In-flight boundary layer investigations on a Airplane Wing using LDA measuring techniques

Research and development work in aircraft aerodynamics to date has been heavily based upon wind tunnel investigations, even though it is known that the test facilities employed impose severe limitations concerning directly applicable design information. For instance, results of wind tunnel studies yielding design parameters for laminar wings have to be verified by flight experiments. This task requires advanced measuring techniques that are nonintrusive and reliable in the harsh environment of flight. Several techniques have been developed in past years for transition detection, e.g., hot film arrays, piezo foils, and infrared cameras. Nevertheless, all of these techniques are only capable of acquiring information directly on the wing surface. Laser Doppler Anemometry (LDA), in contrary, can provide insight into the complete boundary layer and the surrounding velocity field. However, typical LDA instrumentation does not perform adequately in the environment of in-flight testing. In the present paper the development of a dedicated system for in-flight LDA measurements is outlined. Starting from basic design considerations it finally is shown that LDA is well suited for providing the desired local velocity information, but must be specifically adapted for the various spatial, geometrical, and power constraints imposed by the test aircraft. Laboratory and in-flight measurements studying boundary layer transition on an airplane wing downstream of an excitation source were successfully carried out and a summary of results is presented. Finally, suggestions for further advancements of LDA systems are proposed. Fig. 1 Velocity distribution in the laminar boundary layer of the wing glove acqired during inflight tests with the test aircraft GROB 109B