That Raskin page drives me insane. It is the most over-referenced, inane piece of "aerodynamic literature" from a pretentious poseur that I have seen on the Internet. First, check out his curriculum vitae. With the exception of being a partner in a model airplane company for five years, he has no training or experience in aerodynamics. A B.S. in math is enough to get him into trouble, and I suspect he wouldn't recognize the Navier-Stokes equations if they were hanging from his beard. Second, one would expect that from a paper entitled Coanda Effect: Understanding Why Wings Work that one would gain an understanding of the physics involved. We do not. Instead, we get these brilliant explanations.





Now, I don't know about y'all, but when I click on a link with "Understanding Why Wings Work" in the title, I kind of expect a "physical account" of an "experimental fact," not simplistic and incorrect explanations of a simple and applicable experiment. (blowing through a straw over shapes in a box) The Coanda Effect is an observed phenomena with physical underpinnings (pressure, shear stress, momentum) and if one is going to use it as an explanation of why wings generate lift, then one needs to explain why the effect works! Saying that wings lift because of Coanda is like saying that aircraft can fly faster than the speed of sound because of the sonic boom. It's an illogical cause and effect.

Nick, in the other thread and in your first summary point above, you seem to imply that the the pressure field around a wing is not what keeps it in the air. That is, integrating the pressure at each point over the wing panel does not keep the airplane in the air. Am I understanding your statement correctly?

The reason I ask is because that 2% number is another bit of "I read it somewhere" Raskin gibberish that gets tossed around as fact far too often. He determines the pressure differential between the upper and lower wing surfaces in an entirely erroneous manner (by using upper/lower surface length differentials and by imposing a pseudo-Kutta condition that isn't real) in order to show that Bernoulli is erroneous for calculating wing lift. I agree with Raskin in that Bernoulli is not directly applicable for calculating wing lift, but this whole 2% thing appears to lead people to discard not only Bernoulli, but everything else pressure related along with it, including the pressure field around a wing as the physical representation of lift acting on the wing.

The real reason why Bernoulli is not directly applicable (I'll get back to the directly part in a bit) is because the Bernoulli Equation is only applicable after certain simplifying conditions are met. These are,

1) Steady flow - Flow that does does not change with time. That is, flow at a particular point that does not change in speed or direction. Separated and turbulent flows are unsteady.
2) Incompressible flow - Fluid does not change density. This only arises in high-speed aerodynamics, not catamarans, general aviation aircraft or tabletop experiments.
3) Frictionless flow - No viscosity. This means that Bernoulli doesn't work with boundary layers.
4) Flow must be along a streamline A streamline is the familiar smoke trail that we see in car ads when they put the sporty car in a wind tunnel. It represents the path of a particle of fluid past the object.

As you can probably guess, the reason Bernoulli cannot be used to calculate wing lift as Raskin attempted to do is not because it drastically under-calculates the lift needed and therefore must be wrong, it is because it violates restriction number four. The upper and lower sides of the airfoil are not on the same streamline. This appears to be intuitively obvious, but instead of Raskin discarding Bernoulli for that simple reason alone, he goes on and on, confusing the issue with an erroneous calculation based on an erroneous assumption, the length difference. It's ok to simply say that a particular equation does not apply because the underlying assumptions for that equation are not met.

So where is Bernoulli applicable around a wing? Wouter gets into it a bit in the other thread, but it's when you follow a streamline and avoid areas where viscosity is a factor in determining the flow. That is, avoid the boundary layer and separated and turbulent flows. If you went into a wind tunnel with a wing, inserted a smoke trail that passed over the wing (again, staying out of the boundary layer and separated flow regions) and took pressure and velocity readings along the smoke trail, you would find that your measurements would be valid in Bernoulli.

In summary, Coanda explains nothing, 2% is good for milk and Raskin needs his site hacked.