As part of the project, wind-tunnel tests measured propeller efficiency and acoustics while the Royal Netherlands Aerospace Centre (NLR) ran tests examining how propeller noise translated into the cabin.
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UK government-funded research program Impacta began in 2012. Likewise, synchrophasing, whereby blades on twin- or four-prop aircraft turn in close synchronization, helps minimize fuselage noise-effects.” But Dowty’s quest for ever-quieter propellers drives continued aeroacoustic research. “All our propellers are variable-pitch and change blade-angle for different flight conditions, allowing us to tune the noise signature. “Composites afford us flexibility to tailor shapes to address acoustic aspects,” says Chestney. “Regional aircraft require the lowest possible noise signature, both near-field within the fuselage and far-field, in terms of those living close to airfields.” As Dowty worked with progressively larger aircraft generating greater thrust, it evolved means to regulate propeller noise.ĭowty brought the world’s first all-composite blades into service in the 1980s and has made over 23,000 propellers from composites. “Innovation out of necessity remains a trait of the business,” says Dowty’s technical director, Jonathan Chestney. Today as part of GE Aviation Systems, Dowty produces propeller systems for the Lockheed Martin Super Hercules and De Havilland Dash 8. Owty’s R391 propeller system is used on the C-130 Super Hercules UK-based company Dowty Propellers, which can trace its history back to 1937 and produced propellers for the Spitfire WW2 fighter, is working to refine the technology. Blade configurations can be tailored for acoustic performance, but incremental reductions depend on highly accurate wind-tunnel data and cutting-edge digital optimization. A propeller developed for EU research project ARGOS (Aerospace propeller useful for diesel engines with extreme excitation of vibrations) last year produces a relatively tolerable noise-signature because of the low rotational speed of the diesel engine which drives it. “The lower frequency of a Harley-Davidson makes it less annoying than a dentist’s drill,” Pompe says. One way to mitigate propeller noise is to reduce the frequency of sound they emit. If no noise is radiated, no aerodynamic forces exist and no airplane or rotorcraft can fly.” If propellers are our only choice, then their noise can only be mitigated. “But every lifting-surface which generates an aerodynamic force also becomes a source of noise.
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“Aerodynamic forces depend on pressure-changes,” says Czech Republic-based propeller manufacturer Woodcomp Propellers R&D Manager, Vilém Pompe.