Spring cruise control governor for consistent string tension for a kite in gusts

edited September 2014 in Kites
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
and http://brucerail001.webng.com/kitehorizonpoint.html 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.

Comments

  • Interesting suggestion. I do like the spring approach at it is more reliable/predictable than elastic or bungee cord.

    I'll get back to this and read more in details.

    Thanks for posting this.
  • Sounds fascinating and well thought out. I have too many kites to do it on all of them, but I'd like to give it a go on one of them.
  • edited September 2014
    I am very interested to hear more results of this in use. I've considered this as a possible solution for while. I thought a safety line might be needed to limit the maximum extension possible.
  • edited September 2014
    I'm also interested! I looked into bits of elastic a few years ago, but did not find the right material then...
    After reading this post I bought some springs to try on my rokkaku (impulsive action 0.69 euro a piece! :-) )
    They might be a bit too stiff (elongation 6 cm for 2 kg), we'll see...
  • This is cool! I didn't look into elastic years ago because I was hoping someone like Hans would when that discussion first came up. But I've been interested ever since.

    I need to check the springs I have at home and the ones available at the store. Thanks for listing the specific springs you're using. I can order those as a starting point.

    Do you find you have any issues with oscillation? Does your kite ever get into a pumping action with the spring? Or are their fundamental frequencies so different that they don't couple?

    The bridle on my 7.5' rok has some fray points on it, so I was planning to replace it this coming weekend. I might make up two center bridles for it - one with a spring and one without - and try it both ways.

    Tom
  • I considered bungie when I was starting out, and actually built a system on the kite line itself, rather than the bridle to try to damp rig motion. I ultimately abandoned that because of the extra weight and complexity. But maybe putting the spring on the kite deserves another look. I would reccomend McMaster.com for any spring related needs. They provide every dimension you could need, and ship quickly.
  • I was able to get my hands on the springs in the local hardware store and test pull them, but my choices there were only 2 from their whole rack of 50.
    McMaster offers a lot of springs. www.mcmaster.com Here is some of their informaton, and some springs I thought about.
    Extension springs are made of steel music wire (for high strength), spring-tempered steel (for heat resistance), or stainless steel (for corrosion resistance).
    Steel music wire is good enough for a kite.
    They are available with loop and hook ends; I like the full loop end, so nothing slips off.
    Minimum Load—The pounds of force it takes to start to extend the spring.
    Maximum Load—The pounds of force it takes to fully extend the spring.
    Rate—As you stretch an extension spring, it gets harder to pull. The higher the rate, the harder it is to stretch the spring. Shown as lbs per inch.

    Min. Load should be the load it will hold at the length when it is its shortest. When you put on more load, the spring should start getting longer.
    'Overall length' is actually its shortest, measured to the end of the loops.
    Extended length is the manufacturers recommended longest length you can pull it to; its a safe number, for like, maybe 10,000 testing cycles?
    You might find a spring that has a minimum load that is enough to also lift a camera, and not need my pre-load by putting the long dowel inside of it.

    length, od, wire diameter, extnd Length, min load, max load lbs, rate lb/inch
    3 inch 0.313" 0.028" 7" 0.38 2.22 0.46 could barely be dowel pre-loaded to 2 # and have some spare length
    3.3 0.375" 0.041" 4.3" 1.49 3.25 1.76, package of 6, 9654K296, $ 13.50, 1.76 is a stiff rate, otherwise has good loads
    4-1/8 0.734" 0.063" 4.9" 1.55 2.75 1.66 pkg of 3 9654K304 $7.05
    stiff? not much extension but why, its better than the above?
    5 0.219" 0.023" 10" 0.36 1.80 0.27 ? can't be dowel stretched to 2 # and not exceed manufacturers limit ?
    5 0.313" 0.041" 6.7" 1.05 3.91 1.83 pkg of 3, 9654K271 1.83 is stiff, and that's a min for extension 1-5/8 inch ?
    6.5 0.328" 0.035" 7.6" 0.80 1.31 0.51 6 1330K26_ $8.79 poor max load and extension, but a good rate
    5 0.25" 0.031" 8.1" 0.91 3.64 0.88 pkg3 9654K264 $10.56 nice range, rate, min #, 1-1/4" of dowel stretch would get it to 2#
    6 0.25" 0.028" 11" 0.61 2.74 0.41 pkg6 9654K234 $11.36 good rate, dowel stretch needs 3 inches to reach 2, just watch the weight?

