A Wednesday conundrum
Discussion
pdV6 said:
Aaaaaaaargh! Been done to death already!!
It wont take off as there is no lift surely as the plane is static.
Would a car with downforce generate any on a rolling road at max speed? No. Reverse of this principle surely.........
You're getting confused with ground and air in the same way that scorp is.
Car downforce is not a factor of ground speed, but of the airspeed passing over the bodywork, spoilers etc.
Even stationary in a field, a car is capable of producing downforce if a strong enough wind blows over it.
hence why F1 teams use Wind Tunnels...
Moose. said:
A plane is standing on a runway that can move (some sort of giant conveyer belt). 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?
Question is "can" answer is yes, but only with a set of given circumstances...
Lift is a factor of wind speed over the shape of the wing. In a gale, a stationary plane will have lift, the question is how strong does the wind need to be to generate sufficient lift...
p.s. the question does not state it is a jet...
tiga84 said:
It wont take off as there is no lift surely as the plane is static.
This is the problem with your reasoning - simply that the plan is NOT static.
On a 'plane, the wheels turn simply because the airframe is moving and the wheels are touching the ground.
It is not true that the 'plane moves forwards as a result of the wheels turning.
Therefore, the airframe will carry on doing its moving forward and taking off trick whilst all the while the magic conveyor belt will be self-destructing trying to "keep up" with the wheels...
pdV6 said:
Wrong, planes at takeoff use thrust to get off the ground, the wings are ineffective at takeoff speeds.
so why do they need a runway then? put the engine to max and it should shoot stright up into the air by that reckoning.
Inertia.If the wings are "ineffective at takeoff speeds" as you say, then why do 'planes have wings at all?
Why, in fact, is there any such thing as a "takeoff speed"?
Consider the fact that the takeoff ground speed is lower when taking off into a strong headwind (and for extra points, work out that the takeoff air speed is always the same regardless of wind)
I think you've somehow gotten yourself confused in a parallel universe where physics works in a completely different manner...
I did just make a u-turn. I thought the jet engines themselves supported some weight at take off since the airflow isnt that high when your pounding down a runway, do the wings take 100% of the load at that stage?
jacksdad said:
A plane is standing on a runway that can move (some sort of giant conveyer belt). 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?
Question is "can" answer is yes, but only with a set of given circumstances...
Lift is a factor of wind speed over the shape of the wing. In a gale, a stationary plane will have lift, the question is how strong does the wind need to be to generate sufficient lift...
p.s. the question does not state it is a jet...
Nope - you're 1/2 way there but obviously haven't got it yet. The plane moves through the air by means of jet engines / propellors / whatever regardless. The wheels just happen to turn because they're touching the ground at the time.
Moose. said:
A plane is standing on a runway that can move (some sort of giant conveyer belt). 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?
Please define 'the plane speed' i.e. what are you measuring the speed relative to?
Tried to work this through, cheated and googled, came up with this page..
http://homepage.ntlworld.com/grahamlegg/whokeeps.htm
None the wiser.....
http://homepage.ntlworld.com/grahamlegg/whokeeps.htm
None the wiser.....
scorp said:
I thought the jet engines themselves supported some weight at take off since the airflow isnt that high when your pounding down a runway, do the wings take 100% of the load at that stage?
The landing gear takes 100% of the load when stationary, dropping to 0% at takeoff (air) speed, at which the wings are doing 100% of the work.
How, exactly, did you imagine the engines to be "supporting" the plane during takeoff? Does the plane crash to the ground as soon as they're switched off after parking up?
pdV6 said:
The landing gear takes 100% of the load when stationary, dropping to 0% at takeoff (air) speed, at which the wings are doing 100% of the work.
How, exactly, did you imagine the engines to be "supporting" the plane during takeoff? Does the plane crash to the ground as soon as they're switched off after parking up?
When the wheels leave the runway i was talking about
what supports the weight of the plane, is it only the wings or is it a combination of wings and thrust?GreenV8S said:
A plane is standing on a runway that can move (some sort of giant conveyer belt). 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?
Please define 'the plane speed' i.e. what are you measuring the speed relative to?
Its irrelevant!
pdV6 said:
A plane is standing on a runway that can move (some sort of giant conveyer belt). 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?
Question is "can" answer is yes, but only with a set of given circumstances...
Lift is a factor of wind speed over the shape of the wing. In a gale, a stationary plane will have lift, the question is how strong does the wind need to be to generate sufficient lift...
p.s. the question does not state it is a jet...
