Hybrid electric aircraft
A hybrid electric aircraft is an aircraft with an hybrid electric powertrain, needed for airliners as the energy density of lithium-ion batteries is much lower than aviation fuel. By May 2018, there were over 30 projects, and short-haul hybrid-electric airliners were envisioned from 2032. The most advanced are the Zunum Aero 10-seater, the Airbus E-Fan X demonstrator, the VoltAero Cassio, UTC is modifying a Bombardier Dash 8, while an Ampaire prototype first flew on 6 June 2019.
The Boeing Truss-Braced Wing subsonic concept was planned with hybrid electric propulsion. The Diamond DA36 E-Star first flew on 8 June 2011, the first flight of a series hybrid powertrain, reducing fuel consumption and emissions by up to 25%, a technology scalable to a 100-seater airliner. A small and 100 kg (220 lb) lighter Austro Engine 40 hp (30 kW) Wankel engine generates the electricity, supplemented by EADS batteries for silent take off, feeding a Siemens 70 kW (94 hp) electric motor turning the propeller. The AgustaWestland Project Zero was intended to be hybrid-electric.
Zunum Aero, backed by Boeing and JetBlue, is working since 2013 on a family of 10- to 50-seat hybrid electric regional aircraft. On 5 October 2017, Zunum launched the development of a six-to-12-seat aircraft with its powertrain installed on a testbed and flown in 2019. Aiming to fly in 2020 and be delivered in 2022, it should lower operating costs by 40–80% to reach available seat miles (ASM) costs of a 78-seat Dash 8-Q400.
The 1,300-shp GE Catalyst could be used in hybrid-electric propulsion: in late 2016, General Electric modified a GE F110 fighter turbofan to extract 250 kW from its HP turbine and 750 kW from its LP turbine, supported by the USAF Research Laboratory and NASA, developed and tested a 1-megawatt electric motor/generator with GE Global Research, and tested a liquid-cooled inverter converting 2,400-volt DC to three-phase AC with silicon carbide-based switches and 1.7-kW MOSFET power modules.
By May 2018, consulting firm Roland Berger counted almost 100 electric aircraft in development. This was up from 70 the previous year and included 60% from startups, 32% from aerospace incumbents, half of them major OEMs and 8% from academic, government organizations and non-aerospace companies, mainly from Europe (45%) and the U.S. (40%). Mostly urban air taxis (50%) and general aviation aircraft (47%), a majority are battery-powered (73%), while some are hybrid-electric (31%), mostly larger airliners. Industry experts expects a 50+ seat hybrid-electric airliner to debut in commercial operation by 2032 for routes like London-Paris.
The potential of electric and hybrid-electric propulsion remains limited for general aviation, according to Textron Aviation, as the specific energy of electricity storage is still 2% of aviation fuel. An hybrid configuration is needed for airliners: lithium-ion batteries including packaging and accessories gives 160 Wh/kg while aviation fuel gives 12,500 Wh/kg. As electric machines and converters are more efficient, their shaft power available is closer to 145 Wh/kg of battery while a gas turbine gives 6,545 Wh/kg of fuel: a 45:1 ratio.
For Collins Aerospace, this 1:50 ratio forbids electric and hybrid-electric propulsion for long-range aircraft, as a 500 nmi (930 km) mission for an all-electric, 12-passenger aircraft would require a six-fold increase while three-fold increase is projected between 2019 and 2029. Roland Berger observes the best Li-ion batteries achieve 300 Wh/kg by 2019, enough for small aircraft, while a regional airliner would need a 500 Wh/kg battery pack. For MTU, an electric, limited-range Airbus A320-sized single-aisle would need 2 kWh/kg from 0.25 kWh/kg in 2019 to enter service 30 years later.
The EU funded the Hypstair program with €6.55 million over three years till 2016 for a TRL of 4: a Pipistrel Panthera mockup received a serial hybrid-electric powertrain, ground testing a 200-kW motor driven by batteries only, by a 100-kW generator-only and by both combined. It is followed by Mahepa project from 2017, EU-funded over four years with €9 million under the Horizon 2020 research program to reduce aviation carbon emissions by 70% in 2050, till TRL 6 before entering product development. The Panthera drivetrain will be divided in modules: electric motor thrust generator and internal combustion power generator in the nose, human-machine interface and computing, fuel and batteries in the wing. Ground testing is planned for 2019 before flight tests in 2020.
The dual-fuselage, four-seat, battery-powered Pipistrel Taurus G4 received a DLR hydrogen fuel cell powertrain to fly as the HY4 in September 2016, with hydrogen tanks and batteries in the fuselages, fuel cells and motor in the central nacelle. Partners are German motor and inverter developer Compact Dynamics, Ulm University, TU Delft, Politecnico di Milano and University of Maribor. Ground and flight tests should follow those of the Panthera a couple of months later.
