
It takes energy to jump. Where does that energy come from?
The
answer is kinetic energy: specifically that resulting from your forward
motion through the air. Not your motion over the water, and not
relative to the earth. Just relative to the air.
Through the
manipulation of aerodynamic forces, your kite can be used to convert
kinetic energy (an exponential function of airspeed) into potential
energy (a linear function of height). In a particular set of
conditions, there is a maximum possible theoretical height of jump that
can be achieved, due to the physical law that energy must be conserved.
You can't create energy from nothing. With an ideal kite, and ideal
technique, this theoretical height may be approached, but it can never
be exceeded.
I should mention that there are a few other sources of energy that can augment the height of a jump. They are: 1. Kinetic energy related to the vertical component of your speed over the water as you ride up a wave face. 2.
Electrochemical energy stored in your muscles that can be converted by
contracting your muscles to accelerate your mass upwards against the
vertical force of the water against the board. 3. Kinetic energy related to vertical air currents as described in "Kiteboard Theory  Thread 3: Lift, Looping and Hangtime". These sources of energy are usually minor and I won't dwell on them any more in this thread.
Now, back to the conversion of kinetic energy to potential energy: To
achieve the highest jumps, you must start with lots of kinetic energy,
as a function of airspeed. There are two obvious factors that are
relevant to doing this: 1. Ride in strong wind. 2. Maximize your
apparent windspeed at the instant you take off. This means riding fast
in a relatively upwind direction. That's why carving hard upwind
without losing too much speed is vital to good jumps.
Assuming
that all of your basic technique is good, your highest jumps will
result from efficiently converting as much of your kinetic energy as
possible into potential energy. Ideally, at the apex of your jump, you
will have no kinetic energy at all, which means that your horizontal
airspeed will be zero, which means that you will be swinging below your
kite downwind at exactly the same speed as the wind. This scenario can
generally be achieved with the right timing.
Here are the fundamental factors to putting it all together: 1. The windier the better. 2. Just before jumping, ride fast and carve hard upwind to maximize your apparent windspeed. 3. Send your kite back aggressively to take off. The best technique is a matter of trial and error. 4.
While jumping, your kite should be trimmed for "minimum sink rate",
which might be different than the trim for maximum L/D ratio. That
probably means your bar should be trimmed as tight as possible without
inducing any aerodyanamic stall. You can probably only know for sure
that you have ideal trim if you use telltales (which I'll go over in
another thread). 5. Keep your kite back just enough that your
pendulum motion matches as closely as possible to the windspeed and
direction when you are at the apex of your jump. Again, trial and
error. If you actually get it right, and you happen to be smoking at
the apex, you could blow gentle concentric smoke rings. (After the
apex, don't forget to dive your kite aggressively forward, or you'll do
a nasty backflop.)
Here are a couple more thoughts about energy conversion and conservation: 1.
No matter what, you'll never convert all of your kinetic energy into
potential energy because there will always be some losses relating to
aerodynamic drag, which effectively energizes the air by way of
turbulence. 2. As I mentioned before, kinetic energy is an
exponential function of airspeed, but potential energy is a linear
function of jump height. That means that if you double the airspeed at
lift off, your theoretical best jump height will be four times as high.
Or another example: If you have the right equipment, nerve and skill,
you should be able to jump nine times as high in 45 knots apparent wind
as you could in 15 knots apparent wind (1718 mph on iKitesurf), which
could be pretty !@#$% high.


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