• 17 Apr 2024 07:04 PM
• 0

Sikorsky’s HEX Signs

By Frank Colucci
Vertiflite, May/June 2024

Sikorsky Innovations reveals plans for a hybrid-electric tiltwing demonstrator to validate technologies in rotorcraft to come.

On the eve of the Heli-Expo trade show at the end of February (see “Heli-Expo Finale,” Vertiflite, May/June 2024), Sikorsky unveiled renderings of its innovative hybrid-electric (HEX) tiltwing experimental vertical flight demonstrator. The company had first revealed the program a year earlier at Heli-Expo 2023 but had declined to indicate what configuration had been selected for the demonstration aircraft.

This year, Sikorsky president Paul Lemmo told the Anaheim, California, audience, “Tiltwing concepts, tiltrotor concepts have been around for a long time, but I’ve always said they make a lot more sense when you can electrify them.” The 9,000-lb (4,100-kg) HEX aims to test integrated tiltwing dynamics, validate the benefits of electrification and ultimately explore new configurations for future aircraft. Sikorsky’s Innovations rapid prototyping unit expects to power up a HEX sub-scale power system testbed (PSTB) this year and fly the bare-bones uncrewed aircraft system (UAS) in 2025. First autonomous flight of a full-size HEX demonstrator built around a 2,500 shp-class turboshaft and 1 MW-class generator may occur in 2027, depending on PSTB lessons learned.

Refined by flight test data and customer requirements, HEX technologies could scale to a family of crewed and uncrewed aircraft of different configurations. These aircraft will share common power electronics, motor technology and aerostructures, and autonomy and advanced flight controls via Sikorsky’s MATRIX technology suite.

Sikorsky Innovations director Igor Cherepinsky said his team started looking at what configuration made the most sense for a hybrid-electric aircraft with good hovering and highspeed characteristics, and the analysis led them to consider both tiltrotor and tiltwing designs. He said that the physics of tiltrotors are well understood, but there are many aspects of a tiltwing design that are an unexplored space, “so we’re going to explore.”

The 1960s-vintage Canadair CL-84 tiltwing turboprop proved easy to fly but was complicated by mechanical cross-shafting and control mixers. Cherepinsky said, “Suffice it to say that we are not building a copy of CL-84 or any other tiltwing,” which were primarily developed by fixed-wing aircraft manufacturers that did not have Sikorsky’s depth of knowledge regarding rotorcraft systems. He acknowledged, “We are looking at dramatic reductions in operating costs as well as acquisition costs.”

The Canadair Dynavert tilted its wing and 1,500-shp turboprops to take-off vertically on propellers and transition to cruise flight on fixed-wing lift. Cams and levers in a mechanical mixing box enabled pilots to control the aircraft seamlessly through the transition, but the innovative demonstrator never found a sponsor to enter production. At Heli-Expo, Cherepinsky told Vertiflite, “Mechanical complexity is a problem… How do I get rid of mechanical complexity? And again, electrification bubbles right up to the top.”

The largely composite, S-76-size HEX tiltwing uses its hybridelectric powertrain to attain cruising speeds of 200–300 kt (370–550 km/h) and retain the low-speed handling and hover performance of a helicopter. “We absolutely considered a tiltrotor,” acknowledged Cherepinsky. “The interesting thing about tilting [the wings] is we really are trying to gain that last 7–10% of hover efficiency by getting the wing out of the vertical drag in a hover.”

Innovative Demonstrations

The Innovations team will design, build and integrate two HEX tiltwing demonstrators in Stratford, Connecticut, and its other facilities. Sikorsky-designed motors and other laboratory hardware are already running. “We are flying subscale aircraft, and performing CFD [Computational Fluid Dynamics] analysis,” noted Cherepinsky. He added, “It’s important to mention that the primary goal of the HEX demonstrator is hybrid-electric propulsion — the aircraft configuration is a secondary goal.” A notional 12-passenger helicopter, for example, could integrate a HEX-derived hybrid-electric powertrain with an electric tail rotor.

The HEX PSTB is a tubular composite frame structure containing aircraft components and a power source. It is designed to be flown at low speeds from hover to 20 kt (37 km/h).

Cherepinsky said that PSTB “is going to look like the VS-300” — Igor Sikorsky’s first successful helicopter, with exposed trusses and rotors — and will be able to hover and will be used for all the low-speed test work and to prove the 1.2-MW hybrid-electric drivetrain.

