The transition to hybrid-electric propulsion for short-haul aircraft has been has been energised by two milestone developments involving engine manufacturers Pratt & Whitney Canada and Rolls-Royce. The former, which produces engines for medium-to-large turboprop aircraft, has partnered with GKN Aerospace in the Netherlands to develop a high voltage, high power wiring system for a new hybrid-electric powertrain, which is targeting lower CO2 emissions and 30% more efficiency than today’s most advanced turboprop engines. Flight testing is expected to begin in 2024. Meanwhile, Rolls-Royce has just completed the first fuel burn of a gas-powered small turbine for use in the Advanced Air Mobility (AAM) sector and hybrid-electric commuter aircraft seating up to 19 passengers. The new turbogenerator system is designed to provide scalable power offerings for hybrid-electric aircraft, enabling pilots to extend flight range by switching between electric power and either sustainable aviation fuel or hydrogen combustion.

Pratt & Whitney Canada is progressing its expansion into hybrid-electric aero propulsion in partnership with Collins Aerospace, a sibling company within the aerotech conglomerate RTX, and UK-headquartered GKN Aerospace. 

Having integrated a lightweight 1-megawatt electric motor developed by Collins into a high-efficiency fuel-burning engine, Pratt & Whitney Canada is now partnering with GKN Aerospace to develop the high voltage, high power electrical wiring interconnector system (EWIS) for the RTX hybrid-electric flight demonstrator project.

The companies will collaborate on the development, construction and installation of the electrical wiring system on the demonstrator, which Pratt & Whitney Canada expects to achieve a 30% improvement in fuel efficiency and lower CO2 emissions than the most efficient turboprop engines currently in use, delivering better performance during take-off, climb and cruise.

Collins says its 1MW motor is half the weight of the most advanced electric motors now flying, but will deliver four times the power and double the voltage, with half the heat loss. The unit is being developed by the company at its Solihull, UK, facility and tested at the University of Nottingham’s Institute for Aerospace Technology.

Supported by the governments of Canada and Quebec, flight testing of the new powertrain will begin next year on Pratt & Whitney Canada’s Dash 8-100 experimental aircraft.  

The Netherlands division of GKN Aerospace will lead development and design of the EWIS for the hybrid-electric propulsion system, as well a producing and installing the hardware on the demonstrator aircraft.

“Hybrid-electric propulsion technology has the potential to improve efficiency for a wide range of future aircraft applications, supporting the industry-wide goal of achieving net zero CO2 emissions for aviation by 2050,” said Jean Thomassin, Pratt & Whitney Canada’s Executive Director for new products and services. “Our collaboration with GKN Aerospace brings extensive expertise to the project, which will help integrate high voltage electrical systems on our experimental aircraft, as we target flight testing to begin in 2024.”

John Pritchard, President of Civil Airframe at GKN Aerospace, welcomed the new partnership, which follows the company’s design and manufacture of EWIS systems for the all-electric Vertical Aerospace VX4 air taxi and the Eviation Alice passenger and freight planes.      

“This project extends our teamwork in hybrid-electric propulsion technology, which also encompasses the SWITCH project, which is backed by the Clean Aviation Joint Undertaking of the European Union,” he said. 

The Rolls-Royce turbogenerator system, which is part-funded by the German Ministry for Economic Affairs and Climate Action, will allow power to be scaled between 500kW and 1,200kW, enabling hybrid-powered aircraft to fly longer routes or carry greater payloads than all-electric battery powered models. As well as delivering energy to electrical propulsion units, it can recharge batteries in hybrid-electric powertrains.

“The development of the turbogenerator solution brings together Rolls-Royce’s capabilities in designing compact and lightweight high-speed rotating electric machines and highly efficient gas turbines, combined with the expertise to integrate them on a system and platform level,” said Matheu Parr, the engine maker’s Customer Director, Electrical.

In addition to providing more operating flexibility, Parr explained the engine had been designed using novel combustion technology to minimise emissions, not just in the evolving AAM market, which includes electric vertical take-off and landing (eVTOL), electric short take-off and landing (eSTOL), but also potentially for helicopters and auxiliary power units on larger aircraft.

“This significant achievement confirms the effectiveness of the compact, power-dense turbine that will be integrated into a lightweight turbogenerator system,” he said.

“The turbogenerator system will enable our customers to extend the routes that electric flight can support and means more passengers will be able to travel further on low and potentially net zero emissions aircraft. It is well suited to recharge batteries as well as provide energy to electrical propulsion units directly and therefore enables aircraft to switch between power sources in flight.”

He added that since the product had been defined, it had taken just two years to develop then test the new gas turbine. 

“This significant achievement follows the fast-paced development time of the new gas turbine from concept freeze to ‘pass to test’ in under two years,” he said. “Test facilities and equipment, comprising 14 sub-systems in total, were designed, procured, and built – or adapted – by a global team in a record time of just under a year.

