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68 Dossier | Woelfle Engineering/Aixro XF40, XH40 and XP40 Fuel consumption can consequently be inferior, but an inherent power density advantage counters that. Indeed, relative to a four-stroke reciprocating engine, the Wankel is well-suited to the UAV sphere, as it is inherently simple, compact, light and better balanced, with less torque fluctuation and vibration. Woelfle Jnr observes that since combustion cycle processes take place simultaneously on all three sides of the triangular rotor the torque output equates to that of a three-cylinder two-stroke. “To visualise that, the kart engine, which now delivers nearly 50 bhp, uses a clutch that was designed for a 15 bhp single- cylinder four-stroke,” he says. “It follows that peak loads on UAV transmissions are much lower.” From the kart engine foundation, Woelfle Engineering developed the Aixro XH40 intended for use in helicopter UAVs and the XF40 and XP40 principally for fixed-wing UAVs. All three have the same design, sharing modifications made to the kart engine to suit the UAV environment, in particular a lower operating speed – 7000 rpm is the redline, and no more than 6000 rpm is used in cruise mode. Power output is correspondingly less, in the interest of dependability. All three models fit within a volume of 320 x 320 x 260 mm, and all weigh around 25 kg when their exhaust, radiator, reduction drive and other ancillaries are taken into account. The differences are in mounting, plus the XH40 and optionally the XP40 have an integrated centrifugal clutch. Hereafter we’ll refer to all three as the ‘Aixro UAV’ engine. The first flight test of an Aixro UAV was made by a customer from the UK in 2006. The first XH40 was developed and flown in 2008 for a South Korean customer who wanted a UAV helicopter for agricultural spraying. Eccentric shaft and rotor “If you want to fit different belt drives or clutches to customise your engine for different vehicles, that can cause problems for a piston engine, but a rotary has one big, solid shaft that runs in big roller and ball bearings and lasts for years without any problems induced by what’s attached on the end,” remarks Woelfle Jnr. The Aixro UAV’s eccentric shaft runs in a rotor needle bearing plus one ball bearing for its axial loading, and two roller bearings for its radial loading, each roller bearing sitting in the respective end plate. There are counterweights running on either side of the main engine structure October/November 2020 | Unmanned Systems Technology The Aixro UAV engine has a one-piece iron rotor The Aixro UAV engine uses a steel eccentric shaft; here are three ready for assembly into engines A rotary has a solid shaft that runs in roller and ball bearings, and lasts for years without problems from what’s attached on the end