BOUNDARIES FOR WORLD CLASS SAILPLANES

Abstract

This paper examines likely boundaries of span and aspect ratio for sailplanes conforming with the "World Class" specification. Empty masses are in accordance with Stender's empirical formula while the performance is assumed to correspond with the simplest analytical expressions. The variation of aerodynamic characteristics with span and aspect ratio are generally similar to those proposed earlier. By varying a constant in the Stender formula, it is possible to consider "heavy", "medium" and "light" structures. By inspecting earlier data, it is also possible to estimate "high, "medium" and "poor" values of the maximum lift coefficient. Spans from 10 m to 18 m and aspect ratios from 10 to 22 are considered. Boundaries are tlen drawn on span-aspect ratio axes relating to the stall, a minimum lif/drag ratio of 30 and, if relevant, a minimum sinking speed of 0.75 m/s and a reasonable value of the lift coefficient at the minimum sink condition, for interesting combinations of structural mass and maximum lift coefficient. As one would expect, the combination of a light structure and a high Clmax provides the best combination. The span could be as low as ll m but there is then no room for error. Spans greater than 13 m appear to be more satisfactory. The performance improves as the span and aspect ratio are increased, the latter being limited by neither the sall boundary or the Clms boundary. At the other extreme, a heavy structure and a poor Clmax give very little room for maneuver. The span must be more than 17 m and even then there is only a very restricted range of possibilities. While it is unlikely that a designer would deliberately use such an unfavorable combination of characteristics, these results show that one must be very careful not to sacrifice too much to simplicity of construction. A likely combination is a "medium" structural mass and a "high" Clmax. The characteristics of such sailplanes are examined in some detail. It seems that achieving a high Clmax is more important than minimizing the structure weight. Since the specification of the "World Class" imposes no restriction on the span, other than by implication, there is an incentive for designers to consider large spans.