Would It Take Off Or Not?

[Flat Pack]

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

The question is:

Will the plane take off or not?

[Bibbs]

NOOOO !!!!

Was just reading this on another forum ..

I think I should delete this now ..

A plane is sat on a runway. The runway is like a rolling road, as the plane moves in one direction, the runway acts like a conveyor belt and moves in the opposite direction - it can do this instantly and exactly matches the speed that the plane's wheels turn.

The question is:

Will the plane take off or not?

[GIDDLEPIN]

As the power supplied is purely external thrust there is no way on earth a ground moving conveyor could create any type of momentum (forwards or backwards) through the wheels, as they are free running bearings.

The only way it might create any equal balance is if the engines were powering drive through the landing gear, in which case no lift would be achieved and therefore it would not take off.

:P

[Fidgits]

The reason a plane takes off, is, simply wind speed...

take a 747 for instance, fully loaded (passengers luggage and fuel) that needs to be travelling at (IIRC) 300mph to get the lift to be able to lift its front wheel off the ground.

Now, planes need to take off, and land into wind - but the 150mph is the wind speed required to get the lift to be able to lift the nose wheel.

Once the wheel is off the ground, the characteristics change, and at 200mph, the angle of attack and wind speed are such, the plane lifts off.

Therefore - regardless of any outside influences, the only way a plane can take off is if the airspeed over the surface of the wings is that required to create enough lift at rotate speed.

(for a cessna 152, this is 60mph by the way)

[Bibbs]

Les - that's where I got to.

If the wheels are free, and it's external driven, it'll take off as it's just on a frictionless surface.

If the wheels are driven (unlikely on a plane) it'll not.

[Flat Pack]

The reason a plane takes off, is, simply wind speed...

take a 747 for instance, fully loaded (passengers luggage and fuel) that needs to be travelling at (IIRC) 300mph to get the lift to be able to lift its front wheel off the ground.

Now, planes need to take off, and land into wind - but the 300mph is the wind speed required to get the lift to be able to lift the nose wheel.

Once the wheel is off the ground, the characteristics change, and at 350mph, the angle of attack and wind speed are such, the plane lifts off.

Therefore - regardless of any outside influences, the only way a plane can take off is if the airspeed over the surface of the wings is that required to create enough lift at rotate speed.

(for a cessna 152, this is 60mph by the way)

Indeed, but does it take off or not? :)

[Moffmo]

Since the conveyer is set at the speed of the plane(which when being set is sat at 0)

Since the conveyer is moving at 0 miles an hour it makes no diff :P

[Flat Pack]

Since the conveyer is set at the speed of the plane(which when being set is sat at 0)

Since the conveyer is moving at 0 miles an hour it makes no diff :P

The conveyor automatically (and instantly) matches the speed of the plane at all times, except in the opposite direction :)

[chrisb]

NO!

CB

[martin_longbow]

If the plane requires wind speed of say 300 mph to provide the lift to take off, and the conveyor can track the speed of the plane should we not all just flame flat pack for posting such a stupid topic and referring several persons back to the original post?

Will it take off or not.

No it is a hypothetical chicken and egg. There is no answer but designed to get people to think/argue/frustrate

try reading plato

[funkyg]

Is the plane powered?

[Flat Pack]

There is a correct answer :) and it's already been given (not surprising as people have said both yes and no!) can't deny its intended to get people to think though and normally causes quite a discussion ;)

Maybe I should say the conveyor belt immediately matches the indicated groundspeed of the plane, that removes any kind of bizarre chicken / egg type scenario I think.

The plane is powered, by conventional jet or propellor means.

[Fidgits]

The reason a plane takes off, is, simply wind speed...

take a 747 for instance, fully loaded (passengers luggage and fuel) that needs to be travelling at (IIRC) 300mph to get the lift to be able to lift its front wheel off the ground.

Now, planes need to take off, and land into wind - but the 150mph is the wind speed required to get the lift to be able to lift the nose wheel.

Once the wheel is off the ground, the characteristics change, and at 200mph, the angle of attack and wind speed are such, the plane lifts off.

Therefore - regardless of any outside influences, the only way a plane can take off is if the airspeed over the surface of the wings is that required to create enough lift at rotate speed.

