French start-up electric-hybrid aircraft manufacturer VoltAero is preparing to start flight testing its Cessna 337-based "Cassio 1" aircraft with the front engine replaced with propellers driven by electric motors. Meanwhile, it is displaying its "iron bird" mockup incorporating the test hybrid power module at the Paris Air Show (Static A6) and is constructing the Cassio 2 production prototype, which will benefit from the fully validated propulsion system.

The company said it has agreed on a certification plan with the French industry regulator, the DGAC in what is a cornerstone of its strategy—to bring a viable hybrid-electric aircraft to the market where such technology remains very new, in the CS23 category (under 2.5 tonnes maximum takeoff weight).

Cassio 1 is based on a Cessna 337 airframe while Cassio 2 is a new-build based on the initial configuration being tested on Cassio 1, but with an all-composite airframe produced by Aero Composites Saintonge.

VoltAero has defined four configurations in its path to certification, as it gradually introduces and tests the technologies required to Cassio 1. Configuration 1 saw an air data boom added for data gathering; Configuration 2 is being readied for flight with wing-mounted electric motors replacing the front engine; Configuration 3 will test these electric units with the existing rear engine; and Configuration 4 will, by the end of 2019, see the hybrid "power module" replacing the rear engine. At this stage, Botti said, "This configuration will help us go to EASA and then we'll go quickly to the FAA."

The Cassio design will have a pusher prop powered by a hybrid-electric “power module” (including a 60 kW electric motor driving the same shaft as the “thermal” engine), along with two forward-facing props, one on each wing, each driven by two 60 kW electric motors. It is designed to be a four- to nine-seat aircraft with a 3.5-hour duration, with a cruise speed of 200 knots.

Based at Aérodrome de Royan-Médis, north of Bordeaux, the company initially converted a Cessna 337 (Cassio 1) that it unveiled to the media in October last year, while it also has an iron bird development rig at partner Solution F in Venelles, France.

Jean Botti, VoltAero CEO and corporate technical officer, said taking the project into production will require €70 million ($79 million) of investment. The support of the region of Nouvelle-Aquitaine has been key, he said, with the location for series production set to be at a to-be-finalized location in the region. He said deliveries are planned “from 2022, and we expect to produce 150 aircraft annually at full rate, which we hope to reach in 2025-26, three years after we start.”

Botti said the aircraft will have 600 kW of power (almost 800 horsepower) available, 300 kW from electric motors and 300 kW being the thermal portion of the hybrid power system. The main role of the thermal engine is to charge the batteries while providing redundancy for “an unmatched level of safety,” given either thermal or all-electric power could be used if the other has a problem. Thus takeoff and landing would normally be full-electric, making for quieter operation.

For a flight of less than 200 km (108 nm) Botti said pure electric could be used; with “mild” hybrid between 200 and 600 km; and “heavy” hybrid” for ranges beyond 600 km, with a maximum range of 1,300 km. The fuel tanks at present hold 120 liters (31.7 U.S. gallons).

For the demonstrator avgas is being used as the fuel but “the idea is to switch to biofuel,” said Botti, meaning the eventual hybrid engine will run on diesel or kerosene. “We get a 20 percent fuel saving despite having around 400 kg extra weight for batteries,” he added.

Central to the design is the automated power management; the pilot(s) have two levers, one for thermal power and a second for electric. They don’t have to be worried about how to distribute power between the puller and pusher propellers. “So the pilot defines the hybrid strategy,” said Botti.