Optimize the zenith ability and stability

[Regis-de-giens]

Hi all,
Hydrofoiling low end limits are so low that the stability of the kite at the zenith can become a showstopper , for example when you need to swim out from shore before launching the waterstart. Hence my 2 questions :
- what is the root cause of the backstall of the kite (not so obvious IMO)
- how can we increase the minimum wind for our foilkite to stay at zenith without special care nor working the kite.

My point of view :
- trim the kite
- get thin and light lines
- play with the camber of the kite : i.e. extend a couple of cm on B and C connections of the speed system (Mixer test) to reduce backstall, up to the start of kite instability (yoyo / fronstall).

Do you agree ? Any other tips ?
Has someone experimenting that flat camber could on the contrary improve this "zenith ability" ?

[alexrider]

Hi Regis,
The showstopper I initially experienced when hydrofoiling in marginal winds with my FS Sonic FR and even Speed5 was not backstalling (which I can prevent by pulling depower), but tip collapse, ending sometimes with the kite ending in the water. The remedy (only partial with the Sonic FR): shortening B and C (about 2 cm on B), making the profile flatter (not extending as you suggest). The Sonic2 on the other hand is so stable even in 5 knots, that the mod isn't required. If the kite falls out the sky (backstalls), there's just not enough wind to ride.
Rather than light lines, get light kite.

Hi all,
Hydrofoiling low end limits are so low that the stability of the kite at the zenith can become a showstopper , for example when you need to swim out from shore before launching the waterstart. Hence my 2 questions :
- what is the root cause of the backstall of the kite (not so obvious IMO)
- how can we increase the minimum wind for our foilkite to stay at zenith without special care nor working the kite.

My point of view :
- trim the kite
- get thin and light lines
- play with the camber of the kite : i.e. extend a couple of cm on B and C connections of the speed system (Mixer test) to reduce backstall, up to the start of kite instability (yoyo / fronstall).

Do you agree ? Any other tips ?
Has someone experimenting that flat camber could on the contrary improve this "zenith ability" ?

[socommk23]

No real remedy for this. A kite will suffer stalling and instability in low winds simply because it has too low an airspeed.
You can not increase a kites minimum wind speed.
Its all about lift/drag/weight and forward thrust created by the wind. If the wind is too low to produce enought lift and forward thrust then the weight is all that is left and so gravity will bring it back down.
The lighter the kite the lower wind itll fly efficiently in.

[Regis-de-giens]

Regarding the Sonic you are right I suppose ; but in my case I have already optimized the kite weight which is under most (if not all) foilkites and far lighter than speed 5 and sonic 1. So I cannot do much more on that , except using lighter lines, which pushes the limit a bit further (say approx 200 gr which is however approximately the difference between Sonic 1 and 2 ...)

Currently I have no tip collapse, even fully depowered , thanks to a moderate Aspect Ratio. I even fully trim the kite during zenith phases . So ! thanks to my light weight I can start riding when kite hangs "properly" in the air (i.e. with less wind than necessary to maintain a Speed5 in the air even FULLY depowered, I did the back-to-back test).

Side-shore wind would be less a problem, but with my onshore configuration, the main difficulty is keeping the kite in the air the time for me to get away from shore. Body drag is not possible in such light wind, so I need to let go of the bar and swim out rapidly with both hands (on the back), just correcting the kite azimuth now and then to keep it at 180 degrees. This tricky situation last about 1-2 minutes, which increases the risk of a lull, so if I can save some wind for kite hanging in the air I think, I can still ride (with a double loop for waterstart , 30 m lines and big hydrofoil wing). Problem is the same after a fall in the water , the time I recover the Hydrofoil. I have sometime to pull directly on front lines (on top of full depower trim) to keep the kite in the air ( but I loose the kite control during that phase)

Hence my above question : I suppose that more camber will lead to more lift to keep lines taught and would like to get your feedback if some tested the zenith limit of their kite in both configurations (putting aside tips problems) ; or another way to see it : have some of you had to increase the flatness, and noticed how it impacted the backstall when fully depowered ?

[baxterbradford]

Interesting topic. The danger of inducing too much camber is that flow won't stay attached at such low airspeed, the air just doesn't have the energy to move around the foil. In dinghy racing in ultralight airs, flattening the sail is beneficial for some points of sail, until apparent wind kicks in. At the zenith you'd expect the wind to be strongest due to laminar flow over the water.
Agree with statements about weight/lift/drag by socommk23.

[Regis-de-giens]

Thanks ! I also agree with the lift/trust/weight, but would like to understand more in details, since as far as i know ( in enough wind at least) the L/D ratio does not depend on wind speed on a NACA profil for exemple; so the phenomena of backstall if we do not power the kite is not obvious : it should slack the line before backstalling, don't you think ?

