That plane/conveyor problem - filmed solution
Discussion
Helloooo? Why are we discussing a bastard conveyor? And if we're talking about the plane taking off, how can we not be talking about lift?
The plane needs to pass air over its wings to lift. There could be 3000 elephants running under it, or a conveyor or whatever. The planes motion is NOT dictated by it's wheel speed. If you attach a GPS to the plane (fine, use the one thats already on it) and run a conveyor belt under it at 3 gadjillion miles an hour (you would need VERY good wheel bearings) and ensure that as far international grid lines are concerned, the fuselage doesnt move, nothing will happen surely? Thers no increase in air flow over the wings so zilch.
Starting to feel dumb now...
Sure, if the wheel bearings seize (probable at around 1 gadjillion miles an hour) and provided you have immense grip from your goodyears, of course the whole thing will be shot into orbit (0.5 of gadjillion is sufficient to escape the earthe gravitational field - probably).
Im lost.
Im also assuming a plane without its engines turned on. Just sat there, minding its own business.
PB
The plane needs to pass air over its wings to lift. There could be 3000 elephants running under it, or a conveyor or whatever. The planes motion is NOT dictated by it's wheel speed. If you attach a GPS to the plane (fine, use the one thats already on it) and run a conveyor belt under it at 3 gadjillion miles an hour (you would need VERY good wheel bearings) and ensure that as far international grid lines are concerned, the fuselage doesnt move, nothing will happen surely? Thers no increase in air flow over the wings so zilch.
Starting to feel dumb now...
Sure, if the wheel bearings seize (probable at around 1 gadjillion miles an hour) and provided you have immense grip from your goodyears, of course the whole thing will be shot into orbit (0.5 of gadjillion is sufficient to escape the earthe gravitational field - probably).
Im lost.
Im also assuming a plane without its engines turned on. Just sat there, minding its own business.
PB
Cotty said:
My take on this as I never posted on the original thread.
Stationry plane on the conveyor. Switch the conveyor on and the plane will move in reverse. This is down to friction in the wheel bearing and the weight of the plane resting on those bearings. Start the engines and and probably just above idle you would be able to overcome the friction in the wheels.
The plane is now stationry as it has matches the friction in the wheels. Now you can crank the conveyor to whatever you like and the plane will remain stationry as its only fighting friction not the conveyor as its freewheeling. now open the throttles fully the plane moves forward to a point where its speed generates lift and it takes off.
Stationry plane on the conveyor. Switch the conveyor on and the plane will move in reverse. This is down to friction in the wheel bearing and the weight of the plane resting on those bearings. Start the engines and and probably just above idle you would be able to overcome the friction in the wheels.
The plane is now stationry as it has matches the friction in the wheels. Now you can crank the conveyor to whatever you like and the plane will remain stationry as its only fighting friction not the conveyor as its freewheeling. now open the throttles fully the plane moves forward to a point where its speed generates lift and it takes off.
yep, dead right.
edit: except as said, the conveyor only ever tracks the plane's speed and moves in the opposite direction, so it doesn't move until the plane moves forward
Edited by hugoagogo on Monday 11th December 21:10
308mate I think you're misunderstanding. The original question is:
"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?"
What's your answer?
"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?"
What's your answer?
aceparts_com said:
I've just asked my g/friend. She asked me to ask you guys how long the conveyor belt is? She's concerned the plane may fall off the end of the belt 
Yep that is a factor that isnt explained in the original question. It only needs to be the same lenght as the planes usual take off distance..
-DeaDLocK- said:
mackie1 said:
Quite. Although the speed it'd have to run at to generate enough force in the other direction is pretty big! Would it be physically possible to accelerate the wheels so much that they'd counteract 200,000lbs of thrust?
I like it theoretical, so let's not get too bothered about that.
I'm curious that in a model of the plane remaining stationary because the conveyor belt can spin arbitrarily fast, how is the energy from the thrust of the engine expended?
I have been adopting this theoretical approach this whole time but still arrive at the conslusion that the plane would move forward.
For this version of the question the conveyor belt's only means of countering the thrust of the engine and the resultant attempted movement of the plane is via the friction of the wheel bearings, that is where the energy goes. Assuming something like an exponential increase in conveyor speed for each unit that the plane moves away from the reference zero, our infinite speed conveyor will very quickly be powering the aircraft's wheels round at a fantabublous speed, countering the engine's thrust with an opposite force due to bearing friction. Obviously this will cause the bearings to overheat and self-destruct spectacularly fast.
What happens afterwards is open to debate I guess, but I reckon the rubber on the wheels would disappear in a quick smear and the plane would be chucked off the conveyor backwards as metal-conveyor contact occurred.
But that wasn't the original question it seems....
micky g said:
Car on rolling road doing 130 mph, whats the air resistance?
Doh
Doh
Zero
Until you shove a large metal bar in the rollers then it might create some
Actually there might be litte due to the cooling fans but that wouldnt be generated by the car
Edited by Cotty on Monday 11th December 21:25
3200gt said:
oh, and where's the destination? will I need my shades?
It depends on which way the conveyor belt is facing- this lot dont seem to be too concerned with things like this. I mean, if it faces the sun then the air at the front of the aircraft will be hotter (therefore less dense) then the air at the rear of the 'plane. Again, this lot are not taking things like this into consideration...
apache said:
mybrainhurts said:
apache said:
the plane will, at first, be stationary......so will the belt.
No, no...we're talking DRIVEN belt here
I was led to believe that the belt was accelerated to match the forward speed of the plane
Yes, so the plane does 150 mph forward, take off speed, the belt does 150 miles per hour backward, = take off speed, and the wheels just turn twice as fast as they normally do.
Anybody who can see a reason that the plane doesn't take off really needs to go back to school.
Its so bloody simple I can't see why it still takes a gazillion pages every session for the numpties to realise they are wrong.
paulie-mafia said:
OK, so if the plane takes off then why did they make the Harrier jump jet like they did? Why don't aircraft carriers have bloody great big conveyors on them instead of steam rams?
Edited by paulie-mafia on Monday 11th December 22:22
Because the conveyor would have to be just as long as a conventional runway. This would be impractical to fit onto a ship.
In the scenario described, the aeroplane takes off in exactly the same way it would on a conventional runway, except the wheels are spinning much faster.
HTH.
Edited by SamHH on Monday 11th December 22:29
King Herald said:
Its so bloody simple I can't see why it still takes a gazillion pages every session for the numpties to realise they are wrong.
The answer depends on how you interpret the question. End of story. All you can judge someone's answer on is how it relates to their interpretation of the question. there is no black and white, only shades of grey, man.Gassing Station | The Pie & Piston Archive | Top of Page | What's New | My Stuff