Abstract: This paper describes the development of what might be considered the first successful ultralight sailplane. The SWIFT is a high performance foot-launched glider, designed to combine some of the convenience of hang gliders with the soaring performance of sailplanes. It takes off and lands like a hang glider, yet maintains exceptional performance at high speeds, achieving a lift-to-drag ratio of about 25:1. Although it is a fullycantilevered rigid wing with aerodynamic controls and flaps, it is light enough to launch by running from a hillside and is easily transported on the top of a car. This paper describes the design, development, and flying of this unique aircraft. Introduction and Background Pioneers of heavier-than-air flight were inspired with the idea of being able to fly like birds – not for the purpose of efficient, high-speed transportation, but for the shear freedom that such a capability would permit.
This was the motivation for, and is the appeal of, modern soaring aircraft such as paragliders, hang gliders, and sailplanes. Although the performance of sailplanes has increased dramatically over many decades so that lift-to-drag ratios of 60:1 have been achieved and 1000 km flights are possible, certain aspects of high performance sailplanes seem counter to the vision espoused by Lilienthal and others [1]. Especially for a group of graduate students in the San Francisco Bay Area, the cost of sailplane flying, along with the long drive to an airport that supported such activities, meant that achieving the goal of bird-like flight was only somewhat more realizable than it was 100 years ago.
That one was often restricted to flights near this airport also made the reality of soaring somewhat less inspiring than the vision. This, of course, was one of the reasons that the sport of hang gliding became popular. But while hang gliding avoided many of the problematic aspects of sailplane flying, it introduced new difficulties. Hang gliders were inexpensive and could be flown from many local sites, but their performance was such that long distance flights were uncommon. Flying was often restricted to a very small corridor on a ridge, and more often than not, consisted of an unimpressively short glide to the bottom of the hill. Furthermore, the simple, yet subtle, techniques for hang glider control using pilot weight-shift seemed to limit further performance improvements and led to less than ideal handling qualities under some flight conditions. In 1985, I recruited a group of outstanding graduate students at Stanford, many of whom were hang glider or sailplane pilots, to investigate what was possible at the boundary between these two aircraft types.
The idea was to consider the possibility of an airplane that would fly at the speed of birds, permitting launching and landing like a hang glider, yet with the performance and control that would permit extended soaring flights on good days. With affordable computational aerodynamic analysis capabilities improving, composite structures evolving, and with some specific concepts for efficient tailless aircraft configurations, we began the design of a foot-launched sailplane that would eventually become the SWIFT.
Of course, we were not the only ones working on such ideas. Designs such as the Mitchell Wing, the Canard 2FL, and lightweight sailplanes inspired by the SSA’s homebuilders’ workshops suggested that such an airplane might be feasible. Of particular relevance was the work of Brian Robbins, Erik Beckman, and Brian Porter of BrightStar Gliders, just two hours North of Stanford. BrightStar had been developing a rigid wing hang glider, called the Odyssey, which Brian Porter piloted to first place in the 1989 U.S. National Hang Glider Championships. Brian Robbins suggested that the Stanford group might improve the Odyssey's airfoils somewhat; but after several evenings of discussions, we agreed to pursue a radically new design. Four months later, in December of 1989, the SWIFT took to the air over a small hill in Marin County. This paper describes the technical development of the SWIFT, with a focus on the aerodynamics design concepts unique to this configuration and aircraft class. Additional descriptions of the evolution of this design may be found in [2,3,4].
The design of the SWIFT began with a study of the requirements for cross-country soaring. Based on surveys of thermal distribution and strength and interthermal downdrafts from [5], we developed a crosscountry soaring simulation that would permit changing glider parameters and evaluating the effect on the likely achievable soaring distance. Even without this simulation on could see the direction required for extended soaring (see [6]). From data on 76 thermals encountered in Dick Johnson’s flights over eastern Texas and from our own experience in California and Nevada, we created a statistical distribution of thermals that were spaced 1.2 to 12 miles apart with heights of 1600 to 7000 ft. The interthermal sink varied from 1.4 to –0.3 kts with an average of 0.3 kts and with 80% of the cases less than 0.5 kts. Based on this model one could compute the probability of reaching the next thermal – or of flying 100 miles. This is shown in the table below as a function of the effective interthermal glide slope.
