Abstract: An "adaptive synchrophaser" is disclosed for modifying the phase angle relationship between aircraft propellers to reduce cabin noise and/or vibration. Rather than use a constant pre-selected angle for a specific passenger cabin configuration during a particular flight mode (e.g., during liftoff or cruise), the synchrophaser periodically monitors actual operating conditions and modifies the phase angle accordingly. In the preferred embodiment, a plurality of transducers (microphones) are installed at several cabin locations to sample noise periodically. The signals are then transmitted via a signal conditioner, a multiplexer and an analog-to-digital converter to a signal processor. The processor calculates the maximum acoustic noise at each microphone location for all possible phase angles. It then identifies the optimum phase angle that resulted in the lowest maximum noise anywhere in the cabin, and signals a synchrophaser to set that angle.

Abstract: Noise is reduced on an aircraft engine of the type having aft mounted propeller blades 10 which pass through exhaust gas streams 20 expelled from circumferentially spaced nozzles 14. The nozzles 14 are rotated, or the exhaust flow deflected, such that the high velocity exhaust streams 20a expelled from the nozzles 14 are directed away from their normal axial direction to impact the blades 10 at the same relative angle of attack 36a as the relative angle of attack 34 of the low velocity ambient air stream 22 thereby reducing fluctuations in blade lift and the associated noise.

Abstract: The rotation of N-bladed propellers of a multi-engine aircraft is controlled by phase angle synchronizer 30 such that a predetermined phase angle offset .phi.3 is maintained between the blades of a first propeller 10 and the blades of a second propeller 20, at which offset cabin noise is reduced to a minimum and cabin vibration to a near minimum. The controller 40 determines the optimum phase angle offset .phi.3 to be that phase angle of the set of N periodic phase angles: .phi.1+K(360/N), where K ranges in integer steps from 0 to N-1, each characteristic of minimum cabin noise generation, beginning with the minimum first phase angle, .phi.1, lying between 0 and 360/N degrees, which is closest to the second phase angle, .phi.2, characteristic of minimum cabin vibration.

Abstract: An aircraft engine propeller synchrophasing system (4) has a gain stage (8) which varies the gain of the system (4) according to the magnitude of the difference between the preferred propeller relative phase and the measured propeller relative phase.

Abstract: The fuselage (cabin) (15) of a propeller (45) driven aircraft (10) is isolated from vibration associated with the wake (46) of the propeller by vibration isolators (50, 55 and 110) in the wing and tail surfaces of the aircraft.