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