Unmanned Systems Technology 018 | CES show report | ASV Global C-Cat 3 USV | Test centres | UUVs insight | Limbach L 275 EF | Lidar systems | Heliceo DroneBox | Composites

66 Dossier | Limbach L 275 EF (likewise mounted on the airframe). The ECU also incorporates the drivers for the two injectors (one per cylinder). Aside from ignition and injection control, the ECU switches the fuel pump on and off. Throttle operation is governed by the airframe’s flight control unit according to its autopilot. The ECU is Trijekt’s ‘Bee’, modified as appropriate for Limbach’s use. In essence, since the design also caters for automotive use, any hardware surplus to the requirements of aviation is removed, while the software is bespoke. The key inputs to the ECU are from a trigger wheel on the crankshaft, which not only indicates crankshaft speed but identifies its position, and a throttle position sensor. Aside from those speed and load measurements it is provided with readings of air pressure and air temperature. It has altitude compensation and a rev-limiter function. The customer’s flight control system can receive real-time readings of exhaust gas temperature and cylinder head temperature, which are not sent to the ECU. “Our philosophy is that we do not want flight control to interface directly with the ECU,” Limbach explains. “We want the ECU to be a closed system with the engine, so that any kind of failure in the aircraft’s electrical system will not affect the ECU’s operation. So if, for example, you had any cross-talk in your sensor lines generating false signals, that wouldn’t extend to the ECU. Some customers would like to integrate the ECU into the flight control system but I am concerned that we might run into problems.” The generator that most customers currently order is a 28 V unit adapted from automotive use; it is rugged, as reflected in its considerable weight of 4.5 kg. Using a converted automotive generator is cost-effective, Limbach notes. However, he adds that the company is developing a lighter, 14 V starter/ generator that will replace it, together with the electric starter that many customers opt for. It will also negate the need for a DC-to-DC converter to take the 28 V output down to the 12 V required by the ECU and the fuel system. Performance The L 275 EF weighs only 7 kg, albeit excluding the ECU and its wiring, the fuel pump and its supply line, and the optional electric starter and generator. The engine’s output and fuel consumption are impressive for that weight, even though to some extent it is compromised by the exhaust system preferred by most customers. Using a ‘tuned’ exhaust can improve the power output by as much as 20% at 6000 rpm, and by 15% at the 7500 rpm peak power speed, and also improves fuel efficiency. The drawback is the length of pipe required to properly exploit wave reflection, which is 980 mm compared to 200 mm for the standard system (which likewise provides an individual exhaust for each cylinder). “Usually the benefit of the additional performance is more than offset by the additional packaging requirement of this exhaust system,” notes Limbach. “We are looking into other exhaust designs, but regardless of the design there are always weight and bulk trade-offs to be considered.” Another alternative is a system with 400 mm pipes. In this instance the entrance and exit to each cylinder’s individual unit is at the same end, doubling the effective path length. The upshot is 8% more horsepower in the mid-range and 4% at the top end. But not only is that harder to package than the standard system, it is also very expensive. Using the standard exhaust, maximum power is at least 24 bhp at 7500 rpm. Typically the engine stays below 7000 rpm: normal take-off speed is just under that (for about 23 bhp – see the power curve above) then in regular flight it will normally run at between 4000 and 6000 rpm (with torque in excess of 25 Nm/18 lb-ft throughout that range). Most of the engine’s operation is at a steady rpm level, and throttle response is not an issue, Limbach notes. Specific fuel consumption is in the region of 340 g/hph. Clearly the L 275 EF provides the power, fuel efficiency and proven dependability that make it an excellent choice for aviation propulsion, where its low weight and diminutive size are key factors. February/March 2018 | Unmanned Systems Technology Performance of the Limbach L 275 EF