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72 Dossier | Woelfle Engineering/Aixro XF40, XH40 and XP40 provides variable spark timing on a pre- programmed curve. “We are aware that some have a preference for dual spark plugs to give a feeling of redundancy, so that is something we can offer,” he says. “But it makes packaging more complex and we don’t have plug issues such as fouling, so a single plug is sufficient.” He adds that installing a second spark plug specifically into the trailing end of the combustion space, rather than having them side by side, could mean better flame propagation and fuel economy. “Mazda had three spark plugs arranged that way in its Le Mans design, but for our commercial-facing design philosophy, we’re not that hard-pressed to get the last 2% of power out of our fuel,” he says. The ignition system includes a small, 70 W alternator. The starter is separate and pulls power from the UAV’s battery. A starter/generator is an optional extra. Exhaust “Our engine’s performance isn’t really sensitive to how the exhaust is designed; it has some impact but it’s minor compared to two-strokes where you have to match resonances and fluid dynamics carefully to keep the system running well,” Woelfle Jnr remarks. “A muffler’s back-pressure has some effect, though less so than on a two-stroke. “The exhaust does get very hot, owing to constant flow through the peripheral exhaust port; this should be taken into account when considering the packaging design. The exhaust’s dimensions are largely a compromise between targets for noise level, weight and cost, as only stainless steel and titanium can be used [owing to the heat]. “Customers tend to use our head pipe from the engine, and our silencer, but they’ll usually opt to make or at least select everything else in between themselves. Every customer has a different aircraft configuration so it’s really important to leave scope for customising.” Lubrication Oil is pre-mixed with the fuel at 2%. The company recommends using only two- stroke oils that are high-performance, fully synthetic, low-smoke varieties, with minimal ash residue and a low flash point. Woelfle Jnr notes that the Aixro’s four-stroke cycle and high exhaust gas temperatures mean the oil is fully burned. “It only smokes a little when cold,” he says. “Some customers have certain oil preferences and transfer them to our rotary, but most of those oils have a much higher flash point and more particles that result in ash, which can leave deposits on the rotor. Using our recommended oil, this is not a problem.” Cooling The Aixro is the fourth UAV Wankel investigated by UST : issue 3 (Summer 2015) featured an Austro single-rotor, issue 4 (Autumn 2015) a Rotron twin- rotor and issue 7 (April/May 2016) an AIE single-rotor. All three had Fichtel & October/November 2020 | Unmanned Systems Technology The Aixro UAV’s ignition control unit sits on the rear of the engine, directly driven by the eccentric shaft; the adjacent water pump is belt-driven