Unmanned Systems Technology 001 | UAV Factory Penguin C | Real-time operating systems | Hirth S1218 two-stroke twin | Base stations | ASV C-Enduro | Composites | Datacomms

43 at lower rpm. That provides more thrust for a given horsepower, and in practice it is a more efficient solution. “That is an optional extra that we recommend,” notes Kehe. “Customers who accept that option are always happy with the result.” When using a direct-drive propeller, a representative cruising speed is around 4500 rpm. If the engine is used in this way for most of a mission, flight time can be expected to exceed 12 hours. In fact, depending on the UAV in question, its fuel load and mission parameters, Kehe says a representative figure is 14 hours-plus. “Typically a mission will start with 10- 20 minutes of wide-open throttle [WOT] running for the take-off plus the climb to the required altitude,” he explains. “Then the throttle will be backed off, normally to run at a constant speed that will represent something in the region of 65- 80% of maximum power. So it is stressed within that amount of maximum at all times. That is a lot of stress.” The cylinder head operates at a temperature of up to 280 C. Measured 110 mm downstream of the exhaust port window, exhaust gas temperature is typically around 680 C at WOT and up to 720 C at part load. Ignition timing is normally in the region of 30-25 º BTDC, regardless of fuel type. “Advancing the ignition can help, but not beyond 25 º ,” remarks Kehe. “That is a typical two-stroke thing”. He adds that all engines supplied by Hirth are mapped so as not to run into knock. “The mixture preparation provided by our resonance tube technology – and the way the engine is designed and mapped – means we can avoid knock, even when using kerosene-based fuel.” As is typical of a two-stroke, maximum torque for this engine is very close to maximum power – in this case it is just 200 rpm lower, and maximum power is 15 bhp at the 6500 rpm limit. Given that kerosene-based fuel is hard to burn, fuel consumption is a function of the type of fuel used. Using kerosene-based fuel, a representative figure at WOT is around 360-380 g/kWh; in cruising mode it would be more like 280-300 g/kWh. It all adds up to a state-of-the-art UAV engine, and one with a remarkable multi-fuel capability. Owned by Siegfried Göbler, Hirth employs 50 people at its base near Stuttgart in Germany in the business of the design, development, building and testing of two-stroke engines. It supplies engines for UAVs, including aircraft and helicopters, as well as karts, hovercraft and portable fire pumps. Hirth makes most of each engine in-house, including machining structural components from raw castings. The manufacture of built- up crankshafts using the well known Hirth-type couplings is a core technology. These are forged outside the company but are machined and assembled in-house. Hirth offers a 1000-hour warranty for each of its press-together crankshafts, the hand assembly of which – using special tools – is a highly skilled task. Outsourced are altitude chamber (Porsche) and climatic chamber (Beru) testing. In-house facilities include five- axis machining and a quality control room maintained at a constant 20 C. To ensure repeatability, components are kept there for 24 hours before checks. The company has various test cells for sub-assemblies as well as complete engines. All engines are tested on one of the in-house dynos before delivery to the customer. Most are supplied as a package complete with the mounting frame – the approach is that of ‘plug and fly’. Full documentation is always provided with each engine. Hirth the company Unmanned Systems Technology | November 2014 The Skeldar UAV is powered by Hirth’s S1214 water-cooled engine A mission will typically start with 10-20 minutes of WOT running for the take-off plus the climb to the required altitude