Plane

[RichA]

No you've fallen down some rabbit hole of strangeness . I don't even know where to begin to pull you out.

If you follow the advice you have yesterday and actually read the theory until you understand it then I'm confident you'll figure it out...

Why isn't it moving ? Youre just getting confused , it's not a wind up. Take 5 mins to read up on it.

 

[RichA]

It is probably because you lack clarity, and I don't know if you are intentionally doing so? Because, I'm not sure why you are comparing the force of the engines thrust relative to the treadmill speed? The treadmill speed is irrelevant to the forward motion of the plane.

It may be theoretical as nobody has designed such a treadmill. But, it isn't something where the answer is unknown if it was tried, because the mathematical and physical theory is sound. There would be no point in designing a system, because the answer is known.

If a software engineer was asked to developed such a system to operate the treadmill, they would soon realise it is impossible to achieve in practice. They'd end up in an infinite loop. The speed of the wheels is passive, thus an output, and a function of the speed of the plane caused by thrust and the speed of the runway. A programmer would need to set the treadmill speed to an output, to match the speed of the wheels in the opposite direction. So, as engines get plane moving, the speed of the wheels would send an input to the treadmill to move in opposite direction. But, once that is converted into an output in actually moving the treadmill, that is a change in the input that the wheel speed is dependent on. So, the wheels get even quicker. The software will always be reacting to ever increasing inputs from the wheels, but never be able to catch up.

The plane would continue to merrily accelerate down the runway, because it doesn't care one bit what the wheels are doing. It isn't a car. meanwhile the wheels and treadmill would be continually speed up as they are dependent on each other. As speeds get crazy high, the software may crash, mechanics of wheels may get destroyed as they can't cope, or the frictional force exceeded between wheels and ground that they end up just skidding as they grip and re-grip continually. The only way the plane will stop moving forward is if the wheels get destroyed and it crashes

Your solution has you in the mindset that it's a trick question. It isn't. The question suggests that the treadmill opposes the plane wheels' motion perfectly. As long as the plane is on the ground it is stationary relative to the Earth.

 

[Swango1980]

Your solution has you in the mindset that it's a trick question. It isn't. The question suggests that the treadmill opposes the plane wheels' motion perfectly. As long as the plane is on the ground it is stationary relative to the Earth.

Actually, it is a trick question and perhaps we are all confused as to the direction of travel of the treadmill (perhaps it was Crow that came up with one interpretation that fits).

If the engine provides thrust, the plane moves forward and the direction of travel of the wheels against the ground is backwards. So, if the treadmill is programmed to move in opposite direction, it is actually moving in the direction of the plane (in opposite direction you would have at the gym). The wheels actually remain stationary as the plane moves forward, and it takes off as per normal.

So, if treadmill is programmed to move in the traditional direction of a regular treadmill (opposing planes movement), plane will still take off and treadmill will never be able to catch up with that speed. But if treadmill is programmed to run in reverse to oppose direction of bottom of wheels, plane still moves forward but wheels are stationary. In both cases, the plane moves forward and takes off

 

[Deleted member 35927]

If you follow the advice you have yesterday and actually read the theory until you understand it then I'm confident you'll figure it out...

I'm sorry you have lost me, it's not really that complicated.

Treadmill rolling forward - planes moves forward and takes off as normal .

Treadmill rolling backwards - plane moves forward and takes off as normal.

You keep making these sly digs and opaque claims.

It's just basic physics and the laws of motion.

 

[RichA]

View attachment 56870

Ok. Humble pie eaten. The penny has dropped.
The picture doesn't help. The conveyor belt needs to be about 2 miles long.
@hambugerpete @Swango1980 I apologise.

 

[Deleted member 35927]

Ok. Humble pie eaten. The penny has dropped.
The picture doesn't help. The conveyor belt needs to be about 2 miles long.
@hambugerpete @Swango1980 I apologise.

No need to apologise, I now understand the confusion of the picture.

 

[Hobbit]

If the plane is stationary relative to the surrounding air it will never take off (unless it is a helicopter , lifted by a crane or a bomb is placed under it).
Any movement of the runway is irrelevant.

However, and I may be completely wrong, if the engines are sucking air over/under the wings at the required speed to create lift the plane will take off.

Then my head goes all fuzzy again with questions. Can the engines create enough suck to ‘see’ air being pulled over the full length of the wings? What front to back width ratio to length does the wing need to be…? And then my head hurts even more…

For those maybe not understanding how the lift principle might work on a stationary object, think about a model aircraft in a wind tunnel. In effect the tunnel provides the airflow just as the engines on the OP’s plane creates an airflow.

But maybe I’m just totally bamboozled.

