Abstract
A theory for harmonic noise radiation is studied for general guidance to the designer and is applied to some propeller noise problems of current interest. Only the linear sources are studied in detail. The frequency domain results clarify the role of acoustic noncompactness (noise cancellation due to finite chord and span effects). Nondimensional parameters arising from the analysis give design guidance by showing the potential for noise reduction due to changes in airfoil section and blade sweep, twist, and taper as functions of operating conditions. Conventional propellers are shown to be relatively insensitive to variations in blade design. However, advanced turbopropellers (prop fans) currently under development are decidedly noncompact because of their high solidity and speed. Examples of chord wise and span wise cancellation are given illustrating substantial benefits of sweep.
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