Physics Question

[911paramedic]

Originally posted by: giantpinkbunnyhead
You are correct. With jets, virtually NO airflow over the wings is added to by the engines. But on prop-driven, multi-engine, non-centerline thrust aircraft, the propwash does flow around the wings. It creates more lift than a jet would, but still nowhere NEAR enough to be beneficial for anything.

True, but we are not talking about propwash, we are talking about a jet powered plane. To me (moronic as I may be) if the jet engines create force/energy to make the wheels spin at a takeoff speed, say 200mph, then the treadmill is spinning at 200mph. Net velocity is zero, airflow is zero...or next to it.

Why do people make this so hard?

 

[Yossarian]

Would be nice if the airplane did takeoff. We could replace steam catapults on aircraft carriers, and add conveyors to every airport in the world for nice safe zero groundspeed takeoffs.

 

[letdown427]

sorry, the fan was meant to be a serperate analogy, not to do with the plane lol.

thanks for not just instantly attacking me as such. I agree with you that i certaintly didn't think it would fly at first, but I'm just trying to explore the possibility that it could.

see, initially i thought it would stay still, as that seems the most obvious answer. it seems like common sense.

but then like, i thought, am i just thinking that becasue i'm imagining it's the wheels pushing the plane forward, so any effor tthey made against the conveyer would be pointless? like for humans, or cars etc.... so i started thinking about it lol. I'm not saying you're wrong, or i'm right, i don't know at the mo, so please, don't attack me for thinking 'out loud' (yet)

"The engines are only a method to accelerate a plane to a true airspeed "

Yeah, so the engines accelerate the plane, using the air? This must be true, as in the air, when they're doing their flying, there is only air, and i think the engines have a fair bit to do with keeping the plane in the air (didn't mean to sound so sardonic), as they keep it moving forward. Airflow keeps the plane moving upwards against gravity supporting itself, (and provides the engines with their means of moving the plane forwards)

How do planes start moving? Their wheels are not driven. The engines start the plane moving surely?

Would it be reasonable to imagine the engines as being a simple force, pulling the plane forwards? I know this is not how it works, but for the simplicity or trying to explain my opinion, if you consider the engines like a rope, pulling the plane forwards. They use the air to create this forwards pulling. The air is independent of the ground.

Imagine a plane is being towed forwards along a stationary conveyer. it is being towed at 10mph. the vehicle that is towing it is not on the conveyor, as it is independent from it. if the conveyer starts moving opposite to the plane, at 10mph, the plane does not stop moving. it continues forwards, being towed, at 10mph. the wheels move at a rotation equivalent to 20mph.

it's the fact taht the engines use air to create the forwards movement, and that this is not goverend by the conveyer. that is what is bothering me, and making me doubt my original thought of it must b stationery.


see, "if the jet engines create force/energy to make the wheels spin at a takeoff speed" is the kind of thing I thought i might be thinking when i originally thought, yeah it'll be still. the wheels spin because the plane is moving forwards, not because they are pushing the plane forwrds against the tarmac, like in a car.

lol, i hope this stays light hearted, this is the longest 'arguement' i've seen where people are being polite lol

 

[CoveX]

Originally posted by: Yossarian
Would be nice if the airplane did takeoff. We could replace steam catapults on aircraft carriers, and add conveyors to every airport in the world for nice safe zero groundspeed takeoffs.

Obviously, the conveyor will need to be as long as a regular runway (slightly longer in fact because of the added friction), but it will take off.

 

[911paramedic]

Originally posted by: letdown427Imagine a plane is being towed forwards along a stationary conveyer. it is being towed at 10mph. the vehicle that is towing it is not on the conveyor, as it is independent from it. if the conveyer starts moving opposite to the plane, at 10mph, the plane does not stop moving. it continues forwards, being towed, at 10mph. the wheels move at a rotation equivalent to 20mph.

It's being towed at 10mph, how do you get to 20mph? You are adding speeds when they are equal, it is being towed at 10 so the conveyor moves in the opposite direction at 10. You dont add, they are equal, net speed=zero.

 

[giantpinkbunnyhead]

I just saw the link above, and realized this is an issue of semantics.

THe original question says that the conveyor belt matches the speed of the plane. But the proof provided in the link relies on the plane moving forward. Now the conveyor belt is not moving foward... so if the plane is indeed moving forward, the conveyor belt would not be matching it's speed anymore and thus the question is invalidated. The guy who writes the solution in the link hits upon the same point. How are we to define "speed"?

 

[swtethan]

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

yes it can actually, i change my answer.... if both objects (plane and conveyor) accelerate at the same speed then the conveyor would have a speed lag trying to catch up to the plane (plane goes first, system tries to match whie plane is still accelerating).... the plane will not stand still

 

[911paramedic]

Originally posted by: CoveX

Originally posted by: Yossarian
Would be nice if the airplane did takeoff. We could replace steam catapults on aircraft carriers, and add conveyors to every airport in the world for nice safe zero groundspeed takeoffs.

