
Here are some definitions and examples and stuff. I know that none of
this info replaces good technique, but I believe that a decent grasp of
the physical principles at play in any endeavour can help to advance
the boundaries of performance. If any of it sinks in, I guess it's
worthwhile.
VELOCITY
Velocity
is a vector which represents speed and direction. The term is often
used erroneously to represent speed, which is a scalar value (having no
specific direction), but in most discussions that isn't a big deal.
FORCE
Force
is typically measured in pounds or Newtons. Like velocity, force has
both magnitude and direction, but the direction is often ignored in
very simple discussions. Lift and drag generated by a kite represent
force, as does the tension in the lines and what is often erroneously
termed "pressure" on the bar. Force mustn't be confused with pressure,
energy or power. They are all completely different concepts.
PRESSURE
Pressure
is the force per unit of area. In a kite bladder, that's usually
anywhere from about 510 psi (pounds per square inch), which is about
1/3  2/3 of atmospheric pressure at sea level (~15 psi). If your kite
bladder is inflated to 7 psi, that means "gauge pressure" which is the
difference between the "absolute pressure" inside the bladder (~22 psi)
and outside it (~15 psi).
The tension, or force, acting on the
fabric of a tubular bladder casings is proportional to the diameter of
the tube. The fabric and stitching of an 8" diameter tube has to resist
twice the tension, or force, of that of 4" diameter tube, given the
same pressure.
The aerodynamic pressure acting on the surface of
a kite will vary across its surface, but on average, if there are 200
pounds of tension in the lines of a 12 m kite, then the average
pressure acting on the surface of the kite would be about 200
lbs/18,600 sq inches = 0.01 psi. At about 1/1,000 of bladder pressure,
that's pretty light pressure. This is "dynamic" pressure, caused by the
flow of a fluid, as opposed to "static" pressure, such as inside the
bladder.
ENERGY
Energy
can be found in different forms, including kinetic energy (proportional
to mass times the square of velocity), gravitational (potential) energy
(proportional to mass times height), heat energy, wave energy,
electrical energy, chemical energy and atomic energy. It is normally
measured in Joules, BTU's, watthours or kWh. Energy can be converted
between forms, but it is always conserved, and it is a scalar value
(having no direction). Kinetic energy and gravitational potential
energy are most relevant to kiteboarding, although heat and wave energy
also play a role.
When energy is converted from one form to
another, it can be said that "work" is performed. When a force is
exerted, causing a particular object to move, then the energy
converted, and work performed, can be calculated by multiplying the
force by the distance that the object is moved. The energy converted
(from kinetic to potential) when a kiteboarder jumps, can be calculated
by multiplying the force acting on him (his weight) by the height that
he is elevated by that force.
BTW, weight is defined as the mass
of an object multiplied by the force of gravity. The mass of an object
is constant (notwithstanding the effects of general relativity) but
it's weight varies depending where it is. It weighs more on Mount
Everest than it does on the moon, and slightly more again at sea level.
POWER
Power
is the rate at which a device can convert energy from one form to
another, and is usually measured in watts or horsepower. It can be a
confusing term, because the relevant types of energy have to be
defined. A 100 watt lightbulb converts electrical energy to light
energy and heat energy at a
rate of 100 watts, but if the lightbulb is only 20% efficient, that
means it only converts electrical energy to light energy at a rate of
20 watts.
A bouncing rubber ball, and a jumping kiteboarder,
both convert energy alternately between kinetic and potential forms,
but because they aren't sustainable processes, power ratings aren't
relevant in these examples.
In some circumstances, power can be
quantified by multiplying a force by the speed at which that force is
exerted. Here are some examples of power calculations based on specific
examples:
1. Heavy(ish) kiteboarder on freestyle board going
25 mph with 30 lbs resistance (independent of wind speed and kite
size): power = 2.0 hp
2. Light kiteboarder on high L/D ratio board (eg Spleene Session) going 18 mph with 20 lbs resistance: power = 0.96 hp
3.
Same kite as in example (1.), attached to a Honda Civic with tires
overinflated to reduce resistance, on a broad reach on an airport
runway, with wind blowing like !@#$%, chicken loop mechanically
fastened to car (pilot couldn't possibly hold it), going 50 mph with 40
lbs resistance (rolling resistance only, apparent wind over the beam):
power = 5.3 hp
A kite alone does not generate power. Power is
generated when a traction kite and surface vehicle interact as a
system, whether that vehicle is a kiteboard and rider, a buggy and
rider, a car, an ice boat, or whatever.
The maximum achievable
power is based largely on what the vehicle can handle. A given vehicle
will have a maximum practical upper limit to power generation. If the
vehicle is a particular kiteboard and rider, they may have an
approximate upper end of 1.5 hp, regardless of whether they achieve it
with a big kite in light wind or a small kite in high wind.
The
heavier and stronger the rider, and the more efficient the board, the
higher the maximum power will be. A 200 sq m monster kite wouldn't help
a kiteboarder generate more power than with a normal kite, but a
freighter or a heavyweight ice boat could generate huge power with the
monster kite.


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