• 19 Sep 2022 12:00 AM
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Integrated Propulsion: Proving the Promise of Hybrid Air Transport

By Robert W. Moorman
Vertiflite, Sept/Oct 2022

Leading engine manufacturers are developing hybrid-electric powerplants for turboprop and turbofan regional airliners for reduced emissions, fuel consumption and operating costs.

Most new aircraft-related technologies come in stages. Consider the pathway to development of emission-free powerplants for electric conventional and vertical takeoff and landing (eCTOL and eVTOL) aircraft. First came the overarching goal from innovators, airlines, governments and others to significantly reduce operating costs and carbon dioxide (CO2) emissions from aircraft.

Next came the unveiling of short-haul, battery-powered eVTOL and eCTOL aircraft by new players to aircraft manufacturing. This was followed by the measured approach toward battery-powered eCTOL and eVTOL aircraft development by the big established players, such as Boeing, Airbus and Embraer.

Now come four leading turbine-engine manufacturers — Pratt & Whitney/Pratt & Whitney Canada, General Electric (now GE Aerospace), Safran and Rolls-Royce — which are developing hybrid-electric propulsion for commercial airliner and other aircraft applications.

What makes the hybrid-electric propulsion story compelling is that it provides both short and possibly long-term benefits.

“I feel this is a real moment in the sun for an engineer,” said Christine Andrews, GE Aerospace’s Hybrid Electric Systems Leader. Andrews joined GE in Cincinnati, Ohio, following employment with Gulfstream Aerospace as a structural certification engineer. “With the public’s attention on sustainability, I am personally drawn to hybrid-electric technology. Once we prove this [technology] in flight, I don’t [think] aviation will ever go back.”

Competitors agree.

Anthony Rossi, Vice President of Marketing and Sales at Pratt & Whitney Canada, said, “We think hybrid capability is a faster path to getting closer to a zero-emission scenario. That is why we are exploring these types of technologies.”

Grazia Vittadini, Chief Technology and Strategy Officer, Rolls-Royce, took a broad view toward ongoing clean propulsion efforts. “We want to be ready to pioneer sustainability with whatever the future requires, be it hydrogen, electric power, sustainable aviation fuel, or gas turbine efficiency.”

Pratt & Whitney

In July, Pratt & Whitney Canada, a business unit of Pratt & Whitney and division of Raytheon Technologies Corp., announced plans to advance its hybrid-electric propulsion technology and flight demonstrator project as part of a CAD $163M (USD $125M) program supported by the federal government of Canada and the provincial government of Quebec. The Scalable Turboelectric Powertrain Technology (STEP-Tech) is a principal element of P&W’s ongoing commitment to producing sustainable and clean propulsion systems. Rossi said the STEP-Tech program is a civil endeavor primarily.

Rossi was candid when asked if the hybrid-electric sector was beyond the exploratory stage: “I would say that we are at the end of the first stage and people are starting to ask deeper questions on what is and isn’t real.”

STEP-Tech will include complete system capabilities, including an efficient turbogenerator, energy storage, power electronics and modular electrically driven propulsors. It will support high-voltage turboelectric hybrid-electric propulsion concepts in the 100-500 kW class, and up to 1 MW, and support the development of propulsion systems for eVTOL and eCTOL vehicles.

P&WC is working with De Havilland of Canada to install this hybrid-electric technology into a 40-seat DHC Dash 8-100 turboprop purchased in 2019 to become a flight demonstrator. One of the aircraft’s two P&WC PW120 turboprops will be replaced with a hybrid engine, but the turbine engine to be used has not been disclosed.

GE Aerospace

For GE, there’ve been several hybrid-electric-powered related developments.

Collins Aerospace, also a Raytheon Technologies company, will supply an advanced electric motor and controller. The demonstrator will be housed at the Raytheon Technologies Research Center in East Hartford, Connecticut.

