That plane/conveyor problem - filmed solution
Discussion
bobthemonkey said:
A lot depends (we think-Asked hung-over engineers and physicists) on t/w ratio's. if the t/w>1 then the plane should ake off without an lift, at t/w<1 lift is needed to achieve flight. Look at it like this two identical planes A (t/w>1) and B(t/w>1). Point them so they are facing vertically upwards. A will go up, B will not. Assuming (and its a big assuption) the conveyer belt acts in a way to prevent air moving over the wing (ie the plane is stationary) A will take off, B will not. 
Aha yes, you have pointed out a basic flaw in the theory that needs to be clarified from the outset. This scenario can not be applied to something like a Harrier jump-jet because it doesn't give a monkeys what the conveyor belt is doing, it'll take off anyway. So in the initial conditions it has to be stated that the aircraft is of a conventional design requiring that the certain take-off speed has to be achieved before it has sufficient lift to fly.
And no, of course our conventional plane won't take off! Once the wheel bearings seize at a gazillion RPM it'll be thrown backwards off the conveyor to crash and burn.
Having not been very involved in the "big plane/conveyor thread", I'm enjoying the pain of those horrified to see the damned subject return...
nel said:
This scenario can not be applied to something like a Harrier jump-jet because it doesn't give a monkeys what the conveyor belt is doing, it'll take off anyway.
same applies to any plane
that's the whole point
perhaps you should check you have the right question before you defend your answer
"a conveyor that matches the plane's speed in the opposite direction"
if the plane has a speed, it's clearly moving. if the plane isn't moving, neither is the conveyor
nel said:
Once the wheel bearings seize at a gazillion RPM it'll be thrown backwards off the conveyor to crash and burn.
the wheels will only ever spin at twice their normal speed at take off, not a gazillion RPM, not light speed
that's all I'm saying
orgasmicliving will be along in a bit, I don't know what side he's on now
nel said:
And no, of course our conventional plane won't take off! Once the wheel bearings seize at a gazillion RPM it'll be thrown backwards off the conveyor to crash and burn.
Stop being an armchair physicist and go and ask an expert.
I did. Two of them. Both gave near instant answers. And the planes do take off.
hugoagogo said:
"a conveyor that matches the plane's speed in the opposite direction"
Ah so - a common failing not reading the question! I assumed that the conveyor was capable of infinite speeds in its attempt to keep the plane stationary - if it only equals the plane's speed than the wheel bearings should hold out OK until take-off!
This is briiliant!! I wasnt privvy to the original thread but I want to ask those that sound like they know, a question. The lift bit I get (I think). To me, if you could pass air over a Jumbo fast enough, while it might not actually rise, surely if you took the ground away, it wouldnt fall? As the plane's wings attempt to occupy the area of low air pressure above the wings surface and therefore hold it there(?).
BUT, the need for the plane to physically move forward I dont get yet. How do they measure downforce/lift in a wind tunnel where the subject CANT move?
PB - be patient with me, learning and knowledge sharing is the original vision for the internet in any case....blah blah blah whatever..
BUT, the need for the plane to physically move forward I dont get yet. How do they measure downforce/lift in a wind tunnel where the subject CANT move?
PB - be patient with me, learning and knowledge sharing is the original vision for the internet in any case....blah blah blah whatever..
Edited by 308mate on Monday 11th December 16:59
308mate said:
This is briiliant!! I wasnt privvy to the original thread but I want to ask those that sound like they know, a question. The lift bit I get (I think). To me, if you could pass air over a Jumbo fast enough, while it might not actually rise, surely if you took the ground away, it wouldnt fall? As the plane's wings attempt to occuly the area of low air pressure above the wings surface and therefore hold it there(?).
BUT, the need for the plane to physically move forward I dont get yet. How do they measure downforce/lift in a wind tunnel where the subject CANT move?
PB - be patient with me, learning and knowledge sharing is the original vision for the internet in any case....blah blah blah whatever..
BUT, the need for the plane to physically move forward I dont get yet. How do they measure downforce/lift in a wind tunnel where the subject CANT move?
PB - be patient with me, learning and knowledge sharing is the original vision for the internet in any case....blah blah blah whatever..
as it's a serious question.... In a wind tunnel the subject is moving relative to the wind, which is all that is important for measureing air bourne. The only reason for the need of the aircraft to move forward is to create the airflow - if you had a feck off fan blowing the air, the aircraft wouldn't need to move at all.
