Quote by Brian_C
How can that equation be valid at such large angles? I would expect flow separation to come into play long before you reach 90 degrees of deflection.

I can only comment that McCormick have plots of measured and theoretical values up to 90 degrees with no (visible) discontinuities and that this approximation is for the drag coefficient, not the lift which would be a different matter. That said, I too would be surprised if the approximation were to be just as accurate at 90 degree as at, say, 10, but that is just a hunch. And who would want to allow flap deflection over, say, 40 degrees anyway.