Airfoil selection for a foiler ?
Being a total novice and having only just started to play with XFLR5, could someone tell me why foilers use sections like the 64 412 as on the Fastacraft Moths when modern glider sections (shown below) appear to be better ? or am I totally missing something ?
I have played around with optimising a foil for a boat but cannot specifically answer your question without doing more analysis.
I would do the comparison at some realistic design speed. From there, there are interacting parameters that you need to iterate around. These are:
Foil Width - will alter area, AR and bending load.
Depth of operation - unlike air foils you have to contend with wave drag for near surface foils. Placing it deeper reduces wave drag but the suppoting strut is longer so greater drag. There are also near surface affects that reduce lift.
Chord length - will alter Re, plan area, AR and section modulus.
X-section shape - has to be able to carry the load while giving good L/D for the design condition and likely operating range.
It could be that someone found a particular shape that worked well and it has become the standard section. It is unlikely to be the best for all applications. What I do know is that the water/air interface has an influence that you must take into account otherwise you get a highly optimistic result. Hence just comparing two 2D foils at a particular Re may be superficial.
You can certainly get sections with higher lift to drag at these Re such as MA409 or just fiddling with modified NACA 4-series with small noses and high camber. They might have narrower operating AoA and lower sectional modulus but it is a matter of how it all comes together as you iterate and what can actually be built.
Have you checked out the H105 section by Tom Speer-its being used by several foilers and a carbon version is available from John Ilett at Fastacraft.
The 63412 section on pages 522 and 523 of "Theory of Wing Sections" was Ilett's first foil.
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