Aerodynamics for Naval Aviators

Silvaire wrote:

In regard to knowledge applicable to light aircraft design what has been forgotten since the late ‘50s is probably a more extensive list than what may have been learned

Very well put. Also remember that the book is meant for aviators. It’s not an engineering book. It’s not a science paper in cutting edge aerodynamics research, and it’s not a “popular science” book. Also important, it’s a rock solid reference for anyone creating courses and similar.

What is almost always missed on “popular explanations” of lift is the transient phases. What happens when a wing starts from zero velocity and all of a sudden starts to create lift when moving. Well, it doesn’t happen “all of a sudden”. This transition helps to get a mental picture of lift, and in particular circulation.

This is from Fluid Mechanics by Frank M White. One of the most used books about general fluid mechanics at universities, typically second and third year courses (lots of math is needed first).

Clearly four things are involved (at least): Fluid momentum – causing circulation due to the sharp trailing edge for which the fluid simply cannot follow around from bottom to top. Then there is viscosity and the conservation of momentum. It’s this one little detail about shedding of starting and stopping vortices that often is omitted. Conservation of momentum of a wing (or any moving object for that matter) is not fulfilled until the object has come to a full stop and all the starting vortices have been “cancelled” by stopping vortices.

That little starting vortex is what puts everything in motion regarding lift – the circulation. Whenever the lift is increased, vortices are shed, and whenever lift is decreases, stopping vortices are shed. It’s a direct cause-effect relationship with increased/decreased circulation, hence lift. You could say that lift IS circulation, and nothing else. All the “Bernoulli and Newton” arguments are simple effects caused by this circulation. This is not the same as wing tip vortices on finite wings, but related, since both are circulation.

Alpha_Floor wrote:

I am of the opinion that circulation does not explain anything. Circulation is a mathematical concept, not a physical one.
The Kutta condition doesn’t explain anything either, it is more of an empirical observation. Why does there have to be a stagnation point at the trailing edge? And why is circulation such as to move this stagnation point to the trailing edge? It’s not at all trivial.

Circulation is very much a physical concept, just like the rotation of the propeller or wheels. Just think about it for a minute. You are flying in dead still air. How does the air move when an airplane comes flying by? Well, the wing is like a flying vortex of air. We are used to looking at lift animations and videos from wind tunnels where the air is moving, not the wing. This isn’t at all what is happening. In real life, the wing is moving through the air. The air itself has zero velocity.

From a physics or mathematical point of view, it makes no difference if the air is moving, or the wing is moving. It’s the relative velocity that is important. But, I assure you, from a human cognitive perspective, it makes a huge difference. Then you will actually see of the air moves. It starts with zero velocity, the wing comes, and you will see the main movement is circulation and acceleration. I’m sure someone can dig up a YouTube video showing this.

Ted wrote:

Except the air moves well before the wing gets there.

Yes, it’s part of the circulation. It is really hard to explain in words, and must be seen. However this effect is small compared with the acceleration above the wing. This in turn is caused by the difference of sucking and blowing (see below).

Alpha_Floor wrote:

The Kutta condition doesn’t explain anything either, it is more of an empirical observation. Why does there have to be a stagnation point at the trailing edge?

It explains everything, if you let yourself be explained It’s all about fluid momentum. Fluid momentum is NOT like the momentum of a solid body. A fundamental concept of fluid momentum is sucking and blowing are two very different things. This is easy to see using a vacuum cleaner. Put a nozzle on the exhaust and it will blow at least across the room. The same nozzle on the suction end, and you have to be really close to feel the sucking flow. Yet, the momentum at both ends are the same.

In essence, the accelerated flow above the wing (blowing), will blow the initial stagnation point at the top of the wing back to the trailing edge. It will do this because aft part on top of the wing cannot suck the air from the underside to the top, and the blowing direction on the underside is parallel to the underside pointing aft and down. However, a prerequisite for this to happen is that the flow on the top side does not separate. It has to stick to the surface. This is helped by viscosity.