kitexpert wrote: ↑
Mon Apr 05, 2021 12:44 pm
IDK if you have much or at all experience with LEI's but they are easier because they sit deeper in WW. Single skin structure allows more grunt, back stalling later (or not at all) and recovering it faster. Small foil kite needs much more kite skill, you have to be able to feel what kite wants to keep it flying and at the correct part of WW and producing force. With slow high drag board kiting can become jerky, i mean controlling pull can be a challenge.
That´s exactly what I learnt and grew up with in the kite buggy as a child, to steer a rather small non deowerable handle-foilkite where it needs to be to control the power so I actually love that feeling of a foil on the water with TT. But yes you´re right if the waves get high and breaking like in the north sea it gets a bit complicated with small foils on TT as you need quite a lot of power on/off going through the wave above into the next one. Also the relaunch in breaking waves get´s an issue, so thats when I use my tubes. And yes they are easier and above 25/28 knots I´m rather on a tube. But thats another discussion in other topics tube vs small foil.
Here on the (most of the time) rather flat baltic sea I love getting up to speed with one/two power dives and have the pull so much forward still with the foilkite, being able to scream upwind when I want. Ultimately I designed the 6m for HF use but my riding skills aren´t progressing as fast as my sewing apparently
(Otherwise there wouldn´t be a point in making it so light)
To go back to the design thread and also what Rein is experiencing:
We should discuss why that deep in the wind window physically is working so well with tube kites (on TT) and why a foilkite struggles a bit more in that respect as there might be some nice learnings to have foils that have a lot of grunt when powered with the bar but nice stability and high L/D when depowered so here´s my aspects:
-foil kites have much less camber in order to be stable in pitch motion (or not to frontstall in kite therms). They fly further to the edge of the WW (less AoA) and therefore are more prone to get negative or zero A0A and flip over if having too much camber when being depowered. That is just because drag is missing compared to a tube. So one needs to find a way (for freeride TT, slow HF, Waveboard, a race guy would call me stupid) to add drag but add a benefit to it as well (more grunt).
-lift is related to drag so adding AoA (sheeting in) brings the kite further back in WW but then with the average foil we experience earlier backstall as with tube kites. Why is that? My thoughts:
-we induce less camber with foil kites than with SS or Tubes when sheeting in... stall occurs earlier. (attachment points with larger distance from Z to C than from C to B and so on can help adding camber when sheeting in as well as certain mixer settings)
-due to billow we will never get such a smooth LE and uppersail panel like a tube, so also because of that we will always have earlier stall in the slow flying/high AoA domain with a foil as long as that issue isn´t solved. (Double membrane foilkite with smooth upper sail sewed onto the billowed skin, anyone?)
-Profile thickness and AR are two main drivers of drag. If on the very thin/high AR end thats where Rein is at atm. It´s nice though when being up to speed on a HF as you will always have a really far forward flying wing, getting going might be complicated getting up to speed with a race HF wing setup and 5m². But the main reason for Rein was a small and not so expensive/long to build test platform before moving onto a bigger kite project as far as I remember.
The thickness thing is probably one of the main aspects making the Soul so easy to use and accesable for beginners as its thick as, so it sits a bit deeper in WW among other "benefits" (for freeride) a thick foil has.
I´m quite sure and Rein and me discussed a lot in private mails that there is an important relation of profile thickness and flying speed/RE number what a kite has to match for a certain range of use/purpose in order to not fly out of the WW and stay there with not much pull until getting up to speed for slow applications without much apparent wind/downwind angles.
Also interesting to discuss a bit more the Tip collapse.
We´re both using quite agressive aerodynamic washout meaning that when I deveoped the tip profile I added a lot and more than usual reflex in the tips. (High pitch stability in tip). I wanted to get tip collapses off the list, and for my designs it never once occured, although even running same AoA over the whole wing (no geometric washout) as most have to increase AoA in the tips to keep the tips out.
So the biggest difference is the higher AR of Rein.
I also run 10% attachment on A (relatively far aft -> high stability again because counteracting bridle force gets closer to aerodynamic force and therefore not having such a big pitching moment as the lever of the moment is reduced. Don´t remember your( Rein´s) value. I run very stiffly sewed in LE battens in order for that to work and support the forward part of the LE, also distributing the loads of A-risers better
Another aspect could be sewing/billow related differences as it´s quite hard to sew those small tip cells together and small deviations have big effects due to size effects for high cell count, high AR, small wings.
Inflation pressure during flight due to vent layout another point to look for maybe
What we can (can we?) write off the list for tip collapse reasons is bridle layout, because the tips come in because they frontstall, and the bridle cant do much about it because it can only pull in the "wrong" direction for that matter. What the bridle can do though is induce higher AoA or reflex in the tips to resolve this issue.
It should be checked for safety before changing something else though that the bridle is close to "factory" settings after being loaded the first time to ensure that the profile is kept as it´s designed for in the tips and doesn´t have too much camber as B and C might be out of tune for some reason. Small devitions in the tip bridles have big effects again for the short chord.
Another point I could think of (if I remember correctly) is that in the tips your B lines gets the A lines as there is only 3 attachment points in the tips. If you depower B could be released so much that the tips become unsupported so they have no (A- or front line to flag out on). So for a trial connecting the tip bridles on A B and C instead of B C and D could be a thing to look out for (If I recall correctly again).
Let me know what you guys think!