Does Aspect Ratio Matter in Kite Choice?

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.

Albatross
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.

high performance sailplane
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:

The technical formula for AR
For a rectangular outline with an Area = LxW, a Length vs Width determination is in fact proper:

rectangular outline with an Area = LxW
But what about other outlines?
Here is an example of several different outlines with identical AR's:

Examples-of-aspect-ratios
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.

Projected-span-of-a-kitesurfing-kite
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.

SwitchKites Method2 and Nitro3 kitesurfing kites

Method2 (Left) Nitro3 (Right)

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...

4 thoughts on “Does Aspect Ratio Matter in Kite Choice?”

  • kepa duperou

    I have always loved an high AR kite in lightwind, but do not have enough knowledge about different outline. But between flat rectangular outline (genetrix hydra kite) and round outline (north rhino 2009), I prefer the last one, more stable in very very lightwind.
    Today, my favourite kite are method2 and nitro3.
    This nitro 3 is amazing kite, all perfect, fast, power, turning speed, wind range. It just not perfect in waves.
    For lightwind, the high AR it's for me one of the important detail to choice a kite. And with the Nitro 3 I get one of the best :-)

    Reply
    • Bill

      Hi Kepa:
      Thanks! Glad you like the N3!
      Cheers!

      Reply
      • Jon

        Bill
        I was under the impression that the high aspect versus low aspect actually made it harder to fly upwind. For example, in my review of the E3 15m I have flown this winter at Berkeley, I felt the higher aspect was what was preventing me from getting upwind as well as I do on my other kites which are 10 and 13 m edges. Is this not the case? If not, was it just a "feeling"?

        Reply
  • Bill

    Ultimate upwind VMG is a measure of efficiency. Generally, higher AR relates to higher efficiency but only if all other parameters are unchanged. A flatter kite with higher projected area can excel upwind at lower AR than a more 'C' shaped kite with higher AR. Projected AR is an artificial concept as well so really not a proper parameter to judge. There are simply too many variables contributing to efficiency to tie performance to AR alone.

    Reply
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