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Applicability of a numerical code to aerodynamic design of a Prop-Fan is established by precise agreement of numerical results with experimental data, i.e., not only measured integrated performance indices, such as power coefficient or net efficiency but also pressure distribution on the blade surface should agree well with computed results. For this purpose, an Euler Code using the Total Variation Diminishing scheme has been developed. Numerical calculations are performed with this scheme for the SR-7L Prop-Fan at the freestream Mach number 0.5 and 0.78. The computed power coefficient, CP = 1.46 at M∞ = 0.5 shows good agreement with experimental data. At this computed CP, the computed pressure distributions on the blade surfaces show good agreement with the experimental results. For the 0.78M∞ case the computed CP of 0.87 also shows good agreement with the experimental results and the computed pressure distributions are in general agreement with the experimental data.

Airline deregulation and the high cost of fuel have caused a renewed interest in propeller-driven aircraft as a replacement for existing turbofan aircraft. Since passengers on existing turbofan aircraft have become accustomed to lower interior noise than exists in current propeller aircraft, there has been a renewed interest in interior noise control by reduction of propeller source noise, by design of lightweight fuselage soundproofing and other noise reduction concepts. This paper discusses the noise control problem from a source noise and aircraft design standpoint. The existing state-of-the-art is reviewed and the promising strategies for reducing noise in propeller aircraft are discussed.

The initial results of a program to investigate the sources of noise in unshrouded propellers under forward flight conditions are reported. Tests were conducted using a three-blade, full-scale instrumented propeller mounted on a twin-engine aircraft. Measurements included 1) farfield noise at fixed ground stations and at two aircraft wing tip locations, 2) blade surface pressures at seven locations on one of the propeller blades, 3) atmospheric turbulence encountered by the aircraft in flight, and 4) aircraft operating conditions. The results confirm that significantly lower levels of propeller noise are produced in forward flight than at static conditions. The most significant reductions occurred at mid-frequencies which dominate Perceived and A-Weighted Noise Levels. Blade surface pressure data showed the presence of disturbances in the propeller inflow under static conditions which were seen to disappear as the aircraft started its takeoff roll.

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.

Government and industry representatives are currently at work formulating a rule for certifying the noise levels of short takeoff and landing (STOL) aircraft. Conventional aircraft are primarily turbofan propelled and fly over well-defined landing or takeoff paths at conventional airports. In contrast to this, STOL aircraft may be propelled by propellers or rotors as well as turbofans, and will fly over varied landing and takeoff paths at new STOL ports located in or around populated areas. Therefore, it is expected that the STOL noise rule will differ from that issued late in 1969 for conventional subsonic transport aircraft. In this paper, the background information on STOL aircraft noise, STOL port site characteristics and noise evaluation units is discussed.