EVOLUTION OF AIRFOILS FOR SAILPLANES

Abstract

About 110 years ago O. Lilienthal built his gliders with cambered plate wings which were stiffened out by a plenty of strut wires. Analysis of this type of airfoils show their narrow range of angles of attack and their rapid loss of lift at stall conditions. The development of the thick airfoils of the Gottingen series led to the construction of the Vampyr with a strut free wing and a torsion leading edge box. The quality of the surface and the maximum thickness close to the leading edge did not allow reasonable extent of laminar flow. The big advantages of laminar flow airfoils have been discovered in the late thirties and have been systematically investigated by the NACA. But only the combination of an airfoil designed for laminar flow and the excellent surface quality of a sandwich construction of glass fibre reinforced plastics and solid foam made the big success of laminar airfoils for sailplanes possible. This is shown by way of the example of the Ka 6 and the Phonix. Examples of typical airfoils demonstrate further developments: Airfoils without and with flaps; problem of laminar separation bubbles, use of destabilisation zones and use of turbulators. An attempt to assess future possibilities for performance improvements of sailplanes closed the paper.