: A noise component method is presented for calculating airframe noise. Noise from clean wing and tail surface is represented as trailing edge noise caused by the turbulent boundary layer. Landing gear noise is given by an empirical representation of model data. Trailing edge flap noise is modeled as a lift dipole normal to the deflected flap, with amplitude and spectrum given by a correlation of flyover data. Measured flyover data for the Prue-2 sailplane, Aero Commander Shirke general aviation airplane, Lockheed JetStar business jet, Boeing 747 commerical jet, and Convair F-106B delta wing airplane are used as test cases. These spectra are compared with predictions by this method and the NASA ANOPP total aircraft method and drag element method. Methods for reducing various components of airframe noise are examined and evaluated.

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Airframe Noise Prediction Method

A method was developed for predicting aerodynamic noise radiated by an airframe. Separate contributions are calculated for the clean wing, horizontal tail, vertical tail, landing gear, leading-edge slats and flaps, and trailing-edge flaps. Each noise component is predicted using scaling laws appropriate to that component, with amplitudes matched to available data. Spectra calculated by this method, the NASA Aircraft Noise Prediction Program (ANOPP) total aircraft method, and the drag element method are compared with flyover noise data for a twin-propeller lightplane, a business jet, and a jumbo jet. from high bypass ratio turbofans with extensive inlet and exhaust acoustic suppression, at approach power setting. Future certification levels must be based on what can be achieved with economically viable airframes and propulsion systems. Thus it is necessary to understand the fundamental processes of airframe noise radiation. Several methods for predicting airframe noise were examined by NASA as part of the Aircraft Noise Prediction Program (ANOPP). Two airframe noise prediction methods, the total aircraft method for aircraft in the clean con- figuration, and the drag element method for all con- figurations, were recommended2 and were subsequently programmed by NASA Aircraft Noise Prediction Office. Verification by comparisons with measured airframe noise spectra have not been published for the total aircraft method and were available for only a few cases for the drag element method. More recently, a noise component method was developed for the Federal Aviation Administration under Contract DOT-FA76WA-3821.3 The purposes of this paper are to describe that method, including recent revisions, and to evaluate the validity of these several airframe noise prediction methods by comparisons between calculated and measured flyover noise spectra. NASA ANOPP Methods Two NASA ANOPP methods for calculating airframe noise were obtained from NASA within one computer program. Option 1 of that computer program is the total aircraft noise method developed by Hardin 2 for clean air- frames. It was derived from a regression analysis of measured peak OASPL for selected aircraft flyovers. These OASPL's

Noise Component Method for Airframe Noise

Sound generated in the vicinity of the trailing edge of an upper surface blown flap

Portions of a four-year analytical and experimental investigation relative to noise radiation from engine internal components in turbulent flow are summarized. Spectra measured for such airfoils over a range of chord, thickness ratio, flow velocity, and turbulence level were compared with predictions made by an available rigorous thin-airfoil analytical method. This analysis included the effects of flow compressibility and source noncompactness. Generally good agreement was obtained. This noise calculation method for isolated airfoils in turbulent flow was combined with a method for calculating transmission of sound through a subsonic exit duct and with an empirical far-field directivity shape. These three elements were checked separately and were individually shown to give close agreement with data. This combination provides a method for predicting engine internally generated aft-radiated noise from radial struts and stators, and annular splitter rings. Calculated sound power spectra, directivity, and acoustic pressure spectra were compared with the best available data. These data were for noise caused by a fan exit duct annular splitter ring, larger-chord stator blades, and turbine exit struts.

A method for calculating strut and splitter plate noise in exit ducts: Theory and verification

The noise characteristics associated with various upper surface blowing configurations were investigated using a small model consisting of a plate and flap assembly (simulated wing with flap) attached to a rectangular nozzle. Nozzle aspect ratio, flow-run length, and flap-deflection angle were the experimental parameters studied. Three nozzle-exit velocities were used. The normalized noise spectra obtained for different nozzle aspect ratios proved to be similar in terms of Strouhal number based on jet velocity and flow-run length. Consequently, the need for knowing local flow velocity and length scales (for example, at the flap trailing edge) as required in some of the existing noise prediction schemes is eliminated. Data are compared with results computed from three different noise prediction schemes, and the validity of each scheme is assessed. A simple method is proposed to evaluate the frequency dependence of acoustic shielding obtained with the simulated wing flap.

Effects of geometry and jet velocity on noise associated with an upper-surface-blowing model

Methods were developed for predicting externally blown flap (EBF) noise and turbomachinery strut noise. Noise radiated by under-the-wing and upper-surface-blowing EBF configurations is calculated as a sum of lift dipole noise, trailing edge noise, and jet quadrupole noise. Resulting predictions of amplitudes and spectra generally were in good agreement with data from small-scale models. These data cover a range of exhaust velocity, flap deflection, exhaust nozzle position, exhaust nozzle shape, and ratio of exhaust nozzle diameter to wing chord. A semi-empirical method for predicting dipole noise radiation from a strut with incident turbulence was in good agreement with data. Leading-edge regions made of perforated plate backed by a bulk acoustic absorber achieved up to 7 db reduction of strut noise caused by incident turbulence at high frequencies. Radial turbulence in a turbofan exit duct was found to have a relatively high level associated with the mean velocity defect in the rotor blade wakes. Use of these turbulence spectra and a dipole noise radiation equation gave general prediction of measured aft-radiated sound power caused by a splitter ring in a full-scale fan exit duct.

M. R. Fink

Papers

Prediction of Externally Blown Flap Noise and Turbomachinery Strut Noise