Small aircraft propeller noise with ducted propeller

The purpose of this paper was to document the results of initial testing of various configurations of a ducted propeller apparatus. The apparatus was designed based on a combination of acoustic principles and a desire to be able to apply the knowledge gained to a practical application such as an ultralight aircraft in an effort to reduce the overall noise levels emitted. The apparatus consisted of a 35 horsepower ultralight engine, a four bladed ultralight propeller, and a duct constructed of a foam core covered with fiberglass. Initial evaluations compared noise levels from the apparatus both with and without the shroud in place, as well as various engine silencer configurations. The data gathered proved the apparatus was actually about 6 dB louder with the shroud than without the shroud as a result of strong rotor-stator interactions. Based on the initial evaluations, this apparatus demonstrated its potential for further testing and acoustical work in the principles of rotor-stator interactions, short duct acoustics, and active noise control applications with the long range goal being to reduce the acoustic emissions from propeller driven aircraft.