Hybrid "a la carte" - Widerøe er nevnt her - AW&ST

VoltAero Hybrid-Electric Regional Aircraft Development Testing Begins Graham Warwick French startup VoltAero has begun the phased development of a hybrid-electric aircraft that offers a range of propulsion options to optimize energy use depending on the mission profile. “I call it an airplane a la carte,” says CEO Jean Botti, former chief technology officer at Airbus. Available with four, six or nine seats, the Cassio is a series/parallel hybrid design able to fly as a pure electric aircraft over ranges up to 200 km (108 nm) at 200 kt., as a battery-assisted mild hybrid over 200-600 km and to ranges exceeding 600 km as a strong hybrid making more use of the combustion engine. The aircraft has two electric-driven tractor propellers mounted on the wing and a third, pusher propeller mounted on the rear fuselage and driven by a power module that combines an internal-combustion, or thermal engine and three electric motors. The thermal engine is able to drive the propeller mechanically, generate electricity to power the motors and recharge battery packs installed inside the wing. The Cassio will be able fly on batteries only, fuel only or any combination of the two. “The pilot will be able to determine the hybrid strategy based on the mission,” says Botti. The propulsion system will automatically recommend a power management strategy, but the pilot will have two power levers—one for the thermal engine and one for the electric motor—to adjust the balance. To illustrate how this flexibility might be used, Botti cites the example of Wideroe, a regional airline that flies a combination of short-, medium- and long-range flights in Norway—an environmentally conscious country that has set the target of having all domestic flights electrically powered by 2040.

Industrial partner Solution F is developing the hybrid power module, with a thermal engine and three electric motors. Credit: VoltAero

Using Cassio, an airline such as Wideroe would be able to fly short flights on pure electric power, with a small amount of range extension from the thermal engine for safety, he says.  “They would be able to reconfigure the aircraft [between missions] very easily,” he adds. VoltAero plans to develop Cassio in two phases. In the first phase, the company is incrementally modifying a Cessna 337 Skymaster to act as a testbed for the propulsion system. In the second phase, VoltAero will build conforming prototypes and certify the Cassio under European CS23 regulations. In Phase 1, the company has already completed baseline flying of the Skymaster equipped with an air data boom to collect reference data. Electric motors are now being installed in the wing nacelles and flight-testing of this “Version 2.1” will begin shortly, initially without the propellers. Flights of Version 2.2, with the tractor propellers and batteries installed, will follow. At this stage, the “push-pull” Skymaster’s front and rear piston engines are retained. The next step will be Version 3, in which the front engine and propeller will be removed, but the rear unit retained. Version 3 is scheduled to fly in September, to be followed before the end of the year by Version 4, in which the rear engine will be replaced with the hybrid power module and new propeller. This represents the configuration of the Cassio, which will be based on the Skymaster but with a composite airframe.

The Cassio is based on the configuration of the Cessna 337 Skymaster. Credit: VoltAero

Working with Aero Composites Saintonge, VoltAero has also modified a Skymaster airframe into an iron bird for ground-testing of the propulsion system. This will be on static display at the Paris Air Show on June 17-23, with an electric motor on one wing and integrated power module in the rear fuselage. In Phase 1, the power module uses a Nissan high-performance automobile engine. In Phase 2, VoltAero will develop a specific multifuel engine for the production aircraft. Within the module, the thermal engine drives the pusher propeller via the same shaft as the three electric motors. The engine and any or all of the motors can be disconnected from the shaft for safety in the event of a failure, as the dual power sources and independent modes of operation provide redundancy. VoltAero has secured funding for Phase 1, and is beginning fundraising for Phase 2 in which it will build three conforming prototypes and a structural test article. The startup is planning to deliver the first production aircraft by the end of 2022, says Botti. The production Cassio will have 600 kW of power from the thermal engine and five 60-kW electric motors. This will provide a 200-kt. cruise speed. Batteries will be mounted in the nose and, along with fuel, in the wingbox, providing 3.5 hr. of flight time. There is room in the wing for more batteries and fuel, Botti says, which could extend flight time to 5 hr. The 5,000-kg-gross-weight (11,000-lb.) Cassio will have an electric motor in the nosewheel, allowing it to taxi without the propellers turning for low noise. The aircraft will take off on electric power, to reduce noise, and engage hybrid mode above 1,200 ft.

The iron bird will be on display at the Paris Air Show. Note the flight-test aircraft in the background. Credit: VoltAero

Typically, the Cassio will cruise in hybrid mode until the batteries have discharged to a 20% energy level. Then the thermal engine will begin recharging the batteries to at least the 50% level before landing—and to 80% in strong hybrid mode on a long flight. Some recharging on the ground will be required. By discharging the batteries to 20% in flight and extracting the maximum energy, the Cassio can reduce fuel consumption by up to 20%—despite the additional weight of around 400 kg for the battery packs and electric motors. “That is where we make the saving,” says Botti. VoltAero sees two main markets for the Cassio: private owners for the four-seat aircraft and regional/commercial operators for the six-seat and stretched nine-seat versions. “We are starting to have discussions with operators,” Botti says. Cost of ownership will be €290/hr. ($323) and availability 10 hr. per day—enough for about eight daily rotations—calculates VoltAero.