The engineering of stunt kites -- begins with the basic stunt-kite equations 

Also see  More about designing stunt kites  and  About designing FLYING handles

Designing creates appearance.  Engineering creates performance.  More about that.

When you engineer a product, the first step is to investigate its theoretical performance in its theoretical environment. In the case of a dual-line kite, the performance is "speed & pull." The environment is the "flight envelope."

You must evaluate the equations that describe the kite's speed & pull. The equations tell you what must be done to achieve the desired speed & pull characteristics.



Here are the basic speed & pull equations for a dual-line kite being flown from a stationary point F. They describe speed & pull as functions of wind speed (W), kite's location K on flight envelope (<PFK), kite's flight direction (flying along an iso-power line or along a power-gradient line), and the kite's aerodynamic efficiency (L/D = overall lift-to-drag ratio = aerodynamic efficiency with control lines).

Kite's airspeed = apparent wind speed = the wind speed the kite feels = Va = [1/(L/D)][(L/D)² + 1/2]Wcos(<PFK)
when the kite is anywhere on the <PFK iso-power line and flying in any direction.
 
Kite's groundspeed along an iso-power line = the speed you see = Vg-ip = square root of (Va² - W²)
when the kite is flying anywhere along <PFK's iso-power line in either direction.
 
Kite's groundspeed along a power-gradient line = the speed you see = Vg-pg = (L/D)Wcos(<PFK) ± Wsin(<PFK)
when the kite is flying along any power-gradient line when crossing <PFK's iso-power line. The term ±Wsin(<PFK) is positive when the kite is flying toward the power zone, and is negative when the kite is flying away from the power zone.
 
Kite's pull on you = your pull on the kite = (d/2)C_LA[(L/D)² + 1/2]W²cos²(<PFK)
when the kite is anywhere on the <PFK iso-power line and flying in any direction (d = air density, C_L = lift coefficient, A = wing area).

Note: the term [(L/D)² + 1/2] is an approximation that is very accurate for L/D > 2. We use it to clearly show the effects of L/D on performance.

In the illustration, the airspeed and pull equations are tabulated and the numbers are universal: they're good for dual-line kites of ANY constant L/D. For details and graphs of these equations, see Kite Physics 102.

Notice how FLYING a single-line kite is a special case of these equations. It FLIES at U, <PFK = 75° approximately, kite's airspeed = wind speed, kite's groundspeed = zero. Rig it with two lines instead so you can bank it to make it turn, and you have a dual-line kite. The single-liner FLIES at one point on the flight envelope. The dual-liner, in addition to FLYING at that one point, can FLY all over the flight envelope. See how single-line kiting and dual-line kiting are closely related?

What do these equations say?

To achieve the performance favored by the kite culture and required for competition flying -- steady speed across the flight envelope with little edge to power-zone acceleration, low-to-moderate power-zone speed, little acceleration when the wind kicks in, and relatively soft pull changes as you fly across the envelope or encounter turbulence and gusts -- your stunt kite must progressively deform out of shape and lose L/D as it flies from the edge to the power-zone and as the wind strengthens. As the forces of flight grow stronger, your stunt kite must bend & billow farther-and-farther out of shape and progressively lose aerodynamic efficiency like a typical kite does.

And that's how virtually all stunt kites are made. For low FLYING performance compared to what could be achieved. And for a not-so-lively feel. How readily do framed stunt kites deform and lose performance? Perform this simple test. When flying it, you see and feel all that distortion.

To achieve the performance desired by the general public and necessary for exciting fun-recreational flying -- hot edge to power-zone acceleration, strongly-rising pull during that acceleration, high power-zone speed, hot acceleration with sharply intensifying pull when gusts hit, and lively pulses of speed and pull in bumpy wind -- your stunt-kite's L/D must not deteriorate as it flies from the edge to the power-zone or as the wind strengthens. That is, your kite must not progressively deform out of shape and lose aerodynamic efficiency as airspeed and pull increase. As the forces of flight grow stronger, your stunt kite must hold its shape like a rigid airplane wing.

And that's how we aeronautically & structurally engineered WindDance parafoils. For hot FLYING performance. And for an exciting lively feel. See it in the WindDance video. See and feel it when you WindDance.

What else do these equations say? They explain basic skill, as well as passive and active flying. Here's how: Combine the first and last equations to get, pull = (d/2)C_LA(L/D)²{1/[(L/D)² + 1/2]²}Va². This explains how pull varies as the square of your kite's airspeed, and vice versa. More simply, since FLYING can occur only when your kite experiences airspeed, pull = speed = FLYING. That's the theory. The skill? Pull to make it FLY. To the kite, it makes no difference what or who is creating the pull & airspeed. Or which comes first: the airspeed or the pull. Airspeed results in pull. Pull results in airspeed. For each different kite, there is a unique relationship between speed & pull, that is, for each level of pull there is a certain airspeed and vice versa -- no matter what the wind strength is, no matter where the kite is on the flight envelope, no matter how the flyer is controlling the kite in straight flight. You can see that in the equation: there are no terms for wind strength or location on flight envelope. Here is the pull vs airspeed relationship for WindDances. The wind, and your FLYING kite's location on its flight envelope, create pull = speed = FLYING -- this is passive flying. You the flyer, when you generate pull using basic skill, create pull = speed = FLYING -- this is active flying.

We searched for the existence of basic stunt-kite performance equations, and found nothing. So we derived them ourselves. Were we at Seattle AirGear the first ever in the history of dual-line kiting to take this first necessary step in the engineering of stunt kites?

After that first step, the serious engineering began! Aeronautically and structurally engineering the WindDance Wing to require only two rows of bridle lines -- all other parafoils need three or more rows to support the wing -- so that the two-adjuster WindDance Bridle would be possible! Engineering the WindDance Bridle!  Designing and engineering the WindDance Wingtip!  Engineering the product, and the manufacturing processes, in ways that assure consistent manufacturing accuracy! Above all, aeronautically engineering the various types of hot FLYING performance and superb handling -- including the exceptional acceleration and response to the wind, sharp & fast turning, and the natural steering-&-turning feel! Virtually everything about WindDances is new technology developed entirely by Seattle AirGear! Even the trailing-edge mooring loops are a first! All inspired by the Pure Joy of Kite-FLYING!