Cortex HF50 hybrid aircraft combustion engine

The Cortex HF50 hybrid is a series-hybrid aircraft combustion engine from Currawong Engineering: a single-cylinder, 50 cc two-stroke engine with electronic fuel injection and an integrated generator, built as a hybrid propulsion source for small unmanned aircraft. The HF50 designation refers to the variant that runs on UMAN-VP4F heavy fuel, an Isopar-based fuel with a high flash point, while a petrol version of the Cortex 50 is also offered. The engine is derived from the maker's Corvid-50 direct-drive engine.

The engine does not drive the propeller directly. It turns a generator that supplies electrical power to the aircraft's motors and to a battery pack, so the battery can be much smaller than in an all-electric design and endurance rises. Power flows both ways: the battery can feed the engine for self-starting, add a power boost in flight and restart the engine in the air. The Currawong Hybrid Transmuter, based on the maker's Velocity ESC, controls this exchange and decides whether energy moves from the engine to the batteries or the other way round.

The key figures are a 50 cc displacement, a 2.1 kW electrical generator output at 7500 RPM and a fuel consumption of 600 g/kW-hr of electrical output at 5000 RPM and 50% throttle. Short boosts of up to 7.5 kW are possible from the battery, which is discharged during the boost because the generator itself is limited to 2.1 kW. The heavy-fuel version lists a time between overhauls of 350 hours and a recommended maximum take-off weight of 40 kg, and the maker notes that heavy fuel may change the performance numbers.

For integration, the complete engine system weighs 4.7 kg and includes the generator, low-noise exhaust, cooling system, isolation mount, fuel injection with pump, Transmuter and harness. Ignition is by CDI and cooling is by air, with a cooling duct and a cooling control servo. The package also contains the ignition unit, spark plug, fuel injector, crank sensor, throttle servo, cylinder head and manifold air temperature sensors, a manifold pressure sensor and the throttle body with manifold. The Transmuter is specified for a 60 V, 14S battery system, 150 A sustained and 300 A peak current and 7.5 kW of power, with an internal operating temperature of -20 °C to 100 °C. The engine operates at ambient temperatures from -20 °C to 45 °C.

The heavy-fuel version uses the Currawong Triplex fuel pump to atomise the fuel. It has three pistons, delivers high-pressure, smooth flow, primes itself and meters fuel flow with an error below 1%. The maker shows the Cortex 50 installed on small UAV airframes, where it replaces part of the battery mass to extend flight time.

Typical applications are hybrid-electric multirotors and fixed-wing UAVs where the specific energy of liquid fuel extends range beyond what batteries alone allow. Integrators who need a single fuel supply shared with turbine-powered aircraft can choose the heavy-fuel version, and the in-air restart supported by the hybrid power supply lets the battery bring the engine back into operation during flight.