't turn and be launched backwards like every youtube clip of someone on a treadmill.

[chuckskull]

The real question is; can a plane on a treadmill move off it, under it's own power. Which is a very good question

I think it can, if the treadmill is powered to the intended airspeed of the engines, then the planes wheels will spin twice as fast. If not powered, then the treadmill will barely move and possibly move forward, depending on the friction from the wheels/tyres/bearings.

Of course the plane will never take off, because the pilot will be sitting arguing with the co-pilot over whether it can take off

[TeePee]

Of course the plane will never take off, because the pilot will be sitting arguing with the co-pilot over whether it can take off

I fear you may be right...

http://forums.jetcareers.com/showthread.php?t=22291

http://www.pprune.org/forums/showthread.php?t=209859

[BenW]

Here you go, the plane will move forwards: http://videos.streetfire.net/player....D-D6BA1A43A06B

[manwithnoname]

“Imagine a plane is sitting on a massive conveyor belt, as wide and as long as a runway. The conveyer belt is designed to exactly match the speed of the wheels, moving in the opposite direction. Can the plane take off?

I think everyone agrees the plane will take off if the airflow passed the wings is sufficient. I've highlight designed, those who do not think the plane will take off assume the conveyer belt design will stop the plane moving through the air.

I'm in the take off camp, I go along with someone building a massive conveyer belt but there is no way that a conveyer belt could hold the plane stationary relative to the ground - when there is thrust from the engines

[redsky009]

redsky009 - When you have your eureka moment and realise you have got in so very wrong (that's the brilliant thing about this - even bright people get in a muddle and are totally sure they are right. The more they explain it to themselves, the more convinced they get) will you admit it?

Wehey, you said I'm bright I'm a scientist, provide reproducable proof and I will accept defeat

The thing I object to most is the notion that the wheels are somehow a separate entity from the plane and that the treadmill is spinning the wheels - these are complete and utter rubbish in the terms set out by the original hypothesis.

The only forward force here is provided by the engines, the only backwards force is provided by the treadmill. Prove to me (conclusively) that the force of thrust is greater than the force produced by the treadmill on the plane (when the treadmill has been designed to go at exactly the same speed as the wheels, and I will quite happily eat humble pie.

I will not accept conjecture, oh it must be this because etc etc, just because such a treadmill is implausible in our understanding does not change the facts, in the situation of this problem, such a treadmill does exist, and it can keep exactly the same speed as the plane

tyres burning out etc et are not relevant here, neither is the situation of a stationary runway, the friction coefficient of rubber on tarmac is not that high relative to an engines thrust. this is a moving runway, therefore there is an equal and opposite force acting on the wheels of the plane.

Once again, proof gentlemen, show me forces, show me working.

[badass]

Here you go, the plane will move forwards: http://videos.streetfire.net/player....D-D6BA1A43A06B

There we have it - proof!
Particularily note the fact that the speed of the paper towels underneath have absolutely no effect on the speed of the skateboard.