(for a cessna 152, this is 60mph by the way)

Indeed, but does it take off or not? :)

Of course it doesnt!

(wasnt that clear)

relative ground speed is irrelavant - the speed of the air flowing over the wings if effectivley 0mph - therefore no lift can be generated.

its simple aerodynamics?

[Bibbs]

But why doesn't it move? there is no forces applied through the wheels.

[jaxx]

The reason a plane takes off, is, simply wind speed...

or bernoulli's principle, ;) but nothing to do with wind speed, aircraft can take off in stationary air ;)

an aircraft will only leave the ground under motion, due to the fact they generate lift on the wings cutting through air space and creating a different pressure level on one side to the other. The low pressure above the wings litterally sucks it up into the air, but you need that motion in the first place

on a conveyor with the props on full chat it would go nowhere what-so-ever as the wings are not creating the alternative pressure levels so it would stay put on the ground

HOWEVER, if you had a fan for example that could push enough air volume at the correct speed at the leading edge of the wing, then you could create them same differential in pressure and thus lift the aircraft, even with the engines switched off

most manufacturers test 30% scale models in wind tunnels this way

so in answer to your question, as already stated.... NO!

[Bibbs]

I'll say again .. why doesn't the plane move?

It's not pushing against the floor .. the wheels are free.

Jets (and propellers) push against the air (think of releasing an inflated baloon - it doesn't need to be on the floor to fly).

The plane goes foward, the floor goes back and the wheels are running at twice the normal takeoff speed.

If the plane was powered through the WHEELS, you are correct .. but it's not, so you are not.

[jaxx]

I'll say again .. why doesn't the plane move?

It's not pushing against the floor .. the wheels are free.

Jets (and propellers) push against the air (think of releasing an inflated baloon - it doesn't need to be on the floor to fly).

The plane goes foward, the floor goes back and the wheels are running at twice the normal takeoff speed.

If the plane was powered through the WHEELS, you are correct .. but it's not.

ahhh i get what your saying ;)

yes your correct the wheels would have no effect at all as theyre free running and not driving, the engines are working against air pressure which as no direct realation to what the conveyor is doing

the aircraft could move forwards at xxxmph and the conveyor could counter act that as much as it wants, itll just spin the wheels backwards but the craft would still move forwards, AND take off! So i suppose its YES! To contradict myself

All it would do i suppose is create unwanted drag!

[Bibbs]

tee hee .. so yeah, if it was a car with wings it wouldn't do anything.

but a plane does nothing with the wheels .. they are just a way of getting over friction (a sea plane on the ground is a bit rubbish ;))

[jaxx]

correct as the wheels move the car, but if you stuck an engine on that created 'thrust' through air pressure then away you go

[Fidgits]

ahhh, i see...

relative airspeed would be normal.

relative ground speed would be double, but irrelavent..

[Al J]

Yeah, it flies.

Newton's third law gets in there. It's pushing backwards, it goes forwards. As acceleration equals force divided by mass as long as the force creates a great enough acceleration to get the air moving over those wings fast enough (as Jaxx said) it'll get enough lift and off it goes.

[selfbuild]

No it won't take off.

Planes rely on air resistance to get off the ground. Since there will not be enough air resistance to get the plane to lift regardless of how much power is being used, it will not be able to leave the ground (I'm picturing a running machine on a much larger scale).

[jaxx]

but the planes engines also work against air resistance ;) the wheels do nothing for its forward motion for the wings to create lift

the only way it wouldnt work is if the engines were in a vacuum

[GIDDLEPIN]

No it won't take off.

Planes rely on air resistance to get off the ground. Since there will not be enough air resistance to get the plane to lift regardless of how much power is being used, it will not be able to leave the ground (I'm picturing a running machine on a much larger scale).

Harrier

[selfbuild]

The engines (unless they are propeller driven (these maybe able to)) will not be able to move the amount of the air needed across the wings surface to create enough lift to get the plane of the ground. You need air resistance across the entire wings surface (both sides) to create the lift needed to get it of the ground.

Harrier jump jets are the exception as they have directional outlets. No other plane has this capability.