Rolls-Royce Completes First Ground Test of Hybrid-Electric Propulsion System

Intended for use in future eVTOLs and general aviation aircraft, flight testing is slated to begin within two years.

Rolls-Royce eVTOL Aircraft
Rolls-Royce says it plans to begin flight testing of its developmental hybrid propulsion system in 2021.Courtesy Rolls-Royce

Rolls-Royce has begun ground testing a hybrid propulsion system using its M250 gas turbine engine for helicopters that the company says will clear the way for experimental flights on aircraft within two years.

The company said it successfully tested the hybrid-electric version of the M250 gas turbine in a ground demonstration setting using three operating modes: series hybrid, parallel hybrid and turbo-electric. The M250 hybrid is planned as a propulsion plant with power ranging from 500 kilowatts to 1 megawatt. 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.

• Series hybrid: In this configuration, the engine operates as a turbo-generator that charges an onboard 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: In this configuration, 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: In this configuration, 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 Rolls-Royce hybrid electric propulsion system demonstrator integrates the 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 optimizes 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.

Rolls-Royce engineers based in the United States, UK and Singapore worked together to develop the engine into a hybrid-electric propulsion system. The M250 gas turbine engine has powered more than 170 varieties of fixed-wing military, civilian aircraft and helicopters since its initial development. The company selected this engine for its maturity, power-density, ease of maintenance, and high reliability. In the past half-century, M250 variants have logged more than 250 million flight hours and nearly 33,000 M250 engines have been delivered to customers.

Complete engine testing took place at Rolls-Royce’s facility in Indianapolis, where each component and sub-system of the hybrid engine was individually tested for electrical performance. These tests included simulating use across takeoff, 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 4,400 pounds. This would support the Rolls-Royce eVTOL concept that was unveiled during the Farnborough Air Show in England last year.

"Rolls-Royce has always been a pioneer in aviation and one of the key elements of our strategy is to champion electrification across all our businesses," said Dr. Mike Mekhiche, deputy director of Rolls-Royce Electrical. "The successful testing of the hybrid M250 system is an important step forward in providing a hybrid-electric propulsion system that will enable a new class of quieter and cleaner air transport."

"Electrification is one of the most exciting developments in aviation since the birth of the jumbo jet,” Dr. Mekhiche added. “We are determined to use our pedigree in aerospace to be at the forefront of developing innovative propulsion systems to meet the needs of the next chapter in aviation.