The
lift/drag (L/D) ratio of a kite is the most fundamental characteristic
that defines its performance. It is the result of many design
parameters that can be summed up as one simple value, much the same way
that an engine's horsepower is the quantifiable result of a myriad of
design features.

A kite's secondary, but also important
performance characteristic is it's maneouverability, or turning
characteristic. Other charactersitics include range of trim (depower)
and various convenience and safety characteristics which are not
generally related to maximum performance. I won't be focusing on these
other factors in this post.

Now, back to the L/D ratio:
Anecdotal descriptions are often made of kite performance, such as
"quick across the window" and "flying far forward", but these
descriptions are all simply functions of the L/D ratio. The L/D ratio
has two components: 1) lift, which is the useful
aerodynamic force created perpendicular to the free airflow past the
kite, and 2) drag, which is the parasitic aerodynamic force parallel to
the airflow. (It is worth noting that the free airflow over the kite is
exactly the same thing as the apparent wind at the location of the
kite.) Lift is created, as with all aerofoils, by the interaction of
the kite with the airflow, and resulting differential pressures above
and below the kite's surface. Drag is also caused by the airflow over
the kite, generally by surface friction and turbulence. Many design
factors affect lift and drag, and I'll cover some of these in a future
thread. For now, here is a link that I googled, which appears to depict
these principles, although I only glanced at it superficially: http://www.auf.asn.au/groundschool/umodule4.html

Here's
an example: Suppose that you are standing still on a beach, flying your
kite steadily overhead in a steady wind. The kite is exerting about 40
pounds of tension, via the lines, on your bar and harness. If the kite
is really flying exactly overhead, with the lines completely
perpendicular to the airflow, then that means that your kite and lines
have no drag whatsoever, and therefore an infinite L/D ratio (40 lbs
lift, 0 lbs drag). But that's impossible, because even the most
efficient aerofoils generate some drag, and in the case of the kites we
use, the drag is typically between 1/5 and 1/7 of the lift, which
results in L/D ratios between 5:1 and 7:1. Because your kite and lines
have some drag, they will be flying a little downwind of directly
overhead. If there are 8 pounds of drag, which is 1/5 of the 40 pounds
of lift, then the kite will be flying 1/5 as far back as it is high.
So, if it is 100 feet high, it will be flying 20 feet back from
directly overhead. This would represent a L/D ratio of 5:1. Get it?

Ok,
now I'm going to complicate things just a little. Supposing that the
kite and lines weigh 8 pounds, the kite is actually generating about 48
pounds of lift in order to support itself and also exert 40 pounds of
tension on the bar and harness. So, taking the weight of the kite into
consideration, the L/D ratio in this situation is actually about 6:1
(48 lbs lift, 8 lbs drag), even though the angle of the kite and lines
suggests about 5:1. The windier it is, and the greater the lift that
the kite is generating, the less the effect the kite's weight will have
on its apparent L/D ratio.

If you want to compare the L/D ratios
of different kites, try flying them overhead, exactly side by side. The
one that flies farther forward will be the one with a higher L/D ratio.
Also, the trim (depower) of the kite will affect its L/D ratio, and you
can experiment by flying your kite overhead and sheeting the bar in and
out until you find the exact trim that causes the kite to fly the
farthest forward. This brings up a couple of other points:

1) The
apparent windspeed, or airflow past the kite, will also affect its L/D
ratio. Any aerofoil can generate slightly different L/D ratios at
different speeds.

2) The trim that causes the kite to fly farthest
forward is not necessarily the same trim that will result in the most
lift or line tension. For a given kite, it is quite likely that the
maximum force, tension, or whatever you want to call it, is caused by
sheeting in the bar as far a possible without inducing aerodynamic
stall (to be discussed in a later thread). But the best L/D ratio might
result from sheeting the bar out a little, reducing lift a bit, but
reducing drag proportionately more.

