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74 Dossier | Woelfle Engineering/Aixro XF40, XH40 and XP40 combustion. This basic approach has been adopted by all four UAV engines we have investigated, but with fundamental differences in respect of the approach to internal air cooling. Water-cooling the rotor housing in the Norton manner is straightforward. In the case of the Aixro UAV, a water pump is belt-driven at engine speed by the eccentric shaft. The water gallery is such that the coolant is sent in through one end plate and, having cooled the housing, exits through the other, after which an external radiator cools it. The radiator’s exact size and position are application-specific. Woelfle Jnr explains that it is logical to cool only the ‘hot’ side of the rotor housing, between the spark plug and the exhaust port, where the combustion phase occurs. “You always have the intake on one side and the thermal expansion on the other, so any Wankel that runs cooling water through the intake side just ends up heating that colder side, which does not make much sense unless there are manufacturing reasons for it. “We consider it very important to use water cooling, though. Air-cooling of the housing is less efficient and results in thermally induced structural deformation.” The greater challenge is cooling the rotor and its bearing deep inside the engine. That needle roller bearing is the equivalent of the reciprocating engine’s big end, and as such it inherently sees less mechanical stress. Nevertheless, in a reciprocating engine, the heat is generated at the top end and, distanced from the combustion chamber by the length of the con rod, the big-end bearing runs down at the bottom end, where there is a good supply of cool oil. In the Wankel, the combustion face and the rotor’s needle roller bearing are only millimetres apart. In view of this challenge, we saw that the Austro uses a belt-driven fan to force (ambient) air into a hollow end plate, from which it is sent through the centre of the rotor before travelling up through the other end plate and out through a duct. At that stage the flow is that of air- carrying oil splashed from the bearings, and this is discharged into the exhaust. We saw that Rotron’s twin has two options. For one, incoming charge air is ducted through each rotor before flowing into the respective intake port. Each induction stroke is exploited to pull its charge in through the rotor cooling route. For the other, like the Austro there is a dedicated air-cooling system. In this case though, rather than fan delivery, October/November 2020 | Unmanned Systems Technology This diagram represents the situation with the short cut to the peripheral port a little open. As it is opened from about 70% throttle, this would be around 75%. The white arrow symbols are cold air, grey arrows are warmed air. Not shown, the carburettor sits at the air inlet We consider it important to use water cooling. Air-cooling the housing leads to thermally induced deformation