Along their ground handling, scaling to 19- and 70-seat airliners will be studied in two configurations: more of the same size modules for electric distributed propulsion, or larger sized modules extrapolating the flight-test results, powering twin propellers. Flights will test system behavior, measure performance and reliability, and evaluate failure modes. A failure rate of one per 10 million hours is targeted, as low as in airliners, with very reliable components or with redundancy.
Austrian company ScaleWings, developer of a P-51 Mustang scale replica, has developed a hybrid and redundant piston/electric engine, based on independent modules: a 1.15 L (70 cu in) four-stroke V-twin producing 80 and 120 hp (60 and 89 kW) when turbocharged, and electric motors, producing 170 to 350 hp (130 to 260 kW) combined.
VoltAero is a startup company formed in September 2017 by the CTO and test pilot of the 2014 Airbus E-Fan 1.0, located in Royan and established with the support of the French Nouvelle-Aquitaine region. The company is developing a hybrid testbed based on the Cessna 337 Skymaster, which it intends to fly in late February 2019. The clean-sheet, all-composite VoltAero Cassio prototype should follow in 2020, before deliveries in late 2021 or early 2022. It will be powered by two 60 kW (80 hp) electric motors driving tractor propellers on the wing and a 170 kW (230 hp) piston engine and 150 kW (200 hp) motor driving a pusher propeller in the aft fuselage. The combination of fuel and batteries will give it a 1,200 km (650 nmi) range with nine people aboard.
On 31 October 2018, Diamond Aircraft flew the HEMEP, funded by Germany’s economics ministry and the Austrian Research Promotion Agency, reaching 130 kn (240 km/h) and 3,000 ft (910 m) within 20 minutes. It is a modified DA40 with its single piston engine replaced by two Siemens 75 kW (101 hp) electric motors in the nose powered by a 110 kW (150 hp) Austro Engine AE 300 diesel or two 12 kWh (43 MJ) batteries, for a 5 h. endurance or 30 min. on batteries only.
By January 2019, U.S. startup Ampaire was replacing the Cessna 337 Skymaster (a push-pull aircraft) aft piston engine with an electric motor, to fly the prototype on Hawaiian Mokulele Airlines commuter routes operated with Cessna Caravans. Seven other airlines are interested by Caravan or Twin Otter conversions: Seattle’s Kenmore Air, Tropic Air of Belize, Puerto Rico-based Vieques Air Link, Southern Airways Express of Memphis, Tennessee, Guernsey’s Aurigny and Star Marianas Air, based in the Northern Mariana Islands, as well as Norway. Test flights will take place on a 28 mi (45 km) route over 15 minutes between Kahului Airport in Central Maui and Hana Airport on the East side. The hybrid prototype held its first public test flight on June 6, 2019, before scheduled service planned for 2021. Personal Airline Exchange (PAX) became the launch customer for the Ampaire Electric EEL modified six-seat Skymaster, to be certified in 2021, with an order for 50 plus 50 options.
By March 2019, UTC was converting a 39-seat Bombardier Dash 8 Q100 into a hybrid-electric for demonstration flights from 2022 within its Project 804. The 2 MW (2,700 hp) design is similar to the Airbus E-Fan X program, but aims for certification and production for a subsequent commercial offer. One 2,150 hp (1,600 kW) PW121 turboprop will be replaced by a 1 MW (1,300 hp) gas turbine joined with an electric motor of the same rating, powered by off-the-shelf lithium-ion batteries for takeoff and climb. The turbine is used alone in cruise and drives the motor-generator to recharge the batteries in descent. The downsized engine operates at its optimum for 30% fuel savings over 200–250 nmi (370–460 km). Range is reduced from 1,000 to 600 nmi (1,900 to 1,100 km) due to the higher empty weight and 50% lower fuel capacity.
At the June 2019 Paris Air Show, Daher, Airbus and Safran teamed up to develop the TBM-based EcoPulse demonstrator, with half of the €22 million ($25 million) demonstration funded by the DGAC. The maiden flight is scheduled for the summer of 2022 before a hypothetical 2025-30 certification. The aircraft’s existing engine will be supplemented by six 45 kW (60 hp) safran electric motor on the wing fed by a 100 kW (130 hp) APU or batteries. Similar to the NASA X-57 Maxwell, the distributed propulsion reduces wingtip vortices and add low speed lift by blowing the wing, enabling a smaller, lower drag wing.
A mid-May 2019 survey for UBS shows 38% of Americans and Germans said they would be likely to fly in a hybrid-electric airplane, rising to more than 50% for 18-44-year-olds. UBS thinks hybrid aircraft for up to nine passengers over short routes below 250 nmi (460 km) could be available from 2022, and 2028 for regional airliners up to 1 h routes. UBS forecast a market for 16,000 hybrid-electric airplanes and $178-192 billion over 2028-40, mostly in general aviation, light business jets and regional aircraft with 20% lower operating costs than present 50-70 seaters.
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