The way I understood this, it's impossible to generate lift without creating a net motion of air. Pressure is force per area, so saying that there is a pressure difference between the upper and lower side of the wing means that the air is exerting a force on the wing. By Newton's third law, that also means that the wing is exerting a force on the air, which has to move.
I guess you can also see it from conservation of momentum. Gravity acts on the plane, so each second there is a certain amount of downwards momentum added to the system. But the plane is not in fact moving downwards, so the all the downwards momentum has to end up in the air, so it has to move downwards. The net "lift" force is equal to the rate at which momentum is added to air.
If I understand it correctly, you can calculate the lift on the wing either in terms of the pressure on the two sides, or in terms of the downwards acceleration of air--it should be two equivalent views giving the same result. Bernoulli's principle gives a way to calculate the pressures if you only know the velocity of the air (this is a typical situation in a wind tunnel, where you make movies of moving smoke puffs), but it's not a separate effect from the air motion.
Unless I'm mistaken, ori_b is asserting that you can't generate lift without sending air off in another direction (i.e. downwash), and this is false. Again, most lift is generated this way - my posts here seem to be mistaken for arguments that Bernoulli force contributes significantly to the lift. It does not. However, the work done to reconfigure the air surrounding the wing by the Bernoulli principle will contribute to the lift, and while it does result in moving air around (not necessarily down though, actually), it does not impart any net motion to the air after the wing has passed.
Yes, both ori_b and I are asserting that you can't generate lift without sending air off in another direction. If you don't impart a net motion to the air, then by conservation of momentum you also can't generate a net lift.
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The way I understood this, it's impossible to generate lift without creating a net motion of air. Pressure is force per area, so saying that there is a pressure difference between the upper and lower side of the wing means that the air is exerting a force on the wing. By Newton's third law, that also means that the wing is exerting a force on the air, which has to move.
I guess you can also see it from conservation of momentum. Gravity acts on the plane, so each second there is a certain amount of downwards momentum added to the system. But the plane is not in fact moving downwards, so the all the downwards momentum has to end up in the air, so it has to move downwards. The net "lift" force is equal to the rate at which momentum is added to air.
If I understand it correctly, you can calculate the lift on the wing either in terms of the pressure on the two sides, or in terms of the downwards acceleration of air--it should be two equivalent views giving the same result. Bernoulli's principle gives a way to calculate the pressures if you only know the velocity of the air (this is a typical situation in a wind tunnel, where you make movies of moving smoke puffs), but it's not a separate effect from the air motion.
Unless I'm mistaken, ori_b is asserting that you can't generate lift without sending air off in another direction (i.e. downwash), and this is false. Again, most lift is generated this way - my posts here seem to be mistaken for arguments that Bernoulli force contributes significantly to the lift. It does not. However, the work done to reconfigure the air surrounding the wing by the Bernoulli principle will contribute to the lift, and while it does result in moving air around (not necessarily down though, actually), it does not impart any net motion to the air after the wing has passed.
Yes, both ori_b and I are asserting that you can't generate lift without sending air off in another direction. If you don't impart a net motion to the air, then by conservation of momentum you also can't generate a net lift.