Trapped air in double skin foils?

[Regis-de-giens]

There is however one big advantage of entraped air : the internal pressure provide an additional stiffness that allows more efficient foil profile, like less "twisting" of wingsfor exemple. On the contrary on a SS (so, without internal pressure ) you need to impose more Angle of Attack on wings (to avoid their collapse), and you loose some efficiency (drag) and thus less speed and pull during the travel of the kite, very useful in lightwind.

I don't think the stiffness is the main reason for better performance. I am also paragliding and fly single skin paragliders. However, unlike the kite market there are high AR single skin paragliders (AR of 6), I don't know of any high AR single skin kites (not counting LEI, which in fact are also single skins). The main obstacle is the increased drag due to the fact that there is no skin on the bottom for better aerodynamics. Hence single skins just do not fly fast, otherwise they have quite good performance, in some respect even better performances and even better stability than double skins even though they are not at all stiff. Tom

Not the stiffness in itself, I fully agree ; but the fact that designers can use the increased stiffness of double skin (and same for LEI's) to optimise the twist at tips locations, and decrease the AoA of the tips, leading to a better L/D ratio overall, which is called Performance ; for a similar aspect ratio, a double skin will have a better L/D ratio than a single skin (also the turbulence behind the leading edge , it is true); if you take a well deigned double skin and cut the intrados, most will collapse at tips if you do not change massively the local twist ("vrillage" in french) by changing the bridges around the tips .

[slide]

well you can try to be quiet or if not cough at the same time , and then move away very quickly , but 2 Rennie normally sort me out , sugar free of course