Theory of flight part 3 – Apparent wind
The apparent wind
The knowledge of the concept of apparent wind and its effects in the practice of kitesurfing is essential to maximize the potential of the kite.
We begin to introduce the general concept that concerns any sailing boat and then we will see the peculiar aspects for kitesurfing.
The wind that acts on the sails of any sailing boat when it is still, is only the natural wind, but once the boat has reached cruising speed, a part of the wind generated by the wind is added to the natural wind. boat speed itself.
In the following example we schematize a typical situation, with a natural wind of 15 knots and a traverse boat.
When the boat has reached the maximum speed allowed by the conditions, which we assume to be 15 knots, it will not sail any more with the natural wind, but with a so-called apparent wind coming a little further from the bow and of intensity higher than that of the natural wind.
In this case, with wind direction perpendicular to the vessel’s direction, to calculate the wind intensity we can use the Pythagorean theorem: if we consider the natural wind vector and that of the vessel speed as the cathets of a right-angled triangle , the apparent wind is given by the length of the hypotenuse and with the Pythagorean theorem:
The apparent wind has an intensity greater as greater is the speed that can reach a certain boat with the same natural wind.
The apparent wind can be preponderant compared to the natural wind, in the sense that there are boats that can reach speeds higher than those of the wind in which they move. On the frozen lakes the sail sledges arrive at speeds 3 or 4 times higher than the natural wind, given the reduced ground friction to the minimum terms.
With the traction kites this concept is fully applied, but there is a peculiarity not found in any other type of sailing boat.
In fact, kitesurfing is the only sailing sport where the speed of the boat and the sail do not coincide, as we can fly the kite up and down with a sinusoidal movement while the board can hardly touch: in practice there are situations where we travel, suppose, to 15 km / h and the kite flies at 40 or 50 km / h.
In this way we create a wind of our own, which can reach intensity 3 or 4 times higher than those of natural wind, with a consequent increase in traction.
Apparent wind, angle of incidence and power zone
Now that we know the apparent wind, we can answer the question left pending at the end of the 1st paragraph: why does the kite fly at very high angles of incidence without stalling?
Because, compared to the direction of the natural wind has an incidence that can reach around 90 °, we have seen how the kite flights in the apparent wind and therefore the angle of attack with respect to the air flow is much lower.
Let’s take an example:
As can be seen, the angle of incidence respect to the natural wind of 60 °, but at the speeds shown in the figure, the angle of incidence respect to the apparent wind is 17 °.
The tendency of the kite to increase speed as it approaches the power zone, and to reduce it as it moves away, compensates for the opposite trend of the incidence, allowing it to remain on values that prevent the kite from stalling.
This explains why, when the kite ‘sinks’ stalling in power zones without speed, it can no longer fly, even if maybe a few seconds before it flew in the same portion of the window at great speed.