Friday, February 22, 2013

Making a 2m Rokkaku - bow tension and flight

Part 4 of 4

Spar bow lines
A rokkaku is a flat kite which means it has no inherent stability. Some bend in the spars is required. Adjustable tensioning lines bend the spars like an archer's bow.

In the sewing instructions I mentioned how to create an end loop in the pockets for the bow lines. This photo shows how the end loop works.

The lines are about 2.2m long, tied at one pocket, looped through the other pocket then back to a sliding clip.

I made the sliding clip from UPVC plastic which can be heated and bent into shape. A 30mm x 15mm piece is drilled with two holes, rounded off and heat bent with a slight curve.

The clip slides along the line and holds the bow under tension.

The amount of bow can be adjusted to change the flight characteristics of the kite.
More bow gives less pull, more stability and a lower line angle, better for stronger winds.
Less bow gives more pull and a higher line angle but will reduce stability. If the kite is wiggling side to side you need more bow.

Some folks say you must have more bow in the bottom spar than the top, others say they can be the same. Mine seems to fly well either way.

I tend to set up with 6 to 8" of bow in the bottom and a little less in the top.

Angle of attack
The final small loop in the bridle is used to adjust the flight angle. The prussic (prusik) hitch, or double larkshead hitch holds it in place under tension but can be loosened and moved under no tension.

With the loop at the centre point of the bridle the kite will pull like crazy and sit on the ground. Moving the loop up a long way, say 12", will make the kite go up a bit but tip over towards you.

This ideal bridle position is somewhere between these two extremes. Try placing the loop about 8" above centre and see how it flies.

If the kite flies but with not much pull and is luffing or tipping over at the top move it a bit lower.

If the kite is pulling hard and not rising far move the loop a little higher.

When it is in the correct spot the kite will climb up high and sit there with moderate pull on the line. You can now dictate exactly how much line pull you have by moving the loop up or down in small amounts. If you need more pull for lifting an SLR move it down a bit, and if you want less pull for easier flying move it up a bit.

The rest is up to you, go fly the kite and have fun.
I'll make a video showing different bridle positions and the resulting flight characteristics soon, maybe different spar bowing too.


  1. I liked your testing video showing the different positions, very helpful indeed to know how to get my very first rokkaku tuned up. You just can't beat a good YouTube to see how the behaviors are different. I am currently test flying with bamboo spars which don't have such a symetrical curve, will have to order some carbon spars online. If only there were shops to supply materials like these locally. The local model shops only seem to have expensive carbon spars suitable for model airplane construction. Would wooden dowel work at all?

  2. I have had to get all the materials online from US mostly, especially line and spars.
    Wooden dowel works well, just that it's a bit heavier and easier to break. Very cheap to replace however.
    Some kite builders swear by bamboo.

  3. I made the maiden flight with my 2m Rokkaku built mainly based on your information today. Thank you very much for sharing this!

    1. Great to hear Jorg. I'm so glad the info was helpful.