It's better to just remember that energy is conserved. Bernoulli's principle is a real thing and will add to the lift a wing generates without contributing to any net motion of the air upward or downward. The work done to compress the air below the wing and expand it above helps keep the plane in the air.
But, as everyone else responding to the parent has pointed out, the forces generated in this way are not nearly enough to keep the plane in the air on their own.
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.
Bernoulli's principle is a real thing and will add to the lift a wing generates without contributing to any net motion of the air upward or downward.
The compression of the air below the wing moves it downwards, more or less. There are no ways to get around Newton's laws of motion (barring relativity, and planes aren't fast enough for that).
After the wing passes, the air is no longer compressed, and the net motion of the air is unchanged from before it encountered the wing. What then?
Anyway, I'm not saying you're wrong, I'm just saying that thinking about the individual forces involved leads to more complexity than necessary (this is usually the case). It's much simpler to just consider the energy going into the system and derive the net work produced from that.
No, there would be a small amount of lift generated, in accordance with Bernoulli's principle, despite there being no net motion of the air after the wing has passed.
Uh, by the downwash generated by the wing. Obviously. Are you even reading my posts? I am not asserting that the Bernoulli force is the dominating factor, or even a significant factor, in the lift generated by a typical wing. I've made that really clear. Your original post seems to take that even further, however, in asserting that the force does not exist at all, and this is flatly wrong. Have I misread your original post? The Bernoulli force does exist on a wing, and it is in principle possible to make a wing which will generate lift solely by Bernoulli force. Such a wing would likely not be able to lift its own weight, much less carry an airplane.
Bernoulli force is just a way to look at things. It can fully explain the lift - as well as why planes can fly upside down. Alternatively, you can operate with direct Newton laws without referring to Bernoulli effect. You can mix these two explanations :) or have some other one - these are all just different aspects of the same process.
For example, from Newton one can state that the whole lifting force F_lp acting on the plane is opposite to gravity force F_gp acting on the plane, so plane flies horizontally (in the simple case): F_lp + F_gp = 0. Next, from third law of mechanics the lifting force F_lp acting on the plane is opposite to F_da force acting on air downward: F_lp = -F_da. From the second law of mechanics force F_da acting on air causes the air momentum to change: F_da = dp/dt, which means that either the plane pushes a lot of air down or the plain pushes air down really fast, just like article says.
Looking into the picture from Bernoulli perspective, we can talk about flows around the wing. If say we have a wing which is flat underneath and curvy on top, the air is initially pushed upwards until the top point of the wing is reached. From there air continues along the wing profile, which goes down, so air has a chance to accelerate - this is effectively an empty space which becomes available to the air, and air goes from points of higher pressure to points of lower pressure. The air speed increases, the pressure remains less than in (unchanged) air below, so we're getting the Bernoulli effect - lower pressure above the wing.
If the plane is upside down, it won't fly horizontally. It has to fly somewhat upwards - in which case flows on both sides of wings change, with net effect the same - the air is pushed down, pressure under the wing is higher than above, both Newton and Bernoulli explanations work.
A plane travelling in level flight at a constant speed has constant energy, though. Conservation of energy isn't going to explain how it keeps going in a straight line when gravity is otherwise trying to convert its potential energy into downward kinetic energy.
Comments
It's better to just remember that energy is conserved. Bernoulli's principle is a real thing and will add to the lift a wing generates without contributing to any net motion of the air upward or downward. The work done to compress the air below the wing and expand it above helps keep the plane in the air.
But, as everyone else responding to the parent has pointed out, the forces generated in this way are not nearly enough to keep the plane in the air on their own.
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.
The compression of the air below the wing moves it downwards, more or less. There are no ways to get around Newton's laws of motion (barring relativity, and planes aren't fast enough for that).
After the wing passes, the air is no longer compressed, and the net motion of the air is unchanged from before it encountered the wing. What then?
Anyway, I'm not saying you're wrong, I'm just saying that thinking about the individual forces involved leads to more complexity than necessary (this is usually the case). It's much simpler to just consider the energy going into the system and derive the net work produced from that.
Then, there would be no lift generated, since there is no acceleration of mass.
No, there would be a small amount of lift generated, in accordance with Bernoulli's principle, despite there being no net motion of the air after the wing has passed.
As a thought experiment to convince yourself of why this is not the case: How would flying upside down work?
Uh, by the downwash generated by the wing. Obviously. Are you even reading my posts? I am not asserting that the Bernoulli force is the dominating factor, or even a significant factor, in the lift generated by a typical wing. I've made that really clear. Your original post seems to take that even further, however, in asserting that the force does not exist at all, and this is flatly wrong. Have I misread your original post? The Bernoulli force does exist on a wing, and it is in principle possible to make a wing which will generate lift solely by Bernoulli force. Such a wing would likely not be able to lift its own weight, much less carry an airplane.
Bernoulli force is just a way to look at things. It can fully explain the lift - as well as why planes can fly upside down. Alternatively, you can operate with direct Newton laws without referring to Bernoulli effect. You can mix these two explanations :) or have some other one - these are all just different aspects of the same process.
For example, from Newton one can state that the whole lifting force F_lp acting on the plane is opposite to gravity force F_gp acting on the plane, so plane flies horizontally (in the simple case): F_lp + F_gp = 0. Next, from third law of mechanics the lifting force F_lp acting on the plane is opposite to F_da force acting on air downward: F_lp = -F_da. From the second law of mechanics force F_da acting on air causes the air momentum to change: F_da = dp/dt, which means that either the plane pushes a lot of air down or the plain pushes air down really fast, just like article says.
Looking into the picture from Bernoulli perspective, we can talk about flows around the wing. If say we have a wing which is flat underneath and curvy on top, the air is initially pushed upwards until the top point of the wing is reached. From there air continues along the wing profile, which goes down, so air has a chance to accelerate - this is effectively an empty space which becomes available to the air, and air goes from points of higher pressure to points of lower pressure. The air speed increases, the pressure remains less than in (unchanged) air below, so we're getting the Bernoulli effect - lower pressure above the wing.
If the plane is upside down, it won't fly horizontally. It has to fly somewhat upwards - in which case flows on both sides of wings change, with net effect the same - the air is pushed down, pressure under the wing is higher than above, both Newton and Bernoulli explanations work.
After the wing passes, the air is probably warmer than it was before the wing passed through, so the net motion is changed.
A plane travelling in level flight at a constant speed has constant energy, though. Conservation of energy isn't going to explain how it keeps going in a straight line when gravity is otherwise trying to convert its potential energy into downward kinetic energy.