Model Propeller: Noise
ADVANCES in model aviation over the past decade have spurred tremendous growth in the hobby. Each year thousands of people across the U.S. are becoming active participants in the weekend sessions at the local flying field. The greatest influx of newcomers to the sport naturally occurs near large cities where the population is concentrated, so that flying sites tend to be located in suburban areas. While this allows easy access for fliers, it also subjects many an uninterested neighbor to the characteristic whine of the model aircraft.
Model Propeller Noise
THE FIRST article of this series on model propeller noise reviewed basic theoretical concepts concerning thrust, drag, and aircraft performance, then continued with definitions of sound, frequency, sound pressure level, and sound power level. The nonlinear manner in which sound levels combine-a result of the logarithmic nature of the dB scale-was also explained, helping the reader to understand the relative contributions of various noise sources to total sound. Finally, data were presented which clearly demonstrate that, in a typical model aircraft equipped with a reasonably efficient muffler, it's the propeller rather than the engine that dominates the noise signature.
Model Propeller Noise
IN THE LAST article on propeller noise sources, the discussion centered on noise caused by torque loads on propeller blades. This article will address noise due to blade thickness and noise due to thrust loading. Thrust noise is generally held to be a less efficient sound-generating mechanism than both torque and thickness noise. This results from the manner in which the noise is produced, although a complete discussion of that mechanism is outside the scope of this article. Thickness noise is in many respects similar to torque noise.
Model Propeller Noise
THE PREVIOUS articles in this four-part series have shown that propellers are the main source of model aircraft noise and explained the various sources of propeller noise. It was further shown that noise due to torque loading is the most dominant of these different sources. Last month's article concluded with the following comments: "The propellers available are generally very inefficient, and this inefficiency leads to high torque loads relative to the thrust that is produced. "The path to model aircraft noise reduction is therefore clear, although perhaps not easy. Making propellers more efficient would reduce noise levels without compromising aircraft performance."