So, why is L/D ratio so
important? Because a higher L/D ratio results in a kite that pulls in a
more beneficial, or efficient direction. All other things being equal,
it'll let you kiteboard faster, point more upwind, jump higher and
longer, and stuff like that. Suppose that you are kiteboarding at 20
mph, in a 15 mph wind, and you are going 10 degrees upwind with a big,
efficient kite (high L/D ratio) and a Spleene Session or flat plywood
board. This is the example that I presented in my previous thread about
apparent wind, and as I explained in that thread, the apparent wind
would be blowing at an angle of about 33 degrees from straight ahead.

Now consider this:

1)
If you are kiting in a dream world, and your kite has an infinite L/D
ratio (no drag), then the kite would be pulling exactly at a right
angle (90 degrees) to the apparent wind. That would mean that the lines
would be pulling 33 degrees forward from straight sideways, which is
more than enough to make good progress with good edging technique.

2)
But if you are kiting in the real world, and your kite has a L/D ratio
of 6:1, it will be flying about 9 degrees back (derived from simple
trigonometry: Arctan[1/6]) from a right angle to the apparent wind.
That means it will be pulling about 33-9=24 degrees forward from
straight sideways, which is probably about the threshold for
maintaining speed without having to bear off, even with the right board
and technique.

3) If you are flying a lame or poorly trimmed kite
with a L/D ratio of only 4:1, it will be flying at 14 degrees back from
a right angle to the apparent wind, which means 33-14=19 degrees
forward from straight sideways, which won't possibly keep you going
unless you bear off to improve your apparent wind angle, which will
result in going a bit downwind instead of a bit upwind.

4) Bonus
point: Your kiteboard also has a theoretical maximum L/D ratio of its
own, specific to a given speed and perfect riding (edging) technique. I
don't know what the maximum L/D ratio of any board is, but my wild
guess is that they typically fall somewhere between 2:1 and 4:1. I do
believe that my Spleene Session 141 has close to double the L/D ratio
of my first generation (red with yellow sun) Naish TT Sol 125. (Simply
put, the TT Sol is smaller and generates less lift, but because it has
a lot more rocker, it also generates more drag. It's fun and agile, but
like most freestyle twin tips, not very efficient.) If you're into
math, you can deduce that you'd theoretically need a board with a L/D
ratio of at least 2.2:1 in order to make example 2) work, but
considering waves, less-than-perfect technique, and stuff like that,
you'd probably actually need a board with a considerably better L/D
ratio than that.

Another tidbit: The tension or pull from a kite
is actually a combination of both lift and drag, but mostly lift in the
case of high L/D ratios. The higher the L/D ratio of the kite,
typically the bigger kite you can fly to exploit its potential.

And
another: A parachute would have an abominable L/D ratio because it
generates almost all drag and no lift. You could kiteboard with a
parachute, but you'd probably be stuck riding at an angle of about 45
degrees downwind, relative to the true wind direction. That would
result in a lot of walking.

One more thing, regarding the notion
of how fast a kite crosses the "window" or "power zone": Consider
standing on the beach while you fly your kite right across the deepest
part of the power zone, just above the ground, immediately downwind of
you. That's when your kite will generate the most force (and power, if
you want to get technical) because it creates the fastest apparent
wind. If your kite has a L/D ratio of 6:1, it will cross the deepest
part of the power zone at roughly six times the true windspeed (and its
apparent wind will be about the square root of (1^2 + 6^2) = 6.08 times
the true windspeed. The reality is that your kite probably won't go
that fast because it will generate so much force that you'll get
dragged downwind, which has the effect of moderating the apparent wind.
(That is, both your apparent wind and the kite's apparent wind: If the
true wind is 15 mph, and you get temporarily dragged downwind at 8 mph
as the kite crosses the power zone, then the apparent wind passing you
will be only 7 mph, and the kite's speed and apparent wind will also be
greatly reduced.) Kites that seem to cross the window fastest are
generally just the smaller, less powerful ones because they don't drag
the kiter as much. This is frankly more significant than their L/D
ratios in this particular circumstance. If you can stand still while
you fly a small trainer kite across the power zone, you probably know
that that kite will move insanely fast, and generate some serious force.

That's all for now,

James