Theory of flight part 2 – Wind Window

Second chapter of “Teoria e pratica del kitesurf” by Geppy Piloni

Wind window width and window edge position.

During the course you learned the basics about the wind window, but there are some considerations to make to better understand some kite behavior.
The following figure is the one that is generally shown by the instructors:

wind window

It is a plan view, and therefore in 2 dimensions, and it is the one we will use for almost all the following examples, given that for our calculations at the moment this representation is sufficient.
The first thing to say is that a kite in flight, naturally and inevitably tends to reach the window edge, the kite never stops in a position that is not the window edge.

In any other position within the wind window, it passes without stopping, and unless you let it crash to the ground or turn it to 360 ° on purpose, the kite will again reach the edge of the window, slowing down until it stops.
Conceptually, the window edge is that position in which the kite reaches a balance between the main aerodynamic forces, the lift and the resistance.
The position of the window edge is not fixed, but varies according to the aerodynamic characteristics of each kite.
The 15 ° indicated in the drawing can become 20 ° for an inefficient kite or go to 10 ° for a very efficient kite.
Moreover, the wind window of a given kite varies according to the intensity of the wind, it widens when the wind strengthens and narrows when the wind drops: you will have noticed as a gust, the kite on the window tends to advance and vice versa, when the wind drops, it tends to move backwards.
Again, this does not mean that the kite changes position within the flight window, but it is the window itself that widens or shrinks depending on the intensity of the wind and the kite does nothing but chase the new position of the window edge.

If the wind decreases, the kite will retract because in the previous position it does not have enough traction to compensate for the lower speed of the air and therefore withdraws to increase the incidence.
When instead the kite is hit by a gust, it generates greater traction without a significant increase in resistance, and therefore the flight window widens.

Under the same conditions, the width of the window changes with the variation of the incidence, as it is possible to do with the 4-line kites via the depower, but in this case things are further complicated, because, varying the angle of incidence , the aerodynamic characteristics of each kite come into play and the ratio between the coefficient of lift and resistance that varies from kite to kite, based on the
depth of the profile, the size of the inflatable entrance edge, the elongation (or AR, aspect ratio)

In this case, so the increase in lift obtained with a greater angle of incidence, you pay with a noticeable increase in resistance and this causes the kite to have a greater power, but at the expense of a retreat of the window edge .
The important thing is that you do not have to imagine the window edge as something immutable or static, because it constantly changes position, both due to the wind variations and the effect of our adjustments, and indeed, later on, we will see how the edge window moves according to the direction of the table with respect to the wind and what are the consequences of this phenomenon.

Learn more reading more in the next article about the Apparent wind

Source: Geppy Piloni – Teoria e pratica del kitesurf 

By achim Theory