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K^2
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Bernoulli Equation doesn't explain lift at all. It can be used as a good estimate, under certain conditions, but in order to get exact lift, you need pressure at the actual boundary. And at the actual boundary the flow velocity is precisely zero and Bernoulli Effect is also zero.boneh3ad said:Bernoulli's principle is one way to calculate the lift on a wing in certain situations (the wing cannot be separated, for example). Given a velocity distribution over a wing, you can use Bernoulli's equation to deduce the pressures on the wing and hence the lift. It says nothing about how you find sai velocities or the best shape of a wing. Bernoulli's equation is merely a tool; it cannot explain lift completely.
The only reason Bernoulli Equation works to estimate lift is because viscosity of the air is low, and if you are working in regime where air can be treated as incompressible, the pressure gradient near the boundary is low. So you can take air flow near the boundary, and use Bernouli Equation there to say that pressure at the boundary will be the same. At mach number << 1, you will be correct.
There is one more problem with Bernoulli in that it is difficult to implement near the critical angle of attack. As the separation layer creeps up the wing, relevance of Bernoulli Effect to pressure at the boundary decreases further. So you really want a different method of estimating lift, and that's where Kutta Condition and Kutta-Joukowski Theorem come up, as you point out.
But ultimately, it's the pressure differential on the wing surface that's the direct cause of the lift, and you cannot apply Bernoulli Principle at the boundary. You can use it as a motivation to look for an effect, but if you want to actually explain it, you can't do much better than, "Boundary conditions of the problem." Because that's the real reason for the pressure differential.
Please, explain flight of a glider, which generates no thrust.Lunar-Scooter said:Everything.russ_watters said:What does the thrust have to do with the lift?