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55 Cobra Aero A33i | Dossier reversal, and that is really important for fatigue life.” The 8 mm nut on the nose secures the pulley that in turn mounts the propeller and drives the generator. The pulley and the timing wheel behind fit over a front section of the crankshaft that is formed as a three-sided drive spline, avoiding the need for a keyway, which in turn can be a stress raiser. “That keyway is where other manufacturers of this type of engine see a lot of breakages,” notes Hilbert. The crank throw is unusual in that it is formed as an integral web to which a second piece is attached. That banana- shaped piece is bolted to the lower part of the web on one side and follows the outline of its base, so that part of the web becomes a little thicker. Hilbert explains, “That is for variable counterweighting – the second piece can be steel or tungsten, or anything else we want to attain a given balance factor. “Changing the balance factor allows us to tune the vibration of the engine according to the specific application. Depending on where and how the engine is mounted, what orientation it has and what the payload is, we can change the balance factor accordingly. For example, if you are using optics you would like the shaking forces to be within the focal plane rather than at a right angle to it.” The additional loading attributable to the twin-plug head created a lot of work to reinforce the bottom end. “The steel caged needle roller big-end bearing is our own design,” says Hilbert. “We have worked very hard to make it durable in the face of the twin-plug loading.” The outer end of the cantilevered crankpin has a thread that could be used for a big-end bearing retention mechanism. Instead, the current specification is to have a spring-loaded retaining ring fitted into a groove near the end of the crankpin. The con rod is crankshaft guided, hence there are thrust washers sandwiching the bottom end. The I-section, one-piece forged steel rod is shot-peened and copper-coated on its outer surfaces. The side surfaces of the bores are ground. Another steel caged needle roller bearing is used at the small end. The superfinished, uncoated pin it accepts runs directly in its piston bores. It is 10 mm in diameter and through-drilled. The circlips are round wire without a retention peg, so are free to rotate in their groove. The uncoated cast aluminium piston is a custom design in a high-silicon hypereutectic alloy. The single piston ring is steel, 1 mm deep, uncoated and of a semi-keystone type. Crankcase The crankcase is machined from billet aluminium. The one piece approach not only makes for a very robust structure compared with the usual bisected item, it also reduces weight and eliminates leak paths. It is machined from 7075 aluminium alloy and is hard anodised all over, aside from the bearing bores which are masked. The main bearings run directly in the crankcase and are of an undisclosed type. “We are working on a new low-friction idea, but we are not sure yet if we are going to put it into production,” Hilbert says. All lubrication is via fuel-oil pre-mix. There is a lip seal (of an undisclosed type) on one side of each bearing with an integral seal on the other side (where there is only the spacer). “So rather than having four lip seal surfaces, which a lot of the other engines of this type have, Unmanned Systems Technology | February/March 2017 The A33’s crankshaft has variable counterweighting via the piece attached to the rear of the web