Interpretation of Air-to-Air Flyby Noise Measurements on a Piston-powered Propeller Aeroplane.

Abstract

To clarify certain conspicious unsymmetries in the radiated noise from piston-powered single-engine propeller-driven aeroplanes "air-to-air acoustic measurements" were conducted employing an inherently quiet sail plane as a microphone carrier and to fly the very test aircraft at various positions relative to it (i.e. above, below and to the side of). The test aircraft, a Dornier DO 27, was equipped with a 6-piston combustion engine and a 2-blade propeller with an uneven gear-ratio in between, faciliting the separation of propeller and engine exhaust contributions in the combined radiated acoustic pressure time history (PTH). The engine's twin pipe exhaust system was located on the underside of the aircraftsuch that its two orifices were acoustically shielded to some extent from an observer above the aircraft. The purpose of this study was to check on the acoustic consequences of superposing the propeller signal and the exhaust signal, hoping that certain characteristics.