Kiteboard Theory - Thread 6: Aerodynamic Efficiency of Kites

The kites we use provide aerodynamic lift, which is what makes them useful for kiteboarding. Lift is a force generated by any aerofoil, whether a kite, wing, sail, windmill blade, or whatever, and that force is, by definition, exactly perpendicular (90 degrees, or at a right angle) to the free flow of air past the aerofoil. "Free flow" is an important distinction, because the direction of some air near the kite is altered by the kite.

If you are standing still while flying your kite overhead, the lift is a vertical force. If you are flying the kite just above the surface, at the edge of the window, the lift is primarily horizontal. But when the kite is flown in any other position the lift is at some angle from horizontal, with both horizontal and vertical components.

Kites, like all aerofoils, also generate drag, which is a rather self-explanatory, parasitic force which is by definition parallel to the free flow past the kite. The combination of lift and drag creates the combined total force acting on your kite, which is approximately equivalent to the tension, or pull, you feel in the lines. It's not exactly equivalent though, because gravity is involved too. The relationship between lift and drag is represented as the L/D (lift to drag) ratio, and I'm not going to say anything more about it because I already did in "Thread 2".

What I am going to do is describe the general factors that influence lift and drag, especially in relation to kites. Here goes:

FACTORS THAT AFFECT LIFT AND/OR DRAG

1. Larger surface. All other things being equal, a bigger kite creates more lift and more drag, but in roughly equal proportions. So even though a bigger kite creates bigger forces, its L/D ratio doesn't change much.

2. Flatness of kite across its span. A kite with less curve, and therefore more projected area, creates more lift. Among LEI (leading edge inflatable) kites, this is one of the main advantages of a "Bow" design compared to "C". For a given surface area, the "Bow" has more projected area, and therefore more lift, but no more drag than the "C".

3. Aspect ratio. This is generally the ratio of a kite's effective span, from tip to tip, to the average chord, or distance from the leading edge to trailing edge. A higher aspect ratio typically results in more lift, but proportionally less drag. Note that, as in (2.), the flatness of the kite also creates a higher "projected" aspect ratio. Here is a link with some handy diagrams and terminology: http://www.grc.nasa.gov/WWW/K-12/airplane/geom.html

4. Camber. This is the curve of the kite, or "fullness", from the leading edge to the trailing edge. The more camber a kite has, the more the air "bends" as it flows over the surface of the kite. Generally, the more camber a kite has, the more lift it creates, but there is a limit. A kite with too much camber can't maintain smooth, "laminar" airflow over its surface, and the air "detaches" from the surface and causes turbulence, which decreases lift but increases drag, which is of course bad.

5. Aerofoil section. In addition to the kite's camber, the exact shape of the cross section at any point along the kite is important. A "perfect" kite would have rigid top and bottom skins of a specific optimal shape, including the point of maximum thickness, or draft, about 30-40% of the way back from the leading edge. But the LEI kites that most of us fly have a single, flexible skin, with less than perfect shape stability, and a tube across the leading edge which introduces lots of turbulence to the airflow below the kite (which isn't nearly as bad as turbulence above the kite). All of these LEI kite-specific compromises reduce lift and increase drag.

6. Surface. Certain aerofoil surfaces (typically very smooth) are better at maintaining laminar airflow to maximize lift and minimize drag.

7. Bridles and lines. They don't affect lift but they do increase drag. Bridled or fifth-line kites have a slight disadvantage over four-line kites in this regard.

8. AOA (angle of attack). A kite's angle of attack is increased by sheeting in the bar. A higher AOA creates more lift, up to the point that the kite begins to experience turbulence above the upper surface. This condition is a partial or full aerodynamic stall, which decreases lift but increases drag.

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