Theory of flight part 5 – Apparent wind and wind traction
APPARENT WIND AND TRACTION
Read this after reading and understanding the apparent wind blog post.
Let’s try to analyze the combined effect of the apparent wind and the exponential relation between traction and speed. Suppose we are still in the water with the kite standing on the edge of the window, which pulls for 20 kg: we start from the water, we start gliding and we park the kite at the edge of the window to tack at the side at a speed of 15 knots at this point, due to the simple effect of the apparent wind, which means that the wind with which the kite is flying is not more than 15 knots but 21 abundant (see fig. 1), the kite, although still standing at the edge of the window, develops a traction twice that generated before starting to tack.
If then at the speed of the table we also add the speed that we give to the kite when we make it fly up and down in the window quickly, we can arrive at situations in which the traction reaches values that are 10 or 15 times higher than those generated when we are stopped in the water with the kite at the edge of the window.
From a practical point of view this calculation shows us, for example, why with little wind it is very important to try to gain some speed to be able to start planing, even at the cost of let and fly for a few seconds and make the kite fly up and down in speed: once the planing is acquired, leaving and decreasing the pressure on the upwind edge of the board to reduce friction, the apparent wind will give us that increase in traction enough to maintain the glide itself and to tighten the bow even with the kite still. If instead with the kite fixed at the edge of the window we lose the glide, then just let it fly up and down, the apparent wind will give us the traction we need. This is also the reason why, in case you have to choose between two kites and be undecided whether to arm the big one or the small one, it is always advisable to use the smallest. In fact, if by chance we estimate the wind by default and mount the big one, we might find ourselves overpowered: since once we cannot reduce the surface in the water and we cannot do more than hold it, the choice of the small kite would have been the correct one. In the opposite case, on the other hand, that is if we estimate the wind in excess and we find ourselves under-unveiled, we can still generate enough traction to tack even with a small kite, without running any danger.