Due to their high propulsive efficiency, Counter Rotating Open Rotors (CRORs) have the potential to significantly reduce fuel consumption and emissions relative to conventional high bypass ratio turbofans. However, this novel engine architecture presents many design and operational challenges both at engine and aircraft level.
The assessment of the impact of the main low pressure preliminary design and control parameters of CRORs on mission fuel burn, certification noise and emissions is necessary at preliminary design stages in order to identify optimum design regions. These assessments may also aid the development process when compromises need to be performed as a consequence of design, operational or regulatory constraints.
The required preliminary design simulation tools should ideally be 0-D or 1-D (for computational purposes) and should capture the impact of the independent variation of the main low pressure system design and control variables such as: the number of blades, diameter and rotational speed of each propeller, the spacing between the propellers and the torque ratio of the gearbox or the counter rotating turbine amongst others. From a performance point of view, counter rotating propellers have historically been modelled as single propellers. Such a performance model does not provide the required flexibility for a detailed design and control study.
This paper presents a novel 0-D performance model for Counter Rotating Propellers (CRPs) based on the classical low speed performance model for individual propellers and the interactions between them. This model also incorporates a compressibility correction which is applied to both propellers. The proposed model is verified with publicly available wind tunnel test data from NASA.
The novel 0-D counter rotating propeller performance model is used to produce a performance model of a geared Open Rotor engine with a 10% clipped propeller. This engine model is first used to study the impact of the control of the propellers on the cruise fuel consumption. Subsequently, the engine performance model is integrated in a multi-disciplinary simulation platform to study the impact of the control of the propellers on the certification noise.
The results of this case study show that 1–2% SFC savings at cruise are possible and an optimal control schedule is identified. It is also concluded that significant certification noise reductions are possible through an adequate control of the rotational speeds of the propellers.