http://www.tehachapinews.com/home/Blog/Vincente/5673 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/home.htmlCAL-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 thisoscillation 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 atequilibrium 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. 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/home.htmlCAL-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 thisoscillation 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 atequilibrium 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. en-us