Ever since the shrill whine of the Rolls-Royce Dart ushered in the age of the commercial turboprop in the late 1940s, this efficient form of gas turbine has evolved steadily over the last seven decades both in performance and reliability.
But fast forward to 2021, and the signs increasingly indicate that conventional turboprop engine development, particularly that of the larger civil models, may be reaching a crossroads as the focus on sustainability shifts the industry toward a hybrid-electric and turbo-electric future.
- Open fan could advance GE future turboprop plan
- Pratt hybrid-electric ground tests due in 2022
Market dynamics in the regional airliner business have also played a role. Early in the 2010s, engine-makers were busy developing advanced large turboprop engines for the expected development of higher-capacity 90-seat regional aircraft. But the market for large turboprops never materialized. General Electric’s CPX38, a proposed 4,000-6,000-shp derivative of the GE38 turboshaft, and Pratt & Whitney Canada’s even more powerful NGRT (next-generation regional turboprop) competed for concepts that neither ATR nor Bombardier were willing to launch.
Now, as the entire air transport market slowly recovers from the impact of the COVID-19 pandemic, the engine-makers are rapidly adjusting strategic development plans to the demands of a more sustainable future. The main response has been to dramatically expand research into hybrid-electric and all-electric propulsion, along with investments into studies of new combustors capable of handling sustainable aviation fuels (SAF) and even hydrogen.
Yet defining the size and final configuration of the next-generation turboprop, or even a formalized development strategy, remains difficult amid the many uncertainties of the rapidly evolving market push toward a decarbonized ecosystem. Both airframers and engine-makers are attempting to analyze the mid-to-longer-term needs of the changing regional turboprop market while assessing the readiness levels of the new propulsion alternatives.
“We are probably in the largest state of flux that we’ve ever been in,” says Tom Lodge, general manager of Commercial Engines Marketing for GE Aviation. “The one constant our whole life has been jet fuel, and the industry, whether it’s the airframe- or the engine-maker . . . has been driven by the economics of form, fit and function coupled with the ramifications of deregulation.”
All of this is changing as new fuels and technologies are introduced and policymakers and regulators around the world begin to exert an influence on carbon emissions requirements for future aircraft, Lodge says. “Everyone’s trying to figure out how [to] take the right actions and move toward the goal, because we believe in it, and we need to do it. But we have constraints around us, through capital funding, the supply chain and through infrastructure buildout and through the actual manufacture of alternate fuels such as SAF and hydrogen. It can be done, but it has to be built out, and we’re trying to pivot on something we’ve been doing for 100 years.”
Lodge says the bottom line is: “We really have more questions than answers as this evolves. But we know we cannot just sit here and do nothing,”
GE’s recently unveiled RISE (Revolutionary Innovation for Sustainable Engines) technology demonstration program with CFM partner Safran is part of the overall response, says Arjan Hegeman, GE Aviation’s general manager for Advanced Technology. Although the open-fan project is mainly aimed at the future single-aisle jet market, much of the technology could also be applied to future regionals. “The turboprop kind of fell out of favor a little bit because you don’t get the speed and you can’t fly over the weather, but with RISE technologies you are able to do that. The turboprop might get a revival because they are very efficient propulsive machines.”
The wide-ranging RISE technology portfolio covers propulsive and thermal efficiency initiatives from advanced blade and gear designs to hybrid-electric adaptations that can be scaled to the thrust requirements of the regional market, Hegeman says. “We’re looking at the middle of the decade on RISE to demonstrate a lot of the technologies, which include hybrid. The program includes turboprop efficiency and high speed, low acoustics and high-altitude performance,” he adds.
“Toward the middle of the decade, we also have a hybrid-electric demonstrator of very high power and voltage,” Hegeman continues. “These two demonstrators are being used to mature the technologies to be able to put them into a product.” Other supporting GE research efforts include the company’s ongoing Mestang (more electric systems and technologies for aircraft in the next generation) covering development of megawatt-class, more electric power systems. In Italy, GE’s Avio Italy has also worked with Leonardo on advanced propeller studies for the airframe-maker’s large regional turboprop concept.
Pratt & Whitney, the dominant player in today’s regional turboprop market, is also working to protect its lead through work with De Havilland to develop a hybrid-electric propulsion demonstrator based on the Dash 8-100. The C$163 million ($129 million) project targets the start of flight tests in 2024 and is a successor to Project 804, an earlier hybrid demonstrator plan that was subsequently sidelined. Ground testing is targeted for 2022.
Pratt says that while regional aviation is under increasing pressure from governments in Europe to close routes and shift travelers to surface transportation alternatives, “there’s a big opportunity for regional aviation to be one of the first segments to benefit from new, lower-carbon technologies like all-electric, hybrid-electric and hydrogen-powered propulsion.” It adds that although regional aviation represents only a small segment of global aviation CO2 emissions, “many of the technologies we develop and mature for this market will ultimately go on to benefit larger-scale aircraft as well.”
