Arrangement for minimizing buzz saw noise in bladed rotors

Claim 1

A rotor including a plurality of rotor blades, said rotor being adapted for rotation within a duct under conditions in which at least radially outer portions of said rotor blades attain at least sonic velocity thereby producing shockwaves within said duct, said blades being subject to manufacturing tolerances manifested in blade-to-blade variations of at least one selected dimensional characteristic of said blades, which dimensional characteristic has a significant effect on shockwave strengths, whereby said blades generate shockwaves which vary in strength from blade to blade around said rotor thereby causing buzz saw noise within said duct, said rotor being produced by a process in which the blades are arranged in a sequence which minimizes the intensity of at least one selected buzz saw noise tone, said process comprising the steps of: (a) measuring said at least one selected dimensional characteristic of each blade to be assembled into said rotor; (b) notionally arranging said blades around said rotor in a chosen sequence; (c) evaluating the shockwave strength at each said blade as arranged in sequence; (d) calculating at least one Fourier coefficient of said shockwave strengths for said sequence, said at least one Fourier coefficient being selected to correspond to said at least one selected buzz saw noise tone; (e) rearranging said blades around said rotor in further sequences and repeating steps (c) and (d) for each such sequence to determine which one of said sequences produces an acceptable minimum value of said at least one Fourier coefficient; and (f) assembling said blades into said rotor such that they are arranged in that one of said sequences for which said minimum value of said at least one Fourier coefficient is obtained.