Unmanned Systems Technology 036

66 I n the launch issue of UST (November 2014), we featured the S1218 engine from Hirth Engines, a long- established producer of two-stroke engines for aviation and other applications. For a comprehensive examination of Hirth’s progress since then, we present an investigation of one of its newest and most successful models of recent years, the 3507, as well as a brief look at how the S1218 has evolved over six years of development. The 3507’s history The 3507 is a 55.7 hp inline two-cylinder two-stroke engine, spark-ignited and designed to run specifically on heavy fuels. It is seen as a major workhorse in Hirth’s product range, and is the power plant for UMS Skeldar’s V-200 helicopter UAV, which is operated by organisations including the German, Spanish and Canadian navies. It is based on Hirth’s 35 Series engines, which were originally developed for gasoline- powered light sport aircraft. The success of the 35 Series in recreational aircraft, hovercrafts and boats has generated a huge amount of customer feedback, which has been used to develop a version for the large and fast-growing maritime UAV market. Here, heavy fuels are often preferred to gasoline for their comparatively higher flash points, which makes them safer for use on ships. After a request from Saab (co-owner of UMS Skeldar) in the early 2000s, a version of Hirth’s 3503 engine was initially modified to be compatible with heavy fuels, and continued r&d by Hirth and Saab led to the prototype of the S1208 engine, which would later become the 3507. The current version of the engine is the 3507-01M, keeping it in line with its roots in the 35 Series while also being the first of the new 35-HF (Heavy Fuel) series. This inline two-cylinder engine is liquid-cooled and ECU-controlled, and is Rory Jackson traces the development path of this two- stroke workhorse, which is designed to run on heavy fuels Heavy duty February/March 2021 | Unmanned Systems Technology The 3507-01M has its roots in Hirth’s 35 Series, and is designed to run on Jet A-1, JP-4, JP-5 and JP-8 fuels (Images courtesy of Hirth unless stated otherwise)