308mate said:
BUT, the need for the plane to physically move forward I dont get yet. How do they measure downforce/lift in a wind tunnel where the subject CANT move?
A plane only needs to move forward to generate airflow, so that it has lift. In a wind tunnel the airflow is already generated, so it can literally just take off.
a kindly old fellow said:
The only reason for the need of the aircraft to move forward is to create the airflow - if you had a feck off fan blowing the air, the aircraft wouldn't need to move at all.
Right! Ok, so it seems I get that, its all just as I thought. And because for some reason I cant view the original video it all centres around, I cant really see where the confusion lies. Sounds bloody obvious to me!
PB
308mate said:
Right! Ok, so it seems I get that, its all just as I thought. And because for some reason I cant view the original video it all centres around, I cant really see where the confusion lies. Sounds bloody obvious to me!
People get confused with the whole "conveyor belt matching the speed of the wheels" thing. With this statement they can't help but believe that the plane would remain stationary. This would be true the the thrust is coming from the wheels, like in a car, but on a plane it isn't.
Oh god, not again. It would take off unless you subscribe to orgasmicliving's definition of the question which involves an axiomic rule that states that the plane's wheel speed must equal that of the conveyor. In that case it's not a physics question anymore but a trick logic question.
he said:
People get confused with the whole "conveyor belt matching the speed of the wheels" thing. With this statement they can't help but believe that the plane would remain stationary. This would be true the the thrust is coming from the wheels, like in a car, but on a plane it isn't.
So, in the interests of everyone covering old ground just for my benefit:
Regardless of what provides thrust to the wheels (in the case of the plane, nothing. Theyre just there to stop the thing skidding along on its belly) - who cares about a conveyor belt? You could have a conveyor belt doing 400mph....no, 800mph under a stationery wing and nothing will happen because theres no air flow. Yes? (yes yes, assuming the surface of the conveyor has a cd of 0.0 and creates no turbulence)
PB
Edited by 308mate on Monday 11th December 17:19
I always use this analogy when trying to explain it:
Imagine you are on a skateboard which is on a treadmill. The wheels of the skateboard have little motors that drive it along, and the treadmill matches the speed of the wheels. Great. You get on the skateboard by bolting down your feet to it with superglue or something, hit the gas and sit absolutely still as both the treadmill and wheels of the skateboard spin violently at the same speed.
Increase gas, increase speed, no movement. Ad nauseum.
And then your shit-faced little brother comes along, sees you carrying out this ridiculous experiment and shoves you in the back just for fun.
What happens?
Do you still remain stationary? Or do you move forward?
If you can imagine yourself remaining stationary in this scenario, you are completely warped. The natural reaction is to imagine yourself moving forward.
The push on your back is akin to the thrust from the engines of a plane.
Imagine you are on a skateboard which is on a treadmill. The wheels of the skateboard have little motors that drive it along, and the treadmill matches the speed of the wheels. Great. You get on the skateboard by bolting down your feet to it with superglue or something, hit the gas and sit absolutely still as both the treadmill and wheels of the skateboard spin violently at the same speed.
Increase gas, increase speed, no movement. Ad nauseum.
And then your shit-faced little brother comes along, sees you carrying out this ridiculous experiment and shoves you in the back just for fun.
What happens?
Do you still remain stationary? Or do you move forward?
If you can imagine yourself remaining stationary in this scenario, you are completely warped. The natural reaction is to imagine yourself moving forward.
The push on your back is akin to the thrust from the engines of a plane.
Kind of. Basically the thrust of the engine(s) with be able to overcome the slight extra drag created by the runway/conveyor moving at double speed beneath the wheels. It will thus be able to move forward, accelerate to take off speed and lift off.
If you want to get pedantic then issues such as tyres bursting or bearings melting could occur but that's kind of beside the point.
If you want to get pedantic then issues such as tyres bursting or bearings melting could occur but that's kind of beside the point.
The crux of this is that the question isn't sufficeintly clearly written that there can only be one "correct" way of understanding it. If you assume that "matches the speed" means that the conveyor belt moves at -v if the place is travelling at +v, then the plane takes off so long as its wheels can rotate at twice their normal takeoff speed. If you read the question so that it means that the conveyor belt can spin arbitrarily fast in order to overcome the thrust of the engines, then by definiton the plane remains stationary relative to the ground. If this was an exam question, I'd expect the student to state what assumptions they were making about the meaning of the question, and so long as their answer was consistent with their stated assumptions, they'd get the marks.
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