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Propellers and fans have in the past been tested under static conditions to provide test data for comparison with noise predicted using empirical or theoretically based procedures. This has resulted in noise prediction methodology adjusted to correlate with static test data. Recent tests have shown that the noise of propulsors in flight differs significantly from propulsors tested statically. This paper discusses the current knowledge of propulsor noise from an experimental and analytical standpoint in both static and forward flight operating regimes. Also, the advantages and disadvantages of various approaches to conducting definitive forward flight tests are presented. Emphasis is placed on evaluation of propellers and fans operating at subsonic tip speeds where, experimental and theoretical developments can most easily be applied.

Within the next few years tighter restrictions on general aviation aircraft noise are expected. It is anticipated that these noise restrictions, like those imposed on larger transport aircraft now certified under Federal Aircraft Regulations, will be revised downward over a period of time. While it is expected that initial restrictions can be met by the current propeller technology, the larger lower tip speed propellers necessary to meet succeedingly more stringent restrictions may prove difficult to accept. In this paper an alternative to the propeller as a propulsor for general aviation aircraft is discussed. This is the subsonic tip speed low-pressure ratio fan which can be mated to turboshaft, rotary combustion, or reciprocating engines to provide a low noise propulsor in a small package. Information is presented which shows tradeoffs among noise, weight, size, cost, and performance.