Leading-edge Serrations which Reduce the Noise of Low-speed Rotors
National Aeronautics and Space Administration, 1973 - Rotors (Helicopters) - 62 pages
Acoustic effects of serrated brass strips attached near the leading edges of two different size rotors were investigated. The two bladed rotors were tested in hover. Rotor rotational speed, blade angle, serration shape, and serration position were varied. The serrations were more effective as noise suppressors at rotor tip speeds less than 135 m/sec (444 ft/sec) than at higher speeds. high frequency noise was reduced but the low frequency rotational noise was little affected. Noise reductions from 4 to 8 db overall sound pressure level and 3 to 17 db in the upper octave bands were achieved on the 1.52 m (5.0 ft) diameter rotor. Noise reductions up to 4 db overall sound pressure level were measured for the 2.59 m (8.5 ft) diameter rotor at some conditions.
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0012 airfoil section 12 Serration 18 Basic rotor acoustic acoustic data Aerodynamic Ames Research Center angle of attack background noise bands were achieved Basic rotor Basic blade passage frequency brass strips attached chord-based Reynolds number dB overall sound due to serrations effects of serrated electric motor Figure 14 figure of merit frequency rotational noise frequency spectrums ft/sec harmonics of blade high frequency noise indicate that serration large-scale rotor test leading-edge serrations levels at 4.6 levels at harmonics low frequency rotational measured at microphone Moffett Field NACA 0012 airfoil Noise Measurement octave band overall noise levels overall sound pressure reduced the noise rotor A 18 rotor axis rotor Basic rotor rotor blade rotor center serrated feathers serration position serration shape SERRATIONS WHICH REDUCE shown in figure Small-Scale Rotor Test sound pressure level stagnation point Thrust and torque tip Reynolds numbers torque parameter Cq/o versus torque parameter vortex shedding vortices