Under the project, which targets a 30% reduction in fuel burn and CO2 emissions on a 250-mi. sector compared with a current state-of-the-art regional, one of the aircraft’s standard PW121 turboprops will be replaced with a megawatt-scale hybrid-electric powertrain. The Collins-developed electric motor and controller will be powered by a turbogenerator and a battery system.
As with Project 804, for which Pratt planned to use a novel thermal engine, the manufacturer is not revealing details of the Dash 8 demonstrator powerplant other than saying it is optimized for hybrid-electric operation “with the electric motor boosting power during takeoff and climb.” The new initiative does, however, visibly differ from Project 804 by adding a larger nacelle air intake for cooling and relocating battery packs from the cargo compartment to external blister fairings on either side of the lower fuselage.
The engine-maker, which believes hybrid-electric propulsion is the next major technology paradigm for regional aviation, adds that “longer-term, we see hydrogen as a potential path to zero-emissions flight, and we are active on hydrogen-propulsion development.”
Another experimental hybrid-electric regional retrofit in the planning stage is NASA’s Pegasus (parallel electric-gas architecture with synergistic utilization scheme) concept—an ATR 42-500-based demonstrator powered by a megawatt-class distributed hybrid-electric propulsion system. As part of the buildup to flight tests on a dedicated demonstrator, NASA is shortly due to make contract awards for the Electrified Powertrain Flight Demonstration (EPFD) project to flight-test megawatt-class electric aircraft propulsion systems.
Part of NASA’s broader aviation sustainability strategy, the EPFD effort aims to mature propulsion systems for thin-haul, regional and single-aisle aircraft that could enter service by 2035. To accelerate to a maturity level ready to enter product development and certification in the late 2020s, NASA intends to use existing or planned industry flying testbeds. First flight is targeted for no later than March 2024.
At least two awards are expected, and while NASA has outlined a target performance of 1.5 megawatts for a single-aisle-class demonstrator, it is also looking for a 500-kW-capable powertrain for a 19-seat regional aircraft. The targeted mission energy savings through electrification are 4% for a Part 25 commercial aircraft and 10% for a Part 23 regional aircraft.
Honeywell, while currently a relatively niche player in regional transport, also sees the growing hybrid-electric revolution as a potential opportunity to gain new traction in several markets, ranging from long- range advanced air mobility vehicles to next-generation turboprops. The company has begun testing a 1-megawatt generator suitable for use in hybrid-electric aircraft that is 2.5 times more powerful than the previous version developed in 2019.
Following tests of the generator, now underway at Honeywell’s Mexicali facility in Baja California, the unit will be paired with a modified HGT1700 auxiliary power unit to run as a turbogenerator. Although Honeywell earlier completed design work and mating of the HTS900 turboshaft to two 200-kW generators, “there is more emphasis on the megawatt testing,” says Taylor Alberstadt, Honeywell’s senior director of power systems business development. While traditionally the company has segmented its products to service different sectors, it expects the new hybrid-electric capability will expand its market versatility. “We have ‘X’ amount of electric power. What do you want to do with it?” he asks.
In Europe, research into future turbo-props is evolving to encompass more hybrid-electric technology under Tech TP, a Clean Sky engine demonstrator project targeted at development of a sustainable, efficient engine for general aviation and small regional turboprops. First run in June 2019 at Safran’s Tarnos site in France, the engine is now being modified with hybrid-electric elements developed as a subset of the Achieve (advanced mechatronics devices for a novel turboprop electric starter-generator and health-monitoring system) project. Modifications will be completed by the end of this year or early 2022.
While much of the future turboprop focus is on conventional or sustainable hydrocarbon-fueled hybrid-electric propulsion, several hydrogen fuel-cell-based projects are underway in the regional market. U.S. startup Universal Hydrogen (UH2) has signed letters of intent with Icelandair Group, Air Nostrum and Ravn Alaska to retrofit ATR 72 and Dash 8s with hydrogen fuel-cell propulsion systems.
Under the agreements, the aircraft’s Pratt PW124/127 turboprops are to be replaced with electric motors and fuel cells compatible with UH2’s modular hydrogen system. Flight testing, certification and conversions will be led by aircraft modification specialist AeroTEC. UH2 is currently testing a subscale version of the 2-megawatt powertrain, assembled after receiving a minority investment from existing partner Plug Power. Developed with electric motor developer MagniX, UH2 plans to begin experimental flights in 2023, aiming for supplemental type certification and entry into service by 2025.
A similar initiative is underway in Europe where Deutsche Aircraft, which is working to return the Dornier 328 to production, has partnered with German hydrogen-propulsion specialist H2Fly to demonstrate a zero-carbon fuel-cell-powered version of its revamped 320eco regional turboprop.
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