Sunday, April 12, 2015

What's a Stall of a Kite and How to Prevent It

Many times it has happened to me to see kitesurfers stalling their kites.

This occurs especially in low wind conditions, but if your kite has bad trimming, it can happen also with stronger winds, making it much more dangerous.

The video below shows how kite stalling happens:

In many kitesurfing manuals the powerkite behavior is very well explained, but from my experience the concept is rarely transmitted to kiteboarding students and isn't exposed well enough to a huge amount of kiters. The result of that is easily visible on low wind days and, as shown in the video, can lead to dangerous situations both for the kite and the kite gear.

So, what does it mean to stall a kite? 

Well, a powerkite is nothing more than a curved wing held by our lines, which influence its behavior.

The principal way to measure the behavior of the wing in relation to wind is to measure the degree of its angle of attack. Angle of attack represents the angle between the wing chord and the relative wind direction.

Changing the angle of attack we can change the lift produced by the kite: the bigger is the angle, the stronger is the lift, but just up to a critical limit, where the kite (or any wing) starts to stall. Any further increase of the angle of attack above the critical angle will produce the wing stall. Let's see why.

Any wing basically has a lower and upper camber which have different shapes. In the kitesurf case the upper camber has a more accentuated curvature: this creates a longer path for the air passing on the extrados of the wing, forcing it to move at faster speed than the air on the lower side. The diference of speed, acording to Bernulli's laws, creates a depression on the upper side of the wing, which generates the main component of the lift.

The path of the air also depends on the angle of attack. If we increase it, we enhance the difference of length of air paths, resulting in a stronger lift. But this just to a point: it's called the critical angle of attack, beyond which the upper layer of air detaches from the surface, reducing the lift drastically.

Experimental evidence shows what happens: the upper air layer detaches from the wing extradox, creating turbulence, which enhances the pressure on the upper wing's surface, balancing the lower surface pressure, practically nullifying the lift's main component.

So when are we stalling the kite? Now it's easy to understand: when the angle of attack is too big: in other words, when the back lines are too short, due to the excessive pull on the bar or to a bad kite trimming. 

That's what happened to the guy in the video: he was continuously pulling the bar and overpowered the kite.

We can understand the kite is stalling by simply observing it:
1. The kite starts to fall back on the trailing edge, in the powerzone direction just in front of us;
2. The kite's shape in not straight as usual but the tips tend to close behind (see the above image: the purple kite has a more pronounced arc shape, the green is flatter and more open).

What do we do in that case? As usual, we follow one of the best rules in case of control loss: we release immediately the pull on the bar or even let it go completely. By doing so, the kite will immediately stop stalling, flying back to the border of the window. In this movement it will generate power, because as soon as it gets out of the stall, the lift is restored.

The pull will be proportionate to the distance from the border of the window: if the kite was all the way down in the powerzone, the pull will be strong! 

This fact leads us to two final conclusions: 
1. The faster you cease to stall the kite, the slower it will fall in the powerzone, generating a less dangerous pull in case of relaunch; 
2. If you had stalled the kite completely in the power zone - especially if the leading edge is still pointing upwards (as in the third attempt in the video), you might want to activate your quick release in order to prevent the unwanted kite restart and huge pull.

If you have any comments or questions, please share your opinion with us!

Blog post about the stall of a kite: what's it caused by and how to prevent it safely
Date published: 04/12/2015
Date Update: 04/12/2015