Trailing bridle spring tension control. . . . . I put a lot of words in the title so this topic might be easy to search and find.
Some past discussions suggest people do some things with bungee cords to control the way a kite performs in changing winds. They also make spars springy so the kite changes shape to suit variable winds. The kite manufacturers say watch the weather and set up the kite 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 pre- set.
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 releases some load by allowing the tail of the kite to raise and lower the angle of attack.
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.
You can see my longer instructions on my web page:http://brucerail001.webng.com/k1409114spring/a140914.html
for the page about springs
for my index page about kites.
(have patience with Nex Gen, if it doesn't appear; then try again later.)
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 Rok. 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 suitable for my 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.
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. (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 spring is not ideal but 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 the spring will try to satisfy the force diagram. The kites 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 and re-launching it.
The one thing the spring can't do is make wind if there is a lull below the kite's minimum flying speed. So have a good fast reel or run like crazy.
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. 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 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 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. Any one with a Rok / mule can appreciate this. And since the line tension is low and controlled, I can use my 90 lb line instead of the 150, and get more on the reel and have less drag in the air.
I notice the camera is more stable in altitude; instead of flying up an extra hundred feet during a gust, it stays much steadier.
Its also good for kites for kids. Adding just a simple spring on a little kite can tame variable wind conditions for them. It keeps things from getting frustrating because kites that threaten to roll and crash or fly into the ground is not good.