Plane on a conveyor belt

holnrew

HAL9000

Yes. Because the wheels are free-wheeling rather than driven. It's the jet engines that are doing the work. Jets are reaction engines (burnt fuel/air goes backwards - aircraft goes forwards) so it doesn't matter a jot what the wheels are doing.

Drew_TNBD

...

This is dumb. Planes are not cars.

holnrew

I was dumb enough to not see it that way for a good few hours

Drew_TNBD

holnrew said:

I was dumb enough to not see it that way for a good few hours

Welcome to coding!

quarky

If the conveyer belt turns, there is a trade of energy which must come from the plane. Therefore the plane is expending less energy on forward motion than if it was on a runway right? So the interesting question is, is there any situation in which that energy transfer is enough to stop the plane taking off? I guess "yes" if the plane was heavy enough. But the amount of energy transferred in turning the belt must be quite small compared to the energy required for takeoff..

RandallFlagg

Similar question, with a 150mph head wind could a plane take off but appear not to be moving forward?

quarky

RandallFlagg said:

Similar question, with a 150mph head wind could a plane take off but appear not to be moving forward?

Sure, that is just air/ground speed.

cacophony

is anyone here a member of the f365 forum by any chance?. 


jus' askin'...

Ravenous

RandallFlagg said:

Similar question, with a 150mph head wind could a plane take off but appear not to be moving forward?

That's more or less what a kite does, so yes.

lasermonkey

Lift. Think about it.

Gassage

RandallFlagg said:

Similar question, with a 150mph head wind could a plane take off but appear not to be moving forward?

Ummm, no, needs more wind.  V2 (the speed when a plane becomes airbourne) on a fully laden (MOTW) 747 (pictured) is c. 155 Kn (180mph), depending on flap settings. So ....

Whatever the weight

On flap 20, the speed will be lesser but the poz climb rate will be slower.
On flap 10, the speed needed is greater, and therefore climb rate higher.
At 20, the stress on the landing gear is lesser and so is the noise.

So, technically, if the headwind was a mighty 180 mph, the plane would need to be tethered to the ground.

An unladen 747 would need less. c. 140kts I believe.

Yes, I'm plane anorak.

ennspek

I thought this was going to be about the sequel to Snakes on a Plane 

fretmeister

No.

No airflow over the wings. No Bernoulli effect. No lift.

monquixote

Mythbusters actually tried this and showed it could take off.

Gassage

Here you go- unladen 747-400 taking off into an 60mph headwind unladen!

600m take off.

Compare this with Joburg where a fully laden 747 will take 3000m to become airborne.

Maynehead

No it cannot take off.

For there to be any net forward motion relative to a stationary observer, the wheels must implicitly be rotating at a faster speed than the conveyer. This is the case regardless of whether power is being delivered through the wheels, or in this case, through the jet engines.

This more of a thought experiment more than a realistic situation because in reality, it is impossible to create such a conveyer belt that precisely and instantaneously matches the speed of the wheels. However, should such a belt exist, the plane would be kept stationary by the belt through the frictional forces between the belt and the wheels and the wheel bearings, creating a resistive force that matches the thrust delivered by the engine. In reality the wheels would have to be spinning so fast for this to happen that they would probably fail and disintegrate quite quickly as the engine power is increased, putting an end to the experiment, and the plane itself.

Chalky

Wheel speed is irrelevant, and the friction of the wheel bearing would be negligible compared to max engine thrust, even if the conveyor belt was moving at 360kt compared to the 747 airspeed of 180kt at take off. So it would take off with ease.

Sambostar

It can't take off because airspeed is the critical thing to provide lift to the wings.  If air isn't travelling fast enough over them it won't take off.

hywelg

Tosh.

Wheel and bearing drag will be small in comparison to thrust available from the engines. What limits an aeroplanes ability to take off is relative windspeed over the aerofoil sections.

 Since the conveyor belt has no influence on the windspeed (except in the boundary, probably millimetres) and wheel drag is proportional to conveyor speed the aeroplane will move forwards relative to the ground.

Since at the start the thrust will exceed the combined drag of the wheels and win,  it will accelerate up to the point where thrust <= air drag + wheel drag. Before this point the forward ground speed will exceed the speed required to generate sufficient lift to take off and wheel drag will cease to influence the equation. 

rlw

Surely in this scenario, there is no air passing over the wings as the plane is actually static, apart from the wheels rotating.  Bit like a rolling road.