Big Earl - 17-12-2009 at 04:09 PM
Does anybody know a formula for how much force wind will exert on an object? I know the shape of a kite and kite speed greatly affect the power of the
kite. I am thinking about creating a scale for kite strength. Say a flat sheet 4m2 in 10 mph wind pulls with a strength of 40 lbs, 4m2 kite A pulls
with 50 lbs, 4m2 kite B pulls with 60lbs. then we could see an objective "pull scale" for all kinds of kites. Maybe time them across the window and
come up with a similar scale for kite speed. Has anyone done this or something like it?
Any ideas?
Kamikuza - 17-12-2009 at 04:55 PM
I think it'd be a bit more complex than just a single formula - what with projected area, drag etc ... hence my desire for a kite pull test rig 
acampbell - 17-12-2009 at 05:09 PM
The engineers here will do better than I can (Krumly, where are you?), but while data for a flat object might be available, there are so many more
dynamics involved with an aerodynamically sound flying object that the comparison would mean little. Measuring the force on a 4x8' piece of plywood
compared to a kite of similar area would not be useful.
I think what may be more useful and to the point about what you are looking for would be flight polars (charts) that describe the Lift/ Drag (L/D)
ratio over various airspeeds. In my soaring days I could compare polars of various aircraft that were measured both theoretically and from real world
measurements by flying with the right instrumentation. There exists nothing in the kite world like this that I have found. Flexifoil claims to have
used the Cambridge University wind tunnels for research but that data is not public. Peter Lynn literature describes the L/D of a modern high
performance kite at around 6, but when I ask how this was arrived at, the speculation was that it was inferred theoretically from CAD design software.
(sailplanes can achieve L/D's of over 50, last I heard)
I have seen mentions on-line of grad-student papers talking about measuring kite L/D by plotting kite airspeed through the window against the local
true wind speed, but that trail went cold for me.
Great topic, though and I would love to hear more if anyone has modern, pertinent data. the only other attempt I have seen to calculate this in
Kites is in "Stunt Kites II" by van der Horst and Velthuizen (c) 1994, 95. but those kites were complete slugs compared to what is out there now.
rocfighter - 18-12-2009 at 04:00 AM
SEE your math teachers where right. You will need this later in life!!
Erics - 18-12-2009 at 07:01 AM
Can you say that a 4mtr depower has same pull as a 4mtr foil and even then what difference is there between a blade 4mtr and a low lift beamer.
Interesting idea though.
lives2fly - 18-12-2009 at 07:38 AM
Well I know that a 6m High aspect fixed bridle, a 6m depower foil, a 6m Arc and a 6m inflatable will all pull and perform differently but I dont know
exactly why so it would be great if there was an easy way to explain it!!
william_rx7 - 18-12-2009 at 08:40 AM
I've been looking into this lately on my web site, LocalKiteSpots.com
Here's the basic lift equation. Note, that velocity is most significant, but kite size (surface area) and density are factors as well.
Lift=Lift_Coefficient*(1/2)*(Surface Area)*(Density)*(Velocity)^2
Density of air changes quite a bit, especially with temperature, and altitude. At the same wind speed, if you are comfortable on a 12m kite at +20c,
a 10m kite will seem as powerful in -20c.
Since LocalKiteSpots.com has access to all of the data to calculate air density, I created a new statistic, called "Density Boost". It compares
current air density with "normal" air density (sea level, 20c, 50% humidity).
Density Boost is now part of every forecast on LocalKiteSpots.com. See http://localkitespots.com/boost.htm for details, or visit http://localkitespots.com/kitesurfing for your local map, and density boost for your local spots.
Jolt - 18-12-2009 at 10:32 AM
Well it truly depends on the shape of the wing, and how its moving through the air and a lot of other factors :/...
If i were going into mech. or aeronautical engineering i would be able to tell you later on, sadly im only doing electrica.
indigo_wolf - 18-12-2009 at 12:16 PM
Sooner or later, I expect you are going to end up here.
ATB,
Sam