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William Roeseler & David Culp
Kites have several unique and compelling advantages over other sailing rigs, and some limitations. A kite array's traction force can be concentrated at a single attachment point on the hull platform, eliminating much of the strength and rigidity of construction necessary in conventional sailcraft structures. Through offsetting the kite attachment point to leeward and/or the hull's sideforce device(s) to windward, the designer can precisely align the centers of effort of both hull and rig, resulting in no rig induced pitch, yaw, or heeling moments. This balance of forces promotes superb stability on a cruising boat. Kiteboats, when not pushed, feel like they are "on rails". This also allows the designer to push the performance envelope to its limit, carrying massively large rigs on very small hull platforms with only minimal regard to stability. In fact, Roeseler routinely carries 100-130 sq ft of kites on a "hull" consisting of a pair of water skis; displacement = 20 lbs plus pilot, and wetted surface = 1.5 sq ft. Culp carries up to 225 sq ft of kites on his 14' catamaran; displacement = 150 lbs plus pilot, and wetted surface = 8-10 sq ft. Both carry these rigs in true winds as high as 30-40 kts.
Kites fly some 30-100 ft above the water, and thus above the reduced velocity, turbulent boundary layer where most boats sail. This results in 25-30% higher wind speeds and thus >60% more energy in the free air stream than at the surface. Turbulence is hard to quantify; however, while flying kites in the 20-40 kt wind range over open water, the operator perceives significant power losses and buffeting at altitudes below 20-40 ft.
Many kites are capable of flying at 2-4 times the windspeed. By flying the kites in a figure 8, zig-zag, or circular figure, the kitesailor can utilize this ability to maintain the rig at high Vrig/Vwind ratios, sweeping much larger cross sections of the air stream per unit time than ordinary rigs and extracting huge amounts of power while the hull is moving slowly or accelerating.
On the negative side, most currently available kites and certainly all existing efficient kites fly poorly when wet and some will not float. As some of any kite's lift is devoted to keeping the rig aloft, kiteboats have a critical light air threshold where the kites drop into the sea. Re-launch is very difficult without a support boat or returning to shore. In addition, overall boat size (the rig flies up to 300' away from the hull) and height clearance considerations under bridges, over power lines and other boat's masts, adversely affect maneuverability. These limitations are changing as designers are developing kites (some in limited production, some in prototype stages) which float, or fly on short lines, or can be self-launched and/or re-launched wet (Ref. 7). Current technology has tended to limit kitesailing applications to either low technology "pottering" or to labor intensive, structured competition, such as speedsailing.
Existing kite technology worldwide lags behind theoretical ability. Flexifoils®, the most commonly used power kites, are capable of lift to drag ratios (L/D's) of 4 to 5 and have only rudimentary pitch control. Kites with higher L/D's are typically fragile and/or suffer unacceptable light air limitations.
Both of the authors are currently working with Flexifoils, either singly, or stacked into 2-12 element arrays. Flexifoils are very fast, having been clocked at speeds over 100 kts. They are, however, somewhat less stable and more difficult to control than other power kites. This leaves them very responsive, but difficult to fly statically (unattended). Their structure is almost wholly in tension; their gross shape becoming more rigid at higher speeds, not less as with typical delta winged kites.
Flexifoils have no active control of angle of attack; however, the operator can control their power. When flown overhead in moderate winds (like a single line child's kite), a Flexifoil will rise to an angle of perhaps 70°-80° above the surface (Fig 3). At this point the angle of attack is low and lift is only enough to overcome gravity.
The kite is semi-stable here. It can be maneuvered out and down to either side until it approaches the surface at a flying angle of approximately the same 75°, giving e(aero) = 15°. The kite path just described, along with a semicircle drawn on the surface, whose diameter equals the kite line length describe the limits of a gore of a sphere; the surface of which comprises the total possible range of kite flying positions.
The specific aerodynamics of Flexifoil kites are beyond the scope of this paper. The reader is directed to the inventors' patent (Ref 3) for a concise description. Suffice it to say that as the kite approaches its efficiency limit (75° overhead or out to either side), its angle of attack decreases, as does lift and kite speed. As the kite is maneuvered towards its maximum power point (dead to leeward, at the surface), its angle of attack increases. Due to its relatively light weight, the increased lift thus generated results in rapid acceleration, and a large power surge. The apparent wind draws forward and the kite continues to accelerate to its maximum speed, maintaining this speed until it passes the maximum power point, when it begins to de-power and decelerate. Using this knowledge, the sailor can vary the force by an order of magnitude and, with a bit of luck, maintain line tensions and direction of pull suitable for his heading. Since airload varies directly with angle of attack and as the square of windspeed, the relative airspeed of the kite is even more powerful than angle of attack in generating and controlling force.
Others are working on developing kites, some for precision control ie: three or four line bridles rather than two to give pitch control, and some for greater efficiency or ease of launching, particularly over water (Ref. 7). The Force 10, a 15 foot stunt kite from Milwaukee uses camber inducing battens similar to modern sailboard rigs for increased efficiency and tight control.