Noise reduction of a Counter Rotating Open Rotor through a locked blade row

Abstract

Counter Rotating Open Rotors (CROR) have the potential to significantly reduce aircraft emissions. However, there are a number of concerns surrounding their noise levels. In this contribution, we propose a number of novel configurations to reduce the noise of CROR for general aviation aircraft. In particular, we lock either fore or aft blade row on departure and approach, and keep the CROR trimmed at the same thrust as in design configuration (both rotors running). The CROR is switched back to design configuration at cruise to regain the high propulsive efficiency. To further reduce noise, two additional cases are investigated: 1) folded aft rotor; 2) increased diameter fore rotor (with locked-aft blade row). A low-order CROR trim code that has been developed to compute the aerodynamic and aeroacoustic performance of new design configurations is used in the numerical analysis. The results demonstrate significant noise gains for a number of blade count combinations during a constant altitude flyover. The analysis found that using a locked-fore and locked-aft configuration resulted in a maximum noise reduction of around 3 dB(A) (or 3 dB (EPNL)) and 3 dB(A) (or 6 dB (EPNL)) respectively. Folding the aft rotor backward could result in a maximum noise gain of around 12 dB(A). Increasing the fore rotor diameter by 30% was shown to reduce noise by a maximum of 8 dB(A) against an operative CROR of the same tip extension.