Lifting Kite Design Limits

Hello Shane,

I don't know if anyone here has replied to you yet- I've been in the US- coincidentally flying at Doug Hagaman's old stamping ground of Lincoln City.

Anyway, you raise some questions but I think I'll first answer the ones you didn't ask!

The internal mechanism by which single line kites "know" where up is, is the pendulum created by their centre of pressure being above their centre of gravity. As line angles approaches 90degrees relative to the horizontal, this mechanism rapidly ceases to operate. Single line kites therefore cannot usually fly reliably if they have lift to drag ratios above about 4. (L/D for a kite is, accurately, tangent of the line angle at the point it attaches to the kite). Arc style kites appear to violate this 'rule' because they do have higher L/D's- like up to 7 or more, but when I say, "fly reliably" , this means at a zennith exactly downwind from their tether point. Arcs, when flown as single line kites will tend to hang to one side or the other rather than fly exactly downwind- and to the extent that their functional L/D drops back to 4 or so.. Although which side they will hang to is initiated by some minor asymmetry in the kite, that they fly to one side or the other is a result of their upwards sensing mechanism fading out at high line angles. Indeed, all high L/D single line kites that I know of will either lean off to one side in this way or hunt and circle. An L/D of 4 is not a magical number of course, just tends to be for parafoil style kites. Some kite styles can push the edge further than others- from my experience, kites with the form of conventional aeroplanes do the best.

A theoretical answer to making reliable single line kites with higher L/D is to use some other 'upwards' sensing mechanism- like a cosmic ray intensity sensor in combination with some way of 'seeing' the anchor point. We've had many years to think about possible kite auto pilots, have tried a few things- so if you decide to try something along this line, it might be useful to run your thoughts past here first.

The second problem with making high L/D kites is that the max speed that a kite will attain when repositioning itself is the true wind speed times it's L/D- so when a kite with L/D of, say, 7 falls back in the sky for any reason and than climbs to zenith, it's maximum pull during the climb will be approaching 50 times it's static pull. Pull mitigation systems are possible of course- and one of the best in any lifting kite goes back to Rudolph Grund the kite designer at the Lindenburg Observatory during the heyday of high-altitude kite flying before WW1. I've flown one of his original kites (a modified Hargreaves style framed box kite) and it seemed to manage about 3/1 mitigation. Unfortunately, this doesn't help very much when up against the 16/1 max to min pull ratio this kite would theoretically have while repositioning even in steady state wind- but sure is a lot better than having none. Pull mitigation systems for ram air inflated kites seem, so far, not to do much better than 2/1- with more than this they either collapse or don't steer (2 and 4 line kites); collapse or become unstable (single line kites), though I certainly believe that more will be possible, possibly a lot more- how much have you managed to get on the Hagamans? From my experience with Doug's designs, they tend not to be able to accept as low an angle of attack without becoming volatile unstable as more recent parafoil designs- which is a major limitation for pull mitigation.

Again, the pull mitigation systems I've seen with the greatest range have been on aeroplane form single line kites- but these are not normally regarded as suitable designs for lifting payloads (because structural constraints severely limit the max sizes they can be built to within acceptable weigt/area limits and while maintaining adequate robustness).

Steerable kite design yields to theory and analysis, but single line kite design is an exceptionally complex field- I would say not just unknown but to a large extent unknowable- and this is after working in the field full time as an engineer for 30 years so far. Nevertheless, as far as I can tell (not everyone shares information or has revealed what they knew), there are some things known now that weren't, even 5 years ago. I have no doubt that a serious assault on the challenge of creating a better lifting kite could now make some useful progress- but the level of effort required is likely to be in the order of 2 years full time for someone like myself, an investment that wouldn't seem to be supported by the likely market- especially as preventing copying of any improvements that are made is practically impossible.

I'm not sure this will have helped- except maybe if it's filled in a few of the corners for you.

If you still want to try some arcs though, we'll be very happy to oblige- might even be able to find some prototypes that are likely to be more suitable to your purpose than our commercial ranges- Pete, what's around?

Good luck anyway- I'll be here off and on if you have more questions- and I am thinking about the problems you pose- and fully intend to do something in this area when other projects permit.

Regards,

Peter Lynn