Passive design for reduction of high-speed impulsive rotor noise

The high-speed impulsive noise from non-lifting rotors is examined in hover at large tip Mach numbers using an Euler/Navier-Stokes solver, A parametric study is performed to systematically investigate the nonlinear effects of sweep, taper and thinning on high-speed impulsive noise in order to determine the key parameters for the design of a low noise rotor. An untwisted UH-1H rotor blade forms the baseline blade for this study. Forward sweep is found to he more beneficial for noise reduction than rearward sweep, effectively delaying delocalization, At lower tip speeds taper and thinning are more effective for reducing the in-plane noise, However, at high tip speeds phasing effects become more important and sweep becomes more effective at reducing high-speed impulsive noise, Combinations of sweep, taper and thinning are investigated. Furthermore, a dogleg planform, with backward sweep inboard of the forward swept tip, is included in order to place the aerodynamic center near the root quarter chord, In particular the Forward Aeroacoustically Swept Thin and Tapered (FASTT) blade is seen to delay delocalization well past a tip Mach number of 0.95 while providing at least 10 dB of noise reduction from the baseline untwisted UH-1H over the entire tip Mach number range investigated, The numerical results form a rich numerical data base for investigating the ability of simpler methods for predicting high-speed impulsive noise for more complex planforms.