Stupid physics question

[Tom]

Theres an almighty raging war of a debate going on at another forum I infest, and I wanted to know the opinions of the people here.

Obviously being dead clever and stuff I fink I know the answer anyway, but lets see what you lot say:

A plane is standing on a 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 is:

Will the plane take off or not?

 

[Milkshake]

I say no....

There wouldn't be enough wind going over the wings.

I think

 

[lilmissnaughty]

depends is it a harrier jump jet
answer should be no..plane needs to be moving forwards to get the drag/upthrust whatever years since i did physics. bit like a plane couldnt just stop in mid air..

 

[Furr]

no, theres no movement of air over the wings to create the high/low pressure difference to create the forces needs to pull the wings and vehicle the wings are attached too up in the air.

 

[ECA]

No.

 

[Louster]

Ahahahahaaaa.

I'd like to see if anyone realises why this really is stupid.

 

[GekuL]

Spoiler

Propulsion is not provided by the wheels

 

[Paradroid]

Yeah that's the key, and the fact that the planes wheels aren't fixed, so they just spin double their usual rate.

 

[Mazling]

I should really not answer things like this at 8am.

 

[WPKenny]

Spoiler

Propulsion is not provided by the wheels

That's neither here nor there. Fact is the plane won't take off regardless of what speed the wheels spin at (or don't) and that's the question.

 

[tris-]

Spoiler

no

 

[Tilda]

Lol at this question, I saw a similar thing asked on an american forum, most people were like "yeah ofc it would take off!!"
It won't because theres no air moving over the wind, and thus no downthrust.

 

[Mazling]

No air moving over the wind?

 

[tris-]

Lol at this question, I saw a similar thing asked on an american forum, most people were like "yeah ofc it would take off!!"
It won't because theres no air moving over the wind, and thus no downthrust.

i suppose the simple way to put it is, the plane is staying in the same spot.

 

[Tilda]

No air moving over the wind?

haha, i meant over the wing.
A plane creates lift by having wings at an angle that makes the air push down on it. Becase as tris said, its staying in the same place, it has no air flowing over the wings, and thus cant generate lift.
Unless ofc there was a huge fan at the end of the runway!

 

[tris-]

would that be classed as take off, as it probably wouldnt actually move forward. would just jump and down a bit? imo.

 

[caLLous]

If the plane is being pushed forward by the big fuck-off jet engines strapped to it's wings then it doesn't really matter what the wheels are doing, that baby's gonna take off.

 

[Tilda]

If the plane is being pushed forward by the big fuck-off jet engines strapped to it's wings then it doesn't really matter what the wheels are doing, that baby's gonna take off.

Your right, except the plane isn't moving forwards as the conveyorbelt keeps up with it, so it cant move forwards, so it cant take off.
Thus the plane accelerates, the conveyorbelt keeps going faster and faster to keep up with it, and the wheels blow out because they cant keep up. The plane dosn't take off.

 

[WPKenny]

Ah bollox, callous is right. And all the other people that said "yes".


*curses time limited edit function*

 

[caLLous]

the plane isn't moving forwards.

Yes it is.

as the conveyorbelt keeps up with it,

The belt is countering the motion of the wheels, not the plane.

so it cant move forwards, so it cant take off.

Yes it can.

 

[Tilda]

Example

The plane goes forward at 100mph
The conveyor goes backwards at 100mph to compensate.

The plane is still going forward at 100mph because the jets are powering it and not the wheels. The wheels are spinning like mad things (at 200mph in this example) but all they provide to stop it taking off is a bit of friction from contact with the conveyor belt. But it takes off.

Plane Wheels cannot freewheel at 200mph, thus my reply.
Sure the plane is going forwards at 100mph and the belt is going backwards at 100mph.
However, long before it got to that stage, the wheels would blow out, drasticly increasing the friction between the plane and the runway (the wheels arn't spinning so its like the plane is just sat on the belt) and so the plane would shoot off the back of the conveyorbelt. 100mph of jet thrust wont do shit against 100mph of belt, when the plane is sat on the belt with no wheels.