It is clear that inter-thermal glide ratios of at least 15 to 18 in the presence of the assumed 0.5 kts of sink is needed to make this kind of soaring easily attainable. At the time that this study was first made time, only a dozen 100 mile flights had been made by hang gliders. Today, although flights over 300 miles have been made, most pilots (even most advanced pilots) have not flown 100 miles. One of the factors limiting the flight distances of hang gliders is their speed. The effective inter-thermal glide slope in the presence of sink is much lower for slow-flying aircraft. The table below shows the aircraft lift-to-drag ratio required to achieve and inter-thermal glide slope of 18:1 in the presence of a 0.5kt downdraft (or 9 kt headwind).
Furthermore, thermals are commonly encountered for a rather limited time during daylight hours and with average cross-country cruising speeds of less than 20 kts, one needs to fly for five hours to go 100 miles. Thus, extended cross-country soaring requires not only a good enough glide to make it to the next thermal, but a fast enough glide to get there quickly and in the presence of headwinds or sink. This is easily done by making large span sailplanes with high wing loading. But if the glider is to be foot-launched, it must be light (span not too large) and have a low wing loading. More refined studies of Johnson's data and barograp records from George Worthington's Mitchell wing flights in the Reno area suggested that a foot-launched sailplane with the required performance was just barely possible. The following target performance figures were established and work began to define the aircraft geometry.
Target Performance for Foot-Launched Sailplane
1. Minimum Sink Rate in 100' radius turn: 200 fpm
2. Maximum L/D: 20:1
3. L/D at 60kts: 15:1
4. Stalling speed: no higher than existing hang gliders for safe foot-launching and landing
5. Weight: less than 90 lbs
6. Exceptional controllability for safe flight at low speeds
The fourth constraint meant that even with large flaps, the wing area would be 120 to 140 sq ft. With this constraint, the third goal would be very difficult, requiring an unprecedented level of aerodynamic streamlining. To achieve the desired performance, low drag airfoils and an extremely clean pilot fairing would be required. The sink rate polars in figure 2 illustrate the importance of streamlining, especially for light weight gliders at high speed. The figure also shows how the predicted sink rate of the SWIFT compares with other gliders; it is clearly in a class above hang gliders and compares very favorably with the Schweizer 1-26 sailplane at speeds up to about 60 kts.
Conceptual Design / Configuration Concept Unless one does something very wrong, the performance of a glider is determined primarily by its weight, span, area, and streamlining. The selection of the configuration, whether conventional, canard, tailless, or something else, is based more on issues such a s packaging, handling qualities, manufacturability, transportability, etc.. In the development of the SWIFT, several possible configurations were studied. The results indicated very small performance differences between tailless, conventional and canard designs; however, the conventional design suffered some from the short tail length required for landing flair and take-off ground clearance. The directional stability of a slightly-swept canard was poor, and performance was also compromised by the short coupling. The tailless design was statically-balanced (empty c.g. near flight c.g.), compact, and did not pay the weight penalty that would be associated with a tail boom. (Note that even a 5 lb boom represents more than 5% of the empty weight and a very large fraction of the wing bending weight.) For these reasons, and to study several aspects of tailless aircraft design, an aft-swept “flyingwing” design was selected.
One of the first steps in the configuration development was a sizing study that started with a look at the sensitivities of performance to several design parameters (see figure 3). This was followed by a more comprehensive optimization of span and area based on the cross-country soaring simulation.
Figure 3. Effect of design parameters on lift-to-drag ratio.
As the design evolved, several mock-ups were constructed to evaluate visibility and ground-handling. A high wing arrangement was adopted for good ground clearance and pilot visibility, with an arrangement similar to that used by the Odyssey. On the ground, the glider is supported by shoulder straps (Fig. 4).
