In response to speculation about Jesse's recent 22 second jump in the SF Bay area, I posted some comments about the effects of air currents and his multiple overhead kiteloops during his amazing jump:
only way the rider could have stayed aloft that long is with the
assistance of vertical air currents. The air current that Jesse was
riding may have been a thermal bubble, rotor (turbulence), or
combination. Either way, the rising air current would have been moving
downwind within the prevailing air mass (wind), so looping "helicopter
style" probably kept him in that moving column of air. It completely
makes sense, just like glider pilots making tight turns to stay in a
rising column or bubble of air.
2. If he hadn't looped, his kite
would have quickly flown upwind out of the vertical current as the
current drifted downwind with the prevailing wind. His example is a
good lesson. Most of us have experienced short bursts of crazy vertical
currents (usually rotor) but those currents quickly pass. If we quickly
initate a loop when we get hit by one of these updrafts, we might be
able to stay with it and get some otherwise unimagineable hangtime.
Other than staying in a rising air current, there is no aerodynamic
benefit to looping or turning a kite, other than possibly squeezing the
last bit of energy out of the kite (potential energy) and rider
(kinetic energy) just before touchdown. In fact, turning a kite will
generally reduce the kite's aerodynamic efficiency.
4. My guess
is that the sink rate (relative to still air) of a good, well trimmed
kite with a kiteboarder hanging from it is about 500 feet per minute,
or about 5 knots. So, if you can manage to stay in a vertical air
current that is rising at 5 knots or more, you can probably stay aloft
for a long time. Good fixed-wing gliders have a minimum sink rate of
less than 200 fpm (2 kts), so they can gain altitude by finding any
column of air that is rising faster than that, and circling in it like
a bird. I have flown gliders in more than 2,000 fpm (20 kts) vertical
air currents, which is like riding a high-speed elevator, and I had to
be careful not to get sucked up into a cloud as it condensed around me.
A common misconception is that looping a kite makes it fly faster,
which is wrong. The speed that a kite flies through the air during a
jump is dependent on two things: 1) its lift and drag characteristics
and 2) the effective weight of the rider and equipment. It's that
simple. During jumps, kites usually seem slow because they are flying
against the wind. If the kite is flying at 20 kts into a 20 kt wind, it
will appear to be hanging almost motionless, but if the kite is turned
or looped downwind it will still be flying at 20 kts through the air,
but with a 20 kt tailwind, it will be flying at 40 kts over the water.
It will look really fast, but its airspeed will be unchanged, and so
will the lift that it generates.