Hydrofoil evolution challenge #2: surface piercing foil

[lobodomar]

C foil inspiration that just flies fine.
Let's imagine what follows with a kite, the catamaran becoming a catafoil -
Not easy to figure out with the lateral traction of a kite. => Don Montague next challenge.
A tripod maybe would suit KBHF better (2 C foils + 1 T )

Lateral traction (and its consequences to lateral dynamic stability at high speeds), that’s the crux of the matter (for both piercing and retractable foils). Sailboats have access to other ways of providing lateral resistance without compromising dynamic stability at high speeds. But for KBHFs, mmmm...

The "golf cart" with wings has pretty much signaled the end of this thread.

That and the fact that the OP, despite kiting since at least 2005, aims to design a piercing KBHF without even knowing how to trim a kite:

To illustrate, riding really fast on a current generation foil board is like flying a kite that's way too big for the wind speed. In both cases you can compensate by reducing the AoA (depowering), but that results in a drastically reduced L/D ratio, which limits performance...

But that's not right, at least for kites, reducing AoA INCREASES L/D. The "further forward" you can the the kite the better you go upwind/ the better your L/D is. You can see this on the beach by flying a kite overhead and sheeting in and our...

If sheeting your kite out causes it to fly further forward, then it was probably oversheeted to begin with, in which case the lines should be adjusted. Ideally, when your kite bar is pulled all the way to the stop (without any depower) it should be generating maximum lift and L/D ratio, just short of aerodynamic stall.
Sheeting out from that point will generally cause lift to decrease at a faster rate than drag, thereby decreasing the L/D ratio and causing the kite to fly further downwind. A kite that's "way too big for the wind speed", as per my example, will fly at a significant downwind angle when depowered for survival.
You can probably find several useful threads on the main kiteboarding forum if you search "kite trim", "back stalling", etc.

By the way, what makes a kiteboard go downwind is not necessarily a bad L/D ratio of the kite (of course a low L/D ratio does not help holding ground), but simply the sheer size of the aerodynamic force component in the downwind direction, which can no longer be counterbalanced by the hydrodynamic resistance in the same direction, even at the kite’s optimum L/D ratio (most properly designed control bar “depower travels” do not allow AoAs lower than that of max L/D ratio to be reached in steady flying anyway – strutless inflatables being the most evident exception to this general rule).

While understanding this concept may not be of primary importance to designing a KBHF, not knowing how to trim a kite and having such a distorted notion of the effects of AoA on the L/D ratio should raise some red flags as to where this thread could ever arrive (besides the inherent difficulties/impossibilities of designing a piercing/retractable KBHF).

[JS]

Haven't we discussed this stuff before JS? with chemosavi and the like, about 6-7 years ago?

Yes, I think so, around the time the DDWFTW concept was being discussed (which most comments dismissed, but which now exists both physically and in the Guinness Records). The context of our foil board discussion was a bit different then: the status quo then was a twin tip board; now it's a sit-down-hydrofoil-derived foil board. As I see it, the conversation has evolved from foil board feasibility to foil board optimization, analogous to the evolution of sailboards from the original one-design Windsurfer to today's designs. They share the same fundamental configuration, but the performance is immensely improved.

In 2007 I built a fore+aft bidirectional fully ventilated foilboard using aluminium plates. I ground down the topside leading and trailing edges to get knife edge almost. Dangerous, but it was a technical demonstration for my own curiosity. The idea was to force flow separation and full ventilation on the entire topside from the get go. Peak L/D is obviously compromised compared to conventional foils, but the low Cl regime is improved of course. Fundamentally this is what a kiteboard already is, as you have already stated. I just reconfigured the concept from one very low aspect board to two moderate aspect plates.
I could have tried a bit of camber but that would have compromised the bidirectional capability by reducing the practical range of operating AoA considering that incidence is fixed for each foil wit respect to the board. I don't think it was really necessary.

When riding on tow I was able to achieve unprecedented drag angles off to the side of the boat, literally tens of degrees better than any wakeboard or waterski or even a standard foilboard. Especially at higher speeds(25knots+). So far so good , as this is what my father and I predicted from our calculations.

In my opinion, your post is more valuable than everything else in this thread combined, and exactly the kind of insight I was hoping to encourage.

The problem came when kiting in fun conditions. Once the wind is up to 20kt and above the surface conditions become "dynamic" shall we say! Control and comfort became a real problem with the surface piercing foils in these conditions. Tripping was very common. So i gave up on bidirectional capability and adjusted angle of incidence to the ideal 6 degrees for both foils going one way. This helped quite a lot obviously, but the fun was lost completely i found.

Before adjusting the angle of incidence to go just "one way", did you try adjusting the angle of incidence between the two foils: "toe up" so the front foil rides angled up a few degrees and the rear foil angled down (so it's the same going both ways)? And if so, what angle seemed to work best?