Figure 4. Glider is supported by shoulder straps prior to launch. After takeoff, the pilot rotates his or her legs forward into the aluminum cage structure. The pilot is supported by a retractable sling that provides a comfortable reclined orientation. (Fig. 5). The glider may be flown with a fairing that covers the cage structure, or with the pilot exposed to the air. Subsequent SWIFTs have included a small wheel and skid that permits feet-up landing when conditions permit and even towed launches
This was the motivation for, and is the appeal of, modern soaring aircraft such as paragliders, hang gliders, and sailplanes. Although the performance of sailplanes has increased dramatically over many decades so that lift-to-drag ratios of 60:1 have been achieved and 1000 km flights are possible, certain aspects of high performance sailplanes seem counter to the vision espoused by Lilienthal and others [1]. Especially for a group of graduate students in the San Francisco Bay Area, the cost of sailplane flying, along with the long drive to an airport that supported such activities, meant that achieving the goal of bird-like flight was only somewhat more realizable than it was 100 years ago.
That one was often restricted to flights near this airport also made the reality of soaring somewhat less inspiring than the vision. This, of course, was one of the reasons that the sport of hang gliding became popular. But while hang gliding avoided many of the problematic aspects of sailplane flying, it introduced new difficulties. Hang gliders were inexpensive and could be flown from many local sites, but their performance was such that long distance flights were uncommon. Flying was often restricted to a very small corridor on a ridge, and more often than not, consisted of an unimpressively short glide to the bottom of the hill. Furthermore, the simple, yet subtle, techniques for hang glider control using pilot weight-shift seemed to limit further performance improvements and led to less than ideal handling qualities under some flight conditions. In 1985, I recruited a group of outstanding graduate students at Stanford, many of whom were hang glider or sailplane pilots, to investigate what was possible at the boundary between these two aircraft types.
The idea was to consider the possibility of an airplane that would fly at the speed of birds, permitting launching and landing like a hang glider, yet with the performance and control that would permit extended soaring flights on good days. With affordable computational aerodynamic analysis capabilities improving, composite structures evolving, and with some specific concepts for efficient tailless aircraft configurations, we began the design of a foot-launched sailplane that would eventually become the SWIFT.
Design
Objectives
Furthermore, thermals are commonly encountered for a rather limited time during daylight hours and with average cross-country cruising speeds of less than 20 kts, one needs to fly for five hours to go 100 miles. Thus, extended cross-country soaring requires not only a good enough glide to make it to the next thermal, but a fast enough glide to get there quickly and in the presence of headwinds or sink. This is easily done by making large span sailplanes with high wing loading. But if the glider is to be foot-launched, it must be light (span not too large) and have a low wing loading. More refined studies of Johnson's data and barograp records from George Worthington's Mitchell wing flights in the Reno area suggested that a foot-launched sailplane with the required performance was just barely possible. The following target performance figures were established and work began to define the aircraft geometry.
Target Performance for Foot-Launched Sailplane
1. Minimum Sink Rate in 100' radius turn: 200 fpm
2. Maximum L/D: 20:1
3. L/D at 60kts: 15:1
4. Stalling speed: no higher than existing hang gliders for safe foot-launching and landing
5. Weight: less than 90 lbs
6. Exceptional controllability for safe flight at low speeds
The fourth constraint meant that even with large flaps, the wing area would be 120 to 140 sq ft. With this constraint, the third goal would be very difficult, requiring an unprecedented level of aerodynamic streamlining. To achieve the desired performance, low drag airfoils and an extremely clean pilot fairing would be required. The sink rate polars in figure 2 illustrate the importance of streamlining, especially for light weight gliders at high speed. The figure also shows how the predicted sink rate of the SWIFT compares with other gliders; it is clearly in a class above hang gliders and compares very favorably with the Schweizer 1-26 sailplane at speeds up to about 60 kts.
One of the first steps in the configuration development was a sizing study that started with a look at the sensitivities of performance to several design parameters (see figure 3). This was followed by a more comprehensive optimization of span and area based on the cross-country soaring simulation.
Figure 3. Effect of design parameters on lift-to-drag ratio.
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