Unless as you greatly explained Braxterbradford, there is a physical limit where the air cannot follow the camber curve if not enough speed ( intrado you mean I suppose). That may be the key indeed, since the process would become
=> lift decreases (but still high enough to lift the kite and tension the line)
=> leading to the decrease of L/D ratio since Lift decreases while Drag keeps constant or even get higher
=> which puts the kite a bit deeper in the wind window ( arctan of the kite azimuth angle is approx the L/D ratio)
=> which induce a bad dynamic (relative wind on the kite is lower if the kite goes backward in the window) and at the same time an increase of AoA (if the kite gets away from the zenith, it directly increases the AoA due static location of the rider )
=> then the air flow gets even worse ( because of higher AoA and less wind)
=> ect ...= a degenerative circle up to the ground if no corrective action from the rider ...

Does that seem right to you ?

[socommk23]

You must remember EVERYTHING has compromises.
Im sure you could design a kite solely for sub 5kts. However it will not be the fastest....or best upwind....or best boosting. A kite will be designed within parametrs that have to be weighed up against oneanother to make it work well enough for the majority of its market.

[alexrider]

Thanks for rephrasing your question Regis. I realise my reply was not what you were looking for... You already had the lightest kite available. I am curious which one is it? It's got to be a single skin, unless you expect to be able to relaunch it.
Would you be kind enough to share again the list of kite you established some time ago of all the kites according to their weight (adjusted to 10 m equivalent)?
BTW I tried the AEROS NAVY 2 16, considered as one on the lightest. It stayed up in the air alright, and had plenty of power once going, I wasn't get going (foiling) any easier than my Sonic2 13. Perhaps lack of experience with the kite. Must try again, since I'm unable to judge whether it would stay in the air with less wind. Still looking for the lightest relaunchable kite available for 4 knots upwards.

Regarding the Sonic you are right I suppose ; but in my case I have already optimized the kite weight which is under most (if not all) foilkites and far lighter than speed 5 and sonic 1. So I cannot do much more on that , except using lighter lines, which pushes the limit a bit further (say approx 200 gr which is however approximately the difference between Sonic 1 and 2 ...)

Currently I have no tip collapse, even fully depowered , thanks to a moderate Aspect Ratio. I even fully trim the kite during zenith phases . So ! thanks to my light weight I can start riding when kite hangs "properly" in the air (i.e. with less wind than necessary to maintain a Speed5 in the air even FULLY depowered, I did the back-to-back test).

Side-shore wind would be less a problem, but with my onshore configuration, the main difficulty is keeping the kite in the air the time for me to get away from shore. Body drag is not possible in such light wind, so I need to let go of the bar and swim out rapidly with both hands (on the back), just correcting the kite azimuth now and then to keep it at 180 degrees. This tricky situation last about 1-2 minutes, which increases the risk of a lull, so if I can save some wind for kite hanging in the air I think, I can still ride (with a double loop for waterstart , 30 m lines and big hydrofoil wing). Problem is the same after a fall in the water , the time I recover the Hydrofoil. I have sometime to pull directly on front lines (on top of full depower trim) to keep the kite in the air ( but I loose the kite control during that phase)

Hence my above question : I suppose that more camber will lead to more lift to keep lines taught and would like to get your feedback if some tested the zenith limit of their kite in both configurations (putting aside tips problems) ; or another way to see it : have some of you had to increase the flatness, and noticed how it impacted the backstall when fully depowered ?

[Regis-de-giens]

My kite is the Conceptair Pulsion; it is a "standard relaunchable closed cell foil kite" there are two dedicated topics (to avoid pollute too much this thread and derive to brand competition):
- the 12 m ( the first I bought in April ) : viewtopic.php?f=197&t=2392648&hilit=pulsion
- the 15m (the second I bought in December) : viewtopic.php?f=197&t=2393978&hilit=pulsion

You will find videos of static flight in very few wind ; weights for ultralight cloth on my kitchen scale are 1750 gr for the 12m and 1900 gr for the 15m (normally 2.1 kg so comparable to the 12m, but mine is specially customized at my request to same some grams). I have difficulties to upload the images of my weight table from work ( I also have the same for 15 meters), will try to find a way next week and maybe open a dedicated topic to centralized all those weight informations.

Aero Navy 2 is indeed one of the 3 lightest kites I know, and should have a better zenith than the Sonic 2 (which seems quite improved vs Sonic1) if I had to make a guess (but no personal test). Good knowledge of his kite, plus lines diameter and length are important parameter to compare flights in light winds for similar kite weights ...

@ Socommk23 : I agree there are compromises of course ; just wanted here to discuss which camber modification from "standard set-up" ( if it makes sense ...) is in favor of the zenith stability in extreme light wind, for each kite model (even if this kite is not optimized for extreme light wind).

So we have one answer thanks to Alexrider : Sonic 1 & Speed 5 requires to flatten the camber because the limiting factor is their tip stability vs backstall.
But it seems not applicable to my medium AR kite where the behaviour seems more backstall vs "yoyo" of the complete kite upward / backward if too much camber.