Question from William, forwarded to CAL-TECH Kite Obelisk Team:

Vincente; There is something wrong with those numbers.
> Dynamic force at 20 mph is very close to one  pound per square 
foot. If a kite wing of 420 sq.ft. could achieve a coefficient of 
lift of one, it could lift 420 lbs. Very high lift airfoils can achieve 
lift  coefficients of perhaps 2.5, in which case 420 sq.ft. might 
lift 2.5  times 420, or 1050 lbs. Not very close to 6900 lbs.

http://obelisk.caltech.edu/...

CAL-TECH:
Indeed. Parafoils and foil kites have a very high lift-to-drag 
ratio. This can be seen by the fact that if flown as a kite, either one 
will quickly settle into a position close to 90 degrees from 
horizontal above the tether point. At this point the angle of attack is very low 
and so is the lift. If a parafoil's lift at 0 angle of attack was more 
than the weight of the skydiver, the guy would never make it to the 
ground! In fact the guy who sold us that parachute said it would pull no 
more than 200 pounds (it's a 2-person parafoil).

The important fact is that it's not quite convenient to think of 
kites and parachutes as "lift and drag" devices, since both forces 
contribute to the rope tension which in fact provides the weight-raising 
force. So a sail or a drag chute will provide much more useful force than 
parafoil at equilibrium. However the fact that the parafoil is steerable 
affords you some very convenient control.

When the parafoil is launched, its angle of attack is more or 
less 90 degrees, and the force it produces is tremendous. However there 
are dynamic effects that are hard to quantify that contribute even 
more force than the static case of a sail because the parafoil is 
moving along a section of a sphere and thus its area is changing with 
time relative to the wind direction. This effect is similar to 
suddenly unfurling a sail, or a gust of wind, in which case there is a 
momentary surge of force compared to the static case of an inflated sail, 
except that in the parafoil case, it is a continuously variable action 
controlled by steering the foil around. You can think of it as 
combining  the force of total drag (a plate at high angle of attack) with 
the advantages of it being shaped like a wing (thus producing 
considerable force perpendicular to its direction of travel), though I'm sure 
it's not that simple in reality.

So, in short, to get force out of a high-efficiency parafoil or 
foil kite the best thing to do is to fly it in a "figure-eight" path 
to keep the angle of attack high but also keeping the kite moving within 
its "envelope" (the intersection of the sphere defined by the 
tethered point and the rope and the wind field within the region where the angle 
of attack is appropriate).

For the big obelisk (14 tons) we were no longer steering the 
kite, and went instead with an "automatic" system where the steering lines 
were connected to two guide ropes running down wind from the site. The 
hope was that the kite would oscillate up and down: if the kite went 
up, then both steering lines would get pulled, which acts as a brake, and 
it would come down, the steering lines would slacken, and it would 
go back up, etc., thus achieving some of the dynamic effect. However this
oscillation didn't happen because the guide ropes were too slack;
instead all this did was force the angle of attack to be high by 
keeping the kite down. This still provided more force than the kite at
equilibrium but was way less than the figure-eight flying, thus 
with this obelisk it was much more up to the wind whether or not it 
would move---that's one of the reasons it was months before it was 
raised for the first time.

Here is a video capture of the footage of a kite test we did, 
where the wind speed was about 5 mph. The rope was tied to a truck's bumper 
via a spring gauge through an eyelet, a rope clutch, and another 
eyelet. The rope clutch eyelet assembly was weighed down by 320 pounds of 
concrete. Immediately after launch, the whole assembly came up off the 
ground; held only by the friction between the rope and the eyelet. This 
video capture is at the instant before the assembly slipped down the 
rope toward the truck. As you can see the angle between the rope after 
the assembly and horizontal is around 45 degrees so it is safe to 
estimate the kite was producing 500 pounds of force.