AirFoil (Aerofoil) Principles

 

The subject of airfoils often leads to some degree of confusion during kiteboarding lessons - mostly, the trouble seems to be understanding what effect pulling in or pushing away on the control bar has on the kite. Often, people refer to pulling in on the control bar as powering the kite and pushing away as depowering it, but while true in some circumstances, it's the opposite in others. So, for a more consistent understanding, I suggest that you think of pulling in as increasing the kite's angle of attack and pushing away as decreasing it since that is the direct result of the action and is always the case. Now, the kite's reaction to that direct result will vary depending mostly upon the wind speed flowing over the kite's surfaces. In higher winds, the increase in the kite's angle of attack resulting from pulling in on the control bar will increase the lift forces generated by the kite, but in low wind conditions, the increased angle of attack can cause the kite to stall downwind. The angle of attack must match the wind speed and for any given wind there will be a stall point when the angle of attack is too great for the kite to generate adequate lift. 

 

Pushing the bar away results in the kite returning to a neutral angle of attack where the kite produces enough lift to overcome its own drag and little more. In extremely high winds, it may be necessary to shorten the front lines by trimming (sheeting in) the kite to decrease its angle of attack below neutral (thereby decreasing the kite's ability to generate lift). 

 

The simple way to think of it is that the kite is flying passively (neutral) when the bar is pushed away from you, it's flying aggressively (powered) when you pull in on the bar and it's flying weakly when you trim in the front lines.  If you try to make the kite fly aggressively in light wind there will be disparity between the kite's angle of attack and the available wind speed and the kite will stall and fall downwind (backstalling) and if you try to make the kite fly weakly in light wind, the opposite disparity will exist and the kite will overfly you and crash upwind. Finding the balance between angle of attack and wind speed is central to learning how to fly a kite. 

 

The following ancient government educational video actually does a really nice job of explaining the concepts of aerodynamics and lift. Watching it should help you progress faster with less confusion without having to take a physics course in thermodynamics. The most relevant parts can be found from 3:30 through 9:00.

Listen to airfoil TTS rather than reading it - Airfoil TTS
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