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"pulsing" and bridle/frame adjustments

Mark Cottrell, Swept Wing Stunt Kites, 1990.
pages 14...15.

   "So the frame is strong, stiff and light and thus
   should be ideal to hang your sail on, but there is
   one other nasty lurking to catch the unwary - resonance.
   Many a normally well behaved commercial kite will
   demonstrate the effects of resonance as the wind
   speed increases. Resonance effects include wing waggling
   (one wing bends up, the other down & then the cycle 
   repeats), cross spar jumping and pitch shrugging
   (feels like sharp juddering). The problem stems from the
   fact that all structures have a natural resonant
   frequency - that is, the frequency at which the structure
   vibrates in response to incoming energy - and since
   conical rogallo sails are very good vibrational energy
   generators (viz they are very noisy) it is not surprising 
   that so many designs show resonant effects.

   Minimising resonant effects is not difficult. The ground
   rules -

   1. The stiffer the frame the higher the resonant frquency
   viz. the stiffest frame is best. Although a very flexible
   frame would absorb and damp resonance the flying 
   charasteristics would be horrible - if the resonant
   frequency is high enough 1-vibrations are short lived
   & 2-the inertia of the kite masks the effects of the 
   resonance.

   2. Do not allow any frame sub-dimension to be the same
   as or an exact multiple/sub-multiple of any other frame
   sub-dimension exept where absolutely necessary (the sub-
   dimensions are the measurements internal to the layout
   e.g. between the two leading edge connections or the top
   leading edge connection to nose dimension). This needs
   careful design and even a 1 cm variation in a spar can
   change a waggly winged kite into a quiet well-behaved
   kite and vice versa."

That's enough of retyping... 

Buy Mark's booklet, The Kite Store Ltd. sells it for 3.95 GBP
and Kite Lines for 10.95 USD - it's really worth the price!

Smooth Resonances
Simo.Salanne@csc.fi
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