I’d been struggling with if I had anything helpful to say in response to Andre’s (ae’s) posting, and I was going to leave alone. But SkyRag posting has pushed me into saying something.
Andre, in making and flying very lightweight kites indeed: XULs, has gone further than my experience, and therefore has the opportunity to know and the authority to say the difference that weight makes.
Moreover there is a simple scientific argument: Energy is proportional to mass, and drag depends on materials and shape. So one would expect that a SUL version, being very similar to but having less mass than a standard kite, would have less kinetic energy at the start of a slack line trick than a standard, but be subject to the same amount of drag. So the SUL would slow down faster than a standard kite in a slack line trick as a greater percentage of the kinetic energy was consumed by the drag.
But, but, but: My problem is that the simple argument above for standard kites tricking better than SULs is contradicted by everyday personal experience. Any trick I can do, I can do whether the kite is a SUL or a standard. Indeed some tricks are widely known for being easier to learn on a SUL – like the taz, and I also know this from personal experience.
Moreover, experience says that sometimes adding weight calms a kite down – for example I added the tail weight to an Ocius UL the other day and it became noticeably nicer on the ends of the lines. The natural backflip wasn’t as deep, but I could do more with it, like more turns in a multi-lazy before the kite hit the ground or flipped over into a wrap, and being able to pull straight out of the backflip rather than having to go into a fade from a half-rotation.
My personal trouble is, I haven’t figured out the story of how significant kite weight is. There are so many other things that affect the kinetic energy verses drag issue. For example:
When trying to lazy my Talon UL after launching from the tide - the kite now being wet and heavier- it was much easier to execute the rotation than when the kite is dry
The same principle is involved in a side slide - where somewhat a portion of centrifugal force is applied in sliding the kite
…my first thought was that you are flying with a wet sail and so the way the sail catches the wind may have changed, as may have the amount of drag from air passing over the sail’s surface. It would be quite a challenge to devise experiments that separate out and identify the contributions of a) wet sail shape/flexibility, sail weight’s contribution to kite’s inertial mass, and wet sail’s change in drag.
Does Cuben fibre have the same drag as Icarex? (Yes they are both used for spinnaker sails of racing yachts – but those sails are so large, and their job is to pull the yachts along, so maybe a difference in drag would be less noticeable there than with a dual line kite doing a slack-line trick.) What about the relative drag of mylar?I do have direct experience that an Icarex sail with some flight hours flies better (and in lower winds) than a sail fresh and new. We found this out from pairs flying practice. And from comparing brand new and well used Icarex sails, I know it isn't that the sail has stretched. The well used sail feels softer and less crisp. - But does this change the drag due to the wind passing over a sail of the same shape, or is it that the softer sail can more quickly adapt to the optimum sail shape for the conditions, or interacts differently with turbulence??
So in summary, I don’t know. And to me it is very complicated.
Pete...And I do like the discussion on kite design evolution in the thread on "straight or curved leading edge design".