Uncrewed Systems Technology 044 l Xer Technolgies X12 and X8 l Lidar sensors l Stan UGV l USVs insight l AUVSI Xponential 2022 l Cobra Aero A99H l Accession Class USV l Connectors I Oceanology International 2022

85 parts pressed together. If a UAV suffers a hard landing or prop strike, that many crank parts could spin out of control.” To reduce the crank part count, the front and rear crank noses are forged with integral crank pins, with the centre crank pin integral to one of the two mid-section ‘dumb-bells’, totalling four pieces pressed together after being forged from high-tension steel, then machined and superfinished. “It meant slightly higher tooling investments going in compared with using COTS pins, but it’s been great for reducing our part count, simplifying assembly and raising engine longevity,” Hilbert comments. “The only problem was the shaft journals for our main bearings – we couldn’t slide a typical off-the-shelf ball bearing onto the two centremost journals, because we forged the dumb- bells as monolithic parts with the ‘pork chop’ counterweight on either end.” The team’s solution here was to use split needle roller bearings that could be pressed around the two middle journals, designing in-house the necessary external races and assembly technique after initially taking guidance from an existing high-speed, high-load roller bearing element. These are four-piece items (the inner element and outer race each splitting into two halves) with a 32 mm outer diameter and a hole in both the top and bottom race halves, the former for receiving oil and the latter having a stepping pin to fasten the race with the crank. “The centre pins are large in diameter, and we wanted them that way, but they’re also extremely stiff as well as hollow and lightweight,” Hilbert notes. “The ball bearings on the ends and the needle bearings at the middle also have the same through-bore, so we line-bore the entire set of cases in one shot, with just one bore diameter.” Vaglienti adds, “We can’t have the shaft hollows open during operation, because that would lead to ‘cross-talk’ between the cylinders, so machined plugs are pressed in there to prevent air moving between the crankcase sections and the cylinders.” As in the A33, the con rods are single pieces, forged from steel before being shot-peened and copper-plated. Hilbert, Hoag and Vaglienti indicate that they have continued working flawlessly since we reported on them in the A33, so no changes have been made. The pistons (still produced by Vertex) are cast from aluminium, with a few minor changes since 2017. Namely, their height has been reduced slightly, some friction reduction has been carried out, and they are now each fitted with a single piston ring with a semi-keystone cross-section, which helps it shed carbon deposits and prevent build-ups (a bigger problem in heavy-fuel engines than with gasoline, and one that directly shortens TBOs if not addressed). “Using just one ring helps reduce friction too, and other aspects of that goal included improving the lip seals on the crankshaft, along with a lot of other little things,” Hilbert says. Unlike the A33, the cylinders are printed as single pieces with their heads to make the design of the liquid cooling easier, reduce weight on the fasteners and eliminate points of failure (particularly regarding seals). Each bonds to the crankcase via four base bolts and a paper gasket; as in the A33, each one is coated with nickel silicon carbide and features two transfer ports largely similar in design to the A33. “We’ve tightened our honing process to improve the coating of the closed cylinder dramatically, along with how we mask off the head,” Hilbert says. “We start with a rough bore and then coat a masking agent on the head before doing our final bore. We’ve also done a lot of CFD work to improve our loop-scavenging, and we’re about to start testing some new transfer ports that implement some of what we’ve learned there.” Liquid cooling As discussed, the use of liquid cooling means system integrators can enclose the A99 inside their aircraft’s aerodynamic profile, improving their UAV’s drag efficiency by not having cylinders and dozens of cooling fins catching the airflow. That also helps reduce noise further by eliminating the ringing effect of many vibrating fins. Uncrewed Systems Technology | June/July 2022 The exhaust manifold is additively manufactured from aluminium, which helps produce curved, complex, aerodynamic geometries to minimise drag and noise from exhaust gases