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Disruptive technology and design advances speed needed evolution of TPs

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Meeting operating, efficiency, and environmental goals are increasingly important aspects of turboprop operations using a mix of new and repurposed aircraft and powerplant designs.


By Don Van Dyke
ATP/Helo/CFII, F28, Bell 222.
Pro Pilot Canadian Technical Editor

UK/US propulsion developer ZeroAvia converted this Dornier 228 by replacing one of its Honeywell TPE331 engines with a 600-Kw ZeroAvia ZA600 hybrid hydrogen-electric battery system. The ZA600 was ground-run for the first time on Dec 22, 2021, and the aircraft’s maiden flight is currently scheduled for Q1 2022. The goal is to power both engines with a hydroelectric powertrain within a year.
Given the present context, the term commercial off-the-shelf (COTS) refers to aircraft adapted aftermarket to the needs of the operator.

Historically, COTS turboprops (TPs) could – and can continue to – be used “out-of-the-box.” They are usually modified to meet business requirements and be integrated into existing organizational systems.

TPs permit robust, efficient flights over short- to mid-range routes, enabling safe access to regions often too remote, undeveloped, or irregular for other types of air transport access.

Evolving public attitudes have inspired a new era of aircraft design. Materials development, seat design, and rapid advances in avionics and software have converged to offer turboprop passengers impressive cabin comfort and control, amenities, and connectivity.

TPs and climate change

Many of the devastating effects of climate change on the human condition are now recognized and receiving more serious consideration among governments, commerce, and the wider public.

The 2021 UN Conference of the Parties (COP) 26 on climate change concluded with agreement among 196 signatory countries to reduce greenhouse gas emissions to limit global temperature increase this century to well below 2 °C.

The aviation industry, including business aviation, committed to this target in support of the earlier Paris Agreement 1.5 °C goal. However, the timelines to achieve these goals are incredibly short, requiring a fundamental transformation of business as usual across the aerospace community to achieve agreed decarbonization and sustainability targets.

The passenger version of the Textron Aviation Cessna 408 SkyCourier offers a no-frills but stylish cabin design with ample under-seat stowage, overhead bins, individual lighting, and other amenities.

Sustainability of business aviation rests on 3 pillars – social, economic, and environmental (ICAO, 2011). The environmental pillar is most notable when people consider sustainability and accompanying requirements regarding CO2 emissions, noise pollution, runoff, etc.

The goal is to contribute to the economy while meeting environmental needs. Innovations to reduce dependence on fossil fuels and promote alternative propulsion converge with regulatory flexibility to lower operating costs and meet mandatory environmental standards for business aircraft.

The keys to achievement are breakthroughs in engine technology, vision, and leadership.

Propulsion

Today’s greatest source of business aviation propulsion is the venerable family of Pratt & Whitney Canada (P&WC) PT6 powerplants.

General Electric (GE) Aviation’s 1300-shp Catalyst is the first clean-sheet TP engine to enter the business aviation market in more than 50 years, and promises 20% lower fuel burn and 10% higher cruise power than its closest competitors.

Catalyst is also the core of a hybrid-electric propulsion (HEP) system for the XTI TriFan 600 business aircraft currently under development.

Hybrid-electric technologies are highly compatible with sustainable aviation fuel (SAF) and hydrogen, as well as advanced engine architectures.

Table 1Novel powerplants. Table 1 includes several “near-term types” of novel turboprop powerplant with full authority digital engine control (FADEC), HEP, and all-electric designs in various stages of development.

These derive energy from solar, battery, hydrogen, and other sources. For powerplant OEMs, the near-term engine market could grow with a slow decline in gas turbine sales and accompanying aftermarket revenues.

With all-electric engines having fewer moving parts, the nature of service and support revenues will change, and likely with it the need for power-by-the-hour contracts.

Battery technology. All-electric engines are emissions-free and operate with a smaller noise footprint, but they are currently limited by the weight and capacity of today’s batteries, which are largely lithium-ion (Li-ion) based.

Lithium-sulfur (Li-S) batteries, however, are claimed to be lighter and more energy-dense (theoretically by a factor of 5) than conventional Li-ion cells, and and accompanying fire risk is lower. Using sulfur rather than costly cobalt would net likely cost savings of 30%. However, although promising, this technology is not yet sufficiently mature for aeronautical use.

The lightweight 1300-shp GE Catalyst TP engine is the first made with 3D printed components, permitting a claimed parts count reduction from 855 to 12, lowered engine weight, and increased strength and durability. The first flight was conducted in late 2021 on a King Air 350 test bed provided by launch customer Beechcraft.

The migration has begun

Everett WA-based magniX is already using its magni350 and magni650 electric motors to retrofit the Vancouver-based Harbour Air seaplane fleet, certified for short-haul passenger flights.

Similarly, in December 2021, an Air New Zealand product requirements document outlined plans to eliminate carbon emissions on its short-haul domestic network operated with a fleet of TPs.

The 3-phase program envisages an initial adoption stage from 2023–2025 in which cargo carriage or training would be performed with 1- to 9-place aircraft using electric or hybrid propulsion.

From 2026–2030, novel propulsion 10- to 50-place aircraft would supplement existing routes and implement new routes. And zero-emission 50-plus place aircraft would replace existing TPs between 2031 and 2035.

