Kite performance testing by flying in a circle

Abstract

With the advent of recreational sports like kite surfing and buggying, the performance of kites has become a market driven item. Producers increasingly require methods to measure and improve the performance of the kites they manufacture. The Mechanical Engineering Department at the University of Canterbury has been working with a local kite producer to develop testing procedures suitable for kite manufacturers. The primary performance measurement is the lift to drag ratio. An early test rig was mounted in the top of a car, but limitations inherent in the design meant that it lost precision as the lift to drag ratio approached that of more advanced kites. This led the investigators to look for alternatives, and resulted in the development of the circular flight method. This method allows the test apparatus to be tuned to the performance of each kite, significantly improving the precision of the results while reducing the time taken for each test. In their raw form, the L/D results are not quite the same as those of the more traditional methods. But they reflect the underlying aerodynamic characteristics, and when used comparatively they can be used in the kite development process. Alternatively, with suitable processing the circular flight results can be converted to the traditional forms. NOMENCLATURElift force (N) L/D aerodynamic lift to drag ratio (weightless flight surface) L t /D t lift to drag ratio raw data from circular flight testing LTD kite lift to drag ratio (kite weight included) r 'walking radius' or flier radius during circular testing (m) R radius of the flight circle (m) S line length (m) T line tension (N) V velocity or wind speed (ms 1 ) β kite performance angle λ angle between r and S θ line angle relating to lift to drag ratio (LTD) ρ fluid density (kg/m 3 ) ψ angle between r and R Subscripts g properties at the ground k properties at the kite w properties at the flyer (walking) ⊥ properties perpendicular to the line in circular testing THE AERONAUTICAL JOURNAL JUNE 2005

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