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plane question

#41
fr0me0 said:
A plane is standing on runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

The question: Will the plane takeoff?
Click to expand...
Yes.

Let's say there's no wind, and the plane will lift off at 150km/h (= airspeed = solid-ground speed).

This is what'll happen:

First, when the plane stands still, the conveyor isn't moving either. Then the plane engine gives full thrust ahead, and the plane starts accelerating.
The conveyor starts simultaneously and accelerates in the opposite direction, matching the plane speed.

But this will not keep the plane back, it will only make the wheels rotate twice as fast as they would on solid ground.

Then, when the plane reaches 150km/h it lifts off the conveyor, that is moving with 150km/h in the other direction at that point.
 
#42
erik II said:
Yes.

Let's say there's no wind, and the plane will lift off at 150km/h (= airspeed = solid-ground speed).

This is what'll happen:

First, when the plane stands still, the conveyor isn't moving either. Then the plane engine gives full thrust ahead, and the plane starts accelerating.
The conveyor starts simultaneously and accelerates in the opposite direction, matching the plane speed.

But this will not keep the plane back, it will only make the wheels rotate twice as fast as they would on solid ground.

Then, when the plane reaches 150km/h it lifts off the conveyor, that is moving with 150km/h in the other direction at that point.
Click to expand...
Er, no, I'm afraid.
If the plane's thrust is equalling the backwards movement of the conveyor, there is no net movement. If you stood next to the plane you wont see it moving. If there is no wind, there is no way the air will move over the aerofoils and produce any lift! The net movement of the plane and therefore the air over its wings is zero!

Unless you forgot to mention that it overcomes the conveyor, gets onto static ground and then accelerates to its lift-off speed, then yes, it would work, but there'd be absolutely no point in the conveyor.
 
#43
erik II said:
Yes.

Let's say there's no wind, and the plane will lift off at 150km/h (= airspeed = solid-ground speed).

This is what'll happen:

First, when the plane stands still, the conveyor isn't moving either. Then the plane engine gives full thrust ahead, and the plane starts accelerating.
The conveyor starts simultaneously and accelerates in the opposite direction, matching the plane speed.

But this will not keep the plane back, it will only make the wheels rotate twice as fast as they would on solid ground.

Then, when the plane reaches 150km/h it lifts off the conveyor, that is moving with 150km/h in the other direction at that point.
Click to expand...
thank you:smug:
 
#44
Martybass said:
Er, no, I'm afraid.
If the plane's thrust is equalling the backwards movement of the conveyor, there is no net movement. If you stood next to the plane you wont see it moving. If there is no wind, there is no way the air will move over the aerofoils and produce any lift! The net movement of the plane and therefore the air over its wings is zero!

Unless you forgot to mention that it overcomes the conveyor, gets onto static ground and then accelerates to its lift-off speed, then yes, it would work, but there'd be absolutely no point in the conveyor.
Click to expand...
This is why I asked whether the acceleration of the conveyor was correlated to the speed of the plane, or the speed of the wheels
 
#45
The plane will definitely take off. No matter how fast the conveyor moves (barring it moving so fast that the friction force between the wheels and the axles becomes on the order of the thrust of the aircraft). The planes wheels are not driven, so they will just free spin. The only difference will be how fast the wheels are spinning at take off.
 
#46
Martybass said:
Er, no, I'm afraid.
If the plane's thrust is equalling the backwards movement of the conveyor, there is no net movement. If you stood next to the plane you wont see it moving. If there is no wind, there is no way the air will move over the aerofoils and produce any lift! The net movement of the plane and therefore the air over its wings is zero!

Unless you forgot to mention that it overcomes the conveyor, gets onto static ground and then accelerates to its lift-off speed, then yes, it would work, but there'd be absolutely no point in the conveyor.
Click to expand...

Unless the conveyor is able to transmit an equal amount of thrust to the plane, this is incorrect. I don't know how it could seeing as how the wheels rotate freely and will only transmit force to the aircraft through friction.
 
#47
Martybass said:
Er, no, I'm afraid.
If the plane's thrust is equalling the backwards movement of the conveyor, there is no net movement.
Click to expand...
There's no friction between the plane and the conveyor. The conveyor could move ten times the plane speed, and only cause the wheels to spin faster.

The plane's engine uses the air to push/pull the plane forward.

Edit: Geoff has already said pretty much the same...
 
#50
BassManolukas said:
Question about the original question -

Is the converyor belt's speed correlated to the speed of the wheels, or the speed of the plane?

This is very important
Click to expand...

Not really important, but if it were correlated to the speed of the wheels it would be a feedback loop, the wheels spinning faster would make the belt spin faster, making the wheels spin faster, but the plane is still moving compared to the ground that the conveyor belt is attached to.
 
#52
i not positive, but im pretty sure no one here is qualified to post on this. any pilots on the forum? they could comment. any physics professors? they could too.

this sort of thing just has way to many variables. wind, torque, friction, thrust, weight, etc.

my 2 cents is the treadmill logic. if the plane were stationary and the belt was moving, it would roll the plane backwards. forward motion of the plane would counter this, but wouldnt exceed it.

actually, now that ive said that i think the real thing is the landing gear. im not a pilot, but one might be able to answer this. How is it connected? it all comes down to whether or not you could (hypothetically) lift the front end of the plane and spin the wheels. if they effortlessly spin then the plane could probly take off on the belt. if they wont budge without the engine then the plance probably couldnt take off.

but again, so many variables...
 
#53
Grug said:
i not positive, but im pretty sure no one here is qualified to post on this. any pilots on the forum? they could comment. any physics professors? they could too.
Click to expand...

geoff is a professional physicist, i'm a couple years off from being one, ray saloman is a few years from being a rocket scientist (but has been absent from the boards).

aerodynamics isn't my thing. :hmm:

(how about you, geoff? :p )

and apparently gard's dad was a pilot.
 
#56
Grug said:
i not positive, but im pretty sure no one here is qualified to post on this. any pilots on the forum? they could comment. any physics professors? they could too.

It all comes down to whether or not you could (hypothetically) lift the front end of the plane and spin the wheels. if they effortlessly spin then the plane could probly take off on the belt. if they wont budge without the engine then the plance probably couldnt take off.
Click to expand...

Doesn't take any of the above professions to know that the plane's engines are not connected to the wheels, otherwise the plane would slow down after it takes off. All the wheels have on them is a braking system. As for lifting the front wheels off of the ground, that is done by the flaps on the wings, which are in no way influenced by the treadmill.
 
#57
Brad Barker said:
geoff is a professional physicist, i'm a couple years off from being one, ray saloman is a few years from being a rocket scientist (but has been absent from the boards).

aerodynamics isn't my thing. :hmm:

(how about you, geoff? :p )

and apparently gard's dad was a pilot.
Click to expand...

I'm also an Air Navigator in the Air Force. ;)
 

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