A bewildering variety of advances in hybrid, turbo and more-electric power are in development for future airliners, regional turboprops and general aviation aircraft, but virtually none of these propulsion technologies are likely to be applicable anytime soon to the high-speed, long-range mission requirements of the business jet.
As the aerospace industry scrambles to meet new, aggressive sustainability targets, the business aviation sector is therefore challenged by having far fewer go-to lower-emission options than virtually any other arena of civil aviation, particularly in propulsion.
“Business aviation aircraft are a little bit like the sports cars of the aerospace world. The whole design paradigm is predicated on time savings, long range and near-transonic speeds,” says Sergio Cecutta of SMG Consulting. “Time is money when you’re spending $80 million on an aircraft.” Most near-term hybrid-electric or hydrogen fuel-cell concepts, on the other hand, are geared to the lower specific energy needs of smaller, slower aircraft and shorter-range turboprop regionals.
Although alternative lower emission, higher speed concepts such as CFM’s RISE Open Fan demonstrator and advanced geared turbofans are under study for next-generation airliners, the larger diameter of these high-bypass systems—even scaled down—makes them unsuitable for installation on today’s classic tube-and-wing business aircraft configurations. By their nature, business aircraft are also compactly designed and are not provisioned with space for batteries or other energy storage devices that most hybrid-electric systems require.
Business aircraft engine maker Honeywell is among those currently investing in hybrid-electric technology and, following ongoing tests of a 1-megawatt generator, intends to mate the unit with a modified HGT1700 auxiliary power unit to run as a turbogenerator. The power system is expected to find a home on a variety of advanced air mobility, regional and air transport vehicles but not, so far at least, business aircraft. “We haven't felt any immediate traction for hybrid-electric or even hydrogen-powered powertrains in business aviation,” says Taylor Alberstadt, Honeywell’s senior director of power systems business development.
But this could change eventually. “There are a lot of different ways things could go. It's somewhat difficult maybe today to see a hybrid-electric powertrain in that segment in the near to medium term, but [after sustainable aviation fuels] then you get into the more complex solutions like hydrogen-burning cores. Do you move into some form of hybrid like increasing battery content using fuel cells? At what point does battery technology if at all, scale up enough to make all electric feasible? Probably not in the near to medium term, but all of those are kind of unknowns,” says Alberstadt.
“At a minimum we are keeping an eye on all those things, but we have to have a diversified portfolio with respect to what’s going to happen,” he adds.
For the moment, therefore, business aviation’s broader push to reach carbon neutrality is focused on a combination of nearer term identifiable technologies and processes that parallel initiatives underway in the air transport business. These range from encouraging the development and production of 100% sustainable aviation fuel (SAF) and growing the use of carbon offset initiatives, to engine performance improvements and streamlining flight operations.
Beyond SAFs, each of the major business aviation engine-makers continues to improve the fundamental performance of turbofans as a key step toward lower emissions. Many of the more-recent large engines in the sector, such as the GE Aviation Passport, Pratt & Whitney Canada PW800 and Rolls-Royce Pearl, leverage propulsion technology developed for bigger commercial engines. The Passport, which powers Bombardier’s Global 7500, claims an 8% better fuel efficiency over other engines in its class based on the use of the GEnx powerplant originally designed for the Boeing 787. Pratt’s PW800, which powered the Dassault Falcon 6X for its first flight in March, uses the same core as the commercial PW1000G geared turbofan.
Rolls-Royce’s Pearl 10X, now selected for Dassault’s Falcon 10X, is derived from the company’s Advance2 demonstrator core, and is expected to have at least 5% greater fuel efficiency than the company’s BR700 family. The engine is being developed at the Rolls-Royce Centre of Excellence for Business Aviation Engines in Dahlewitz, Germany, and is currently undergoing a comprehensive test program, which includes the capability to operate on 100% SAF.
While the larger scale of the higher thrust business jet turbofans makes it relatively easier for engine makers to leverage technologies from commercial powerplants, this is not always practical for the smaller turbofans in the sector. However, further fuel burn and emissions improvements are still possible in the lower thrust arena, some of which are under development by Honeywell as part of the FAA’s long-running Continuous Lower Energy, Emissions and Noise (CLEEN) Program.
Launched in 2010, the CLEEN initiative forms the FAA’s main environmental effort to accelerate the development of technologies to reduce aircraft noise, emissions and fuel burn. According to the Transportation Department, technologies already developed under the first two phases will save an estimated 36 billion gallons of fuel by 2050.
While most of the CLEEN focus has been on air transport propulsion and airframes, a significant element has also supported advances in business aviation engines. Under recently awarded Phase III public-private partnership contracts, Honeywell will develop a more-efficient engine fan, combustion system, compressor and turbine.
CLEEN-based technologies have found a home in Honeywell’s HTS7000 turbofan series, which, to date, has found applications on the Bombardier Challenger 300/350, Cessna Citation Longitude, Embraer Legacy 450/500 and Praetor 500/600, as well as the Gulfstream G280. The latest work builds on Phase II work, much of it still underway, which focused on tests of a compact, lightweight combustor, development and endurance evaluation of a low-leakage turbine air seal, tests of a new low-noise fan and liner, and an advanced high-pressure (HP) compressor.
The HP compressor project aims to lower fuel burn by reducing the weight of the module while simultaneously increasing overall pressure ratio, efficiency and temperature capability. The unit is shortened by use of an axi-centrifugal configuration with increased stage loading, and reduced tip clearance sensitivity.
Together with improvements to the blade outer air seal—which is designed to smooth out blade-to-shroud flow interactions and increase durability—the compact compressor is expected to result in an aircraft mission fuel burn reduction of 22% compared to earlier production engines. Combined with non-CLEEN, Honeywell-developed technologies, the program anticipates up to a 23.1% fuel burn reduction. Target entry-into-service is 2025.
At the heart of the engine work is a compact combustor that integrates advanced aerodynamics and fuel injection improvements to reduce weight as well as emissions of nitrogen oxides (NOx). The goal is to reduce NOx emissions by more than a 50% margin relative to the international CAEP/8 standards and to make the technology applicable to next-generation turbofans, turboprops and turboshafts for entry-into-service by 2025.
Work to reduce noise is also encompassed by the initiative, which includes tests of advanced acoustic fan rotor and liner technologies. Aimed at cutting noise by 2.5 EPNdB, and an additional 1.5% reduction in fuel burn, the advanced liner, bypass and center-body acoustic panels are designed to reduce both tonal and broadband noise.
A complete engine demonstration test incorporating the new liners, advanced HP compressor, outer air seal and compact combustor is scheduled for the second half of 2022, clearing the way for follow-on work to take the technologies to production readiness.
