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70 aluminium castings, machined to the necessary tolerances. The end plates are a customised hypereutectic alloy. “The high [unspecified] percentage of silicon makes the alloy more wear- resistant, which means we don’t need to coat the end plates [inner surface],” Woelfle Jnr explains. “If something undesirable should happen – if there’s not enough oil in the engine, leading to some scratching, for example – the end plates can simply be remachined, because there’s no expensive coating there that needs to be redone. Also, coating adhesion is not an issue on the flat side surfaces if there is no coating.” Sealing The Wankel inherently has two key problem areas: sealing and cooling. The sealing issue was fundamentally overcome by the development of a nickel silicon carbide coating for the trochoid surface. That breakthrough was by Mahle (which called it Nikasil) originally for the NSU Wankel car engine of the 1960s, and it was adopted by Norton for its 1970s engines. Various other companies offer comparable coatings nowadays. Woelfle Jnr declines to reveal the supplier of the Aixro UAV’s nickel silicon carbide trochoid surface coating. He does say though, “We go for somewhere around 400-800 microns thick, rather than the 30-50 micron coating you’d see in a two- stroke, because in those cylinders you only have the piston rings going past the coating; on the rotary you have the apex seals moving against it. “You want low friction so that there’s low wear. The other thing is the centrifugal force of the apex seal against the trochoid surface – if that force is too high, the seal will start ‘digging’ in, partly because the aluminium underneath isn’t as hard. So it’s critical that the coating is thick enough and that the seals aren’t too heavy. “Also, you have to find a really reliable coating process and partner, because if it isn’t done right, lack of adhesion will cause it to come off really easily. That’s why everyone in this industry tends to be very secretive about coating details.” As to the technology of the apex seals, Woelfle Jnr says this is another key consideration, since if they fail to effectively seal the three ‘chambers’, exhaust gases might leak past, contaminating the fuel-air mixture and reducing power output. Furthermore, he says, given that they are consistently exposed to combustion events as well as interacting with the trochoid surface and also the end plates, they must have very low friction but must also be very durable. They must also exhibit little or ideally no thermal distortion. To meet these qualities, today’s Aixro UAV engines use ceramic apex seals (the exact ceramic being undisclosed). The seal supplier took the company’s previous generation of iron-based seals and recreated them in ceramic. “It was far from easy to make them, particularly the curved radius we have at the tips of the seals, which have to be clean and consistent to the micron,” Woelfle Jnr reports. “Manufacturing them at the quantity and quality we needed was a steep learning curve. “Single-piece would have been easier to make than the two-piece seals we use, but their tolerances are more demanding in terms of their required width precision. If the seal is too wide, you will lock it against the enclosures, especially if the aluminium housing has been subject to October/November 2020 | Unmanned Systems Technology Aixro UAV engine rotor and seal sets, including apex and side seals, and corner pieces plus attendant springs You have to find a really reliable coating process and partner, otherwise lack of adhesion will cause it come off really easily