Theory of Flight part 1 – How Kites fly

This is the first part of an interesting guide (Geppy Piloni – Teoria e pratica del kitesurf), modified a bit and traslated in english for you! Hope you find it usefull!

Theory of Flight part 1 – How Kites fly

Immediately after the kitesurfing course, in that phase during which you can ride but still can not return to the starting point, I began to wonder why some kite behaviors and in general what were the rules that govern the kitesurf practice.

I progressed rather quickly, but I could not fully understand all the theory and physical phenomena that determined the techniques of conducting the board and the kite: I was learning without knowing why. At that time, there were very few very good practitioners and even fewer experts: every time I meet another kiter, I was bombarding him with questions but almost always the answers were unsatisfactory or even conflicting or inaccurate, so I started to think , to make sketches and calculations.

When I thought I had found a satisfactory explanation for the questions, I thought that maybe my notes could be useful to someone else and I decided to publish my conclusions on the Internet.

In order to fully understand the concepts expressed, the reader should be a kitesurfer who knows the basic terminology and flight techniques, who has done a proper course at a good school and maybe some session independently.

To avoid a purely theoretical discourse, I tried to describe some common situations in the practice of kitesurfing to realize the reasoning.
Premise that I am not an engineer and therefore I hope that the most prepared will forgive me an inaccurate terminology and the approximation in expressing the most complex concepts.
If someone found errors, inaccuracies, or considered totally wrong my conclusions, I warmly ask you to send me a report or a criticism by the contact form, it would be very welcome.

My hope is that by comparing the first practical experiences with the concepts illustrated below, we can better understand the forces that come into play and therefore on the one hand improve the technique and on the other hand have greater control in emergency situations.
Most of the concepts described apply to any traction kite, but personally I have direct experience of kitesurfing inflatable kites only, so everything you read, unless otherwise specified, is intended only and exclusively for this kind of kites.

1. How kites fly

A kite flies according to the same physical laws that regulate the flight of birds and airplanes and we can therefore compare it to a wing.

theory kite flight

As such, by opposing its surface to the wind flow with a certain angle, said angle of incidence (or pitch), forces the moving air to follow two different paths, one below the surface and one above of it.

The air that follows the lower path, thanks to the concave profile of the kite, travels a shorter route than the one that runs on the back, which in turn increases the speed to recompose the disturbed balance by the presence of the kite.
In this way, a difference in pressure is formed between the lower and upper surfaces according to the relationship theorized by Bernoulli with his famous theorem (greater speed = lower pressure and vice versa) and the kite is pushed by the high pressure created on its back (or sucked up by low pressure, it’s the same thing).

If we use the convention used to explain the lift of a wing, we can break the traction into a vertical component, called a lift and into a horizontal, called resistance. In the case of kites, not being like a wing bound to the fuselage of an airplane or the body of a bird, this representation is not totally correct, because in the case of a kite in full power zone on the water, all the traction would be considered resistance and therefore useless and harmful for the flight. In fact it is not so, thanks to the lines that, regardless of the vertical angle of the kite, still transmit the traction to our body allowing us to move forward.
I think it is more useful for our purposes to break down the traction in vertical and horizontal components and we will see later the practical implications of this aspect.

The pull of a kite depends on the pressure difference between the inside and the outside and this magnitude increases with increasing angle of incidence.
If the angle of incidence increases too much, however, the air above the back of the kite, which normally follows a so-called ‘laminar’ flow, can no longer follow its profile, begins to detach and form vortexes that transform the flow into ‘turbulent’, generating the phenomenon called ‘stall’.

Each kite has a critical angle of incidence beyond which it stops flying and its amplitude depends on the depth of the profile, the elongation, the structural rigidity and other factors of minor importance but we can still quantify it on average around 15/20 °.
In practice, we have to deal with the stall every time we go in the water with gusty wind, when in the wind holes the kite does not even generate enough lift to support its weight or when the wind begins to drop letting us soak in water or when for a pilot error we send the kite out of the window.

With the experience you can guess when, due to a drop in wind or a driving error, you are approaching the stall, but at the beginning you can not understand in time what is happening and when the stalemate is now evident it’s too late to react.
In the event of a weak wind the kite starts to retreat into the window, first slowly then faster and faster until it falls into full power zone without any traction.

With 4 lines kites with depower, instinct leads us to do the opposite of what would be right. In fact, the first thing that comes to mind is to pull the bar towards your body: unfortunately this operation is successful only if the kite has a low angle of incidence or if at the same time we can make it turn and then accelerate because otherwise we do nothing but worsen the situation, increasing the angle of incidence beyond the critical value, bringing the kite to stall if it was not yet or making it deeper than it already was.

In this way the kite retreats into the window, further increasing the angle of incidence which increases the resistance, starting a vicious circle that leads the kite to fall back towards the power zone without responding to our commands.
In these situations, we must weaken the kite as soon as we realize that the stall is near or just started, so that the laminar flow around the kite recomposes itself, decreasing the resistance and restoring the lift.
Who has already tried this technique knows very well that instantly the kite starts to fly forward, accelerates generating enough speed to keep the flight and return to being responsive to the controls: of course if you wait too long to weaken or if the wind it’s really weak, there’s nothing to be done, because there’s still not enough energy to keep the kite in the air.
But if the angle of incidence, as they said before, must not exceed 15/20 °, then how is it possible that the kite can fly in power zones, with angles of incidence close to 90 °?

To understand the reason we must learn the concept of apparent wind, which we will do after having deepened the knowledge of the wind window.

Next chapter

Source: Geppy Piloni – Teoria e pratica del kitesurf 

By achim Theory