Rolls-Royce has successfully completed ground testing of a hybrid-electric propulsion system using the M250 gas turbine normally used to power helicopters. The company says the tests pave the way for experimental flights on aircraft in 2021.

The UK firm indicates that the M250 hybrid is planned to be used as a propulsion plant with power ranging from 500kW to 1MW. The system will be used across a range of transport platforms to enable distributed electric propulsion, including EVTOLs (hybrid electric vertical take-off and landing vehicles), general aviation aircraft and hybrid helicopters.

Complete engine testing took place at Indianapolis, U.S. Tests included simulated take-off, cruise, landing and taxiing and confirmed the system’s suitability for a range of transport platforms including aircraft with a range of up to 1,000 miles and weighing up to 2,000kg.

The Rolls-Royce hybrid electric propulsion system demonstrator integrates a M250 gas turbine engine with a high-energy-density battery system, electric generators, power converters and an advanced power management and control system. The power management system optimises overall propulsion performance in order for the system to be suitable across a variety of platforms, including EVTOL, while delivering efficiency gains, reduced noise and lower emissions.

Tests were successfully carried across three system configurations:

• Series hybrid: the engine operates as a turbo-generator that charges an on-board battery system, and does not contribute to thrust directly. All power required for thrust and other onboard systems is supplied by the battery.

• Parallel hybrid: the platform thrust is supplied by a combination of the engine (mechanical thrust) and the electrical system (electrical thrust), while the other power needs of the aircraft are met by the battery.

• Turbo-electric mode: the battery system is redundant; the engine operates as a pure turbo-generator supplying electric power for thrust and any other power needs on the aircraft.

The M250 hybrid power pack complements the AE 2100 2.5MW system being developed for larger aircraft, including regional aviation, which is being tested with Airbus on the E-Fan X demonstrator platform.