    o.dia wire d extended / min load / max load / rate qty per bag
    these are all 11 inches long, and I removed the listings with min load under 1 lb. or max over 3 lb.
    0.125" 0.025" 12.8" 1.38 3.72 1.30 5 9640K89 13.36 skinny, maybe a short pull range for such a long spring
    0.375" 0.037" 24" 0.82 4.07 0.25 1 9640K151 2.92 0.8 + 4 inches pre-stretch totals 1.8 and the rate is low,
    watch the weight, its 2 feet long

    I don't know the bridle on your kites, but on my delta conynes, the reel line tension is about double what the spring gives me.. Except Roks are harder to figure. i have done ok with 4-inch springs on the delta conynes that are 4 feet long, and a 6-inch long on the 80" rok.

    I thought about a 'safety line' in case something goes wrong with a spring, but decided if the spring didn't break or permanently distort while pulling it by hand to 4 inches longer, it would be ok. i use the 'safety line' with a slip knot, so when I land the kite, I have some evidence of how much the spring moved. Don't tie it too short, let that kite tail move at least 4 inches.

    It is interesting to watch a kite in severe winds. I flew in cold fronts a few times to really test it. When I didn't have a need for KAP, I did attach a water bottle to simulate a camera's weight.
    The wind was 'choppy', like micro-bursts that occur under low rolling clouds that are at 1000 feet elevation, and then a wispy low cloud passes closer yet. The wind meter recorded 22 mph maximum, the average was 14, and I had set the bridle for low wind because I needed it to get the kite up from ground level. There was almost no wind on my street due to the trees. It really climbed when it got above
    the tree-tops. So my bridle was set for one extreme, light wind, even though I knew the winds were stiff up above. I could see the clouds move fast. The trailing edge of the fabric ruffled or flapped during the bursts. The kite nose looked like it would drop 3 inches, yet it didn't change altitude. This 'nodding' happened as often as every 2 seconds until the bursts passed. Yet the kite and bottle hardly changed altitude.
    I also flew some KAP flights in some wicked winds, too, and that kite went side-to-side 30 feet during the bursts. I really shouldn't have been flying but I was a long way from home and really didn't want to waste the trip. The kite stayed level during the side-to-side movement, and it and the camera stayed at the same altitude. I knew it was being buffeted. I was worried that maybe I should release line, but then I held my ground and the gusts passed in 30 seconds. The reel tension went over 6 lbs, for a spring that was set to give 2. The camera swung sideways about 4 feet until the bursts passed, but I still got good pictures.

    I know that without the spring and with my bridle set for light wind for an easy launch between the buildings, that should have made it drop by one wing and circle down to try to get out of the wind. That's when most people have to release line to help the kite recover, and as you all know, if you get hit with another burst and all your line is already out, you don't have much control left. The spring reacts so fast that I can watch the motion, but by the time I decide what to do about it or could guess when the gust would pass, the spring already controlled the kite.
    In normal consistent winds, I haven't noticed any oscilation at the camera. And I know when strings can oscilate, I used to fly flowforms (stick-less kites) and have seen oscillating tensions that pull the line 2 inches every 2 seconds if there's a flutter in the center of a 3-rib flowform. Now I like kites with sticks. I'll let someone else argue that one, its been discussed in this forum before.
  • Very interesting. We had a presentation by Bernard-Noel Chagny at KAPiFRANCE125 last year - or was it at KAPiNED in 2010? He uses bungee on his roc which he uses for archeological KAP to very good effect.

    I found the Century C-193 extension spring on Amazon UK

    http://www.amazon.co.uk/Century-Spring-C-193-Extension-Springs/dp/B000BQK5M6/ref=cm_cd_ql_qh_dp_t

    For £6.07 free delivery and two items in the bag

    Technical Details
    Part Number C-193
    Item Weight 32 g
    Product Dimensions 1 x 5.1 x 15.2 cm
    Item model number C-193
    Item Package Quantity 1

    Product Description
    Model No. C-1 thru C-243: 2-pieces per bag. Measurements are outside diameter

