Questions about the reasons aircrafts fly are frequent among scientist. Since the time I was in high school, even if I now work on the other side of the fluid world (Low $Re$ regime), I've kept asking my professors, advisors, colleagues, what was their own explanation of flight.
I know about the controversy about the push downward, considered a common fallacy by the NASA website, and about the Anderson argument, denying the erroneous principles of equal times, and the overstimated role of the Bernoulli theorem. My best overall and simplest explanation is taken from Anderson, and consists in the following:
Somehow the air reaching the first edge of the wing, after the interaction with it, is going donward. This must be the result of some kind of force, and therefore, for the third Newton's Law, there must be an opposite force of equal strenght in the opposite direction, which pushes the aircraft up.
First: why does the air go down? Answer: the angle of attack and shape of the airfoil, together with simple pressure and stagnation arguments.
Second: which is the role of the Bernoulli theorem here? If the air is pushed down by means of the "geometry", we don't need the difference of velocity between the upper and the lower part of the wing, but we have this just as a consequence of the change of pressure (due to the shape). Is that right?
My second question, actually, is about the most common and sophisticated explanation: the Starting Vortex Balance.
The main argument is: due to the Kutta condition (a body with a sharp trailing edge which is moving through a fluid will create about itself a circulation of sufficient strength to hold the rear stagnation point at the trailing edge) the vorticity "injected"by means of the viscous diffusion by the boundary layer generated near the airfoil, to the surrounding flow, transforms in a continuum of mini-starting-vortexes. This leaves the airfoil, and remains (nearly) stationary in the flow.It rapidly decays through the action of viscosity.
By means of the Kelvin Theorem, which in the 2D case is nothing but the statement that the vorticity is constant along every particle's path, this vorticity must be balanced by the formation of an equal but opposite "bound vortex" around the airfoil. This vortex, being causes an higher velocity on the top of the wing, and a lower velocity under it, causing the arising of the lift by means of the Bernoulli phenomenon.
Now, I wonder:
1) We are assuming that the vorticity leaving the wall (the no-slip condition make the airfoil wall a sheet of infinite vorticity) by diffusion, leaves the boundary layer and enters in the region where the Reynolds number is high enough to allow us to apply the Euler Equation, then the Kelvin Theorem (valid only for inviscid fluids). I usually explain this using the vorticity equation, that is a local version (in 2D) of the Kelvin Theorem: $$\partial_t\omega+\boldsymbol u \cdot \nabla \omega=\nu \nabla^2 \omega$$
in the boundary layer the viscosity terme dominates, whereas in the outer layer it can be neglected. When the vorticity arrives in the outer layer, vorticity is conserved and we can say that the structures arriving from the boundary layer must be balanced (in terms of vorticity) by the fluid in this region. And we can do this only because the circulation, which is the actual constant, is a line-integral, and if we don't cross the airfoil/BL region we don't have problem of any sort. Is this correct?
2) In this explanation the Bernoulli theorem seems to be a cause, generating the lift through the difference of velocity. Is this right?
Thanks in advance. I feel always a kid, asking about that issue. And at the same time, I feel an actual ignorant but curious scientist.
Best Answer
My apologies, I won't be reading your entire question.
But still I will provide an answer. Why is that? Because flight does not require any of the things you talk about.
You could build an airplane that would fly with no "airfoil" shapes. You could build an airplane that would fly with completely flat rectangular wings made out of plywood. The important thing would be the angle of attack of the wings to the air. Consider a flat piece of wood, like plywood. Push it through the air in a direction exactly parallel to its flat dimensions and it develops no lift. Tilt the wood so the leading edge is "up" compared to the direction it is moving and you can feel the lift.
The lift can be thought of a few ways. Think of the air molecules hitting the surface of the wood. They bounce off, in a downward direction. Well if we are pushing air downward, we must have an equal and opposite force, which is the lift. Or another way: we are gathering air under the board, it gets a little pressurized. The pressure is pushing up on the wood. This is really the same picture as the first if you think about it.
All the rest with airfoils and so on, all this has to do with developing lift efficiently, developing lift while minimizing drag. An airplane with flat plywood wings would fly, but it would have a lot of drag and would therefore be very inefficient.