A unified approach is used to derive many of the current formulas for calculation of discrete frequency noise of helicopter rotors and propellers. Both compact and noncompact results are derived. The noncompact results are based on the solution of Ffowcs Williams-Hawkings (FW-H) equation. The compact formulations are obtained as the limit of noncompact source results. In particular, the linearized acoustic theories of Hawkings and Lowson, Farassat, Hanson, Woan and Gregorek, Succi, and Jou are discussed in this paper. An interesting thickness noise formula by Isom and its extension by Ffowcs Williams are also presented.

Linear Acoustic Formulas for Calculation of Rotating Blade Noise

The aerodynamic, acoustic, and aeromechanical performance of counterrotating blade concepts were evaluated both theoretically and experimentally. Analytical methods development and design are addressed. Utilizing the analytical methods which evolved during the conduct of this work, aerodynamic and aeroacoustic predictions were developed, which were compared to NASA and GE wind tunnel test results. The detailed mechanical design and fabrication of five different composite shell/titanium spar counterrotating blade set configurations are presented. Design philosophy, analyses methods, and material geometry are addressed, as well as the influence of aerodynamics, aeromechanics, and aeroacoustics on the design procedures. Blade fabrication and quality control procedures are detailed; bench testing procedures and results of blade integrity verification are presented; and instrumentation associated with the bench testing also is identified. Additional hardware to support specialized testing is described, as are operating blade instrumentation and the associated stress limits. The five counterrotating blade concepts were scaled to a tip diameter of 2 feet, so they could be incorporated into MPS (model propulsion simulators). Aerodynamic and aeroacoustic performance testing was conducted in the NASA Lewis 8 x 6 supersonic and 9 x 15 V/STOL (vertical or short takeoff and landing) wind tunnels and in the GE freejet anechoic test chamber (Cell 41) to generate an experimental data base for these counterrotating blade designs. Test facility and MPS vehicle matrices are provided, and test procedures are presented. Effects on performance of rotor-to-rotor spacing, angle-of-attack, pylon proximity, blade number, reduced-diameter aft blades, and mismatched rotor speeds are addressed. Counterrotating blade and specialized aeromechanical hub stability test results are also furnished.

Experimental performance and acoustic investigation of modern, counterrotating blade concepts

Open rotors are once again under consideration for propulsion of the future airliners because of their high efficiency The noise generated by these propulsion systems must meet the stringent noise standards of today to reduce community impact In this paper we review the open rotor noise prediction methods available at NASA Langley We discuss three codes called ASSPIN (Advanced Subsonic-Supersonic Propeller Induced Noise), FW - Hpds (Ffowcs Williams-Hawkings with penetrable data surface) and the FSC (Fast Scattering Code) The first two codes are in the time domain and the third code is a frequency domain code The capabilities of these codes and the input data requirements as well as the output data are presented Plans for further improvements of these codes are discussed In particular, a method based on equivalent sources is outlined to get rid of spurious signals in the FW - Hpds code

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Open Rotor Noise Prediction Methods at NASA Langley- A Technology Review

Acoustic measurements are presented to show that active cancellation can be used to achieve significant reduction of blade passage noise in a turboprop cabin Simultaneous suppression of all blade passage frequencies was attained The spatial volume over which cancellation occurred, however, is limited Acoustic intensity maps are presented to show that the acoustic input to the fuselage was sufficiently non-localized so as to require more judicious selection of cancellation speaker location

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Active attenuation of propeller blade passage noise

An analytical method is described for prediction of the interior noise levels for propeller-driven aircraft, given the exterior noise signature, its harmonic spectrum, and a description of the fuselage sidewall structure and various candidate "add-on" noise-control elements. The structural response is described by the theory of Koval, but simplified to consider the stiffeners as "smeared" elements. The incremental transmission loss (TL) due to add-on noise-control elements is derived from the Beranek and Work method. Comparisons between experimental data and the theory are presented. The method is reasonably accurate below the ring frequency, but is somewhat conservative at the normal incidence angle. This method is, however, expedient computationally, is economical, and permits rapid comparisons of noise-control penalties for various treatment concepts.

Analysis of Interior Noise-Control Treatments for High-speed Propeller-Driven Aircraft

This paper describes recent research on noise generated by high-speed propellers, on noise transmission through acoustically treated aircraft sidewalls, and on subjective response to simulated turboprop noise. Propeller noise discussion focuses on theoretical prediction methods for complex blade shapes designed for low noise at Mach = 0.8 flight and on comparisons with experimental test results. Noise transmission experiments using a 168-cm-diam aircraft fuselage model and scaled heavy-double-wall treatments indicate that the treatments perform well and that the predictions are usually conservative. Studies of subjective comfort response in an anechoic environment are described for noise signatures having combinations of broadband and propellertype tone components.

Interior noise considerations for advanced high-speed turboprop aircraft

This paper describes recent research on noise generated by high-speed propellers, on noise transmission through acoustically treated aircraft sidewalls and on subjective response to simulated turboprop noise. Propeller noise discussion focuses on theoretical prediction methods for complex blade shapes designed for low noise at Mach = 0.8 flight and on comparisons with experimental test results. Noise transmission experiments using a 168 cm. diameter aircraft fuselage model and scaled heavy-double-wall treatments indicate that the treatments perform well and that the predictions are usually conservative. Studies of subjective comfort response in an anechoic environment are described for noise signatures having combinations of broadband and propeller-type tone components.

R. Prydz

Papers

Interior noise considerations for advanced high-speed turboprop aircraft

Interior noise considerations for advanced high-speed turboprop aircraft