Because of their considerably higher fuel efficiency compared to turbofans, turboprop aircraft are the best choice for short and middle-haul flights. Yet, propeller acoustic emissions need to be reduced to comply with future noise certification standards, and to improve the comfort of passengers and crew.
The CFD solver of the University of Glasgow, HMB3, was first validated for propeller aerodynamics and acoustics against JORP and IMPACTA wind tunnel data, and then employed for comparing different innovative designs and installation options to identify the quietest solution. OSPL and frequency tonal spectra were directly computed from (U)RANS results. Cabin noise was estimated via experimental transfer functions.
The design of the propeller is the key to decrease the emitted sound at source level. A blade design that moves the loading inboard and operates at lower rotational speed yielded relevant noise gains (up to 6 dB in OSPL) without strong performance penalties. Hub configurations meant to redistribute the acoustic energy over more frequencies did not clearly appear more pleasant for passengers.
The presence of the airframe modifies the propeller inflow, and causes additional noise sources as well as sound waves reflections. The need of simulating the whole airplane in real operating conditions to accurately estimate in-flight noise was shown. For a twin-engined high-wing aircraft with propellers in phase at cruise conditions, the counter-rotating top-in layout was found the quietest, with a benefit in interior OSPL of more than 4 dB compared to co-rotating propellers. The inboard-up propeller rotation led louder noise because of the higher blade loading on the fuselage side, and of constructive sound waves interferences. The latters are instead used favorably from propeller synchrophasing, promoting noise cancellation. This strategy was shown to provide more than 3 dB of OSPL noise reduction inside the cabin on co-rotating propellers, whereas propellers in-phase appeared the best operating option for the counter-rotating top-in layout