 

[Swango1980]

Having a slow work day, so tried out an old GCSE type maths question. If a programmer was silly enough to set a treadmill to run a speed equal to to the speed the planes thrust provides, but in opposite direction, how fast would the treadmill be going at take off? The plane is a jumbo jet, thrust of engines provide an acceleration of 2.5 m/s and it reaches take off speed in 35 seconds at around 200mph. The program is set to only measure the wheel speed every 1 second, and adjust treadmill speed accordingly.

After 35 seconds, I worked that the treadmill would be going 1575 mph as the plane takes off. I didn't fancy working out the speed if the software was set to detect and adjust the speed of the treadmill a few billion times per second

 

[Arthur Wedge]

However, and I may be completely wrong, if the engines are sucking air over/under the wings at the required speed to create lift the plane will take off.

Then my head goes all fuzzy again with questions. Can the engines create enough suck to ‘see’ air being pulled over the full length of the wings? What front to back width ratio to length does the wing need to be…? And then my head hurts even more…

For those maybe not understanding how the lift principle might work on a stationary object, think about a model aircraft in a wind tunnel. In effect the tunnel provides the airflow just as the engines on the OP’s plane creates an airflow.

But maybe I’m just totally bamboozled.

Would it make any difference if it’s a plane with the engines on the back wing ?

 

[Deleted member 35927]

The confusion arises because we think of it like a car which puts the force via it wheels and the treadmill can counter that force.

If I sped a treadmill to 100mph and asked you to jump on an remain stationary, you'd fail and go backwards and get injured !

If I ask you to repeat it but this time stand on a skateboard you'd be able to achieve it easily, the treadmill would simply turn the wheels as they offer little resistance.

Then ask yourself if I then gave you jet while on the board and said power it up to enough thrust to go 100mph , do you truly think you'd still be stationary.

 

[Swango1980]

However, and I may be completely wrong, if the engines are sucking air over/under the wings at the required speed to create lift the plane will take off.

Then my head goes all fuzzy again with questions. Can the engines create enough suck to ‘see’ air being pulled over the full length of the wings? What front to back width ratio to length does the wing need to be…? And then my head hurts even more…

For those maybe not understanding how the lift principle might work on a stationary object, think about a model aircraft in a wind tunnel. In effect the tunnel provides the airflow just as the engines on the OP’s plane creates an airflow.

But maybe I’m just totally bamboozled.

This isn't how a jet engine works?

It sucks air in from the front using a fan, and blasts it out the back (not over the wings). Blasting the exhaust out the back propels the aircraft forward (Newtons every action there is an equal and opposite reaction). It is then the forward movement and the design of the wings which causes air to move faster over the wing and slower under it, creating lift.

The engine itself does not directly blow air over the wings to create lift. It is sucking air into itself, and thrusting it out behind the wing. Propellors also create thrust, just using a different method. But they are still designed to thrust the plane forwards, not directly move air over the wings

 

[Hobbit]

Would it make any difference if it’s a plane with the engines on the back wing ?

Where the engines are placed in relation to the wings makes huge differences, assuming my theory is right. The further back they are, the wider the suck. Ideally, in terms of where the greatest airflow is, front or back of the engine, then an engine placed well in front of the wings will create by far the greater airflow.

 

[Voyager EMH]

Having a slow work day, so tried out an old GCSE type maths question. If a programmer was silly enough to set a treadmill to run a speed equal to to the speed the planes thrust provides, but in opposite direction, how fast would the treadmill be going at take off? The plane is a jumbo jet, thrust of engines provide an acceleration of 2.5 m/s and it reaches take off speed in 35 seconds at around 200mph. The program is set to only measure the wheel speed every 1 second, and adjust treadmill speed accordingly.

After 35 seconds, I worked that the treadmill would be going 1575 mph as the plane takes off. I didn't fancy working out the speed if the software was set to detect and adjust the speed of the treadmill a few billion times per second

I like that one.

Now I am imagining the plane being lowered onto the treadmill that is moving at 1575mph. The plane is lowered so that its wheels touch the treadmill and cause them to spin, but the crane holds the plane in that position.
The wheels are moving fast enough to keep the plane stationary on its own, but only if there is no friction/drag or whatever.
The engines run at low power - enough for a taxiing speed of about 10mph.
If the crane lets go, the wheels have no reason to slow down, there is enough power from the engines to overcome friction/drag and the plane does not move backwards, but the additional force from the engines cause it to move forwards with the wheels actually increasing their spin.
If the engines increase their power the plane will move forwards and continue to do so right up to take-off speed.

Unless I'm wrong.

 

[Bunkermagnet]

This isn't how a jet engine works?

It sucks air in from the front using a fan, and blasts it out the back (not over the wings). Blasting the exhaust out the back propels the aircraft forward (Newtons every action there is an equal and opposite reaction). It is then the forward movement and the design of the wings which causes air to move faster over the wing and slower under it, creating lift.

