Turboprops in sustainable transition
Advances in propulsion, materials, and equipage enable turboprops to offer ever greater gains in performance, economy, and capability.
By Don Van Dyke
ATP/Helo/CFII. F28, Bell 222
Pro Pilot Canadian Technical Editor
Business and commercial aviation seeks to provide a rapid transportation network which facilitates domestic, international, and global trade and tourism. It comprises an industry offering economic growth responsible for 3.5% of the global GDP, with employment currently representing 65 million jobs.
Increasingly, aviation’s evolution drives emerging technologies and innovation while seeking to mitigate its environmental impact.
Turboprops by definition
Historically, turboprops (TPs) are defined as aircraft that combine the use of turbine engines with propellers.
Benefits of this combination include impressive short takeoff and landing (STOL); a lower fuel-per-seat-mile requirement for short-duration flights than turbojet or turbofan aircraft of similar size; rough field performance, enabling passengers/cargo to be transported to destinations previously thought inaccessible; and lower CO2 emissions per seat/nm than those of regional jets.
However, as other propulsion systems and technologies reach the market for propeller-based aircraft, the accepted definition of a TP will likely require revision to include energy sourced by electric propulsion units (EPUs).
Critical TP roles
TPs are efficient at lower flight speeds (less than M0.6). When these aircraft are used over relatively short distances, these cost and performance benefits offset the lower speed. TPs minimize environmental impact, enable more direct flights, and provide essential regional connectivity.
Finally, TPs play a crucial role as leading-edge platforms for testing and bringing disruptive technologies to market.
TP market
These factors are driving a resurgence of interest and programs among the notable airframers listed in Table 1. The TP market is moderately fragmented, with relatively few OEMs accounting for significant market shares.
The military segment accounts for the major market share owing to deliveries of large TPs such as the Airbus A400M Atlas and Lockheed Martin C-130J Hercules.
As pandemic restrictions ease, the global civil TP fleet is expected to increase from 1950 (2022) to 2450 by 2041.
Market segments. The civil TP market may be segmented according to application (business, regional/short-haul airline, general aviation, etc) or seating (<15, 15–19, 20–40, 41–60, 61–80, 81–100). Military, public service, and special mission applications are not addressed in this article.
Currently, the sole companies manufacturing regional TPs are ATR and de Havilland (whose TP products were formerly manufactured by Bombardier).
Market drivers. Principal TP market drivers include average fleet age, current regulatory, technological, and sustainability environments, and fuel prices, as well as carbon offsets and taxation.
Additional considerations include the current pilot shortage, industry consolidation and collaboration, and public perception.
Competitive landscape. Table 1 lists notable TP original equipment manufacturers (OEMs) that provide wide-ranging products and services, including many in refurbishment and post-warranty aftermarkets.
Global market forecasts. Table 2 provides summary estimates of independent civil TP global market forecasts (GMFs) for the period from 2022 through 2041.
The forecasts generally agree on similar futures, identifying the largest TP markets for the period as Asia Pacific and Europe.
Sustainability
While efforts to reduce the effects of flying on the environment are critically important, many activists view business aviation as a major carbon contributor.
According to a 2010 GAMA-IBAC study, aviation – in all forms (including scheduled airlines) – represented 2% of all global CO2 emissions, and business aviation represents 2% of all aviation emissions. ATR estimates that replacing regional jet capacity with TPs on US routes of less than 500 nm would immediately reduce CO2 emissions by 28%. However, aviation is today the fastest-growing source of greenhouse gas emissions. By 2050, aviation’s share of climate impact is expected to be 25–50%.
In 2022, the 41st ICAO Assembly adopted a long-term global aspirational goal (LTAG) for international aviation of net-zero carbon emissions by 2050, supporting the UN Framework Convention on Climate Change (UNFCCC) temperature goal.
Energy cost as a percentage of aircraft operating costs is growing. This motivates ongoing research into creating clean, renewable sources of energy, as well as promoting eco-friendly awareness and activities.
Contrails may have up to twice the effect of CO2 on global warming. TPs typically cruise at lower altitudes, actively addressing climate challenges by operating below the contrail formation zone on regional and short-haul routes.
In the next 20 years, demand for new aircraft will shift progressively from fleet growth to accelerated replacement of older, less fuel-efficient aircraft.
And sustainability will influence the entire aviation market as a key consideration for aircraft owners who wish to reduce their carbon footprint, waste, and noise emissions.
In the short to medium term, this will be supported by the introduction of sustainable aviation fuel (SAF) as part of the effort to decarbonize. In the medium term, the operational global fleet must be renewed, and further opportunities for decarbonization will be explored. The long-term goal is to operate with carbon-neutral aircraft.
Growing demand for TPs comes with challenges. The global commitment to the 2050 net-zero carbon emission target must be achieved through revolutionary propulsion systems, next-generation aircraft designs, and alternative energy sources.
Turboprop powerplants
Powerplant technologies currently under consideration for TPs include advanced turbine, hybrid-electric, all-electric, and hydrogen-hybrid.
Turbine. In the 2010s, Pratt & Whitney Canada (P&WC) developed a new demonstrator known as the Next-Generation Regional Turboprop (NGRT) engine for future 90- to 110-seat TPs, such as the proposed Embraer Turboprop Next Generation (TPNG).
