Airplane on a Conveyor conundrum

[TonyN]

Give a scientific explanation how a conveyor belt, acting solely through a set of free spinning wheels, is able to excerpt any force upon the airframe of an aircraft that will prevent it from moving forward

Friction and gravity.

in which case you are needlessly complicating the system.

Thats Matt!!

[Slugsie]

Friction and gravity.

The only problem is that the aircraft has to overcome them anyway during a normal take-off. So while the conveyor will increase friction within the bearings etc, unless the aircraft is running virtually on its limit during normal take-offs, the increase will be negligible.

[Chillitt]

Friction and gravity.

Are you saying that things on a conveyor belt are heavier when the belt moves?

[dieselboy]

what im trying to say is the plane is moving forward at 10mph, the conveyor is moving backwards at 10mph, the wheels will be turning at 20mph as they are free wheeling on the conveyor belt,
therefore the plane is actually moving (not staying still) and it will create lift due to the air moving over and under the wings, and so it will take off,
when its moving at 20mph, the conveyor is moving at 20mph in the opposite direction, the wheels will be turning at 40mph due to the speed of the plane and the speed in the oposite direction of the conveyor,
the plane is physically moving through the air, and will take off when it reaches take of speed,

read that again.

so, the conveyor is moving in one way at 20mph
the plane is going forward at 20mph.

lets exaggerate and say the plane needs a forward speed of 20mph for it to take off,
you would expect that it now has double the ammount of speed to take off?
wrong,
because the plane is actually not going anywhere at all.

the air is still, and so is the plane. even if the jets were full blast, if the conveyor could go backwards fast enough to keep the plane in the same place - it would not take off.

the plane actually needs wind to take off.

so another scenario:
the plane is still on the ground and needs 20mph of wind speed to take off
(this would normally be acheived by the plane moving forward at 20mph)

except there is a constant wind at 20mph going head on to the plane. the plane will take off and appear to hover above the ground.
inside the plane, on the instrument cluster, it will show the plane travelling forward at 20mph on the wind speed measurement only. the actual ground speed will be 0mph.

[Chillitt]

r
you would expect that it now has double the ammount of speed to take off?
wrong,
because the plane is actually not going anywhere at all.

the air is still, and so is the plane. even if the jets were full blast, if the conveyor could go backwards fast enough to keep the plane in the same place - it would not take off.

t

How exactly, is the conveyor belt 'holding' the plane still? the only point of contact is a free wheeling,er, wheel.

[dieselboy]

In fact, to restate the original question:



So I even say that the aircraft is moving forwards.

yes but the plane relative to the air around it are both stationary

the only thing that gets a plane into the air is air itself.

this is why when you fly london to new york it takes longer getting home flying the other way! (or vice versa..)

[BUSHWHACKER]

Thats because the pilot/passengers don't really want to go back to crappy old London after being in a cosmopolitan city like New York.

[dieselboy]

How exactly, is the conveyor belt 'holding' the plane still? the only point of contact is a free wheeling,er, wheel.

doesnt really matter here. there is nothing holding the plane still.
the conveyor is going one way and the plane is moving a tiny ammount of air through its jets (and only its jets, there is none over the wings) to counteract the force acting on it from the conveyor.

if you had an RC car on a conveyor trying to hold that stationary you would have a hard time keeping that in a straight line

the only reason i think a runner dont go off sideways on a running machine is because of the way we balance using the fluid in our ears

[dieselboy]

Thats because the pilot/passengers don't really want to go back to crappy old London after being in a cosmopolitan city like New York.

yer!

[Slugsie]

so, the conveyor is moving in one way at 20mph
the plane is going forward at 20mph.

lets exaggerate and say the plane needs a forward speed of 20mph for it to take off,
you would expect that it now has double the ammount of speed to take off?
wrong,
because the plane is actually not going anywhere at all.

Nope, the plane is moving forward. So it has AIR-speed (that is all that is important as you state below), to it takes off

the air is still, and so is the plane. even if the jets were full blast, if the conveyor could go backwards fast enough to keep the plane in the same place - it would not take off.

Go back to the original scenario. The air is not moving relative to the ground. If the aircraft has a ground speed it also has an air speed. Also, the scenario did not state that the conveyor would go backwards at a sufficient speed to halt the forward speed of the aircraft, just that it would match the forward speed of the aircraft. If the conveyor was allowed to move at say 5000mph, they yes it could possibly generate enough friction in the bearings to cause a problem.