The engine itself does not directly blow air over the wings to create lift. It is sucking air into itself, and thrusting it out behind the wing. Propellors also create thrust, just using a different method. But they are still designed to thrust the plane forwards, not directly move air over the wings

You are overlooking the OP scenario. The conveyor belt is programmed to match the plane wheel speed. Since a plane on the ground has to accelerate viw the thrust of the engines as the plane moves forwards the conveyor belt will move in the opposite direction, therefore the plane on the ground cannot go forward with enough speed to create the needed lift by the air passing over and under the wings.

 

[Swango1980]

You are overlooking the OP scenario. The conveyor belt is programmed to match the plane wheel speed. Since a plane on the ground has to accelerate viw the thrust of the engines as the plane moves forwards the conveyor belt will move in the opposite direction, therefore the plane on the ground cannot go forward with enough speed to create the needed lift by the air passing over and under the wings.

See my post No. 346.

The post you replied to wasn't overlooking anything, because I was simply replying to a post about how a jet engine works. It was a side not, and not really related to the question in the OP

 

[Deleted member 35927]

You are overlooking the OP scenario. The conveyor belt is programmed to match the plane wheel speed. Since a plane on the ground has to accelerate viw the thrust of the engines as the plane moves forwards the conveyor belt will move in the opposite direction, therefore the plane on the ground cannot go forward with enough speed to create the needed lift by the air passing over and under the wings.

This is incorrect. I'm order to stop the plane moving forward there has to be a force applied to counter the jet engine. We know this and can thank sir Isaac Newton.
Where in your explanation is this force coming from.

 

[Imurg]

Mornington Crescent........for those of a certain era

 

[RichA]

You are overlooking the OP scenario. The conveyor belt is programmed to match the plane wheel speed. Since a plane on the ground has to accelerate viw the thrust of the engines as the plane moves forwards the conveyor belt will move in the opposite direction, therefore the plane on the ground cannot go forward with enough speed to create the needed lift by the air passing over and under the wings.

Part of that matched my thinking for the last 24 hours.
Forget planes and engines for a sec. Imagine a shopping trolley on an airport travellator.
If you're pushing the trolley and you're on the travellator which is oppositely matching wheel speed you'll go nowhere.
If you step off the travellator and walk alongside it pushing the trolley then the travellator and trolley wheels can do whatever they like. The trolley will move along at whatever speed you're going. There will be no skidding or friction; wheel speed and travellator speed will continue to be matched as you push it along, even if somebody is randomly changing the travellator speed.
The trolley is the plane. The geezer pushing with no impact from the travellator it is the jet engines.

 

[Swinglowandslow]

Part of that matched my thinking for the last 24 hours.
Forget planes and engines for a sec. Imagine a shopping trolley on an airport travellator.
If you're pushing the trolley and you're on the travellator which is oppositely matching wheel speed you'll go nowhere.
If you step off the travellator and walk alongside it pushing the trolley then the travellator and trolley wheels can do whatever they like. The trolley will move along at whatever speed you're going. There will be no skidding or friction; wheel speed and travellator speed will continue to be matched as you push it along, even if somebody is randomly changing the travellator speed.
The trolley is the plane. The geezer pushing with no impact from the travellator it is the jet

So the travellator moves alongside you at the speed you are walking, does it?
As well as the trolley you have hold of.?
If that’s the scenario, then replacing your pushing power of say 2 mph with the thrust of the planes jets to say 180 mph will cause the plane to lift off ( with some flap)
But where the hell in the image is there any suggestion that the treadmill(travellator, whatever) that the plane is shown to be on, is meant to be moving (at 180mph).? What indication is there that the treadmill, together with what it’s built on, is capable of moving forward, let alone at 180mph and for a distance of a mile or thereabouts.
The image to me (and others, I think) indicates that it is fixed to the ground.
And if it is, then no air is passing over the wings to give lift.
Nothing is moving but the wheels and the treadmill they are on. In opposite directions, and so cancelling each other.

 

[Swango1980]

So the travellator moves alongside you at the speed you are walking, does it?
As well as the trolley you have hold of.?
If that’s the scenario, then replacing your pushing power of say 2 mph with the thrust of the planes jets to say 180 mph will cause the plane to lift off ( with some flap)
But where the hell in the image is there any suggestion that the treadmill(travellator, whatever) that the plane is shown to be on, is meant to be moving (at 180mph).? What indication is there that the treadmill, together with what it’s built on, is capable of moving forward, let alone at 180mph and for a distance of a mile or thereabouts.
The image to me (and others, I think) indicates that it is fixed to the ground.
And if it is, then no air is passing over the wings to give lift.
Nothing is moving but the wheels and the treadmill they are on. In opposite directions, and so cancelling each other.

If you are taking the image literally, then as soon as thrust is given by the engines then the plane will just fall off the front of the treadmill.

So, you have to really assume the treadmill is the entire runway, or just respond the plane crashes to ground in front of treadmill