foilholio wrote:I found this interesting to explore on the keyboard again. Thanks.
Absolutely. These are complicated questions. There is so much changing parameters and feedback loops etc.
foilholio wrote:From my understanding slight negative camber or reflex has the best L/D.
Airfoils with negative camber are inefficient, they have low L/D. Negative cambered airfoil has negative to zero lift at low AoA, then L/D is also 0 or below. At higher AoA's they work but are inefficient.
Reflex airfoils may have quite a lot camber, they are a compromises between efficiency and stability. They are often called auto-stable airfoils, because they resist low AoA's which otherwise may lead to front stall, with kites it means frontal collapse of the wing.
Best L/D is achieved with high camber airfoils. However there is two main reasons why they are not suitable for modern (depowerable) kites. 1) they are too unstable, kite with high cambered airfoil can not be used at low AoA. If wind is gusty it behaves aggressively, overflies and collapses. 2) they produce a lot of lift even at low AoA's. So kite with high cambered airfoil does not depower.
If high cambered kite is forced to low AoA by the bridle, to get it depower, it collapses. So they are made fixed AoA kites, at stable enough AoA. Then they produce very strong pull, stronger than depowerable kite at same AoA.
It is interesting question when kite has highest L/D, at which AoA. According to Peter Lynn it is near 10 degrees, so very well powered up. It is remarkable that it goes down slowly, it is still high at 20 degrees, near or past stalling. I don't know how much this changes for different kites but my own experiences (very long snowkite upwind legs) do support it.
Nope all depower foils like this produce negative camber until Z engages. Preventing positive camber actually requires large adjustments to disable Z being tensioned or tensioned much.
Like Speed? Produce? You can trust me, foilholio: there is no traction kite with negative cambered airfoil. I have studied several of them and designed some
Speed has cambered airfoil, actually I was surprised how much of it it has. Airfoil shape can be seen roughly from the inflated kite.
When kite is depowered fully it flies mainly on A-lines. Then airfoil shape is the designed one and undistorted. It usually has moderate camber and some reflex, to allow enough depowering and for the needed stability. When kiter wants more power he sheets in and kite turns to higher AoA, increasing lift and L/D. Quite often kite is designed to increase camber simultaneously when AoA gets higher. This distorts kite, but at higher AoA's benefits are greater and there is no fear for instability.
For example, you have a new FS kite, correct lenght bridles and even mixer setup. Then you have an idea: "I want to make it more stable, let's pull more B-C". This means you disagree with the designer how cambered kite's airfoil should be. Now when fully depowered some B-C pull may exist and really decrease the camber, but then does the kite fly as low AoA than before? Is there any gain in depower? When starting to sheet in adjustment kicks in: kite has less pull, it is slower, worse upwind, it does not fly as far in the window and is more stable and has softer feeling. There is some more distortion also at low AoA's. Overall result is lower performance, but in very gusty conditions it may be ok.
It makes more sense to try to increase the camber for the final sheet in like Diablo-line does. At high AoA distorsions don't matter much, later stall and increased lift are valuable then. But this comes at the cost of increased bar pressure.
Increasing camber universally makes kite unstable and harder to depower.
Remember foilholio that primary function of the mixer is to change the AoA of the foilkite, because it does not have chordwise rigidity to do it without it. Depowerable kite works very well without any adjustments or designed changes to camber there, but they are useful options of course.