Noise and performance of propellers for light aircraft : final report

Abstract

Introduction and Summary: The project "Noise and Performance of Propellers for Light Aircraft," Contract #NASl-15154 between NASA Langley and MIT, has now been completed, and the main results obtained are summarized in this report and its appendices. The primary practical objective of the study was to explore the possibility of reducing the noise from a general aviation type propeller without altering significantly its aerodynamic performance or the engine characteristics. After an extensive study of this question, involving aerodynamic and acoustic theory, design, construction and wind tunnel testing of model propellers, design and manufacturing of full scale propellers and, finally, flight tests, we are pleased to report that for one of the propellers tested an overall reduction of 4.8 dBA as measured in a flight test was achieved.

The theory deals with aerodynamics and acoustics of lightly loaded propellers with subsonic tip speeds and includes studies of the effects of sweeping the blades, altering the radial load distribution, and changing the number of blades. These studies lead to new insight into the general problem of sound generation from moving bodies. Of particular value are the algorithms, which are well suited for computer coding. The wind tunnel tests involved three propellers, 1/4 scale, including a replica of a fixed pitch propeller used on a 150 HP single engine airplane. The other two propellers were designed to have the peak radial load distribution shifted inboard. The acoustic wind tunnel which was used in these tests enabled measurement not only of the radiated sound field but also the thrust and torque of the propeller. In addition, the load distribution was determined indirectly from wake surveys.

Sound pressure signatures were obtained at different locations and speeds (up to a tip Mach number of 0.75) and compared with theoretical predictions in which only the shape and motion of the propeller were needed as input parameters; no empirical adjustments were made. Agreement to within a few percent was obtained throughout except in the presence of a transonic "buzz" instability which was encountered within a narrow speed range. On the basis of the theoretical analysis and its verification in the model tests, a two-bladed fixed pitch propeller was designed for a 150 HP single engine airplane. Flight tests with this propeller indicated about the same performance as the production propeller for that airplane, but the maximum sound level during a full power flyover at 1000 feet was found to be 4.8 dBA lower. A second propeller, with three blades and fixed pitch, was designed for the Ohio State University 180 HP single engine airplane.

Flight tests of this propeller have not yet been made at this time.