came across this site ,airfoiltools.com/airfoil/details?airfoil=e325-il enables you to design your own foil, i came across a similar foil(Lessacher carbon assy i think)if it wasn't one of his i'm starting too understand his double concaves, has anyone tried a similar foil
"edit" have to get firie bob to sort it out (404 error)

You mean this weird looking thing?



Must admit I don't understand this foil at all, my understanding says the concave should be on the opposite side.
Can anybody explain?

that's it phil in the gustbusters has a foil the same , it looks like a concave at the base at the leading edge, and for 1/3 then it has a concave at the trailing edge 1/3 from the tip , he hasn't had any great wind to test it but its very interesting, theres two concaves , the bottom on one side then from the tip on the other, it is weird

decrepit on your diagram there's a concave from 0 to 0.2

Not my diagram keef, I just googled, copy and pasted it. I know nothing about it.

But it looks way too blunt to be much good as a speed fin.

im sure its wolfgangs assy weed or maybe someone out there can explain, im starting to get the idea with the concaves but im sure you need carbon for the weedies for 0 flex, ive got a couple of fins done with the concave either side, e,g one at the base trailing edge and the other side the tip , as wolfgang said it needs to be like a shovel , ive got from now till January to sort it out

I am pretty sure most eppler airfoils are designed for aircraft. That one definitely looks like its an aircraft foil to me with what they call reflex in the aircraft game, designed for a flying wing without tail surfaces.

I may be wrong though.

No one has yet explained the theory behind concaves that makes sense to me. Phill's lessacher fin has concaves on both sides so would cancel each other out (and cause more turbulence and drag??). An assy fin with a concave on only one side would maybe have advantage on one tack but wouldn't really work on the opposite tack.

On a sym fin.....The concaves are not symmetrical, ie they are staggered so as not to cancel each other out.

to quote from http://www.cameraid.com/lessacher/tech.html:
"A concave surface on the leeward side keeps the flow relatively straight and keeps the high pressure from moving. This provides the best possible circumstances for one direction. Concaves on directly opposite sides of a fin do not work, but when transferred diagonally from side to side, everything is fine. The concave reduces the low pressure and as a result the fin causes less drag."



Don't know about assy fin though.
My head is starting to hurt

Lessacher 22 doubleassy has been to 50 knots. I have used Daffys 22 doubleassy to 38 knots. It has an unusual attribute in that it hardens up at 2 distinct angles of attack. At 36 knots under light foot pressure it seems efficient & low drag then when overloaded it lets go then hardens up strongly at a greater angle of attack with noticible increased drag. This gives a good feeling of confidence. It can also track back upwind on the second step fairly well. I think Daffy set his world mark 1 hour on this fin.

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Wineman said..






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I have tried these concaves, what I found is you can get a really fine trailing edge with less flex , if you concaved both sides the same you could sand through to the other side, so far the fin looks good just need some wind for a test run
REDhat is sailquicks fin the same as in the pic with the opposing concaves and cutout, the foil i'm referring to was like the pic below with a concave at the leading edge at the base

Mike the Eppler 325 foil is a flying wing profile. That is , as mentioned by Mark, designed for tailless planes where the reflex at the trailing edge is used to counteract the normal rotational moment (nose down) that cambered foils develop. It is a type of profile that was experimented a lot in the 1940's (Northrop) but it's main use is hang gliders.


However it may be making a comeback with Stealth tailless UAV.



It looks like it would be crap for fins in water.

The concave at the base should be bigger (60%) The concave from the middle to the tip (40%) That are cm?. At the board bottum have you
air and waves. So the concave have to be bigger.They are always behind the thickest point of profil. Only diagonal. Vis a vis dont work.20 years ago
worked I a board with Acryl bottum,80cm long. There could I see all it under water.All problems comes from the nose. The half concave leewards
make,that the water cannot go behind the fin to the other side. Other fins do that in an angle of good 30?. Thats a big brake. The other concave, for the way back, reduce the big brake. I thought that she should work only at the way back, but she reduce a little of vacuum. All asymm.fins are
leewards flatter than on the other side,but th other side is the vacuum side with the normal profil. A big brake. But nobody want to know that.
The nose is leewards sharper than on the other side,where she is thicker . Leewards is the profil good 2-3mm thinner. Wolfgang

The concave makes that the water cannot go behind the fin to the other side. That makes without concave sometimes trouble,so the water cannot go from the nose along the profil,on the vacuum side.So can water that comes from leeward go along the profil on the vacuum side to the nose and
the rotation round the nose to leeward start. Spin out. With concaves ,no spin out. Wolfgang