Issue 54 Uncrewed Sytems Technology Feb/Mar 2024 uWare uOne UUV l Radio and telemetry l Rheinmetall Canada medevacs l UUVs insight DelltaHawk engine l IMU focus l Skygauge in operation l CES 2024 report l Blueflite l Hypersonic flight

67 competitors make four-strokes that produce high horsepower by spinning fast, with a transmission to convert that into torque for turning a propeller. Instead, we made a two-stroke V4 with the firing pulses and mass air flow (MAF) of a four-stroke V8, and so we redline at 2600 rpm – whereas an equivalent fourstroke V8 would redline around 5200 rpm – but we produce 492 Nm of maximum torque at 2600 rpm. “That’s enough for a large propeller, with speed and power pulses low enough that the propeller doesn’t endure the punishment typical of high compressionratio diesel engines.” The V4 architecture offered more compactness than the boxer configuration more commonly seen across UAV engines, with one key benchmark for the DHK180’s design being the Lycoming IO-360 – a spark-ignited, four-cylinder four-stroke of similar power, which DeltaHawk aimed to be smaller than. “The V4 also makes sense operationally; horizontally opposed engines generate huge vibration from cylinders firing against each other, but a vee’s cylinders don’t do that,” Webb adds. In most tractor-propeller applications, the engine will be mounted as an inverted-vee. This means that highspeed, high-pressure air from the frontmounted propeller backwash can run over the topside of the aircraft for proper aerodynamics, and either the pilot can see ahead without cylinders blocking their view, or in the case of UAVs, the propeller will not hang low within the viewpoint of any payload gimbal carried under the aircraft. For a pusher-propeller integration, either the conventional or inverted-vee can work. Four mounting points sit on the base plate, such that in the invertedvee, they are atop the engine, and on the bottom for the regular vee (the engine can also run vertically for helicopter applications). Several hundred engines’ worth of components have been produced for repeated experiments, testing and iteration – such is DeltaHawk’s satisfaction with the performance and reliability gained by these design properties, and by others to be discussed henceforth. Starting power and fuel The engine starts using a conventional 24 V aviation-style Sky-Tec starter from Hartzell, although a 70 A brushless DC alternator from Plane Power is also integrated as standard (driven by an automotive-type poly vee belt, chosen for long life and reliability, compared with other belts). “The starting method is very traditional: the starter pulses around 200 A from a typical aircraft-type starter battery to get the crankshaft Uncrewed Systems Technology | February/March 2024 The DHK180’s engine block is cast as upper and lower halves from a variation of A356 aluminium alloy, with proprietary additives for high strength and durability for the engine’s high torque and operating pressures, with high-strength studs bonding the halves together. The plenum is integral to the cast block (and the supercharger mounts under the plenum, assuming an inverted-vee installation), as are the cylinders, each of those having a replaceable steel liner inserted, which runs the length of the cylinder. The piston crown is also steel, as is the fireplate that sits inside the cylinder head (hence all combustion-exposed surfaces are steel). The liners are machine-cut with the required lengths and port apertures, and bonded to the block via an interference fit, during which the block is heated and then cools around the liners. The cylinder heads are machined from high-strength aluminium billet, anodised and then bolted to the cylinder atop the fireplates to bond them in place, with seals between the fireplate and liner (each head also features a coolant inlet and outlet, which line up with jackets in the cylinder for water-glycol circulation). The piston bodies are cut from aluminium billet for weight saving (as the reciprocating mass of a totally steel piston would have increased wear and parasitic losses for the engine) and the gudgeon pin is of machined steel. While the steel piston crowns were previously threaded directly on to the aluminium piston bodies, they are now screwed together with custom bolts. The connecting rods are forged from titanium, with a cap then bolted on to each one at the big end, before the U-shaped small end is bolted to the wristpin. The choice of titanium came down to increasing strength and reducing weight compared with other options, particularly in the hunt to remove mass from reciprocating elements, and thereby reduce counterweighting and balancing mass needed at the crankshaft. Most of the crankshaft is forged and subsequently machined to close tolerances as one piece from nickel-chromium-molybdenum steel. The shaft’s four counterweights feature pendulum dampers designed to tune out the engine’s vibration and prevent undesirable frequencies from reaching the propeller; each one moves on pins bonding it to the counterweight, thereby smoothing out the crankshaft’s motion amid torque pulses from the firing order. The camshaft driving the fuel pumps is of machined steel. The turbocharger’s turbine housing is of cast iron, its compressor housing is cut from aluminium, and its compressor impeller is machined 2000-series billet aluminium, while the turbine impeller is cast Inconel. The components are bolted together, with a full-length O-ring for sealing, and then bolted to the plenum. Anatomy