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 nonli near effects of sweep, taper and thinning on high-speed impulsive nois e in order to determine the key parameters for the design of a low noi se rotor. An untwisted UH-1H rotor blade forms the baseline blade for this study. Forward sweep is found to he more beneficial for noise red uction than rearward sweep, effectively delaying delocalization, At lo wer tip speeds taper and thinning are more effective for reducing the in-plane noise, However, at high tip speeds phasing effects become mor e important and sweep becomes more effective at reducing high-speed im pulsive noise, Combinations of sweep, taper and thinning are investiga ted. Furthermore, a dogleg planform, with backward sweep inboard of th e forward swept tip, is included in order to place the aerodynamic cen ter near the root quarter chord, In particular the Forward Aeroacousti cally Swept Thin and Tapered (FASTT) blade is seen to delay delocaliza tion well past a tip Mach number of 0.95 while providing at least 10 d B 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.