In mid-May this year, P&WC selected Swiss company H55 S.A. to provide battery systems for its hybrid-electric flight demonstrator program.

Ground testing of the full propulsion system is slated for the end of 2023, with first flight in mid-2024, said Rossi. A principal goal of the P&WC demonstrator program is to achieve a 30% reduction in fuel burn and CO2 emissions as compared to a turboprop-powered, regional airliner.

In 2019, GE began testing of its megawatt-class, multi-kilovolt hybrid-electric propulsion system at NASA’s Neil A. Armstrong Test Facility in Sandusky, Ohio. The facility houses the NASA Electric Aircraft Testbed (NEAT), which allows developers to test their new propulsion systems. NEAT is run by NASA’s nearby Glenn Research Center in Cleveland, Ohio.

These tests were part of a multi-year program under a so-called Space Act Agreement between NASA and GE that utilized Glenn Research Center’s ability to test full-scale propulsion systems at simulated high altitudes.

In October 2021, the program transitioned to the five-year Electric Powertrain Flight Demonstration (EPFD) program (Everett, Washington-based magniX also received funding in a separate contract). Ground and flight tests of the megawatt program will begin by the mid-2020s. Potential entry into service of the hybrid-electric powerplant is slated for the mid-2030s. Like NEAT, EPFD is part of the agency’s longer-term Electrified Aircraft Propulsion (EAP) research project.

GE’s partners Boeing and its subsidiary Aurora Flight Sciences will modify a test plane, a 30-36-seat Saab 340B, to be powered by GE’s CT7-9B turboprop engines. The aircraft will be used to test the hybrid-electric technology, which includes motors, generators and power converters. NASA is funding GE with $179M over the five-year EPDF project.

Boeing and Aurora Flight Sciences were selected in early February 2022 to provide GE with system integration, flight testing and airplane modification. This includes flight deck configuration design and software, nacelle manufacturing, aircraft performance analysis and systems integration services.

BAE Systems will provide the battery and cabling to store electricity and drive the GE-built motor and generator. In addition, BAE will provide controls and cables for the demonstrator’s power management system to be tested on the CT7-9B engines.

Weight trade studies are being done for various segments of aviation as part of the EPFD program. “So, you can include helicopters, eVTOL aircraft in that space,” said Andrews. “Ultimately, hybrid-electric technology is another tool in our toolbox to apply to the e-market sector.”

At the Farnborough Airshow, GE announced the completion of the world’s first test of a megawatt-class and multi-kilovolt hybrid-electric propulsion system in simulated altitude conditions of 45,000 ft (13,700 m).

The US Army Research Laboratory, part of that branch’s Combat Capabilities Development Command (DEVCOM), awarded GE a $5.1M research and development contract this past March for the Applied Research Collaborative Systematic Turboshaft Electrification Project (ARC-STEP). This project, with application to helicopters and eVTOL aircraft, augments GE’s efforts to develop hybrid-electric engine technologies for military applications. Tests will take place at GE’s research campus in Niskayuna, New York.

The Army’s ARC-STEP and NASA’s EPFD program help GE’s ongoing efforts to produce hybrid-electric systems and electrical power generation for civil and military transport aircraft.

ARC-STEP will use a CT7 turboshaft engine (the commercial version of the T700), combined with GE-provided electric machines and power electronics.

“GE has a record of developing innovative products that make a difference to the US Army and we believe this is another opportunity to build on that legacy,” said Harry Nahatis, Vice President and General Manager of Turboshaft Engines at GE.

The ARC-STEP contract came shortly after NASA selected GE for the EPFD project. The company noted that it had been studying hybrid-electric propulsion systems since 2009 and opened its Electrical Power Integrated Systems Center (EPISCenter) in Dayton, Ohio, in 2013. In 2016, GE demonstrated an electric machine consisting of a 1-MWclass motor/generator, electrically powering an 11-ft (3.4-m) diameter propeller on a test stand.

