Arrangement for minimizing buzz saw noise in bladed rotors

• US 4,732,532 A
• Filed: 11/12/1982
• Issued: 03/22/1988
• Est. Priority Date: 06/16/1979
• Status: Expired due to Term

• Current Assignee: Rolls-Royce PLC (Rolls-Royce Holdings Plc)
• Original Assignee: Rolls-Royce PLC (Rolls-Royce Holdings Plc)
• Inventors: Schwaller, Peter J. G., Newby, David R.
• Primary Examiner(s): Scott, Samuel
• Assistant Examiner(s): Flanigan, Allen J.
• Application Number:
  US06/441,152
• Time in Patent Office: 1,957 Days
• Field of Search: 415/119, 415/181, 415/DIG. 3, 415/DIG. 5, 416/203, 416/223 A, 416/500, 29/156.8 B, 73/116
• US Class Current: 415/119
• CPC Class Codes: B64D 2033/0206 comprising noise reduction means, e.g. acoustic liners
• CPC Class Codes: B64D 2033/0286 for turbofan engines
• CPC Class Codes: B64D 33/02 of combustion air intakes air intakes for gas-turbine plants or jet-propulsion plants per se F02C7/04; air intakes for combustion engines in general F02M35/00
• CPC Class Codes: F02C 7/045 having provisions for noise suppression
• CPC Class Codes: F04D 29/663 Sound attenuation
• CPC Class Codes: F04D 29/666 by means of rotor construction or layout, e.g. unequal distribution of blades or vanes
• CPC Class Codes: Y02T 50/60 Efficient propulsion technologies, e.g. for aircraft
• CPC Class Codes: Y10S 416/50 Vibration damping features

Abstract

The invention minimizes selected tones present in buzz-saw noise, otherwise known as multiple pure tone noise. This noise arises from the production of unequal shockwaves during the rotation at sonic or supersonic velocities of a bladed rotor, the dimensional characteristics of whose blades are subject to manufacturing tolerances. Minimization of selected buzz-saw noise tones is achieved by arranging the blades around the rotor in a sequence which minimizes selected Fourier coefficients of the shockwave strengths; alternatively, the blades are arranged to minimize selected Fourier coefficients of the dimensional characteristics of the blades, such as thickness, t, stagger angle θ, and camber. The invention is particularly applicable to the fan propulsors of turbofan aeroengines.

Claims

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.

2. A rotor according to claim 1 wherein its process steps of production further include the step of:

arranging the blades with respect to each other such that differences between the stagger angles of neighboring blades are not more than a predetermined value, thereby minimizing buzz saw noise tones at frequencies other than rotor rotational frequency.

3. A rotor according to claim 1 wherein its process steps of production further include the step of:

arranging the blades with respect to each other such that the moment weight imbalance of the rotor is not more than a predetermined value.

4. A rotor according to claim 1 wherein said rotor is the fan of a turbofan aeroengine.