Maximizing Sailplane Average Cross-Country Speed through Winglet Optimization

An optimization algorithm is used to design winglets for a Standard Cirrus, a high-performance sailplane with a 15m wingspan, such that the average cross-country speed is maximized. The average cross-country speed is described... [ view full abstract ]

An optimization algorithm is used to design winglets for a Standard Cirrus, a high-performance sailplane with a 15m wingspan, such that the average cross-country speed is maximized. The average cross-country speed is described as the average speed of a sailplane over a segment of cross-country soaring, which includes gliding and thermaling as seen in Figure 1. The aircraft must glide distance d, and climb in a thermal to regain the lost altitude h in the segment. The difficulties in sailplane winglet design is largely due to the sharply contrasting sections of the cross-country segment; gliding distance d is done at higher speeds, and by extension, a low lift coefficient. The thermaling to regain altitude h is done at low speeds, and comparatively high lift coefficients. The designed winglet must then provide a benefit at low speeds for thermaling, without providing a significant parasitic drag penalty at higher speeds during the glide portion of the segment. A multi-objective optimization process is used to design such winglets for the Standard Cirrus. The primary objective is to increase the cross-country speed over a range of various thermal strengths, namely 2 m/s, 5 m/s and 8 m/s core strengths. To mitigate the parasitic drag penalty of the winglets at higher speeds, the drag coefficient at a high-speed cruise of 100kts is also minimized, as well as the bending at the wing root in an attempt to account for the structural weight penalty of the winglets. The purpose of this study is to validate a winglet design process that is significantly more time-efficient than that of traditionally designed winglets, usually made by a single, experienced designer. The flight performance of the winglet designs are evaluated using a higher-order potential flow method that uses elements with distributed vorticity. Results from this performance model, when analyzing a Standard Cirrus, are compared to empirical flight data and agree quite well. Validation of the optimization process include maximizing the span-efficiency factor of a fixed wingspan rectangular wing through altering the chord distribution, and ensuring the resultant lift distribution is elliptical. Results of the winglet optimization are hand-selected for further analysis. They are compared to a traditionally designed winglet for the same aircraft, designed with the same objectives in mind. The results indicate an increase in the average cross-country speed of the sailplane by as much as 1.5% when compared to the traditionally designed winglet. The chosen final designs provide an increase in average cross country speed of 1.5% at lower thermal strengths, and 0.4% at higher thermal strengths when compared to the traditional design. The final designs, along with the traditional design, do not have a crossover point in the flight envelope of the Standard Cirrus; they provide a benefit across all airspeeds.