Safran

In a related development, last year GE and Safran, 50/50 partners in engine maker CFM International, launched the Revolutionary Innovation for Sustainable Engines (RISE) program, which features an open-fan technology with carbon fiber rotor blades.

The RISE program is exploring hybrid-electric propulsion and will incorporate data from the EPFD program. CFM Rise will also test hydrogen power.

In addition to the CFM partnership, Safran may have been the first engine company to test a hybrid-propulsion system of significant size. In July 2018, it began testing a 100-kW system. The test run took place at a Safran Helicopter Engines test facility near Pau-Pyrenees Airport in France. The demonstration was conducted by Safran Helicopter Engines, Safran Electrical & Power and Safran Power Units, in conjunction with Safran Tech, the Group’s research & technology center.

In this distributed hybrid-electric propulsion system, a turbogenerator (a gas turbine driving an electrical generator) was coupled to a bank of batteries. The system powered multiple electric motors turning propellers to provide propulsion. “The power is efficiently distributed by a new-generation power management system, and the motors are controlled by a fully-integrated smart power electronics assembly,” the company press release stated.

In April, Safran Electrical & Power and AURA AERO, a French electric aircraft startup, announced an agreement to work together on the architecture and electric propulsion systems of two aircraft: AURA’s all-electric INTEGRAL E training aircraft and the 19-seat, Electric Regional Aircraft (ERA) hybrid-electric turboprop.

Safran has been working with numerous eVTOL and eCTOL companies. It was originally a partner for Bell’s hybrid-electric Nexus 6HX eVTOL concept for Uber Elevate. Safran has also been supporting Colorado-based Bye Aerospace for its two-and four-seater eCTOL aircraft. In July, French startup VoltAero announced its prototype Cassio 330 eCTOL will utilize three of Safran Electrical & Power’s 100-kW+ ENGINeUS 100 smart electric motors in the aircraft’s parallel electric-hybrid propulsion system.

Rolls-Royce

Meanwhile, Rolls-Royce is also involved in clean propulsion programs aimed at the advanced air mobility and airline markets.

Rob Watson, Director of Rolls-Royce Electrical, has ambitious plans: “Rolls-Royce will be the leading provider of all-electric and hybrid-electric power and propulsion systems for advanced air mobility and will scale this technology over time to larger platforms.”

The company is testing its hybrid-electric propulsion system at its facility in Bristol, England. The 2.5-MW Power Generation System 1 (PGS1) program is being tested on the AE2100 engine. This engine powers the out-of-production, 50- 58-seat, twin-turboprop Saab 2000 commercial regional airliner, and the Leonardo C-27J and Lockheed Martin C-130J military transports.

The turbo-generator technology includes a small engine designed for hybrid-electric operations. Like other efforts, this technology will extend an aircraft’s range beyond short-haul fights when compared to a battery or engine alone.

“As the battery technology develops the range, the markets in which these aircraft can operate will open up,” said Rolls-Royce Electrical Customer Director Matheu Parr. “This is why we are also developing new turbogenerator technology for this market. It will be designed for hybrid-electric applications and will have scalable power offerings.”

In 2017, Rolls-Royce LibertyWorks conducted a NASA study for its EVE hybrid-electric turbofan concept. The company studied parametrically optimized engines with hybrid climb and cruise segments, under NASA’s EAP project.

Rolls-Royce Electrical was formed in 2019 after the acquisition of the electric and hybrid-electric aerospace propulsion activities of Munich-based Siemens’ eAircraft business unit.

Rolls-Royce Electrical has picked up customers and partners along the way. Siemens/Rolls-Royce provided the propulsion system for the uncrewed Airbus CityAirbus demonstrator (the heaviest eVTOL flown to date), which successfully completed its flight demonstration in Manching, Germany, (near Munich) last year.

Bristol, UK-based Vertical Aerospace has selected the company’s electric propulsion unit for its all-electric VX4 aircraft, which will fly later this year. Eve Air Mobility, a spin-off of Embraer, selected Rolls-Royce’s propulsion system for its eVTOL air taxi.

