Postby cyclone » Sat Mar 27, 2004 10:36 am
Everyone with half a brain should know that 2 skinned foils are more efficient than a single skin because the flow is controlled/manipulated/optimized by intended design over both sides of the foil. This IS NOT THE POINT but is often mistaken or misunderstood to be so. The following attempts to explain why things are so between foils and LEI... like better hang for FS but better speed and height for LEI etc
Here are some facts worth noting...
Minimal lift induced drag is usually associted with maximizing the laminar flow distance over the wing chord on both sides of the foil. This minimises the votices generated which is the main component of lift induced drag. On the top surface, the flow WILL ALWAYS seperate at some point and (hopefully) reattach before the trailing edge... As someone stated before, turbulators control where the seperation occurs on some foils so that it may reattach further along the chord as desired.
A single skin foil can only optimize the flow over the top surface BUT the flow over the bottom surafce is much less critical at the angle of attack kites fly at optimally. The flow under the single skin LEI style foil in this case is affected by the reduced pressure above the foil and in this case the turbulance behind the LE bladder is less than you would think due to the negative pressure which explains why LEI are not as inferior to foils aerodynamically as you would think but still lack a little in this area hence a better L/D for the foil but not by much and this helps hangtime when the kites airspeed is slow.
Apart from lift induced drag, there is also profile drag and parasitic drag. Profile drag is related to the frontal cross sectional area (profile depth x span) presented to the direction of travel and parasitic drag on kites comes from the bridal and or flying lines. As you you would expect, bridals create much more parasitic drag than just 4 flying lines which goes in favour of LEI and ARCS and profile drag is also in favour on LEI`s and is also why they are reducing the diameter of these bladders as time goes on.
These types of drag are related to the airspeed and increase exponentially as airspeed builds therefore becoming exponentially more important at higher speeds, whereas lift induced drag is related to the amount of lift generated regardless of speed. LEI are fast arent they, this is why, less profile and parasitic drag which results in better performance at higher speeds.
You should also know that lift increases exponentially as airspeed increases for a given AOA and wing, it has already been discussed many times on this forum. So, if LEI can fly faster, they can develop exponentially more lift and thats why they jump so high but dont hang as long after the kite has slowed down.
Apart from all the theory, the way they fly, turn, speed, pop, feel etc not just the measureable lift, drag and laminar flow etc, is very complicated and very dynamic and i doubt these aspects will ever be able to be evaluated better in a wind tunnel as opposed to just getting out there and flying them even though its subjective. Some wind tunnel testing IS of course valuable and should be used in conjunction with real world testing.
Its not the theory that makes LEI kites great, nor all of PMU`s futile arguments, its just the way they fly... at least for the time being...