Open rotor technologies offer the potential for significant reductions in fuel burn and CO2 emissions relative to turbofan engines of equivalent thrust.
Higher propulsive efficiencies are achieved for turbofans by increasing the bypass ratio through increases in fan diameter but there is a diminishing return to this improvement as nacelle diameters and consequently weight and drag increase. Open rotor engines remove this limitation by operating the propeller blades without a surrounding nacelle, thus enabling ultrahigh bypass ratios to be achieved.
Further improvements in propulsive efficiency can be gained for open rotor engines by using a second row of propeller blades rotating in opposition to the front row to remove the spin from the column of air, thereby producing a more direct thrust. The technical challenges of counter rotating open rotor engines are many, but are principally:
- Reduction of the noise created by the propeller blades to counter the loss of attenuation provided by a turbofan nacelle.
- Definition of the propeller system to reduce the noise created by the counter rotating blades (counter rotating slotted discs in close proximity is the basis siren design and blade interaction noise was the principal issue noted when the open rotor engines were demonstrated in the 1980s).
- Complexity of communications and blade pitch control through the counter rotating power transmissions system.
- Installation of the open rotor engine on the airframe: In conventional engine-to-wing configurations with turbofans, the engines are isolated from the airframe by the nacelle. However, with open rotors, the airflow through the propellers interacts with the supporting airframe structure in a different manner, hence the installation impacts on the engine system noise and efficiency.
Rolls-Royce has developed an open rotor propeller design to minimise noise and has demonstrated the effectiveness of these designs through scaled rig testing in the FP7 DREAM programme.
The SAGE1 project plans to acquire technology for the propulsor system, increasing the Technology Readiness Level (TRL) to TRL 5.
Collaborating in the SAGE1 project under Rolls-Royce leadership are GKN Aerospace and ITP.
Examples of technology maturation studies are the successful Rig 145 “installed and uninstalled” high-speed testing of the Rolls-Royce second generation CROR Blades with Airbus in 2011, and the Z08 wind tunnel test successfully performed pusher/puller configurations with Airbus (Q2 2012).
After the strategic decision of Rolls-Royce to focus on Lean Burn Combustion in SAGE 6, the activities in SAGE 1 were re-scoped in 2012 and then focused on completing the work in 4 main areas:
- Validation of a fast Computational Fluid Dynamics (CFD) solver tool for the Open Rotor case. This work was completed in 2015 and has focused on validating extreme blade angles of attack against rig Z08 blade surface measurements.
- Component integrity and evaluation of blade design and material options, including a sample test programme under high strain rate, during shear extension experiments. This project was completed in 2015.
- Validation of methods for stability, flutter and forced response against available rig data. This programme was completed in 2015.
- Validation of far-field and near-field noise methods for open rotor designs using available rig data. This programme was completed in 2015.
The programme of work was focused on the R&T activities in place at universities to ensure continuation and completion of their research.