by Dan P.
(Lexington Park, MD, USA)
Kite flying at steady state
As part of my job I've been working on a catenary model to be integrated in a computational fluid dynamics code. For those who don't know (like me when I first heard the term), a catenary is something like a rope, cable, or string hung between two points whose shape is determined by gravity and tension.
Well naturally, being a kite flyer working on such a thing, a big goal was to use it to do a kite simulation! I modeled my version of the multi-dowel barn door kite, which is about 8 feet wide and 7 feet tall.
In the simulation it is assumed to be rigid with some bow built into the main spar. It weighs 18 oz., and I tested it in a 7 mph wind. The wind is assumed to be completely steady except for in the vicinity of the kite, which causes some unsteady vortex shedding. It uses a 5-point bridle, and there's also a tail just for some added stability (which is not actually needed but makes the video more interesting).
The first attempt did not work out so well. I didn't know exactly where the center of mass (CG) should be. However, I was kind of under the impression that an aft CG would be more stable, so I placed it farther back than I thought it really would be, close to the lower bridle attachment point. Apparently that was wrong, because as soon as the upper bridle points tugged to one side or another, the kite quickly spun around and entered a death dive! See the video below:
For the next attempt I moved the CG forward so that it was between the upper and lower bridle attachments but closer to the upper one. This seemed to work much better. The catenary reports the tension at each point in the line. It was about 4 lb at steady state for this flight condition. See the improved video below:
As mentioned earlier, there's another alternative to towing indoor kites if it's just not possible to fly outdoors...