Systems for simulation of aircraft-propeller noise in the laboratory have been developed at LTV Vought Aeronautics. The basic unit of these systems is an electronic propeller-noise generator that was designed and built as a Company-sponsored R and D program. This unit produces a voltage signal proportional to the rotational and vortex sound pressures generated by aircraft propellers. The rotational noise is made up of the fundamental 4P blade-passage frequency plus several significant harmonics. Adjustments are possible on frequency, amplitude, and relative phase of these discrete-frequency components. Vortex noise is random in character and adjustments of over-all level, spectrum shape, and frequency-band limits are possible. In a final stage, the rotational- and vortex-noise components are mixed to produce simulation of the complete propeller-noise signal. This voltage signal is used as an input to a high-intensity progressive-wave sound system. In this system, sound-pressure levels of 150–170 dB are generated for testing of structural panels. Another use for this signal is as an input to a high-fidelity speaker system that operates at SPL's of 70 to 125 dB. This system provides simulated propeller noise for interior-fuselage acoustic investigations and human-factors studies. Equalization of either system operating into various impedances of different panels and enclosures can be accomplished by appropriate adjustment of noise components produced by the propeller-noise generator. Typical results are shown in slides illustrating measured waveforms and spectra. These results are compared to propeller noise measured from aircraft propellers.
Farfield noise radiated from low-tip-speed propellers on the YO-3A quiet observation aircraft and measured in flyover tests exhibited unexpected levels and trends that have been called the “quiet airplane paradox.” Levels of rotational propeller noise were found to be higher than predicted by conventional axisymmetric theory, and as propeller tip speed was decreased below a given value, at constant thrust, both harmonic and broad-band noise levels actually increased. Thus a “bucket” was formed in the SPL vs tip speed curve. In addition, the harmonic character of the flyover propeller noise was markedly different than that observed in static propeller tests. These data were declassified in 1973, and a program was conducted to investigate the paradoxical experimeutal results and to develop a theoretical explanation. The theoretical study considered propeller blade aerodynamics, nonuniform inflow through the propeller, chordwise blade loading, and, finally, propeller blade wake/wing interaction. It was concluded that the principal explanation of the paradox lies with the non-uniformity of inflow through the propeller disk causing circumferential variations of blade loads. The blade wake/wing interaction rated second in important. [Original experiments with the YO-3A aircraft were supported by the Army and this research program was supported by the Air Force.]
An experimental quiet aircraft has been developed by LMSC for research in quiet aircraft technology. This company-owned aircraft, designated the Q/Star, had minimum acoustic signature as its principal design goal. To achieve this goal several significant noise-reduction features were employed. A relatively clean aerodynamic design reduced aerodynamic noise. A “Wankel”-type rotary combustion water-cooled engine was selected to avoid noise-reduction problems associated with air-cooled engines. The muffler system was designed to be most efficient at cruise flight conditions and the system was “tweaked” in the field for maximum performance. The propeller is driven through a 5.34:1 reduction system to provide propeller tip speeds less than Mach 0.2. Over all, these design features have produced an exceptionally quiet airplane. A series of flight tests have been conducted using a variety of propellers including one designed specifically for acoustic noise performance. Data are presented to show aircraft flyover acoustic noise signature as functions of propeller configuration, engine rpm, and air speed.
Aerodynamic noise experiments utilizing a two-dimensional cambered airfoil extending through both the potential core and the mixing regions have been conducted in the quiet free jet facility at the Lockheed Rye Canyon Research Laboratory. Farfield noise measured in these tests differs significantly from the noise reported by other investigators in similar experiments. Specifically, the pure-tone phenomena previously observed with airfoils subjected to subsonic flows of 100–200 ft/sec have been absent in these tests. Instead the measured noise is broad band, extending from 100 to 16 000 Hz, and appears to be random in character. Two peaks are observed in the spectra near 400 and 6000 Hz. At present, it is uncertain whether the source of lower band peaking at 400 Hz is in the potential core, the mixing regions, or perhaps both. However, considerable evidence has been obtained that indicates that the source of the higher band peaking at 6000 Hz is a dipole radiation caused by fluctuating surface loads in the turbulent boundary layer of the airfoil in the potential core. This high-frequency band of noise was predicted before these experiments were conducted by the theoretical work discussed in Dr. Revell's paper.
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