Luc, I've been flat out developing the foil software. I wanted to do that before I got into building. Anyway, I've got some more results. But firstly, I'll complete the point re 50 knots.
The graph on this web page explains the problem: Hydroprops
Conventional foils (particularly those with camber) are hard pressed to operate at a Cp min of greater than 0.5 at any angle (positive is downward in this graph). The challenge is to design foils that have a sufficiently low Cp min to operate in the desired speed range at the necessary angles of attack. Thanks to Rick for emphasising this point. Note that the AC72's seem to reach 40+ knots without much trouble but can go no further. Whether foils are the limiting factor I'm not sure but basically you have to shift to wedge foils of some description at this point, whether super-ventilated (like C-Fly) or base vented.
Results are attached. Request attachments from me if you don't receive them. In order to reduce trailing vortex issues, the Y30 foil (red in Lifting Foil Sections.jpg) has a much finer entry than the other candidate foils. Referring to Lifting Foil 4M.jpg, the Y30 (yellow) has better L/D ratios at higher Cl (main graph) and at low Cp min (bottom left). All foils have a similarly low Cp min but the Y30 has a wider angle of attack than the P30210 (bottom right). I have not designed this foil to have low drag at zero lift as it is to be raised when not required.
I have a claim that needs testing: the increase in Cp min that occurs when the attack angle reduces toward -1 degrees occurs on the underside of the foil, thereby disturbing suction on the lower surface. My claim is this will reduce the downward suction whilst not affecting the remaining lift generated at the top of the foil - increasing lift and allowing the foil to operate at lower angles of attack. Or at least the foil is not detrimentally affected by the cavitation on the underside, particularly given that the flatter sections behind the leading edge may enable flow to recover.
Posted by: cruisingfoiler@yahoo.com.au
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