After the PSTB, the two flying prototypes will be composite airframes with a lot of systems innovations. For example, the electric motors will have reduction gearboxes and the team will 3D print the aircraft’s gearboxes, dynamics system and hubs, and the HEX “doesn’t have any forgings or castings.”

“It’s really a new way of building an aircraft,” added Cherepinsky who believes the age of 15-year development is probably over and the industry needs to find new ways to build a civil- or military-certified product in a five-year cycle, which will be different to how aircraft have been developed in the past.

The Sikorsky Innovations team has itself been augmented for electrification. “Not so much for HEX, but for electric propulsion,” Cherepinsky noted. “We have brought experts in power electronics and magnetics to our team from other Lockheed Sikorsky Innovations director Igor Cherepinsky said his team sought a hybrid-electric aircraft configuration with good hover and high-speed performance. (VFS photo by Ken Swartz) Martin branches as well as from outside.” Electrification is just one of Sikorsky’s current technology pillars (see “Pillars to Products,” Vertiflite, March/April 2024). According to Cherepinsky, “When you say electrification, we’re looking at actuation. A lot of components that are built in-house — including actuators, computers and power electronics — can make aircraft more electric in general.” He continued, “The second one is autonomy… We’re going to apply MATRIX [autonomy] to UAS, which is the third pillar. We’re going to go make larger UASs that can be certified in the National Airspace [System] and with the” US Department of Defense.

Sikorsky Innovations previously developed the MATRIX autonomy system that will integrate the HEX demonstrator power and flight controls. MATRIX software, processors, sensors and actuators have already flown on ten different rotary- and fixed-wing aircraft. Demonstrations continue with the S-76 Sikorsky Autonomy Research Aircraft (SARA) and the UH-60 Black Hawk optionally piloted vehicle (OPV). The MATRIX open architecture is also compatible with third-party software and human machine interfaces. Cherepinsky noted, “The HEX prototypes are unmanned, which is letting us run fast and take greater risks. The emerging product line… is going to consist of both UAS and crewed platforms — including nine-to-12-passenger vehicles, etc.”

The HEX tiltwing is sized to model Sikorsky products under study. At Heli-Expo, Cherepinsky said, “Whether there’s going to be a product following it or not, we don’t know yet. We’re out there to learn, to figure out how the physics of all this works.”

Volts Have Value

According to Cherepinsky, military contested logistics and commercial intercity transport missions alike call for payloads around 2,500 lb (1,100 kg) and ranges greater than 500 nm (930 km). GE Aerospace will provide the CT7 turboshaft and the 1.2-MW generator to power the HEX demonstrator. “The power size was determined by relevance to larger products that we are looking into.”

Pure electric solutions limit speed and range. At Heli-Expo, Cherepinsky noted, “Batteries are not viable for these machines… There are lots of developments in fuel cells, batteries, and even in turbines that burn hydrogen and turbines that burn SAF [Sustainable Aviation Fuels].” He continued, “So what we’re developing is an aircraft architecture that’s completely agnostic to the source of power… We’re very interested in power distribution, power consumption.” The PTSB will be compatible with different power sources.

Cherepinsky conceded, “For the time being, using a turbine as a power producer is the most pragmatic option available.” He added, “As usual on new configurations, weight control and thermal management for electric system are the biggest challenges.” The HEX hybrid-electric propulsion system builds on work done by GE Aerospace for NASA and the US Army.

NASA Glenn Research Center is sponsoring work at GE and the University of Illinois aimed at megawatt-class motors with power densities two to three times the current state of the art. NASA Glenn plans a 1-MW hybrid-electric demonstration in 2025 under its Electrified Powertrain Flight Demonstration (EPFD) program. Boeing and its subsidiary Aurora Flight Sciences are partnering with GE to support the flight tests using a modified Saab 340B aircraft powered by GE’s CT7 engines.

GE advances in silicon carbide (SiC) semiconductors meanwhile aim to make power systems lighter, smaller, cooler and more efficient than previous silicon-based technology. The GE Global Research Center is developing a lightweight SiC-based inverter for MW-class hybrid-electric applications.

The HEX demonstrators will give Sikorsky Innovations platforms on which to evaluate hybrid electrification. They give the parent company paths to very different vertical lift solutions. Paul Lemmo said at HAI, “Our future is not confined to rotor blades. There is a wing in the mix to meet customer needs for vertical flight. We envision a family of systems that are safer, more reliable, easier to maintain and can fly farther and faster at lower cost.”