ESDU 90023:2008

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.

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