Obviously, the conveyor will need to be as long as a regular runway (slightly longer in fact because of the added friction), but it will take off.

Catapults move MUCH FASTER than the plane, hence the thrust. Please dont mix apples and oranges it just confuses the topic.

 

[giantpinkbunnyhead]

Originally posted by: 911paramedic

Originally posted by: letdown427Imagine a plane is being towed forwards along a stationary conveyer. it is being towed at 10mph. the vehicle that is towing it is not on the conveyor, as it is independent from it. if the conveyer starts moving opposite to the plane, at 10mph, the plane does not stop moving. it continues forwards, being towed, at 10mph. the wheels move at a rotation equivalent to 20mph.

It's being towed at 10mph, how do you get to 20mph? You are adding speeds when they are equal, it is being towed at 10 so the conveyor moves in the opposite direction at 10. You dont add, they are equal, net speed=zero.

I think he's trying to say that the wheels would be rotating the same speed as if the plane was being towed at 20 on a fixed surface.

 

[CoveX]

Originally posted by: 911paramedic

Originally posted by: letdown427Imagine a plane is being towed forwards along a stationary conveyer. it is being towed at 10mph. the vehicle that is towing it is not on the conveyor, as it is independent from it. if the conveyer starts moving opposite to the plane, at 10mph, the plane does not stop moving. it continues forwards, being towed, at 10mph. the wheels move at a rotation equivalent to 20mph.

It's being towed at 10mph, how do you get to 20mph? You are adding speeds when they are equal, it is being towed at 10 so the conveyor moves in the opposite direction at 10. You dont add, they are equal, net speed=zero.

The speed of spinning wheels of a stationary airplane on a 10mph treadmill: 10mph.
The speed of spinning wheels of a plane moving at 10mph on a stationary surface: 10mph.
The speed of spinning wheels of a plane moving at 10mph (relative to absolute ground) on a 10mph treadmill: 20mph.

 

[911paramedic]

Originally posted by: swtethan

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

yes it can actually, i change my answer.... if both objects (plane and conveyor) accelerate at the same speed then the conveyor would have a speed lag trying to catch up to the plane (plane goes first, system tries to match whie plane is still accelerating).... the plane will not stand still

Again, dont read into the question. (haven't your instructors taught you anything?) It moves at exactly the same speed.

 

[swtethan]

i think my 2nd answer is correct

 

[911paramedic]

Originally posted by: CoveX

Originally posted by: 911paramedic

Originally posted by: letdown427Imagine a plane is being towed forwards along a stationary conveyer. it is being towed at 10mph. the vehicle that is towing it is not on the conveyor, as it is independent from it. if the conveyer starts moving opposite to the plane, at 10mph, the plane does not stop moving. it continues forwards, being towed, at 10mph. the wheels move at a rotation equivalent to 20mph.

It's being towed at 10mph, how do you get to 20mph? You are adding speeds when they are equal, it is being towed at 10 so the conveyor moves in the opposite direction at 10. You dont add, they are equal, net speed=zero.

The speed of spinning wheels of a stationary airplane on a 10mph treadmill: 10mph.
The speed of spinning wheels of a plane moving at 10mph on a stationary surface: 10mph.
The speed of spinning wheels of a plane moving at 10mph (relative to absolute ground) on a 10mph treadmill: 20mph.

absolute ground = oranges

 

[letdown427]

Originally posted by: 911paramedic

Originally posted by: letdown427Imagine a plane is being towed forwards along a stationary conveyer. it is being towed at 10mph. the vehicle that is towing it is not on the conveyor, as it is independent from it. if the conveyer starts moving opposite to the plane, at 10mph, the plane does not stop moving. it continues forwards, being towed, at 10mph. the wheels move at a rotation equivalent to 20mph.

It's being towed at 10mph, how do you get to 20mph? You are adding speeds when they are equal, it is being towed at 10 so the conveyor moves in the opposite direction at 10. You dont add, they are equal, net speed=zero.

I'm going to imagine that you and I are walking through an airport together (lol, heerer we go). you are a young mischievous chap(oh those brits) and you're on your skateboard. we come up to one of those wonderful moving conveyer things, that people like to stand on and get whisked to the other end of that long corridor. i opt to walk alongside it, you opt to ride your skateboard over the conveyer. say it is doing 5mph, but in the opposite direction to which way we're travelling (i said you were mischievous ) i'm walking along side it, at 5mph, and i offer to give you a tow by holding onto you. i walk forwards at my same 5mph, and so, relative to the fixed ground, you too are travelling at 5mph yes? as you are remaining the same distance from me, as i am towing you. now, the conveyer is doing 5mph in the opposite direction, so the wheels on your skateboard are rotating as if they were doing 10mph, they are not stationary. that's why i added the speeds.


the other guys said it much simpler, and without the weakly homoerotic analogy, but hey, i had fun.


i think this little thought exercise is meant to introduce people to the idea of reference frames, perhaps prior to a more indepth study of relativity.