Nope - you're 1/2 way there but obviously haven't got it yet. The plane moves through the air by means of jet engines / propellors / whatever regardless. The wheels just happen to turn because they're touching the ground at the time.
My point, if the wheels are turnng on a conveyor belt, and the belt speed in constant to the plane, then surely the effect of that movement is neutral. If so, then the only effect is the wind speed over the wing?
off now to a meeting to dicuss duration reduction through currency hedging and Credit Default Swops on EM debt, far simpler than this...
scorp said:
The landing gear takes 100% of the load when stationary, dropping to 0% at takeoff (air) speed, at which the wings are doing 100% of the work.
How, exactly, did you imagine the engines to be "supporting" the plane during takeoff? Does the plane crash to the ground as soon as they're switched off after parking up?
When the wheels leave the runway i was talking about
The wings are the ONLY thing supporting the plane in flight.
The wings generate lift by virtue of their shape and the speed of the air passing over them.
Turn the engine off and the plane doesn't plummet out of the sky, it glides, maintaining forward speed by trading potential energy (height) for kinetic energy (speed) until it runs out of height by meeting the ground.
pdV6 said:
The wings are the ONLY thing supporting the plane in flight.
The wings generate lift by virtue of their shape and the speed of the air passing over them.
Turn the engine off and the plane doesn't plummet out of the sky, it glides, maintaining forward speed by trading potential energy (height) for kinetic energy (speed) until it runs out of height by meeting the ground.
Im now enlightened.
pdV6 said:
The landing gear takes 100% of the load when stationary, dropping to 0% at takeoff (air) speed, at which the wings are doing 100% of the work.
How, exactly, did you imagine the engines to be "supporting" the plane during takeoff? Does the plane crash to the ground as soon as they're switched off after parking up?
When the wheels leave the runway i was talking about
The wings are the ONLY thing supporting the plane in flight.
The wings generate lift by virtue of their shape and the speed of the air passing over them.
Turn the engine off and the plane doesn't plummet out of the sky, it glides, maintaining forward speed by trading potential energy (height) for kinetic energy (speed) until it runs out of height by meeting the ground.
I thought it was thrust versus height that kept it going forward and Kinetic energy is stored within the metal itself?
Aircraft fly because they have wings - not because they have engines (gliders anyone?). The key to an aircraft getting off the ground is the flow of AIR over and above the wings. When the correct AIRSPEED (not groundspeed) has been achieved the wings will start to generate lift and the aircraft will leave the ground. The necessary airspeed for takeoff will vary depending on the weight of the aircraft and the air density on the day. That is why airliner pilots have to carry out careful calculations to work out what the correct takeoff speed will be - they have to factor in things such as the weight of the aircraft, the altitude of the airfield, the air pemperature and density and the thrust available from the engines given the conditions. Navy catapult operators have to be even more careful because, if they get the catapult settings even slightly off, they may end up with a very expensive splash.
The number of aircraft that can lift off or climb using thrust alone is very few. The Harrier and other VTOL planes are one category that can. The F-15 and Su-27 fighters can accelearte vertically on thrust alone because at maximum power, the engines push out more thrust than the weight of the airframe.
The most obvious "flying" machine that relies on thrust alone is a rocket - which is the reason why most rockets have no wings at all - they don't need them.
The number of aircraft that can lift off or climb using thrust alone is very few. The Harrier and other VTOL planes are one category that can. The F-15 and Su-27 fighters can accelearte vertically on thrust alone because at maximum power, the engines push out more thrust than the weight of the airframe.
The most obvious "flying" machine that relies on thrust alone is a rocket - which is the reason why most rockets have no wings at all - they don't need them.
jacksdad said:
My point, if the wheels are turnng on a conveyor belt, and the belt speed in constant to the plane, then surely the effect of that movement is neutral. If so, then the only effect is the wind speed over the wing?
Again, you're 1/2 way there. Yes, "the only effect is the wind speed over the wing" - that is correct.
What you can't do, however, is tie that airspeed to the wheels; they're not linked at all.
What will actually happen is that the conveyor belt will never "catch up" with the wheels as the plane will be moving forwards regardless.
Imagine a plane that has an electromagnetic system to keep it off the ground when stationary instead of landing gear with wheels attached (think bullet train) and then get it to take off when the runway is moving underneath it. Now that the 2 aren't physically touching, can you see that the movement of the runway is irrelevant to the forward movement of the plane?
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