Helicopter Main Rotor Noise Reduction by Use of Leading-Edge Serrations
Authors:
Evan Bosque, Clayton J Lay, Tuan A Le, Everth A Moraga, Ross RandallMentor:
Ali R Ahmadi, Professor & Chair of Aerospace Engineering, California State Polytechnic University PomonaA main cause of helicopter rotor noise is due to vortex shedding at the tip of the blades. Standard noise-control methods for this phenomenon rely on reducing the load of the blades or minimizing the intensity of tip vortices through the use of geometry. The particular method chosen here to reduce rotor noise was through the addition of leading-edge-serrations on the rotor blades. Serrated-leading-edge (SLE) rotor blades tend to delay flow separation on the rotor blades, which can inhibit the formation of air turbulence. The acoustic output of SLE rotor blades was investigated and compared to conventional straight (CS) rotor blades as they show promise of lowering acoustic output with little to no decrease in aerodynamic performance. The design of the blades consisted of a mathematical model of a rotor blade with a Hughes Helicopter, HH-02 airfoil. This yielded a 3-D point cloud that was used to create a Computer Aided Design (CAD) model, which was used to manufacture the rotor blades by rapid prototyping. Finite Element Analysis (FEA) software was used to analyze the rotor blades before physical structural testing. Computational Fluid Dynamics Analysis (CFD) was used to compare aerodynamic characteristics between the SLE and CS rotor blades. The blades were then mounted on a T-Rex 600 ESP model helicopter for experimentation. The testing was conducted at Azusa Pacific University’s recording studio where a semi-circular 5 microphone array was used to measure and record radiated noise in decibels (dB). After analyzing the results, the SLE rotor was found to be on average about 5 dB quieter than the CS rotor, with a maximum of 16 dB quieter at 60° below the horizontal. The reduction in radiated noise is consistent with the theory of SLE rotor blades and seems to be a promising method for helicopter rotor noise reduction.