Flap Noise Prediction Method for a Powered Lift System

A method is presented for estimating the noise generated by deflection of the engine exhaust for under-the-wing and over-the-wing versions of an externally blown flap configuration for powered lift. Correlation equations and curves are given for the OASPL and directivity and for spectra scaled to a high bypass 25 000-pound thrust size engine. Data are r-taken from TF34 engine tests and from large cold flow model tests. The f» correlations are empirical, and thus application of this prediction procei dure is limited to geometrically similar configurations. Application of the method is illustrated by calculated sample footprints. Introduction Current interest in aircraft for short-haul applications includes those utilizing externally blown flaps for powered lift. The nature of the operation of such aircraft, as well as the possibility of more restrictive noise goals, require that they be considerably quieter than conventional (CTOL) aircraft. • The noise resulting from the powered lift can be the dominant noise source. For the externally blown flap (EBF) configurations, noise is produced as the engine exhaust flows over either the upper or lower surfaces of the wing-flap system. This noise is referred to herein as "flap" noise; it includes leading and trailing edge noise, scrubbing noise, and redirected jet noise. Extensive data for flap noise have been obtained recently in both cold flow model tests and tests with an actual engine. Results of these tests are being reported separately.(1~8) Noise considerations in aircraft design require correlation of these fragmentary data and a method of making consistent noise predictions. The method of flap noise prediction of this paper consists of equations and curves for overall sound pressure level and directivity and spectrum shape. By means of these, the spectrum of flap noise, or the perceived noise level, can be predicted for any distance and for a range Aerospace Engineer, Applications Analysis Office. 2 Head, Section B, Jet Acoustics Branch. 3 Aerospace Engineer, Jet Acoustics Branch. of direction angles from the source. Sample footprints for under-thewing and over-the-wing configurations are presented to illustrate the results of this prediction method. Data Sources The curves and equations for predicting the flap noise are based primarily on data from two sources: TF34 engine tests(l~3) at the NASA Flight Research Center; and large cold flow model tests at the Lewis Research Center.(4-6) Both tests were conducted using under-the-wing (UTW) and over-the-wing (OTW) configurations. Small-scale (2-in. nozzle diam.) cold flow test data reported in references 5 and 7 are used for comparison to help in interpreting the large model and engine test results. Geometrical relations of the engine, wing, and flaps used in the TF34 engine tests are shown in figures 1 and 2 for UTW and OTW configurations. Fan and internal core noise were highly .suppressed,(1) leaving the flap noise as the dominant noise source. In the UTW configurations, data were taken with a separate-flow (coannular) nozzle at flap angles, 1(1, of 0 , 40°, and 55°, where the flap angle refers to the trailing flap. In the OTW configuration, noise data were taken with an internal mixer nozzle on the engine exhaust, and with a simulated flap angle of 40°. A deflector was used on the engine exhaust for the OTW case to obtain good flow attachment to the flap upper surface. The aspect ratio of the wing section tested was about one. Large model data were obtained in tests with a conical or a coannular nozzle fed from a muffled cold air supply. Both UTW and OTW configurations, shown in figures 3 and 4, were tested. Flap angles were 0°, 20°, and 60°. The coannular nozzle (fig. 3(a)) was roughly a one-half scale model of the TF34 separate flow nozzle of figure 1. The wing section aspect ratio was 1.3. . The test configurations for which data were used for this paper are summarized in Table I. The range of effective exhaust velocities used in the correlations is listed for each test configuration ("effective" velocity will be defined later). Data were not available for all test variables.