Kite Drag
by Chris Calthrop


I reach this conclusion partly from experience in Maui and partly through science:

Ignoring launching and landing practicalities and rider skill limits, the wind limit of a kite is dictated (among other things) by the total drag on the kite. The total drag  is made up of  the skin friction drag, which can be directly related to the surface area, and dynamic drag which can only be measured in a wind tunnel or estimated with a finite element analysis program. (I used both these at university on a windsurfing fin design study back in 1990).
Anyway not to get to technical: 

Skin Friction Drag:
I estimate the skin friction drag on, say, a Blade size 5 m2 is about the same as an inflatable with the same surface area, 5 m2. Both have two surfaces and a similar frontal area. The skin friction may be less on the Blade as it has stiffer surfaces due to the pressure inside but this will be offset by the drag on the bridle lines.

Dynamic Drag:
The important thing here is how much usable power is created for how much dynamic drag. Dynamic drag can be simply understood as the disturbance the kite makes in it's trail or wake when it creates power. It consists of vortices or swirls of wind that fly off the kite surface. It is wasted energy that out-weighs the energy wasted by skin friction drag many times over. The less a kite makes of these swirls as it passes through the wind the better. 
This is where the inflatable loses out big time. Although the Blade generates huge power it does not disturb the air that much. In contrast an inflatable with its arc shape creates forces in the opposing sides that actually pull it apart. This is good for re-launching off the water as one side lifts clear on it's own, however in the air these forces don't pull the rider in the direction he wants to go. Not only that, even more dynamic drag is created by the fight between the two opposing sides, severely limiting the top speed.

To go into further detail, when the kite is maxed out and the kite is turned hard, one side of an arc shaped kite creates more power than the other as it's angle of attack to the wind increases. The forces in the kite then start to unbalance and the kite starts "bouncing". As winds increase inflatables start bouncing around the sky uncontrollably, in the meantime the Blade becomes more and more solid. This turning acceleration of the arc shaped kite helps the really big inflatables turn faster, but it is a nightmare on the small inflatables. In general the Blade is nice because when you stop turning it, it stops turning, whereas the inflatables keep going until you turn them back. This is exaggerated by their extra weight/momentum. Kites will always fall toward the ground a bit, the heavier ones are worse. Blades are the lightest kites there are.

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Copyright © 2001, Chris Calthrop, Last Updated - 19/02/02
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