Tip Vortex Study of a Rotor with Double-Swept Blade Tips

An experimental aerodynamic study was conducted, analyzing the wake structure of a four-bladed model rotor with a forward-backward swept tip geometry inspired by the ONERA-DLR "Etude d ' un Rotor Aeroacoustique Technologiquement Optimise"-design. Similar tip designs are used on some modern helicopter main rotors. The experiments were conducted at the Rotor Test Facility Gottingen (RTG) in hover-like conditions using a stereoscopic high-speed particle image velocimetry system. The results are compared with those of a reference rotor using a conventional parabolic blade tip. The test parameters include both constant-pitch cases and pitch-oscillating cases with a cyclic swashplate input. The constant-pitch tests show a two-step stall behavior for the double-swept tip, with the first step characterized by a reduced thrust slope and a reduced rotor efficiency. This effect is explained by a repositioning and a structural change of the tip vortex generation. The pitch-oscillating cases show that the double-swept tip results in an earlier tip stall compared to the parabolic geometry while maintaining high thrust levels. The dynamic tip stall yields a break-down of the wake's tip vortex system, which is replaced by a spanwise band with small-scale turbulent structures and trailed vorticity, but no large-scale vortices.