3503 liquid-cooled two-stroke inline aircraft engine

The Hirth 3503 is a liquid-cooled, two-cylinder, inline two-stroke aircraft engine from Hirth Engines, rated at 51.67 kW (70 HP). It is a piston engine for light aviation, gyroplanes, helicopters, hovercraft and other installations where weight matters, and it belongs to the maker's 35 Series of water-cooled engines for manned and unmanned aircraft.

The main figures help to size the installation. The measured power at full throttle is 51.67 kW (70.27 HP) with a torque of 73.30 Nm at 6700 RPM, and the specific fuel consumption is 436.43 g/kWh, measured at 6500 RPM and also at the best point at 70 to 80% load. The displacement is 625 cm³, with a bore of 76 mm and a stroke of 69 mm. The weight is 33.5 kg without exhaust, and the exhaust adds 2.5 kg. This gives the highest output per kilogram that the maker offers among its two-stroke engines. The maker rates the time between overhauls at 1,000 hours at 75% power.

For the installation, the engine measures 456 mm in length, 396 mm in width and 411 mm in height. The 3503E version has an electric starter, a single ignition system and a clockwise running direction, and it runs on petrol. Options include dual ignition, separate lubrication and the G 50 reduction gearbox, which is offered with ratios of 1:2.16, 1:2.29, 1:2.59, 1:3.16 and 1:3.65, so the propeller or rotor speed can be matched to the 6700 RPM engine speed. Liquid cooling needs a radiator and coolant circuit in the airframe.

Hirth Engines lists an ISO 9001 certificate for the company.

Typical applications are light and ultralight aircraft, gyroplanes, helicopters and hovercraft. The same engine family is also used in unmanned rotorcraft such as the Skeldar V-200, which flies with the heavy-fuel 3507 variant of the 35 Series. Technical documentation, including a product brochure and a basic service manual, is available from the maker's repository.