UK airframer Britten-Norman is to collaborate with a shipping firm to advance the prospect of hydrogen-powered flights to the Isles of Scilly. The Isles of Scilly Steamship Group, which operates the local airline Skybus, sold a 1994 Britten-Norman BN-2 Islander turboprop to research firm Cranfield Aerospace Solutions to be retrofitted with hydrogen fuel cell technology.

Entry into service (EIS) is expected by late 2023. Cranfield Aerospace Solutions is pursuing the modification under Project Fresson and plans to apply it to more than 700 BN-2 Islanders.

In November 2021, the Textron Aviation Denali completed its first flight powered by a GE Aviation Catalyst turboprop engine. The aircraft is scheduled for certification in 2023.

At the end of 2021, Spanish electric aircraft specialist Dante Aeronautical and London-based Monte Aircraft Leasing announced a collaboration on the development of hydrogen fuel cell powertrains.

The partnership will offer finance and conversion packages to existing operators for retrofit of the zero-emission powertrain.

Furthermore, an all-electric version of the Dante system using batteries and a magniX motor is currently under test with Australian operator Sydney Seaplanes.

The future

In late 2021, NASA announced new research partnerships with GE Aviation and magniX to launch a 1-megawatt (MW) class hybrid-electric technology demonstrator for ground and flight tests by the mid-2020s.

After years of developing and refining individual components of a hybrid-electric system (motors, generators, and power converters), GE will mature systematically an integrated hybrid-electric powertrain to demonstrate flight readiness for single-aisle aircraft.

Dante Aeronautical is expected to begin conversions of Cessna Caravans in 2024 to use a hydrogen fuel cell to power the cruise phase, augmented by batteries to provide a boost during takeoff and climb. Fuel cells and hydrogen tanks will go in the sub-fuselage cargo bay, while the motor and battery will replace the existing TP engine in the nose.

The OEM will demonstrate electric aircraft propulsion (EAP) technologies to be used in commercial aircraft, including short-range and regional passenger TPs.

The NASA project goals are based on current regional aircraft power demands, which correlate roughly with a requirement for a 2-MW engine for takeoff and climb, reducing to 1 MW for cruise and descent.

United Airlines has concluded an agreement with UK/US propulsion developer ZeroAvia for up to 100 ZA2000-RJ 2-MW-plus hydrogen-electric engines to be retrofitted to its Bombardier CRJ550 fleet by 2028, essentially converting the jets to zero-emission 50-seat propeller aircraft.

Similarly, ZeroAvia concluded a memorandum of understanding with de Havilland Aircraft of Canada for line-fit and retrofit of the ZA2000 powertrain to the Dash 8-400.

At the same time, Alaska Air Group has contracted for 50 kits to convert regional aircraft to hydrogen-electric power using the ZeroAvia system.

ZeroAvia is preparing to continue ground- and flight-tests of its ZA600 hydrogen-electric-powered Dornier 228, aiming to introduce that powertrain into commercial service by 2024.

The magniX Magni650 EPU provides 850 shp produced by 3-phase power. If one phase is lost, the motor will continue to deliver 75% power. The magniX EPU line has powered several all-electric types, including DHC-2 Beavers and Cessna 208B Grand Caravans.

Conclusion

Insofar as possible, compliance with tomorrow’s operational and environmental requirements should be accommodated with drop-in solutions.

This will serve to lengthen the service lives of current airframe designs and secure owners’ investments by ensuring that the asset is not rendered worthless by newer technology.

As battery technology continues to improve and electric power matures, drop-in solutions will become increasingly attractive, both operationally and economically. Schemes involving one-by-one conversion to electric power, or new models replacing conventional-engine aircraft, will evolve to meet dynamic market demands.

While this revolutionary technology has promise, in-service experience and exposure to the rigors of flight will identify those components and connections in need of ongoing maintenance, repair, and overhaul (MRO).

Recognizing that electric motors generally require less maintenance than gas turbines, AIAA’s paper Maintenance Considerations for Electric Aircraft and Feedback from Aircraft Maintenance Technicians, by Ryan Naru and Brian German (arc.aiaa.org/doi/10.2514/6.2018-3053) identifies candidate components (batteries, cables, high-speed bearings, windings, magnets, etc) requiring regular inspection and possible cleaning, refurbishment, or replacement.

Not all of these affected components and interconnections will be field-maintainable and may require transfer to dedicated maintenance centers. On the other hand, given the utility of 3D printing, it may be possible to largely avoid conventional spares provisioning by printing replacement components.

In the near term, line-fit and retrofit of all-electric and hybrid-electric motors will likely begin with 2-seat trainers, progressing to 4-place personal aircraft, through 9-place regional aircraft, then on to utility aircraft, like the Cessna Caravan, and beyond.

Every airframer has a plan to investigate and implement electrification of wide-ranging designs, especially to accommodate the transport needs of environmentally conscious regions.
The path and pace of migration will prove global aviation’s commitment to drastic but needed change.


DonDon Van Dyke is professor of advanced aerospace topics at Chicoutimi College of Aviation – CQFA Montréal. He is an 18,000-hour TT pilot and instructor with extensive airline, business and charter experience on both airplanes and helicopters. A former IATA ops director, he has served on several ICAO panels. He is a Fellow of the Royal Aeronautical Society and is a flight operations expert on technical projects under UN administration.