Noise will continue to be a major problem within open rotor design development, writes Aimée Turner.
At the Institution of Mechanical Engineers' recent sustainable propulsion event, Josef Trchalik, performance engineer with Dowty Propellers, reminded delegates of the fate of the General Electric GE36 unducted fan programme of the 1980s, cancelled due to noise levels that would have exceeded requirements for new aircraft.
Noise generated by an aircraft is categorised as near-field (heard from the cabin), or far-field (heard from the ground). Near-field sources break down further: airborne noise that travels to the fuselage through the air and structure-borne noise transmitted through the airframe. Soundproofing is one option, but can increase weight by up to 2% and makes a rear-mounted pusher engine the ideal option.
Structure-borne noise can be generated directly by vibration through the engine mounting and in downstream surfaces by the swirl in the propeller wake.
"Contra-rotation is successful in removing constant swirl that would otherwise reduce engine efficiency. But it is only partially effective, dependent on blade count, in removing random swirl," says Trchalik.
The structure-borne element of near-field engine noise can be reduced if the wake of the contra-rotating blades does not interfere with the airframe. Structure-borne noise can be transmitted directly via the engine mountings, although high-frequency modes could be limited by mass dampers.
Far-field noise levels on take-off and landing are crucial when contra-rotating propellers generate higher noise levels. Noise here is mainly generated when the wake of the front rotor disc passes through the rear rotor disc. Interaction between the rear blades, turbulent flow in the wake, tip vortices shed from the front rotor, yields in pressure fluctuations, all generate noise.
Trchalik says blade-vortex interaction can be reduced by using a rear rotor with a smaller radius that does not hit tip vortices from the front row. Pressure pulses generated when blades in the front row pass blades in the rear row also creates noise.
"Mismatching blade numbers so there is only one blade interaction at a time can also reduce noise level results in smaller pressure pulses of higher frequency that do not propagate through the air so easily," says Trchalik.
Source: Flight International