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.

Noise Source Analysis in Counter-Rotating Open Rotors

“…For subsequent indices, the amplitudes of the fore rotor are generally greater than those of the aft rotor. This highlights the significance of the upstream interactions and agrees with previous findings on the importance of the potential interactions for low-solidity design CROR [6,7,20,57].…”

“…Both studies showed that the CROR follows the same relationship as the isolated rotor where lower tip speeds and larger blade counts are favourable for reduced noise. Smith et al [20] performed a parametric study of the effect of the blade count when either fore or aft blade row was locked to reduce noise. Whilst the authors demonstrated that the noise could be reduced through locking a blade row, ultimately the lowest noise was found for a CROR with the greatest blade count combination.…”

Parametric study of aerodynamic performance of equivalent ducted/un-ducted rotors

“…In the present work, the far-field acoustics is efficiently calculated using the FW-H equation [28], following the Farassat Formulation 1A [29], taking as input CFD solutions of the surface pressure fields. The formulation has been widely used for far-field noise predictions of aircraft, wind turbines [30], and coaxial propellers [ 31]. The Farassat Formulation 1A [29] solves surface terms of the FW-H equation, i.e.…”

Preliminary Design Framework for the Power Gearbox in a Contra-Rotating Open Rotor

“…The potential performance benefits of a CROR relative to single propeller engines and conventional turbofans have been extensively discussed in the literature, from acoustic [11]- [19], aerodynamic [11], [15], [20]- [23] and operability [13], [20]…”