Acoustics of ultralight airplanes

Abstract

In view of the stringent noise limits that have been imposed on ultralight airplanes, an experimental and theoretical research program was initiated to investigate specifically the noise sources of ultralight airplanes. For this purpose, flyover and ground static noise measurements on tractor- and pusher-propeller-driven airplane configurations and wind-tunnel noise measurements on isolated full-scale ultralight airplane propellers were conducted. The study showed that pusher-propeller-driven ultralight airplanes were 5-15 dB noisier than those equipped with tractor propellers. Engine noise—even if well muffled—was frequently found to equal propeller noise in magnitude. The experiments have established propeller blade-tip speed and thrust as the important parameters of propeller noise, rather than flight speed or number of propeller blades. Also, at the relatively low operational helical blade-tip Mach numbers, propeller broadband noise, rather than propeller harmonic noise, determines ultralight airplane propeller noise. To comply with stringent noise limits, ultralight airplanes must be of clean aerodynamic design (to minimize drag for low thrust requirements), be equipped with thoroughly muffled engines, and their operational propeller blade-tip Mach number must be limited to values well below 0.5.