Climate change has accelerated research on alternatives to fossil fuels, with electric propulsion viewed as a leading contender. While the fixed-wing and advanced air mobility sectors are blazing the trails, research and development into electric propulsion for helicopters is also in progress, although to a much lesser degree.
Currently, electric helicopters account for only about 5% of all publicly known electric aircraft developments, according to the Roland Berger Electric Aircraft Database. As Nikhil Sachdeva, project manager and lead for electric propulsion at the London-based consulting firm explains, the electrification of a helicopter simply requires replacing the fuel tank and turboshaft assembly with a completely electric power train comprising a battery, power electronics, electric motors, and the necessary cabling.
“These subsystems are already seeing success in small general aviation aircraft and in urban air mobility, and we expect them to be relevant for small to medium-sized helicopters,” Sachdeva says.
One constraint on electric propulsion for all vehicles is the current limits of energy density, which is the amount of energy stored in a battery per unit volume. Sachdeva adds that further improvements are constantly being made in battery energy densities and costs, primarily driven by the automotive sector, that extend the range of electric-powered helicopters and enable larger helicopters to be electrified.
All-Electric versus Hybrid
As with automobiles, helicopters in the future could be offered with an all-electric or a hybrid solution. An all-electric rotorcraft would derive all of its propulsion energy from a battery that drives electric motors directly powering the rotors. This eliminates the need for an engine powered by avgas or jet fuel, reducing both emissions and noise.
In contrast, a hybrid rotorcraft can have multiple configurations, but the key aspect of this solution is that at least some of the energy comes from a conventional fuel-powered engine.
One example of a current hybrid rotorcraft project is Bell’s electrically distributed anti-torque (EDAT) aircraft. As Eric Sinusas, program director, light aircraft, at Bell in Fort Worth, Texas, explains, this helicopter uses a conventional fossil fuel–driven engine to power both the main rotor and the electric generators. “The generators then provide electric energy—the same way a battery would—to power electric motors that drive the anti-torque system.”
The EDAT’s anti-torque system, says Sinusas, is unique in that it controls thrust by changing the rpm of the fixed-pitch anti-torque blades. In comparison, a conventional helicopter controls thrust by dynamically changing the pitch angle of each anti-torque blade while the blades rotate.
Currently, the EDAT helicopter is flying as a technology test bed, utilizing a seven-passenger Bell 429, configured with an EDAT tail boom encompassing four sets of shrouded anti-torque blades. Bell, reports Sinusas, isn’t disclosing plans for production of an EDAT helicopter but, rather, is testing technology that could be applied to a future production aircraft. The OEM, he says, is currently “continuing to expand the flight envelope” and gather data at its Mirabel facility near Montreal.
The major technical challenge to making an all-electric or hybrid propulsion system viable is the development of batteries with the required power density. In tandem with that, Sinusas says, there are range and payload issues.
“The first all-electric and hybrid-electric helicopters will probably have to make some trade-offs of range and payload capabilities due to the weight of the existing batteries, at least for the short term,” he says. “Battery technology will have to make significant improvements in terms of being lighter weight and, therefore, comparable in weight to gasoline-powered systems.”
In addition, unlike liquid fuels, which are consumed during combustion, thereby increasing payload capability, a battery essentially weighs the same whether empty or full. (Using the familiar E = mc2 formula, Tesla estimates the difference between the weight of a full and discharged Model S battery, for example, to be the equivalent of a grain of sand.) Fuel calculations for electric aircraft will look very different from those powered by avgas.
Also required, reports Sinusas, will be built-in safety provisions that are needed to protect a high-power electric system from lightning strikes. And since any next-generation electrically powered helicopter will likely be all digital, anti-hacking features will be needed. Other factors to consider, he adds, are vibration as well as the impact of HIRF (high-intensity radio field), a radio-frequency energy of a strength sufficient to compromise the performance of a device.
“With HIRF, the energy emitters of concern tend to be large ground-based systems, such as radio and TV transmitters, and radar,” he says. “The concern with airborne systems is how that energy may affect sensitive electrical components.”
But once the engineering challenges are met, says Sinusas, electrification is expected to result in lower ownership costs, given that there would be fewer moving parts and lubricated components. “For an all-electric helicopter, there are fewer items to wear out. For example, gearboxes and driveshafts [would be] replaced by electric wiring.”
Looking forward, Sinusas ventures that lightweight helicopters are the most likely candidates for an electric propulsion solution, which would initially be hybrid. “The hybrid-electric would have more range than an all-electric helicopter, at least in the near term,” he notes. “In fact, at Bell, we believe we can retrofit existing engines for application to a hybrid-electric helicopter, because there is nothing stopping them from integrating with a hybrid-electric system.”
