HOW DOES IT WORK ?
Take a 2-line C-shape kite
(1) or a 4-line one with straight tips
(2) or round tips
(3) : the line joining the center of pressure of the profiles, shown here by the red line, is a straight line seen from top.
The leading and the trailing edges are convex in the three cases.
Let us put in some sweep. The trailing edge becomes concave. The aerodynamic load developed by the tip surfaces will not be used to deploy the complete frame of the kite but mainly its back part. One sees on the movie that the more the tips are moved back, the more the trailing edge is flat.
Why ? As one sees it on the left diagrams below, the forces exerted by the tips merge with the line joining the center of pressure of the profiles on (2) whereas they only tighten the trailing edge on (4). On the right-hand diagrams, the forces exerted on the sections corresponding to the red dotted lines clearly show why the trailing edge is flatter on (4) than on (2).
One can deduce that on the other hand no aerodynamic load helps the leading edge to keep in shape and it would take an excessively curved form if not supported (A).
It is there that the inflatable tube and the bridles intervene. The length of the bridles and the shape of the tube are chosen so that the leading edge has the same form as the trailing edge (B).
One thus obtains an aerodynamically harmonious shape.
It is then necessary to create a bridling which allows at the same time:
- a significant rotation of the profile, to get a large depowering,
- a twisting of the kite to allow to steer it.
A simple bridle, in bundles, leads to a wing with null or limited de-power. A well-conceived bridle makes it possible for the various bridles to slacken in a uniform way.
Why an arch bridling ?
Compared with a classic bridle in bundles, the length of used line is definitely shorter, leading to inferior aerodynamic drag and cost, as well as less tangles.
The BOW concept can come with various shapes and bridles. What differentiates the Bow kites from other kites is the concave trailing edge and the bridling mainly positioned on the leading edge that is designed to allow large de-powering. The more sweep the wing has, the more flat and powerful it is.
Consequently, any wing with a supported leading edge, which does not have a concave trailing edge, will be closer to a traditional C-shape kite than of a flat kite.