In addition, Rolls-Royce is working with Italian airframer Tecnam and Austrian piston-engine manufacturer Rotax on flight-testing a hybrid-electric P2010 aircraft. The four-seat Tecnam P2010 H3PS is powered by a 138-shp (104-kW) Rotax 915 IS engine coupled with a 30-kW Rolls-Royce electric motor, totaling the output of the aircraft’s standard 180 shp (134 kW).

The P2010 H3PS (an acronym for “High Power High Scalability Aircraft Hybrid Powertrain”) made its first flight in December and is the first of its kind to fly in a fully integrated parallel hybrid configuration. The H3PS project has highlighted the benefits of having a hybrid-electric system for emergency power.

In addition, Rolls-Royce is working on plans to develop the all-electric P-Volt commercial aircraft, based on the 11-seat Tecnam P2012 Traveller, with Widerøe — the largest regional airline in Scandinavia.

Watson noted that many of Rolls-Royce’s business units are important players in the company’s global vision. “I would like to thank the German government for their support. As part of our strategy, we are looking at offering the complete sustainable solution for our customers. This means extending routes that electric flight can support through our turbogenerator technology. This will advance hybrid-electric flight and mean more passengers will be able to travel further on low-to-netzero emissions aircraft,” Watson said. “Rolls-Royce is also set to build on our existing network to offer maintenance services for electrical systems.”

The company has tapped into its Power Systems business unit, which operates under the brand, mtu solutions; the name derives from the legacy engine company, MTU, originally standing for Motoren- und Turbinen-Union, based in Munich, Germany. “Rolls-Royce Power Systems is able to offer mtu microgrid solutions to support fast charging of electric aircraft and deliver reliable, cost-effective, climate friendly and sustainable power to vertiports,” he concluded.

Clear Need

Determining exactly how much harmful carbon dioxide (CO2) comes from various aviation sectors varies.

Domestic and international air travel account for around 2.5% of global CO2 emissions, according to Our World in Data, an online scientific publication that focuses on global challenges. But aviation’s overall contribution to climate change is higher when other pollutants are added, according to various sources.

The International Civil Aviation Organization (ICAO) in its “ICAO Environmental Report 2022,” noted that hybrid-electric aircraft’s lower fuel burn would improve local air quality. That said, relying on lithium-ion batteries to power aircraft could pose a good-news, bad-news scenario. Lithium batteries contain toxic and corrosive material, which could “pose threats to health and the environment if improperly disposed,” noted the Montreal-based ICAO. “Nevertheless, there are opportunities for improvements in the batteries’ life-cycles that will reduce possible impacts to the environment.”

Several airlines and other industry leaders, including GE Aerospace Executive VP and Chief Commercial Officer John Slattery, have committed their companies to net-zero carbon emissions by 2050. Decarbonizing aviation “is our industry’s moonshot,” said Slattery.

While much of hybrid-electric technology remains focused on fixed-wing commercial airliners, development of these powerplants will benefit the evolving eVTOL and rotorcraft sectors, and vice versa.

As of now, there is hopeful, yet cautious, optimism regarding hybrid-electric propulsion systems as an interim and possibly long-term solution to lowering aviation’s carbon footprint. At present, there is a lot of experimentation on increasing powerplant efficiency and reducing emissions. For fixed-wing aircraft and light rotorcraft, the hybrid system could be an effective way of dealing with range limitations of battery-electric engines.

Douglas Royce, Senior Aerospace Analyst with consultancy Forecast International, may have said it best: “With any new technology, the commercial viability depends on the outcome of experimentation. If the manufacturers come up with an engine that is more fuel-efficient and doesn’t cost more to maintain or acquire than standard turbine or piston engines used today, then they will be viable in the marketplace. If not, then the manufacturers will go back to the drawing board.”