Plane on a conveyor belt

Sambostar

It's more about the wording of the question.  As I understand it, as the engines provide forward thrust and ground speed the conveyor speed increases so the plane has zero ground speed and zero air speed over the wings.   It could max out it's engines but air speed will still be zero.  Wheels aren't driven, but engines are only there to propel the thing against the air to provide lift for the wings.  If there is no airspeed, there is no take off.  It's like maxing out an engine in a workshop, it won't take off.

Sambostar

Ah, but could a plane take off with a 570 mph tailwind?

Chalky

If wheel friction was significant then when a 747 landed, that friction would cause a moment, the lever being from the 747 c of g to the wheel, causing the nose wheel to slam on the runway.

Sambostar

Why do they have windsocks?

Hertz32

Sambostar said:

Ah, but could a plane take off with a 570 mph tailwind?

No. Thats why airfields have multiple runways that they can use with different wind directions so youvtake off and land into wind (hopefully) 

Sambostar said:

Why do they have windsocks?

So they can see wind direction and pick a runway

viz

Sambostar said:

Ah, but could a plane take off with a 570 mph tailwind?

If that means the 747 is actually travelling at 1140 mph ground speed then yes. Otherwise no. 747s aren't VTOLs. They have to be moving forward (in air). 

Sambostar

Could a passenger board a plane in a 570mph tailwind?

Maynehead

Friction is not what stops the plane taking off, friction is what prevents both the conveyor and the wheels from accelerating to infinity as soon as the engine produces any amount of thrust.

Anyway, that's not the main point of concern here. The critical thing you have to understand is that for there to be any net movement of the plane in the forwards direction (and therefore receive airflow over the wings), the wheels MUST be rotating at a FASTER rate than the conveyor belt is moving in the opposite direction. However, by definition of this thought experiment, this is the one thing that is not possible.

lasermonkey

monquixote said:

Mythbusters actually tried this and showed it could take off.

That was done with an ultralight and i suspect that the propwash provided enough airflow over the aerofoil to generate enough lift. It's a power-weight thing.

Emp_Fab

No.  It's nothing whatsoever to do with speed relative to the ground (or conveyor belt).  It's the speed of the airflow over the wings.  If you blew a 150mph wind at a parked plane, it would take off.

Gassage

Sambostar said:

Ah, but could a plane take off with a 570 mph tailwind?

Yes theoretically- a 747 would need to achieve a V2 speed of 785mph...to do so.

Which it is incapable of.

DiscoStu

No, it can't take off. As has been said before, without forward movement there is no airflow over the wings and therefore no lift.
The plane is stationary but potentially at full thrust, which now begs the question could it take off if the windspeed was high enough, and what would that windspeed need to be?

ICBM

Of course it could take off. Anyone who thinks it can't doesn't understand basic physics.

The wheels are irrelevant, they just support the weight of the plane when it's on the ground. The engines generate thrust and the plane moves forward, the wings generate lift against the air and it takes off. The wheels could be turning forwards, backwards or not at all and it makes no difference.

I actually thought this was going to be about something much cooler, the huge conveyors at the Boeing (and presumably Airbus, although I've never seen pics) factories which move the part-finished planes around when they're being assembled. Disappointed...

Maynehead

ICBM said:

Of course it could take off. Anyone who thinks it can't doesn't understand basic physics.

The wheels are irrelevant, they just support the weight of the plane when it's on the ground. The engines generate thrust and the plane moves forward, the wings generate lift against the air and it takes off. The wheels could be turning forwards, backwards or not at all and it makes no difference.

I actually thought this was going to be about something much cooler, the huge conveyors at the Boeing (and presumably Airbus, although I've never seen pics) which move the part-finished planes around when they're being assembled. Disappointed...

And anyone who thinks it can doesn't understand the basic question. RTFQ!

I quote myself from my previous post:

The critical thing you have to understand is that for there to be any net movement of the plane in the forwards direction (and therefore receive airflow over the wings), the wheels MUST be rotating at a FASTER rate than the conveyor belt is moving in the opposite direction. However, by definition of this thought experiment, this is the one thing that is not possible.

Sambostar

The question basically states that the conveyor matches the ground speed of the wheels, with zero headwind ground speed equals airspeed.  I don't understand what's not to understand unless it's a trick question.  This is like the 11 plus and GCSE geography all over again. 

If you have four clear marbles, but one of them is silver which one is a jelly bean?

ICBM

Maynehead said:

And anyone who thinks it can doesn't understand the basic question. RTFQ!

I quote myself from my previous post:

The critical thing you have to understand is that for there to be any net movement of the plane in the forwards direction (and therefore receive airflow over the wings), the wheels MUST be rotating at a FASTER rate than the conveyor belt is moving in the opposite direction. However, by definition of this thought experiment, this is the one thing that is not possible.

Bollocks.

The only thing that matters is that the plane is being propelled forwards through the air by the engine thrust. The conveyor could be set to run so the wheels are turning backwards and it would make no difference.

Sambostar said:

The question basically states that the conveyor matches the ground speed of the wheels, with zero headwind ground speed equals airspeed.  I don't understand what's not to understand unless it's a trick question.

What you don't seem to be understanding is that the wheels are not driven - they freewheel, and provide nothing other than support for the aircraft when it's below flying speed. If they were needed to drive the plane forward, how would it fly once it's off the ground?

CabbageCat

Surely if the conveyor was matching the speed of the wheels at all times then the plane would be stationary. Even if the thrust is coming from the jets. The conveyor would effectively be dragging the plane back by the wheels with equal force to the jets. The landing gear would probably be ripped off.

Maynehead

Whether the wheels are driven or not is irrelevant. For the plane to move in relation to the ground there must be a speed differential between the rotational speed of the wheels and the rotational speed of the conveyor belt in the opposite direction. This is ruled out by the definition of the question.

Sambostar

I see what you're getting at, but the way I understand the question is that the conveyor increase exactly the same amount as the ground speed increases.  Eventually all that would happen is the bearings would burn out.
They couldn't build a conveyor that big anyway that is capable for doing 570mph..

CabbageCat

ICBM said:

The wheels are irrelevant, they just support the weight of the plane when it's on the ground. The engines generate thrust and the plane moves forward.

How does the plane move forward if the wheels and the ground are moving at the same speed?