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).
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
