Theory of flight part 4 – Kite’s traction
With the previous figure we have shown how the apparent wind can multiply the natural wind intensity by 3 or 4.
From the beginning, the experience has taught us that obviously the kite is faster and more pulls, but the feeling is that the power in the power zone is well above three times as much as the one generated at the edge of the window and is actually just like that, because the speed-power relation is not linear but exponential, so the pull of a kite is a function of the square of the wind speed.
This simple rule affects all aspects of the practice of kitesurfing and is what allows us to start, glide with little wind, to make spectacular jumps and is always the one that, unfortunately, creates many of the danger situations that your instructor has illustrated during the course.
Let’s take an example: enter the water with your kite which, in 10 knots of wind, generates a traction of, suppose, 20 kg: after a few minutes the wind strengthens up to 20 knots.
Commonly it is believed that when the wind doubles doubles the traction, but in fact it is not so: in fact the traction does not rise to 40 kg, but increases according to this calculation:
With a doubling of the wind intensity, the new traction will therefore be 80 kg, the quadruple. If we assume a triple speed, the traction will increase by 9 times, if the speed quadruples, the traction will be 16 times higher and so on.
This means that an absolutely manageable kite with 10 knots will most likely become impossible to control with 20, since it can develop traction that exceeds our body weight and can literally lift us from the ground.
If we consider that the more we learn and the more we tend to go out with big kites, it is clear that a small distraction is enough to accelerate the kite by obtaining a traction that can even be twice our body weight, transforming us, if we do not have adequate control , in fluttering puppets without any chance to react.
Now it should be clearer how it is possible that a kite, which when stationary has just enough traction to stretch the tops of the harness, in speed can lift us from the water to leave or jump a few feet in height.
This also explains the great influence of the apparent wind on the traction: taking the example of the previous paragraph, if a kite with a natural wind of 10 knots can develop an apparent wind of 30 knots, it means that it can develop a traction 9 times higher than that generated by stationary, at the edge of the window.
Now think back to the last time you entered the water above, with the kite that tended, from a standstill, to drag you downwind, and now think about what could happen if for any reason you could escape in power zone … you would pass in 1 or 2 seconds from, suppose, 20 to 180 kg of traction, with the consequences you can well imagine!
Incidentally, this situation (kite that for distraction or driving error ‘escapes’ in the power zone while still on the ground or near obstacles) is the one that statistically procured the greatest number of accidents and the worst consequences.