Speed-to-Fly Errors Arising from the Quadratic Fit of Nonquadratic Polars

Abstract

Speed-to-fly theory, based on quadratic polynomial estimations of a sailplane speed polar, has provided a strong foundation for the optimization of sailplane flight. Through simple analysis, it has long been believed that flying at the incorrect speed does not significantly degrade the average cross-country speed performance, unless the error is significant. This understanding is shown here to be incomplete as the robustness to speed-to-fly errors decreases with inter-thermal airmass lift and increases in sink. Secondly, additional speed-to-fly errors occur when assuming the quadratic fit of the sailplane polar is correct. If the shape of the sailplane polar is not well described by a quadratic polynomial, flying the incorrect inter-thermal speed is demonstrated to cause an additional achieved average cross-country speed error of ∼ 2% for an ASW-24W. This is considered to be significant because of the narrow spread in speed between top contest finishes. Lastly, it is shown that experimental polar measurement errors from a typical flight-test evaluation are unlikely to induce significant speed-to-fly errors when the underlying polar is actually quadratic.