    Fly (not quite so) High

    Sue
  • edited September 2014
    Hi All,

    - Reference: For those who understand French (or are able to interpret Google's translations): Bernard-Noel's experiments are described in detail here: http://photocerfvolant.free.fr/phpBB2/viewtopic.php?t=2318


    - I did a brilliant discovery while falling asleep: when you put two springs in series with (elongation 6 cm for 2 kg) you get a spring with (elongation 12 cm for 2 kg). It is also possible to use two parallel springs resulting in (elongation 6 cm for 4 kg) Patents pending ;-)

  • edited September 2014
    The compensation is lowering the incident angle of the kite, lessening the pull. However the wind being stronger, the drag is still more important and this is lessening the line angle. So the compromise between the compensation and keeping kite elevation is done through the design of springs or elastics systems.

    Equalize the stress through compensating bridle was used by Emile Wenz for kap as he described it page 44 in Le cerf-volant magazine n°4; November 1909.
    image
    He sets as many rubber rings as necessary. When the stress is low, the extension is only a few millimeters. When it is high, the extension should fit the expected compensation. Since there have been many designs applied, and bungee bridle is still used by some French kiters in strong winds.

    In the same way, any stiff spring can be used.
    image
    On the extension spring shown here, 12 mm diameter, 20 mm long, 1,6 mm wire, it extends of 10mm under 5 kg load. However, under 1,5 kg load it extends of only 1,5mm which is barely noticeable. This is because the coli is so tight that it needs some minimum load before it starts extension. So it is usable as is. If the spring was longer, the extension would be proportional and couldn't fit on some kites.

    I wished to be able to set a preload before the compensation would start and to limit the value of the length extended under the highest stress so the kite incidence can be decreased to the right value, but no more.
    To do this I have designed both spring and elastic compensators.
    image

    I have been prone to use compression springs instead of extension springs because it is easier to adjust the length of the spring to get the desired length of the bridle for a given load. Also set the preload is simple. Herebelow are the pieces needed.
    image
    And this is the pre-assembly with the spring been preloaded. The part S is only fitted for tests only. When its length is confirmed, it can be removed and the part B is shortened of the length of part S.
    image
    The spare spring is displayed to show the compression of the preload. On this example, the spring is 10 mm diameter, 64 mm long, wire size is 1mm, coil pitch 4,5 mm. The preload is 1,5 kg and extension is limited to 25mm which is done at 3,5 kg.

    The bungee design is a hollow elastic as used for fishing. The design is simple and efficient. It is really easy to adjust. Firstly, for different free length of the hollow elastic, the value of extension at the desired value of stress is measured until the correct value is obtained. The free length is the distance between the two bushes. The hollow elastic is fitted on the bushes over 25 mm. Then the length of the stem (bamboo stick) is set to the preload value.
    image
    The side line is limiting the extension to its maximum.
    On this example, the preload is 1,5 kg. I have verified that the extension is onlt 1 mm under 2 kg load. The maximum extension is 40 mm obtained at 4kg load.

    The bare spring as shown above weigths 7 g. This elastic compensator weights 10 g its overall length is 165 mm. The spring compensator weights 18 g its overall length is 110 mm.

    In addition to this, I would like to show you this compensating system. It has been designed by Geo Mati and published in Le Cerf-volant magazine n°18 in January 1911. Tje elastic is in EF. B and C are pulleys. I made a model, it really works fine but I haven't tried it yet on a kite.
    image
  • edited September 2014
    I should mention that the WW maxi-dopero modification with a bungee between the lower keels is a special type of self-limiting dynamic governor on the angle of attack. And it really helps with overall stability in gusts and changing wind conditions.

    http://arch.ced.berkeley.edu/kap/discuss/index.php?p=/discussion/2488/kite-repairs-dopero/p1
  • There's a change in my message that started this topic. My web page with the spring details had to be moved to the new 1app web host free site that replaced the nexGen free site.
    Now my page with more details is:
    Kite Spring Tension Limiting Control
    http://kitehorizonpoint.1apps.com/k1409114spring/a140914.html
    and my kitehorizonpoint index page is:
    http://kitehorizonpoint.1apps.com/index.html
    but I don't have many of my own KAP pictures on there yet. Try my link to my railroad hobby page and some KAP pictures there.

    I see several replies to this topic that use rubber bungee cords. My favorite material is a steel spring. That is easy to pre-load, and it will not change its springiness with age.
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