PHENOMENA OBSERVED DURING AEROFOIL RAMP-DOWN MOTIONS FROM THE FULLY SEPARATED STATE

Unsteady pressure data from seven aerofoil models performing constant pitch rate ''ramp-down'' motions from the fully separated state to low incidence have been analysed. It is observed that the time delay betw een attachment at the leading edge and the occurrence of minimum norma l force is, to a first order, independent of reduced pitch rate. As a result, at sufficiently high pitch rate, negative normal force may be generated at large positive incidence. In contrast to this, the re-est ablishment of attached flow to the upper surface is strongly dependent upon pitch rate. The paper also contains a description of smoke flow visualisation tests. These were at a much lower Reynolds number than t he pressure data tests, and consequently are of qualitative interest. Results of flow visualisation show that during a ramp-down test the ne ar wake is much larger than in the corresponding static case, and that attachment is significantly delayed.