About two years ago I went to San Diego. Had a couple of great days down there. Hard not to. Its climate heaven. I was standing on five acres of manicured grass with an uninterrupted flow of air from the ocean. The wind was 5 to 7 - on shore, 69 degrees with dozens of big puff ball clouds passing through. Not bad for New Years day 1998. Right where I wanted to be, doing precisely what I wanted to do. Fly. Had a kite dialed in real good but in keeping with the curse of West Coast fighter fanaticism, I decided to tweak it a little. Started playing with the spine/keel shape.
Tried it straight: Skittish and difficult to keep on track but fast with an awesome spin. The kite would not leaf (float) on it's belly. Had a tendency turn a wingtip toward the ground and knife down sideways, out of the sky. Tried it with varying amounts of curve from nose to tail: Speed and spin retarded incrementally the more the curve. The shallower adjustments showed a kite with more control. The radical adjustments demonstrated sluggish movement, no spin but pin point control and rock solid cobras. Tried it bent backwards, concave to the wind: Did not fly. Tried a straight spine with the nose turned back: Again, incrementally less spin, more control but a little less wiggy on a straight track to the horizontal. Tried a shallow but increasing curve from tail to bow with the nose turned back:
My spine shape evolved to this configuration and is more or less what the shape of the spine was at the beginning of the session. I did learn however that even small adjustments to the "basic" shape give evident results. Granted the conditions were perfect and likely assisted a great deal noticing small changes in flight characteristics. Kept on playing with it until I was satisfied the kites spine was tuned to the best average adjustment. The short story being, I could tune the kite to do a specific task very well sacrificing other flight characteristics. The best adjustment being one that allows the kite to perform the widest range of maneuvers. As with most flying objects, its a give and take thing.
Dinked around with it quite a while. On the way back to the hotel I sashayed up the strand and grabbed a cup of joe n' I'm all like whoa! Thirtyseven inch bustlines and nineteen inch biceps as far as the eye could see. Ahh, the California mating ritual. "Do your own thing as long as you don't up-stage me." ( I wrote that, you can use it if you want ) I notice two kids heading my way carrying surfboards. Surfing and surfers are double cool so I'm checking them out. I'll-be-damned if the centerline down the belly, nose to fin on the surfboards was the exact spine curve I had settled on earlier!
Surfboard shapers have a design component they call "rocker". If a board is floated on the surface of a swimming pool, rocker is the relationship of distance from nose/tail to waterline. Rocker helps to determine where the deepest part of the curve is along the nose/tail line. The rocker of a kite spine can be observed by placing the it on a flat surface.
"So uhh, fat kid - does that mean if I set my spine just like Mr. smarty pants fighter guy my kite will have optimal performance?" "No." It means try it out and see what happens. As always I encourage experimentation. No one can teach you. You must learn for yourself. In a few days I go over a spine shape for a high wind scenario. Totally different deal, same goal.