Trim Drag, Tail Sizing, and Soaring Performance

Abstract

The requirements of longitudinal stability and trim necessitate small horizontal tail surfaces on conventional sailplanes. Although the penalties in weight and drag associated with sailplane trim requirements are smaller than for those of most other aircraft, they can be significant to the soaring performance of modern sailplanes. In this paper, the increments in "trim drag" due to induced drag, increased wing profile drag, and wetted area changes are evaluated for standard-class sailplanes. Airfoil section data, combined with a simple method for computing trimmed induced drag of wing/tail systems, are used to compute total trim drag over the entire flight regime, illustrating the effects of circling flight, airfoil pitching moment, and static margin changes. An investigation of the effects of tail size, aspect ratio, position, and wing lift distribution shape, suggests methods by which trim drag may be reduced. Finally, the paper considers the potential of unconventional configurations for reducing trimmed drag.