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
kite. Of course, these instructions could also be applied to a range of
other spar materials and situations.
The photo below is the finished horizontal spar for my first Dowel bowed
kite, the Dowel Diamond. In the instructions further down, the letters DL stand for Dowel Length,
which for the MBK Dowel kites is 120cm (48"). For example, 0.5DL means
'half a Dowel Length'. Useful for those who want to scale to some other
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 mid-point,
which is needed to balance the finished spar.
- 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 0.05DL
(6cm, 2 1/2") from one tip, on the side facing up. Not too many dowels are perfectly straight!
- Make another similar mark at the other end of the dowel, ensuring
that the 2 marks line up. For example, if one mark is exactly facing the
ceiling, so should the other one.
- 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 1/3 of the diameter of the dowel is OK.
- Do exactly the same for the other end of the dowel. Like you did for
the marks, make sure the 2 notches are facing the same direction.
- Take a length of flying line 0.6DL (72cm, 29") 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.
- Tie the other end of the line around the dowel, seating the line in
one of the notches. Use 3 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.
- 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.
- 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 4 times and tie
off with 3 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.
- 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 pit-falls
before the kite has its first flight...
Checking Cross Spar Curvature
- While the spar is still bowed, lay it down on a smooth flat surface. Concrete or cement is ideal.
- Using a piece of colored chalk, trace around the tips and all along the convex side
of the spar. Keep the spar firmly in place with your foot in the
middle. Trace lightly so nothing moves, and don't worry about tracing
the area under your foot.
- Swap the tips around, carefully lining them up with the chalk marks
at each end. The curvature of the spar should follow the chalk line you
- Take another piece of chalk, of a different color. Again, except for the bit under your foot, trace along the entire convex side of the spar.
- Move the spar away from the chalk lines and look carefully. If one
side is bending even a little more than the other, it will be clear!
Parts of the first chalk line will show, above and below the second
line, as in the diagram above.
- Figure out which side of the spar is bending less than the other, from what you have just done.
- While the spar is still under tension, file up and down the concave
side of the dowel, taking off a small amount of wood from the stiffer
side. Do the chalk test all over again. Repeat until the 2 chalk lines
line up much better. They will never line up exactly, since the correction is being done very crudely, but you should be able to get a big improvement!
Note: If no cement is handy, a table-top and colored
pencils could be used. That is, if you don't mind erasing pencil marks
from the table afterwards!
A Handy Short-Cut...
If your kite has a fully exposed upper cross spar, like a
Diamond or a Rok, you can take another approach. Simply note which side
of the spar is on the outside of the loop when the kite loops
around in fresh wind. Take the kite down, carefully remove some wood
from that side of the spar without damaging the sail, and test again.
Repeat until the kite is perfectly trimmed for straight flight!
Checking Cross Spar Balance
- 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.
- 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.
- 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 un-necessarily? All other things being equal, the lighter the kite, the better!
The Final Touch
Remove the Toggle from its loop, and check that every Half Hitch is
tight, in all 3 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.
Hope you found these instructions on making a bowed cross spar handy!
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Oct 27, 16 03:22 AM
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