Bridal spring control for consistent string tension for a kite in gusty wind

I had this topic on the old Kap forum in 2014, but it has become hard to find and update.
So, I updated my links and made this new message in 2019.

Some past discussions suggest people do some things with bungee cords to control the way a kite performs in changing winds. They also make cross spars springy so they change length, the dynamic spreader, and the kite changes shape to suit variable winds. That's a good way if you can make it consistent.
The kite manufacturers say watch the weather and set up the kites bridle for the one condition. But when I fly, the kite travels through a variety of winds at the many different altitudes. If I had all day to experiment, I could tune a kite on the ground and re-launch it and it could fly great, but just for the conditions that I preset.
My method relies on something consistent and repeatable with the string and bridle. I put a spring in the trailing bridle of a 2-string-er. It is easy, and it releases some load by allowing the tail of the kite to raise and that reduces the angle of attack during a gust of wind.
I prefer to use metal springs because the physics is predictable, and they age well, compared to rubber. I like equipment that can come out of storage and be flown without concern for temperature or age.
Five years ago, someone also commented that some rubber strap and a gear rack was used by a French kite flyer, over a century ago.
You can see my longer instructions on my web page:
Kite Spring Tension Limiting Control to tame a kite in gusty winds.
http://trainweb.org/brucerail10/w190912spring/b190912spring.html

You could also see some simple pictures and short descriptions that I put here:

www.flickr.com/photos/21982607@N00/15306588881/ a pre-loaded spring in the trailing bridle line of a kite
www.flickr.com/photos/21982607@N00/15122976009/ dowels before installing inside the spring
www.flickr.com/photos/21982607@N00/15309384102/ detail of a simple spring tied in the trailing line of a kite
www.flickr.com/photos/21982607@N00/15123156338/ a simple spring on a delta kite

This spring control only works when you can tie a spring in the trailing line of a 2-line bridle. I have done it on delta
conynes, and a Rokaku (before the 2-string bridle goes into the 4 attachment points).
Some kites don't have that kind of bridle, such as when the fabric has the yoke built-in, like some sleds, or deltas with a fabric keel instead of 2 strings.
Most springs that are suitable for my KAP kites have some limitations. A spring that is close-wound has some initial tension. They start moving at about 1/4 lb of pull. That's good enough to fly a kite, but not enough to also generate some pull on the string to lift a camera. I like a line pull about twice what my camera weighs, so there is some performance left to lift a camera to altitude quickly.
And the spring needs enough stretch to allow for another 4 inches of movement during flight, for a 4-foot long kite.
So it needs to be fairly springy or flexible, preferably with a spring rate of under 0.8 lbs per inch. Otherwise, as a wind gust comes along, the spring moves but also gets much tighter, negating what I want it to do. This is discussed on my web page.

To get a useful minimum line tension, I pre-load the spring by pulling it and holding that tension by putting a wood
dowel inside of the spring. (see my web page for the force diagram or calculations) I want the spring to always try its
best to hold the kite at a strong enough angle to climb and provide the tension that I pre-set, and no more. A steel spring is not ideal but it is inexpensive. It will pull harder as it extends, just don't let it be too much pull in those first 4 inches that controls the kite's angle.

It works good, at the expense of 2 ounces of spring and dowel. But I make up for that by setting the bridle lark head line attachment for light winds, so the kite is easy to launch in light wind at ground level, and then the spring changes the kite's angle to control it when it's up in high winds at altitude. What counts is that the spring will try to satisfy the force diagram as shown on the paper. The kite's maximum allowable wind speed isn't increased, its just that the spring does the adjustment in the air rather than me retrieving the kite and making an adjustment of the bridle attachment and re-launching it.
The one thing the spring can't do is to make wind if there is a lull below the kite's minimum flying speed. So have
a good fast reel or run like crazy to pull in the slack.

Limiting the line pull can keep you from pumping a line to flex the kite to get some lift; the spring will compensate.
But I make up for that by setting the bridle for light wind, so the kite can fly up into the good wind above.
Since I limit the pull available on the string, it changes how a camera hangs under a kite. Often, the camera is
nearly straight under the kite. A very stable position, like an instrument package under a balloon. The picavet still
works. And the line from the ground to the kite isn't pulled as tight. Without a spring, and with a stiff wind, and maybe
too big a kite for conditions, the line angle might be 70 degrees. With the spring, my angle goes down to about 45 degrees.
The kite can fly in a bigger variety of wind speeds without me bringing it back down to change settings. And it is
easy to pull it back down. Especially if I fly a larger kite than I should have, and discover variable winds up above.
Anyone with a Rok that pulls like a mule can appreciate this.

I notice the camera is more stable in altitude; instead of flying up an extra hundred feet during a gust, it stays at a much steadier altitude.
Its also good for kites for kids, if you want a 'tame' kite that is harder to crash. Just use a spring, with no need for a load holding dowel since they aren't lifting loads. It keeps things from getting frustrating because kites that threaten to roll and crash or fly into the ground are not good.

Comments

  • I've been looking at this approach for a long time but never had the time to actually try it. With your information, I might give it a try. Thanks for sharing.
  • Excellent work. I remember the 2014 post including the detailed work by Christian (French KAPer) on this subject.

    I have experimented a bit in this area but need to return to my experiments!

    WW
Sign In or Register to comment.