Driver commented he had to do everything possible to stop me pulling the stern around! The lift angle here is around 60-70deg to flow direction. I did better though when I used greater heel angle, but then the loads went right up and I couldn't sustain it..

That picture is great. Worth a thousand words. Thanks for posting

Best regards,
James

[Aummm]

By the way, what makes a kiteboard go downwind is not necessarily a bad L/D ratio of the kite (of course a low L/D ratio does not help holding ground), but simply the sheer size of the aerodynamic force component in the downwind direction, which can no longer be counterbalanced by the hydrodynamic resistance in the same direction, even at the kite’s optimum L/D ratio (most properly designed control bar “depower travels” do not allow AoAs lower than that of max L/D ratio to be reached in steady flying anyway – strutless inflatables being the most evident exception to this general rule).

obodomar, good explanation So would you shed more light and explain why the Strutless inflatables
are exception to this general rule from your perspective,
since In KF you see a lot of bias toward Strutless idea and Why you see all kind of nonsense physics of theory

[lobodomar]

obodomar, good explanation So would you shed more light and explain why the Strutless inflatables
are exception to this general rule from your perspective,
since In KF you see a lot of bias toward Strutless idea and Why you see all kind of nonsense physics of theory

Hi Aumm, that’s because, due to canopy “luffing”, their L/D ratios start to deteriorate at considerably higher AoAs (or, in simpler terms, much "sooner" as the kite is "depowered").
This is definitely bad for competitive racers for example, but great for DTL waveriders who like (or need, depending on the wind/swell directions) a kite that drifts a lot (that's why the Cloud's control system for example actually allow, by design, very deteriorated L/D ratios to be reached).
So I wouldn’t say it’s a matter of good/bad gear, but of using the right gear for the right discipline/conditions/style, and also taking into consideration all other design variables. But, in general terms, yes, the strutless tubes are kind of “niche” kites IMO.

[RalfsB]

I have been thinking about the design of a surface piercing foil for kiteboarding, and there are few aspects. We want it to be self-stabilizing and simple. This is nothing new in the industry, there are plenty of time-proven designs for foiling motor boats, the most typical being variations of U / V shapes. They work fine for motor boats but kiting is different; we need to edge against kite's pull (unless we want to go strictly downwind), so the U / V designs do not seem to be a good ideas since their stabilizing effect goes against edging. So some kind of W or inverse V shape seems to be more appropriate. What I would like to try is a 'coat-hanger' shape (image below), but I have not had time to build it yet.

[Bille]

...

This thread (concept #2): SURFACE PIERCING FOIL

As speed increases, incrementally less foil would remain submerged, and all or part of it might transition to a ventilating condition, submerged on the lower surface only.
...

This would be a Great place to start research on your Thread ; l'hydroptère had
one burst of speed to 56.3 knots ( 64.8 mph) using conventional sails and a surface
piercing foil.
https://www.google.com/?gws_rd=ssl#q=hy ... e+trimaran

After 9 pages on this subject , i personally think it's time to actually talk about
the subject that the OP had requested ?

I researched this tri in detail ; if anyone wants to talk about how the ventilation on
this machine , was kept under control , or what the expected results would be if a
more efficient power-source might be , (like a kite or rigid wing) ; then lets have a
discussion about it !!


Bille

[alexrider]

Interesting idea indeed.

Although a newcomer to HFing learning with a large area wing, I was thinking about a possible solutions to the desired requirement to reduce the wetted area of foils as speed increases (and incidentally make the ventilation induced stall less abrupt). The first one that came to mind was a biplane or multi-plane (multi-wings?) equivalent, the upper wing(s) getting out of the water as speed increases. Thechnically, it wouldn't be surfing piercing foils, since only the bottom one will stay permanently submerged. A drawing is not necessary to illustrate the principle.

Where is the other thread, btw?

Foil boards are in their relative infancy with respect to potential performance.

By example: if a hydrofoil that can carry its rider efficiently at 8 knots, then it could theoretically carry nine times as much weight at 24 knots, which means it's about nine times bigger than necessary when traveling at that higher speed (or 16 times at 32 knots). That wasted potential lift comes with immense parasitic drag, which is a fundamental impediment to hydrofoils reaching extremely high speeds.

I can think of two obvious design concepts to overcome this speed-limiting handicap. I'm starting two separate threads, to solicit and share ideas on each of the two concepts. I expect at least one of them to develop into a much faster, more versatile future generation of hydrofoil. So...

This thread (concept #2): SURFACE PIERCING FOIL

As speed increases, incrementally less foil would remain submerged, and all or part of it might transition to a ventilating condition, submerged on the lower surface only. This might be akin to classical v-foils that have been around for decades, which have been primarily used only at the front of hydrofoil craft to modulate altitude.