This is a head spin. I came across KF foils late last night having never heard of them before and wondered exactlt the same thing. !!! Spooky. !!!
I watched a few videos on RC model sites which seems to be the only application that has endured . The most common feedback seems to be high lift, very high stall angle of attack but very low lift to drag ratio. The low lift to drag is apparently why they haven't been used commercially.
There are some very cool wind tunnel testing videos around of the 3 or 4 common KF foils used in RC planes. The suggestion is that the high stall angle is because the step causes a vortex to get trapped which keeps pulling air down onto the back of the wing so flow separation happens only at much higher angles of attack. It got me wondering if you could get the same affect but putting a vortex generator on a normal wing ( like you see on some sports cars too) this does the same thing - creates some turbulences that pulls air down onto the trailing part of the wing. ? ???
The RC guys seem to find it appealing because the ease of building it. A flat sheet with an second, leading edge piece glued on (so no grinding to remove material).
Not a good candidate for speed because of the drag ( although the KF 4 is a symmetrical version used in acrobatics rc planes ) but wonder if surf foiling might be a good application ? Spees less important and wing stalling on the waves because of everything going on 50cm inside the wave is the biggest issue I'm having with my diy foil in the surf.
I think if content with speed of ~20kts or so you can save wing shaping time by simply using a flat or nearly flat bottom surface (main wing). Has been done reportedly many times with good results.
The KF foil as described in the video is for compressible flow, probably the key to it working is that the air that trips into the hollow part expands to whatever volume it "needs" to allowing the wing to act like it is fully foiled. Water can't do this so air will be sucked down onto the wing, making more drag, which does sound inefficient, but maybe results would be ok.
That said, it still would be easy to save a lot of time by only foiling the low pressure side of your wings, if you are not concerned with super high efficiency and top end.
A few stepped hyrofoils have been developed but not ever widely adapted to my knowledge, they look more or less like this (my sketch, from memory....):
steppedfoil.png (3.56 KiB) Viewed 1159 times
This one was for a foiling dinghy, the goal was to avoid crashing i believe, idea being it might be smoother to have a couple small ventilation areas instead of one huge one, so if the wing surfaced, it would not crash completely at once. Don't think it worked too well, not sure if merely too slow or just didn't work.
I am getting quite keen to have a go at making one and trying it out. I've got 3 weeks off work coming up and I've added this to my summer project list. I'm part way through making a profiled wing with roughly the same outline and dimensions so that will be useful for comparing them.
From what I understand from studying at the University of Google the point BWD made about the water being pulled down on the trailing part of the wing is actually one of the reasons that these wings have very high stall angles. As the fluid passes over the step the sudden drop in pressure causes vortexes to form which flow down on to the wing. Flow separation from the wing after the thickest part of the profile as the angle of attack increases is the main reason a wing stalls. Vortex generators on aircraft and sports cars do the same thing only they cause vortexes that runs chordwise instead of spanwise.
Whether the fluid is air or water you still drag along a pocket of close to stationary fluid so drag will always be an issue (however if this caps the speed at 20 knots I'm good with that cause I crap myself at any more than that ). Another issue is whether or not a high stall angle of attack is a benefit to hydrofoils. In flat water conditions I'd guess the answer is no because the angle of attack is always pretty shallow otherwise you're launching it or nose diving it. However, in the surf I reckon its a different story with all the turbulent flow and rotation and movement of water inside a wave the effective angle of attack could be much larger. Also, I think that in a wave the relative speed of water over the wing could be quite slow at some points ( I always seem to have the foil stall halfway down the face of a wave). The vortex generator effect at low speeds might help reduce the speed the wing stalls - maybe.
Anyhow, the prospect of quicker build for wings and a chance to experiment is enough to get me interested in this and some of the other flavours of KF profiles.