Does Aspect Ratio Matter in Kite Choice?
This entry was posted on April 7, 2014 by Switch HQ.
Aspect ratio has become a common term amongst kiters and sometimes, like it or not, a determining factor to some in pre-judging the viability of a particular kite. The general consensus is that higher aspect kites will be more efficient in terms of lift vs drag and correspondingly better upwind with increased glide and hang time in jumps, etc. But, how important is aspect ratio in choosing a new kite?
The idea that high aspect kites perform better is mostly rooted in basic aerodynamics where the drag associated with turbulent vortex generation at the wingtip becomes a smaller portion of the overall drag determined by the combined wetted surface, form and induced drag components.
The idea that high aspect kites perform better is mostly rooted in basic aerodynamics where the drag associated with turbulent vortex generation at the wingtip becomes a smaller portion of the overall drag determined by the combined wetted surface, form and induced drag components.
That is, the turbulent wingtip flow on a high aspect wing is a smaller portion relative to the stable flow. Hence, given all else same, the higher aspect wing must be more efficient. This is best illustrated in an Albatross with an AR of 18 or high performance sailplanes with AR's of 40-50.
In a simplistic way, AR is often thought to be a length vs width comparative measure of geometry. In many cases, that is enough to make a rudimentary or intuitive judgment of performance for similar outlines.
The technical formula for AR is:
For a rectangular outline with an Area = LxW, a Length vs Width determination is in fact proper:
But what about other outlines?
Here is an example of several different outlines with identical AR's:
Clearly, using AR as a means for pre-judging performance becomes less reliable as the outline diverges from a simple rectangle or ellipse. In kites, which are more complex 3-dimensional shapes than mostly horizontal aircraft wings, the matter of projected area and projected span must also be considered.
In this case the projected AR becomes a ratio of virtual measurements with equally virtual meaning. And, in general, kites (except for some paraglider-based foils) are relatively low aspect in the 3-6 range with much lower projected numbers. Given that the portion of drag due to tip turbulence is a factor in overall efficiency, one must also consider the other contributors to drag which can vary greatly from one kite to the next. Tube size, profile, 'coning,' and outline all contribute to drag and at low AR's become more determinant. As there is no 'across the board' standard for them, two kites with identical AR's may have vastly different efficiency. And conversely, two kites with different AR's may have similar efficiency. Yet, for many kiters, the AR remains the 'holy grail' of perceived performance which is natural since the outline either flat on the ground or in the sky is the most obvious.
In the end, using AR as the decision-making criteria for choosing a particular kite is risky because a high aspect ratio design may suffer in other areas like turning speed and re-launch and actually may not perform better either. If you are interested in a high aspect kite, an extensive test ride is in order as it should be for any kite choice decision. But, aspect ratio does contribute to the vernacular of 'beach talk' and in a way, it is cool to discuss it in a technical way while waiting for wind...
No comments:
Post a Comment