• D. Marsden


Aerodynamics, Structures, Design


The estimation of the performance of a modern sailplane should be easy because of the relative simplicity of the sailplane configuration and the generally good quality of aerodynamic surfaces used. Evaluation of the glide perfornance of a sailplane essentially consists of an estimation of drag as a function of forward speed. In this paper, the parameters contributing to drag have been separated out. Some of these are easy to evaluate on theoretical grounds, while others, such as wing-body interference drag for exanple, are very uncertain. Fortunately, the largest contributions to drag are the ones that can be evaluated with good accuracy, and if average values based on analysis of the performance of known sailplanes are assumed for the more uncertain parameters, quite good glide polars can be obtained for any sailplane with not much more input data than is usually given in sales promotion literature. One of the advantages of the drag equation presented here is that it identifies the parameters contributing to drag in such a way that the relative importance of each of these is shown. This equation is particularly useful in the analysis of flight test results because it makes it possible to spot the most likely sources of excess drag if the perfomance is not as good as it should be. An analysis of the published flight test data for a number of modern sailplanes is presented in order to establish average values for the nore uncertain parameters and to show the probable scatter in value of these parameters to be expected. Some further examples of the analysis of flight test data are given.