Thus powered and paired with advanced propellers and integrated nacelles, the TPNG would have 35% better fuel efficiency than a turbofan-powered airliner on a typical 500-nm flight.
The NGRT remains in ongoing development and seeks to deliver 20% improved specific fuel consumption (SFC) over state-of-the-art models, with power ranging from 4500–8000 shp, and suitability for re-engining existing airframes.
GE has revamped its small TP line, incorporating technology developed during the design and testing of the Catalyst – a new 850- to 1600-shp centerline engine for single and twin TPs. Produced almost entirely in Europe, the Catalyst is designed to compete with the P&WC PT6.
Supply chain issues often continue to be problematic, with long lead times for certain parts. Addressing this concern, GE has 3D-printed 12 titanium alloy parts, replacing 855 components and reducing engine mass by 5%. The engine is likely to be certified in 2023.
Traditional TP development, particularly that of large civil models, may be reaching a crossroads, as focus on sustainability shifts the industry toward an all-electric, hybrid-electric, and hydrogen-electric future.
All-electric. Conventionally-powered aircraft face increasing expense, mainly due to carbon offsets and environmental penalty costs. All-electric engines are among the most promising technologies for the 1- to 19-place market.
Longer battery life, lower electric motor maintenance cost, low community noise during takeoff and landing, and decreased electricity prices have led to better economics over time.
In 2020, Rolls-Royce partnered with Italy-based Tecnam to develop an all-electric 10-place aircraft known as the P-Volt, which is based on the 11-seat Tecnam P2012 Traveller aircraft.
In 2021, Scandinavian regional airline Widerøe committed to introducing the P-Volt into commercial service beginning in 2026.
In early 2023, a trademark was filed with the US Patent and Trademark Office on behalf of Tesla to manufacture electric motors for aircraft. However, no further related announcements have been made.
Hybrid-electric. The next major technology paradigm for regional aviation is hybrid-electric propulsion. In late 2022, Raytheon Technologies announced the first successful engine test run of its hybrid-electric de Havilland DHC-8-100 flight demonstrator. The propulsion system integrates a Collins Aerospace 1-MW electric motor with a P&WC fuel-burning engine.
Kirkland WA-based Zunum Aero was backed by Boeing HorizonX and JetBlue Technology Ventures to develop hybrid-electric regional aircraft for 12–50 passengers.
Owing to disputes among the company and its backers, Zunum’s headquarters closed in 2019. The company is reportedly currently fundraising for its next round of activity.
Hydrogen-Electric. ZeroAvia notes that hydrogen-electric powertrains represent a viable and scalable solution for zero-emission aviation, because their cycling costs are lower and their specific energy is up to 30 times higher than lithium-ion batteries.
US startup Universal Hydrogen (UH2) is partnered with hydrogen fuel-cell producer Plug Power and electric motor OEM MagniX to produce a 2-MW, zero-emission powertrain for retrofit into 40- to 60-seat regional aircraft, beginning with the de Havilland Dash 8 Q300.
UH2 has received letters of intent from Icelandair, Air Nostrum, and Ravn Alaska to replace PW124/127 TP engines with hydrogen fuel cell propulsion systems. Supplemental type certification and entry into service is expected by 2025.
ZeroAvia is a British/American hydrogen-electric aircraft developer with similar plans, but using Dash 8 Q400s for testing and trials. ZeroAvia intends to enable scalable, sustainable aviation by replacing conventional engines with hydrogen-electric powertrains.
Dual-fuel combustion. Interest in hydrogen as a potential alternative to SAF is growing, particularly liquid hydrogen (LH2).
The International Council on Clean Transport (ICCT) found that, while LH2-combustion aircraft do not perform as well as their jet fuel counterparts, they could play an important role in meeting the 2050 climate goals, because LH2 emits no CO2 during combustion, and can be produced with near-zero carbon emissions if made using renewable electricity (green hydrogen).
Benchmarked against the ATR 72, a suitable airframe entering service in 2035 could contribute to aviation’s 2050 climate goals despite having to accept performance penalties relative to fossil fuel aircraft.
Conclusion
Both airframe and powerplant OEMs are attempting to quantify the needs of the evolving TP industry as well as the maturity of energy alternatives. They remain focused on the 2050 LTAG.
Given the uncertainties of the technologies pushing toward a decarbonized ecosystem, the size and configuration of next-generation TPs and appropriate, viable, and reliable energy sources, as well as related development strategies, remain unclear.
Major labor migrations have left aviation with shortages of requisite expertise and an aging workforce needing to be replaced with qualified people. Attracting and retaining talent, however, also remains a significant challenge, and corresponding solutions are elusive.
Given that supply chain and labor shortages continue to plague the industry, some industry consolidation is possible. This awaits greater clarity on the extent to which aviation recovers from Covid-19 pandemic restrictions.
The unique characteristics of TPs are most important in ensuring reliable access to areas where extended range and high speed are not primary requirements. TPs are relevant, and will continue to serve the operational niche they occupy as proud partners in the aviation community.
Don 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.