 

[swtethan]

Originally posted by: 911paramedic

Originally posted by: swtethan

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

yes it can actually, i change my answer.... if both objects (plane and conveyor) accelerate at the same speed then the conveyor would have a speed lag trying to catch up to the plane (plane goes first, system tries to match whie plane is still accelerating).... the plane will not stand still

Again, dont read into the question. (haven't your instructors taught you anything?) It moves at exactly the same speed.

answer me this..... the system cant move the runway if there is no movement of the plane correct? then how can the system catch up to the plane once the plane is accelerating? the plane will always be moving faster than the runway can compensate

 

[JustAnAverageGuy]

No, unless it's a VTOL jet.

 

[swtethan]

its just so simple you dont even understand.....


1)plane starts accelerating

2)system tries to catch up


plane will always be moving faster than system


done

 

[Cerpin Taxt]

Originally posted by: 911paramedic

Originally posted by: swtethan

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

yes it can actually, i change my answer.... if both objects (plane and conveyor) accelerate at the same speed then the conveyor would have a speed lag trying to catch up to the plane (plane goes first, system tries to match whie plane is still accelerating).... the plane will not stand still

Again, dont read into the question. (haven't your instructors taught you anything?) It moves at exactly the same speed.

Yes, but "exactly the same speed"... as what? The plane and not the wheels. Trying to say that the conveyor matches the speed of the wheels leads to a paradox. If the plane is traveling east at 10mph, the conveyor moves west at 10mph. The wheels spin 20mph and forever twice the speed of the plane until the plane takes off.

 

[letdown427]

If the plane is traveling east at 10mph, the conveyor moves south at 10mph.


oh düde, edit that! lol

 

[Cerpin Taxt]

brainfart fixed

 

[giantpinkbunnyhead]

Yep... this is exactly the problem we have... how is speed defined for this problem? The OP states merely that the conveyor matches the speed of the plane. Not the clearest way to lay it out.

Regarding the conveyor always catching up to the speed of the plane... I think for the purposes of this problem, it's assumed that this detail is nonexistent. Correct me if I'm wrong, however. That's just how I took it.

 

[letdown427]

lol, an i didn't even need a compass!

yeah, i agree that the lag is most probably ignored in this. physics does tend to simplify things to make them more ocmplicated!

 

[911paramedic]

That is correct letdown (ironic name, eh?) but the conveyor beneath the plane moves at the exact wheel speed of the plane. We know that from the question. We are not asking what the difference is (double whatever speed you are going) we are asking if the plane moves AT ALL because that is whats needed for take-off. (wind over the airfoils)

If the jets push (or pull, as some of you would have it...) at 200mph on the conveyor, the conveyor EXACTLY matches the wheel speed. That would mean the conveyor was moving at 200mph in the opposite direction, that means the plane appears to be standing still, even though its instruments say 200mph. The sum of the difference may be 400mph, but thats not the question! The question is will the plane take off. No, net speed is ZERO. This is physics 101 isn't it???? Should I go back in my Delorian and bitch-slap my physics teacher?

 

[dawks]

The plane will take off.

I've seen this discussed on several forums now. I was initially like "WTF no it can't take off.." but I've since changed my tune.

The key is that the wheels are free spinning. It doesn't matter how fast the conveyor spins, the wheels will match it. This pretty well makes it equivilant to actually being in the air.

A turbofan airplane moves forward by taking air in the front through the fan blades, compressing it highly, and blasting it out the back. If the plane moved forward by being driven by the wheels, like a car, it would be an entirely different story. But its pulled forward by sucking air in the engines and blasting it out the back.

 

[swtethan]

Originally posted by: Garth

Originally posted by: 911paramedic

Originally posted by: swtethan

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

yes it can actually, i change my answer.... if both objects (plane and conveyor) accelerate at the same speed then the conveyor would have a speed lag trying to catch up to the plane (plane goes first, system tries to match whie plane is still accelerating).... the plane will not stand still

Again, dont read into the question. (haven't your instructors taught you anything?) It moves at exactly the same speed.

Yes, but "exactly the same speed"... as what? The plane and not the wheels. Trying to say that the conveyor matches the speed of the wheels leads to a paradox. If the plane is traveling east at 10mph, the conveyor moves south at 10mph. The wheels spin 20mph and forever twice the speed of the plane until the plane takes off.

but youre trying to tell me that the conveyor knows the instant moment the plane will move.... the plane will always have the speed advantage even if its by 1/100000th of a second, the plane is still moving forward, remember the system is trying to match the speed, but how can you match a plane speed when its still accelerating