“With this achievement, we have proven we can apply our expertise to novel designs and are able to test them on a very quick timescale. This capability will help Rolls-Royce to deliver the products that will help us on our path to net-zero within the ambitious industry timelines of the Advanced Air Mobility market.” 

Image: The RTX hybrid-electric flight demonstrator

Tony Harrington

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American Airlines will join an advisory panel established by aircraft manufacturer Embraer to help define its new Energia family of low-or-no-emission regional airliners. The Brazilian airframer is considering four aircraft concepts, ranging from 9- to 50-seat capacity, powered by a mix of electric, hydrogen and hybrid propulsion systems. Initially, the airframer is focused on 19-seat and 30-seat versions of two proposed aircraft, one powered by a hybrid-electric propulsion system, the other hydrogen-electric. Embraer is targeting technology readiness of the Energia Hybrid E19-HE and E30-HE aircraft by the early 2030s, and the Energia H2 Fuel Cell models E19-H2FC and E30-H2FC by 2035. American is a major operator of Embraer jets, as is another recent addition to the Energia advisory group, US contract carrier Republic Airways, which operates over 200 Embraer jets on behalf of American, Delta and United.

While most public discourse on sustainable aviation concentrates on reducing the carbon emissions of large, medium-to-long-range aircraft, predominantly by increasing the use of sustainable aviation fuels, it is the short-haul sector in which novel propulsion technologies are being developed, tested and certified, before being evolved for use in larger planes.

Currently, most focus is on retrofitting existing turboprop aircraft with new electric, hybrid-electric or hydrogen-electric powertrains, which are faster and far less expensive to certify and introduce than all-new aircraft models. But while providing low-emission propulsion for in-service aircraft, the new powertrains will also help pave the way for a new generation of ‘clean sheet’ aircraft such as Embraer’s Energia range.   

“American operates the youngest mainline fleet and the largest regional fleet among US network carriers,” explained Embraer, “and in 2022 consumed more than 2 million gallons of sustainable aviation fuel. It has also invested in the development of hydrogen-powered propulsion and infrastructure.”

Arjan Meijer, CEO of Embraer Commercial Aviation, said American had signed a MoU to help define the performance and design characteristics of the Energia aircraft concepts as part of a broader advisory group, with members including airlines, aircraft lessors, engine manufacturers and other suppliers. “It’s essential for Energia’s success that we involve a wide variety of air operators,” he said. “American joining our advisory group, with their huge presence and expertise, is a significant step for the Energia project. This group has developed into a core part of the programme because of the breadth and depth of the expertise the different members bring to the table. We’re excited to work together with a world-leading airline on the biggest challenge our industry faces.”

American’s participation in the Energia project closely followed an order for seven new Embraer E175 jets, to be operated by its wholly-owned subsidiary Envoy Air. Deliveries will begin in Q4 this year, increasing Envoy’s all-Embraer fleet to 141 jets by the end of 2024.

“As the operator of the largest US regional fleet, we believe industry collaborations aimed at advancing decarbonisation technologies are critical to helping aviation reach its goal of net-zero emissions by 2050,” said American’s VP of Sustainability, Jill Blickstein. “We look forward to working with Embraer and other members of the Energia Advisory Group to develop next-generation, zero-emission aircraft.”     

Although the Embraer Energia project initially is focused on two concepts, the airframer’s broader ambitions feature four.

The 19- and 30-seat Energia hybrid-electric concept aircraft are planned to operate up to 500 nautical miles (926 kilometres), and achieve CO2 emission reductions of 30% if operated with Jet A1 fuel, or up to 90% with SAF, while the Energia H2 Fuel Cell models are being designed to operate over 200 nautical miles (370 kilometres).

Embraer is also considering a 9-seat all-electric version, the E9-FE, again with a 200 nm range, while the largest concept under consideration, the Energia H2 Gas Turbine aircraft (E50-H2GT), will use either SAF or hydrogen to power a gas turbine. The dual fuel options for modified gas turbine engines would offer operational flexibility and lower aircraft weight, enabling a flight range of 350-500 nm (648-926 km) with 35-50 passengers, and technology readiness by 2040. 

To progress its zero-emission aircraft ambitions, Embraer has also joined forces with UK-headquartered GKN Aerospace to help accelerate the introduction of hydrogen-powered aircraft, exploring innovative hydrogen fuel cell technologies, optimised integration of these propulsion systems into airframes, and even the potential for a hydrogen flight demonstrator.

Through its H2Gear programme, GKN is already exploring development of a fuel cell system which converts liquid hydrogen to electricity. Initially, it is targeting use in sub-regional aircraft, but with potential to upscale for larger planes. 

Image: Embraer’s Energia Family concept

Tony Harrington

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