Putting aside the technology and engineering challenges, however, Sinusas adds a cautionary note. “There is the question of whether an electric or hybrid-electric helicopter will be treated as a new concept by the regulatory and certification authorities, and how comfortable they’ll be with a system that really has no precedent to date.”
The Hybrid Solution
According to Luca Cossetti, Airbus Helicopters’ innovative power solutions expert, the first step toward helicopter electric propulsion will be hybridization. As he describes it, an electric propulsion system will be combined with a current thermal (fossil fuel–powered) engine, in order to supply additional power to the rotors during specific flight phases.
“For helicopters, the four major challenges for electrification are the batteries, the low power and energy density of current cell technologies, a difficult integration within the helicopter airframe, and certification requirements that are still under definition,” Cossetti says. “Also, the electric propulsion chain needs to show sufficient availability/reliability for helicopter applications and expected functions. This is not an easy task with current technology standards for batteries, power electronics, and e-motors.”
Cossetti also stresses that, unlike fixed-wing aircraft, which are more tolerant of increases in empty weight and integration of additional equipment, the balance between empty weight, payload, and fuel quantity on a helicopter is very delicate.
“Even small mass variations can have important consequences for mission capabilities and performance,” he cautions. “Batteries would for sure increase the empty weight of a helicopter—in the case of both hybrid and, even more, in full electric versions. This is why we need a significant increase in battery power and energy densities in order to limit the degradation on payload.”
Cossetti explains that at Airbus, the philosophy is to “start with a small degree of hybridization and increase progressively.” In fact, he notes that the OEM has been engaged in hybrid projects since 2011, when an electric backup system test campaign was carried out on an H125 Ecureuil light helicopter test bed.
“Since then, a lot of progress has been made regarding batteries and electric engines. In the coming months, we’re planning to do another flight test campaign on an H130, with lighter batteries and smaller electric engines,” Cossetti explains. “The aim of the project is to provide power to the rotor system for up to 30 seconds in case of an engine failure. In such circumstances, the system would make autorotation safer by maintaining the rotor’s constant rotation speed.”
Light single-engine helicopters have been deployed at Airbus as test beds, because, according to Cossetti, they’re a logical starting point. “With a light single-engine helicopter platform, hybridization could bring benefits and new functions without unacceptable performance degradation,” he says.
Asked about the potential for easier maintenance with hybrid helicopters, Cossetti says that will depend on system configuration.
Airbus Helicopters has been engaged in hybrid projects since 2011.
“Hybridization, by default, means adding stuff, such as batteries, electrical motors, and controllers, which means more parts to maintain,” he says. “New regulations concerning hybrid-electric propulsion systems could also add requirements that could have a direct impact on maintenance tasks. However, hybridization could bring some significant advantages in terms of maintenance cost reduction over the life of the helicopter, which is one of the key parameters for a helicopter operator/user.”
In the case of full electric propulsion, says Cossetti, electrical components are generally less demanding in terms of maintenance compared with an internal combustion engine, which could partly alleviate the additional complexity and allow for decreased maintenance costs.
Hervé Blanc, executive vice president and general manager, Power Division, Safran Electrical & Power in France, says that while an electrical solution is easier and less costly to maintain than a conventional piston or turbine powerplant, a hybrid solution represents a different dynamic.
“A good hybrid solution increases maintenance costs with the addition of the electrical system but reduces drastically the most expensive part, which is the thermal engine, by optimizing the use and the conditions of operation of the thermal system,” Blanc explains. “If the turbine is less stressed due to the electrical system, then the total maintenance cost is reduced.”
Electrical Solutions for Gas Helicopters
Blanc says there are many opportunities to introduce electrical solutions to conventionally fueled helicopters. They include a full electrical tail rotor, already successfully flown with one helicopter manufacturer, and a “stop and start” solution to safely restart a turbine in flight now in development. “We’re also developing a solution to provide additional power to the main gearbox of the helicopter to ensure greater performance,” he says.
While the electrification of the entire propulsion system is the ultimate goal, Mike Mekhiche, deputy director, Rolls-Royce Electrical in the United Kingdom, ventures that considerable value can be achieved via the partial electrification of a conventional propulsion engine. He terms this solution a “mild hybrid” propulsion system.
“This involves integrating an electric generator-motor—and the associated bidirectional power electronics converter with a sensibly sized battery—into the turbine engine of the aircraft,” Mekhiche explains. “The purpose is twofold. Firstly, using the battery power, the generator-motor can provide additional torque to boost the aircraft thrust and supplement its ability to handle larger payloads and/or enable the aircraft to take off faster under normal operating conditions.”
