Kiteboard Theory - Thread 3: Lift, Looping and Hangtime

Postby James on Mon Jul 30, 2007 11:06 am

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:

1. The 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.

3. 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.

5. 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.

Cheers,
James
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James

Re: Kiteboard Theory - Thread 3: Lift, Looping and Hangtime

Postby James on Mon Jul 30, 2007 2:52 pm

James wrote:Most of us have experienced short bursts of crazy vertical currents (usually rotor) but those currents quickly pass.


"Rotor" represents a type of wind turbulence that is common downwind of obstructions such as mountains, trees and buildings. Think of a rotor as a series of huge barrels of air (without the actual barrels), rolling along the surface. The whole bay at Squamish was full of huge rotor turbulence on Sunday. The gusts at the surface were generally the result of air rolling or tumbling down from higher altitudes on the downwind side of the bluffs, to the West of the Spit.

Image

When rotor turbulence rolls over you, normally you will at first experience a lull and reduced lift from your kite, followed by an elevator-like gust that sucks your kite upwards. If you loop your kite at the right moment, you might be able to ride the rotor turbulence as it rolls along the surface, just like the dude in SF apparently did.

I find that the best way to visualize wind turbulence, or tidal current eddies for that matter, is to imagine the way water flows over and around rocks in river rapids, and then think on a way bigger, slower scale. It's fundamentally the same thing (although pressure and temperature differences in the atmosphere do make things more complicated).

Don't kill yourself,
James

Edit: PS: Here is a link to the guy in SF looping his kite along the back of a rotor:
http://www.youtube.com/watch?v=-bcy44aIr9k

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