Originally Posted by HN39
Not exactly. Shevell's depends on the angle of attack.
Good point! I'd missed that.
The aerodynamic significance of the crest, according to Shevell, is debated: "largely an empirical matter". One idea is that, in the transonic regime, supersonic flow forward of the crest does not contribute much to drag, whereas supersonic flow behind it would contribute a lot to drag, because of the "resulting low pressures (suction) on the aft-facing surface". Another possible explanation is that the crest represents the minimum distance between airfoil upper surface and (undisturbed) free stream, and if Mach > 1 at this point the "flow will accelerate in the diverging channel behind the crest, [leading] to a high supersonic velocity, a strong suction and a strong shock".
This suggests that the AoA-dependent definition of "crest" is aerodynamically significant in the transonic regime. Whereas I don't see at present that my second definition has much aerodynamic meaning at all - but I am willing to be educated!
PBL