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77 Woelfle Engineering/Aixro XF40, XH40 and XP40 | Dossier the cost and logistical benefits, and concedes, “While with a different cooling approach more power could be attainable, that would require additional components and add weight. In terms of power-to-weight ratio I believe we are on the same level.” Time between overhauls In terms of engine life, Woelfle Jnr observes, “We use the same components in our higher-rpm kart engines, so mechanically we have a lot of safety. However, despite the robustness of the engine it is important for us to tell customers that this is an engine, not a computer. So if something has been out of range, it is necessary to check the engine; you can’t just restart it and keep going. “For engine monitoring, the customer should have a bearing temperature sensor and a coolant temperature sensor. Engine speed and throttle position should preferably also be monitored to get a good understanding of what the engine has done in flight. “What we call the ‘bearing temperature’ sensor is sensing the temperature of the inlet charge after passing through the rotor and before the side intake port. That temperature reading is a good indicator of load on the engine and also engine condition. If the rotor bearing gets too hot, for example because the customer has forgotten to put oil in the fuel, it will begin to wear. “The good thing about the rotary is that you still have some time to react in case the temperature does get out of range. Even with an overheated bearing, the engine will not just seize, it will lose a little power and keep going, so there would be sufficient power to land safely.” Woelfle Jnr argues that the integrity of the Aixro UAV engine has been well-proven, given that “some have flown for over 1000 hours. Others are used much less, as some customers just fly a couple of hours a year. Maybe on average we have 150 hours per engine, so we would be approaching around 700,000 hours of use.” On top of that, Woelfle Jnr says that around 5000 examples of the engine have now been deployed – a substantial number justifying the 700,000-hour claim. “Many of our UAV engines go to a customer in the UK who buys engines in batches of 50 units,” he reveals, adding that all three variants have been used by a good number of other customers, mainly based in the Far East. “The TBO depends very much on how an engine is used,” he says. “Generally, we recommend having it checked every 200 hours; we also encourage customers to check their engine every two years just to make sure all is well.” A complete strip, check and rebuild of the housing and rotating parts can be accomplished by experienced personnel in 3-5 hours. “A decent check typically includes replacing the water pump belt and seals, the inlet manifold rubber and possibly the main needle bearing, shaft gaskets and spark plug. After 400 hours the end plate bearings and some rotor seal springs would sometimes need replacing. “To be honest, we don’t know the ultimate lifetime of the main rotary components. Mechanically, they undergo very little stress and are robust by design, so unless the engine has been operated outside the recommended parameters, we don’t ever see them needing replacement.” Unmanned Systems Technology | October/November 2020 “There have been many discussions about how Wankel capacity equates to piston engines,” Woelfle Jnr says. He notes that Karl Ludvigsen’s 1975 presentation on Wankel engine displacement (and the paper he wrote as a follow-up to enable further study) still provides perhaps the best visualisation to date of the similarities between Wankel chamber displacement and three-cylinder reciprocating engine displacement. In that paper, Ludvigsen uses line charts plotting displacement (measured in cc) against shaft angle, as well as side-by-side drawings of Wankel and piston engine chambers with their shafts moving analogously with one another, to show that in both cases, displacement is defined by “the maximum volume variation of all the pumping organs of a given engine”. Therefore, just as a reciprocating engine defines its displacement by the maximum volume between a piston’s top dead centre (TDC) and its bottom dead centre during each crankshaft revolution, a Wankel’s displacement is similarly defined by the maximum volume between each rotor face and the inner housing wall during each eccentric shaft rotation. Woelfle Jnr notes accordingly, “Effectively, the displacement is similar to that of a two-stroke piston engine, with 294 cc being displaced per output shaft rotation.” Wankel displacement The good thing about the rotary is that you still have some time to react if the temperature gets out of range; the engine will not just seize