Also, a simple law of physics states that any action has an equal and opposite reaction. So if the jets are going full blast pushing air backwards, then they are also pushing the airframe forwards through the air. Unless the brakes are on (and they wouldn't be in any aircraft that is actually attempting a take-off) then the wheels - running at any sensible speed - will do nothing to halt that forward movement.

the plane actually needs wind to take off.

Yup, and a conveyor can do nothing to stop that wind.

so another scenario:
the plane is still on the ground and needs 20mph of wind speed to take off
(this would normally be acheived by the plane moving forward at 20mph)

except there is a constant wind at 20mph going head on to the plane. the plane will take off and appear to hover above the ground.
inside the plane, on the instrument cluster, it will show the plane travelling forward at 20mph on the wind speed measurement only. the actual ground speed will be 0mph.

Yup, nothing wrong there. But this has no relevance to the scenario, other than to demonstrate that the wheel speed (zero in this case) has no relevance to take-off. Don't believe me? Turn the plane around and imagine the same plane attempting to take off with a 20mph tail wind. It now needs to travel such that the wheels are spinning at 40mph before it achieves take-off. So, again, the wheel speed has proved to be irrelevant.

[Sancho]

This is what happens when I go on holiday......

The answer is that it would depend on the aerodynamic relationship between the engines and the wings. If the engines were able to create sufficient thrust to suck/blow air over the wings at a speed equivalent to when it is at 200mph groundspeed it would take off. If not (for example if they were very high up the tail and didn't affect airflow over the wings), it wouldn't.

All you clever buggers talking about your laws and your physics, you needed a fcukwit to come along and go back to basics.

[Slugsie]

yes but the plane relative to the air around it are both stationary

Nope, that isn't stated anywhere in the scenario.

the only thing that gets a plane into the air is air itself.

this is why when you fly london to new york it takes longer getting home flying the other way! (or vice versa..)

That is caused by a combination of wind speed and the earths rotation. Not relevant to this situation.

[Slugsie]

doesnt really matter here. there is nothing holding the plane still.
the conveyor is going one way and the plane is moving a tiny ammount of air through its jets (and only its jets, there is none over the wings) to counteract the force acting on it from the conveyor.

if you had an RC car on a conveyor trying to hold that stationary you would have a hard time keeping that in a straight line

the only reason i think a runner dont go off sideways on a running machine is because of the way we balance using the fluid in our ears

What do you mean there is nothing holding the plane still? If there is nothing holding it still, then it will move forward and fly.

The car on a conveyor and runner on a treadmill are totally different situations. They rely on forward motion by directly pushing against the conveyor/treadmill. An aircraft doesn't move that way.

The only way the jets can be moving a tiny amount of air through them is if ther are running at idle. In which case the aircraft isn't trying to take-off. Running at full power, there will be a huge amount of air running through them, and the laws of physics mean that the aircraft will be pushed forward.

[Slugsie]

This is what happens when I go on holiday......

The answer is that it would depend on the aerodynamic relationship between the engines and the wings. If the engines were able to create sufficient thrust to suck/blow air over the wings at a speed equivalent to when it is at 200mph groundspeed it would take off. If not (for example if they were very high up the tail and didn't affect airflow over the wings), it wouldn't.

All you clever buggers talking about your laws and your physics, you needed a fcukwit to come along and go back to basics.

Nope. Normal aircraft don't rely on the engines directly pushing air over the wings. The engines push the aircraft (doesn't matter what it's sat on) through the air which creates the lift.

Any aircraft that is capable of normal take-off/flight is capable of conveyor belt take-off/flight.

[Chillitt]

doesnt really matter here. there is nothing holding the plane still.
Yes it does! this is the one key point!!!
the conveyor is going one way and the plane is moving a tiny ammount of air through its jets (and only its jets, there is none over the wings) to counteract the force acting on it from the conveyor.
Why? the same engines move same amount of air no matter what the plane is on!

if you had an RC car on a conveyor trying to hold that stationary you would have a hard time keeping that in a straight line
But a car is powered though its wheels, a plane is not
the only reason i think a runner dont go off sideways on a running machine is because of the way we balance using the fluid in our ears

And how is that relevant to anything???