Airplane on a Conveyor conundrum

[Sancho]

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.

That's my point. For your plane to take off, the engines would have to directly push air over the wings at 200mph or create sufficient air movement that air would indirectly go over the wings at 200mph anyway.

[dieselboy]

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.

no it wont because the plane would be counteracting the backwards motion of the conveyor.

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.

correct, but there is still a backward pull from the conveyor, albeit only a small ammount. therfore the jets will only need to be on a small ammount.

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.

even if the aircraft was held still, and it had 20 jet engines on full blast, the plane will not take off. it needs air over the wings, not the jets.

If The aircrafts speed is measured relative to fixed point on the ground, and not relative to the moving conveyor

if this is true then it is a trick question.

because no matter how fast the conveyor moves, if the plane is going forwards at 10mph its because its moving compared to the fixed point on the ground.

but then in this case the conveyor would only be going 10mph too..


we all know that in real life the plane would take off. this is because the air is stationary, and the jets would move the stationary air through them to make thrust.
so as long as the engines have always been preventing the plane from moving from the start (speed relative to ground speed) then the jets will overpower the drag by the conveyor and start to move forward.

the only way this wouldnt happen is if the plane engines were off, and the conveyor started to move backwards, lets say up to 10mph (keep it small) then when you switch on the engines, they will first have to slow the plane from going backwards and will then bring it stationary to the ground before then moving the plane forwards.

are you saying that if this was happen, that as soon as the plane engines were switched on, that the plane would immediatly shoot forward? absolutely not. it would first have to slow itself down, but at the same time accelerating the air through its jets.

your first post does contradict itself,
if the plane is moving forwards at 10mph relative to the ground, and then the conveyor is going the same speed but the opposite direction, then the conveyor isnt actually moving at all!!!
lol
the conveyor will be going the same speed as the ground the speed is measured from, and this means its not actually moving at all...

[dieselboy]

oh, and that would mean that in the 2nd situation, where this is a trick question, and no matter what speed the plane is going relative to the ground, the conveyor will be going the opposite direction
this means not actually itself moving relative to the ground therefore:
the conveyor and the ground are going the same speed i.e. 0mph

the plane will take off.


thank you, good evening and goodnight

lol

[dieselboy]

And how is that relevant to anything???

because your asking me how its keeping still
i thought you were mixing up real life situations to made up ones like this thread.

at the end of the day, if a plane was having a force acting on it to go backwards via a conveyor, then the plane would have to provide the same force just to keep it stationary relative to the ground.
the only way to do this is via its jets, and the forces would have to be equal

i logically think that a conveyor providing a force on the plane through its unpowered, free-wheels would only be able to provide a small force to the plane.
therefore you would only need a small force to keep it stationary relative to the ground.
any force provided by the jets in excess of that would then move the plane forward relative to the ground

[Chillitt]

oh, and that would mean that in the 2nd situation, where this is a trick question, and no matter what speed the plane is going relative to the ground, the conveyor will be going the opposite direction
this means not actually itself moving relative to the ground therefore:
the conveyor and the ground are going the same speed i.e. 0mph

the plane will take off.


thank you, good evening and goodnight

lol

If the conveyor is moving, how can it be doing the same speed as the ground?
You still haven't told us how the conveyor is hold the plane 'still' what with the engines pushing all that air through them...

[Chillitt]

That's my point. For your plane to take off, the engines would have to directly push air over the wings at 200mph or create sufficient air movement that air would indirectly go over the wings at 200mph anyway.

The usual arangement is if the engines are doing 200 mph, the the wings are too...

[Chillitt]

because your asking me how its keeping still
i thought you were mixing up real life situations to made up ones like this thread.

at the end of the day, if a plane was having a force acting on it to go backwards via a conveyor, then the plane would have to provide the same force just to keep it stationary relative to the ground.
the only way to do this is via its jets, and the forces would have to be equal

i logically think that a conveyor providing a force on the plane through its unpowered, free-wheels would only be able to provide a small force to the plane.
therefore you would only need a small force to keep it stationary relative to the ground.
any force provided by the jets in excess of that would then move the plane forward relative to the ground

And if they upped the throttles it would take off...

[Slugsie]

OK, I'm sorry DieselBoy, but most of your last few posts don't make any sense, so I can't begin to know how to show where you have gone wrong.

[Slugsie]

i logically think that a conveyor providing a force on the plane through its unpowered, free-wheels would only be able to provide a small force to the plane.
therefore you would only need a small force to keep it stationary relative to the ground.
any force provided by the jets in excess of that would then move the plane forward relative to the ground

And that is kind of the point. It only needs a small amount of force to overcome the friction/drag of the conveyor. Given that the aircraft is actively trying to take-off it'll be running its engines at full throttle, so there will be no problem what so ever overcoming the friction.

Remember, the plane is trying to take off, it is not trying to keep station on the conveyor.

[Slugsie]

if the plane is moving forwards at 10mph relative to the ground, and then the conveyor is going the same speed but the opposite direction, then the conveyor isnt actually moving at all!!!

How can something that is moving, be not moving? If the conveyor is going the same speed, then it is moving. The end result is that the plane and the conveyor have a relative speed to each other of 20mph (in this case).

[Sancho]

The usual arangement is if the engines are doing 200 mph, the the wings are too...

Not quite what I meant but point taken. I suppose what I'm saying is that the engines would need to be running faster than normal to achieve the take off.

[Slugsie]

Not quite what I meant but point taken. I suppose what I'm saying is that the engines would need to be running faster than normal to achieve the take off.

Yes, that's true, but only slightly. Either that or the aircraft would take slightly longer to achieve take-off. But it will do it.

[dieselboy]

The usual arangement is if the engines are doing 200 mph, the the wings are too...

what do they flap or something?

all jet engines do is pull air through them.
if there is wind going 200mph anyway, the engines wont provide any extra force if all they can pull is 200mph.

do you understand this? im rubish at explaining sorry

[dieselboy]

And if they upped the throttles it would take off...

but thats not what the original post says!

[dieselboy]

If the conveyor is moving, how can it be doing the same speed as the ground?
You still haven't told us how the conveyor is hold the plane 'still' what with the engines pushing all that air through them...

when you walk along the ground, does the ground move?

no, but it still goes backwards the same speed your walking.

do you understand this?