scope:

INTRODUCTION

Airframe noise is caused by airflow over aeroplane surfaces. It does not include powerplant noise and therefore sets a lower limit below which reductions in engine noise emission have no significant effect on the total noise level from the aeroplane. The level of airframe noise is dependent upon the aeroplane configuration. In basic terms, an aerodynamically 'clean' aeroplane produces less noise in the airflow than a 'dirty' one. The landing configuration with slats extended, flaps down and undercarriage lowered is therefore considerably more noisy than the clean configuration. An illustration of the magnitude of this increase in noise level for the aeroplane described in the example in Section 8 is shown in Sketch 2.1. The solid curve in Sketch 2.1 represents the predicted noise spectrum for the aeroplane overhead, in the landing configuration, and the dashed curve is for the aeroplane in the clean configuration at the same location and flying at the same airspeed. The breakdown of the airframe noise spectrum for the landing configuration into its constituent component spectra is shown in Sketch 2.2.

This Item provides a computerised method for estimating airframe noise in the far-field. The prediction method, which is a semi-empirical one, has been developed from that proposed by Fink (Derivations 2 and 5) with changes to directivity and spectral functions based on recently available data. The aeroplane geometry and operating conditions are required input data. The program permits the estimation of the OASPL and of one-third octave band sound pressure levels within a frequency range and over polar and azimuthal angular ranges set by the user. The output values are for free-field and still, lossless atmospheric conditions. Procedures to correct for atmospheric attenuation, ground reflection and lateral attenuation in still atmospheric conditions may be found in References 8, 9, 10 and 12. To account for wind and temperature gradients the method described in Reference 11 may be used.