Bell’s electrically distributed anti-torque (EDAT) system, shown being tested on a Bell 429, controls thrust by changing the rpm of the fixed-pitch blades.
And, he says, in case of an engine-out situation, instead of relying on the autorotation to land the aircraft—a maneuver that requires quick reactions and appropriate flight inputs by pilots—the electric subsystem would provide enough electric torque to maintain a controlled rotation of the helicopter rotor and therefore enable the aircraft to “land safely in an area of the pilot’s choosing.”
The sizing of the battery in a mild hybrid system, says Mekhiche, will be determined by the available space within the aircraft and the minimum torque required to enable the aircraft rotor to continue to operate for a handful of minutes in the case of an engine failure. “The electric machine and the associated power-conversion electronics can operate as a motor or as a generator,” he says. “The latter operating mode enables battery charging via the engine upon depletion.”
Issues to Solve
Reaching the goal of an electrically powered helicopter will require sufficient battery energy density to meet the unique power needs mandated throughout a helicopter mission. Those needs, unique to rotorcraft, pertain to sustained hover as part of the mission profile, according to Jonathan Hartman, Sikorsky Innovations’ disruptive technology lead for Sikorsky (a Lockheed Martin Company) in Stratford, Connecticut. (Sikorsky hasn’t publicly announced a hybrid or full-electric helicopter project at this time.)
“The end-of-flight hover and landing is the most onerous part of a helicopter mission because you would need to pull a lot of power at the time the energy stored in the battery would be at its lowest level,” says Hartman. “To address this issue, the performance of batteries has to improve.”
At present, reports Hartman, there are a number of “interesting energy storage technologies” being developed—all at various stages of maturity—that could have a substantial impact on future aircraft design. “But it’s still too early to know if one technology will meet the rigorous performance requirements for certified flight more affordably or reliably than any other,” he stresses, adding that how the electric propulsion technology will be applied could vary significantly depending on the helicopter mission requirements.
“The answer will come down to payload and range, which is why, at this time, the technology for some type of an electric helicopter looks very promising,” Hartman remarks. “Specifically, that would be smaller vehicles flying shorter distances, carrying smaller and lighter payloads.”
Marc Brodeur, VP, military and commercial sales, at MD Helicopters in Mesa, Arizona, emphasizes that in order to make electrically powered helicopters viable, battery technology needs to reach a “breakthrough” beyond what’s available today. “What we have to do is look for a next-generation battery solution. In addition, there are other challenges involving cost and performance,” Brodeur says. MD Helicopters, he discloses, isn’t pursuing a hybrid or all-electric helicopter project currently.
Another hurdle to scale, says Brodeur, is a redesigned infrastructure required to support a fleet of electric helicopters. “You would definitely be looking at a special supply chain, new kinds of tooling, as well as battery storage and charging facilities. But I don’t believe that this would be a showstopper that would make electric helicopters totally impractical.”
Brodeur adds that, in his opinion, the first steps toward an electrically powered helicopter will take place in the unmanned aerial vehicle sector.
“This is where the technology is likely to be perfected,” Brodeur says. “From there, it would transition to very light utility helicopters. It’s going to be a crawl-to-walk kind of progression.”
Rolls-Royce’s Mekhiche agrees that, based on current battery and electric motor technologies, a helicopter’s payload capacity would have to be significantly reduced in order to accommodate a full hybrid or a full electric propulsion system. “The very limited real estate of the helicopter makes it much more challenging to physically accommodate the components of such a propulsion system,” he notes.
Yet, he adds, there are opportunities where a hybrid or an electric helicopter could be repurposed to a mission profile that fit its new or modified ratings. “This would be the case where a quieter operation [would enable] greater operational flexibility, longer operating hours, and superior point-to-point access in heavily regulated areas, such as densely populated urban areas.”
Mekhiche stresses that cell technology is improving at an accelerated rate, with energy density expected to more than double by 2030. “As the cell technology improves, larger aircraft with longer travel distances and greater payload capacity will transition from a hybrid propulsion solution to an all-electric power train over time.”
Airbus plans another series of flight tests of its electric backup system, this time on the H130.
But what about acquisition costs compared with a conventionally driven helicopter? This is hard to guess, says Professor Rolf Henke, member of the Board for Aeronautics and Technology of the German Aerospace Center (DLR) in Cologne, Germany, since an electrically driven helicopter needs to be developed and certified and new production lines installed.