A Perfect Bowed Cross Spar

Make it From Dowel and Kite Line

This page shows you how to make a bowed cross-spar for an MBK Dowel Series kite. Of course, these instructions could also be applied to a range of other spar materials and situations.

 

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Attaching the Cross Spar Bow Line

Here is the procedure I used to make a bowed cross-spar, after the dowel had been cut to length and the tips rounded off with a wood file. Also, a mark had been drawn all around the spar at the exact midpoint, which is needed to balance the finished spar.

1. Suspend the spar in the middle, over one of your fingers. If there is already a slight bow in the spar, it will roll around so the tips end up a little lower than the middle. Make a small mark exactly 0.05 DL (6 cm, 2 1/2 in.) from one tip, on the side facing up. Not too many dowels are perfectly straight!
2. Make another similar mark at the other end of the dowel, ensuring that the two marks line up. For example, if one mark is exactly facing the ceiling, so should the other one.
3. Using the edge of your wood file, put a notch into the wood at one of the marks. Strength is not an issue out near the tip of the spar, so you can make the notch fairly deep. Up to a third of the diameter of the dowel is OK.
4. Do exactly the same for the other end of the dowel. Like you did for the marks, make sure the two notches are facing the same direction.
5. Take a length of flying line 0.6 DL (72 cm, 29 in.) long and tie a Loop knot into one end, making sure there is ample room for a short piece of dowel to fit through the loop.
6. Tie the other end of the line around the dowel, seating the line in one of the notches. Use three Half-Hitches. Slide them down tight, positioning the knot on the opposite side of the dowel from the notch. The loop should just reach the exact center of the spar.
7. Take another length of flying line, also about 0.6DL (72cm, 29") long. Take a 0.01DL (1.2cm, 1/2") length of dowel, notch it in the middle with the file edge and tie the line to it. Just like before, use several Half Hitches. Let's call this the toggle.
8. Thread the toggle through the Loop knot of the first line. Bend the spar until you have slightly more than the recommended amount of bow, for the kite you are working on. Loop the line through the remaining notch, wind it round four times and tie off with three Half Hitches. At first, it took me several attempts to succeed—so persevere. This is definitely the hardest part! It's like stringing a bow.
9. Double check that the required amount of bow remains in the spar, by laying it down with the bow line parallel to a straight line. The edge of a table works well.

Tip: If you secure the knot and then discover that there is not quite enough bow, don't worry! Just tie a knot in one of the bow lines, which will shorten it a little. Keep doing more knots until the curvature is correct.

A non-symmetrical bow in the cross spar will show up sooner or later as wind strength increases. The kite will tend to lean in one direction or the other, severely limiting how high it can fly. Very annoying! Balance is not quite so important, but it also has an effect. Why not take a few minutes to eliminate both these potential pitfalls before the kite has its first flight.

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Checking Cross Spar Curvature

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Checking Cross Spar Balance

1. While the spar is still bowed, place the concave side on a sharp edge, right over the center mark. That mark had better be dead center! Double check it if you like, with the spar out straight of course. A cutlery knife is sharp enough to do this balance check.
2. If one end of the spar droops a bit, get your wood file and remove some wood from the heavier tip. File on the concave side of the dowel, between the bow line attachment and the tip.
3. Keep checking the balance until it gets hard to tell which end of the spar is heavier. That will be quite accurate enough!

Note: Just adding a little insulation tape on the lighter tip might be easier, but why add weight unnecessarily? All other things being equal, the lighter the kite, the better!

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The Final Touch

Remove the toggle from its loop, and check that every Half Hitch is tight, in all three sets. That's one at each end of the cross spar, plus the toggle.

With the dowel now straight again, put a generous drop of glue on every set of Half Hitches to secure them permanently. It's a good idea to slip a piece of paper under each knot to catch any excess glue.

I hope you found these instructions on making a bowed cross-spar handy!

 

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