 

[caLLous]

I deleted that reply because I didn't think it was very clear.

How do you know plane wheels can't freewheel at 200mph? I think the question is assuming that the wheels can take the strain.

If they achieved completely frictionless wheels then from the side it would look like a normal take off. I think.

 

[Louster]

Ahaha. Tilda, you're inventing new parameters within the scenario that're completely beside the point. You can apparently accept the idea of a gigantic conveyor belt that can spin at 100mph, but not undercarriage wheels that can endure the requisite amount of heat. Can you not see that this is just a teeny bit preposterous?

 

[xane]

The plane is not "moving forwards" because what matters is its motion relative to the air, not the ground.

If I run at 15 mph on a treadmill I don't get a 15 mph wind in my face.

The plane does not get 100 mph of air across the wings, therefore no lift is being generated, therefore no take off.

 

[caLLous]

If you put a rocket on the back of a car and then put it on a conveyor belt and used the rocket to power the car rather than the wheels - the car will move forwards (no, it won't take off).

 

[xane]

Flight (lift) is generated by the movement of air across the wing surface, which because of the design, generates a higher pressure below it than above it, you must have air move across a wing surface to generate lift.

It does not matter what the speed is relative to the ground, it matters what the speed is relative to the air.

A plane will fly in a wind tunnel with absolutely no propulsion or forward movement at all, gliders dont have engines, it is the movement of air across the wings that generates the lift.

No lift, no takeoff.

 

[~Yuckfou~]

The original question infers that the planes forward thrust is delivered through the wheels. If this was so then the plane would not lift off. In the same way that an F1 car would not gain any aerodymanic downforce.
If however the plane is powered by it's flight engines then the speed of the conveyor is irrelevant, the plane will move forward. Planes engines generate air speed, ground speed is not the same. If the plane is travelling forward at 300mph airspeed in to a 300mph headwind then it's groundspeed is 0mph but it will be flying.
Contrary to an earlier post lift is generated due to the shape of the wings and not the "angle". The shape which is typically flat and horizontal on the underside and curved on the topside generates high pressure on the underside and low pressure on the topside when air passes over the wing. This pushes the wings up, and the planes lifts.

Hope you were paying attention there will be a test.

 

[~Yuckfou~]

I hate when that happens :/
Xane I hate you, thanks.

 

[Tilda]

Ahaha. Tilda, you're inventing new parameters within the scenario that're completely beside the point. You can apparently accept the idea of a gigantic conveyor belt that can spin at 100mph, but not undercarriage wheels that can endure the requisite amount of heat. Can you not see that this is just a teeny bit preposterous?

The question states that theres a conveyor belt. Thus its ok to accept this. However, i assumed we were talking about a normal plane, which i very much doubt could take off.
But if you're talking about an imaginary plane, with frictionless wheels that can freewheel at 200mph without burning out, locking, melting or anything else, then we get to this;

Plane engines - 100mph
Belt speed - 100mph
Amazing tire speed - 200mph

Total forward plane speed - 100mph

Thus the plane takes off in your imaginary world yes. In the real world, where plane wheels would blow, melt and lock before they got to 200mph, the plane wouldnt take off. The act of the wheels blowing re-connects the plane and the belt (which where disassociated because the wheels were freewheeling) once the now locked tires hit are on the belt, the friction is far higher, and so the tires, the struts get sanded down/melted.

 

[xane]

If the plane is travelling forward at 300mph airspeed in to a 300mph headwind then it's groundspeed is 0mph but it will be flying.

A plane travelling 300 mph forward into a 300 mph headwind has a ground speed of 300 mph and an airspeed of 600 mph, it flies.

A plane travelling 300 mph forward with a 300 mph tailwind has a ground speed of 300 mph and an airspeed of 0 mph, it will crash (this is known as wind sheer and happens for real on takeoff and landing).

In the conveyor belt example there is no "wind" at all, no lift can be generated.