Division Engine installation correction ΔI

An aircraft in flight is a complex sound source. Not only are the engine (and airframe) sources complex in origin, but the airframe configuration, particularly the location of the engines, influences the noise radiation patterns through the processes of reflection, refraction and scattering by the solid surfaces and aerodynamic flow fields. This results in a non-uniform directionality of sound radiated laterally about the roll axis of the aircraft, referred to here as lateral directivity.

There are significant differences in lateral directivity between aircraft with fuselage-mounted and underwing-mounted engines and these are allowed for in the following expression:

where Δ _I (φ) is the correction, in dB, at depression angle φ (see Figure 2.7.m) and

For propeller aircraft directivity variations are negligible and for these it may be assumed that

Figure 2.7.n shows the variation of Δ _I (φ) about the aircraft roll axis for the three engine installations. These empirical relationships have been derived by the SAE from experimental measurements made mainly beneath the wing. Until above-wing data have been analysed it is recommended that, for negative φ, Δ _I (φ) = Δ _I(0) for all installations.

It is assumed that Δ _I (φ) is two-dimensional; i.e. it does not depend on any other parameter — and in particular that it does not vary with the longitudinal distance of the observer from the aircraft. This means that the elevation angle β for Δ _I (φ) is defined as β = tan^–1(z/ℓ). This is for modelling convenience until there is a better understanding of the mechanisms; in reality, installation effects are bound to be substantially three-dimensional. Despite that, a two-dimensional model is justified by the fact that event levels tend to be dominated by noise radiated sideways from the nearest segment.