Acoustics of Side-by-Side Synchrophased Rotors

The design and characterization of a new laboratory-scale rotor test stand for assessing the aerodynamic and acoustic performance of side-by-side synchrophased rotors is described. Each rotor is powered by a dedicated motor that communicates with other motors through a master/slave arrangement. The hardware/software combination is capable of holding angular phase synchronization to within an accuracy of 0.2 deg over sufficiently long periods of time required to capture statistically converged laboratory data. Acoustic measurements using two, side-by-side 18-inch diameter rotor blades (APC model 18×5.5MR- P with two blades per rotor) spinning in a counter-rotating configuration are then captured at four microphone observer locations. This comprises, two observers located at the ro- tor disk plane perpendicular to- and parallel to the multi-rotor axis, and two located 45 degrees below, also perpendicular to- and parallel to the multi-rotor axis. The findings demonstrate a significant change in the first few rotor harmonics as sound waves progress from causing constructive interference to destructive interference at the different observer locations. Sound pressure levels associated with the first four rotor harmonics are then evaluated during steady operations of the rotors and for all observer positions during systematic changes to the rotor phase over 180 degrees in increments of 10 degrees. The first blade pass frequency harmonic exhibits the most significant change in sound levels with a 30 dB reduction to the rotor disk plane observer when the synchrophased rotors are shifted by 90◦, and a 20 dB reduction for the observer located below the rotor disk plane with a 125◦ phase shift. Albeit, the second and fourth rotor blade pass frequency harmonics are shown to fill in where the first and third rotor harmonics fall off. Thus, the acoustic benefit in the form of an overall noise reduction may not be as great with delayed phase-synchronized rotors. However, the benefit may come more in the form of a broadening of the sound pressure spectrum and an overall flattening of the tonal components that produce impulsive like signatures that are believed to drive community annoyance.