Unmanned Systems Technology 015 | Martin UAV V-Bat | William Sachiti | Sonar Systems | USVs | Desert Aircraft DA150 EFI | SeaCat AUV/ROV | Gimbals

55 Desert Aircraft DA150 EFI | Dossier Why a horizontally opposed twin? “Vibration. Whether it is powering a model airplane or a UAV, it is in a lightweight airframe. In the model world that can be a 30 lb airplane, 13-15 hp and trying to keep that much power within a balsa wood structure means it has to be smooth running. There is no mass in the airplane to dampen it. “If it were an engine in a trials bike or a go-kart then vibration wouldn’t matter so much, whereas we really have to focus on it, and the opposed format really helps by neutralising the combustion forces.” Thus the DA150 has a true boxer configuration with both cylinders at TDC at the same time and firing simultaneously. However, Johnson notes, “The shake that is hard to get away from is the power pulsing, the kickback that rocks the whole airplane the other way on each revolution.” How do you counter that? “You can do it a bit through ignition timing and the control of the combustion event. That depends on compression ratio and octane rating. If you can actually have more of a spread in the pressure wave, less of a sharp impact, that helps. It isn’t, ‘This is rough and this is smooth’; it is more subtle than that but you take what you can get. “We use real-time combustion pressure measurement. On the dyno we run an in-cylinder pressure sensor that on every combustion cycle allows us to map the explosion. We can see where we ‘light the fuse’ relative to TDC and you measure the cylinder pressure off there as a curve. “You can play with the timing and the compression, and that curve might start looking like the Matterhorn and if you back off from there you can get more of a boom rather than a crack – it is like different types of thunder: we want the rumble rather than the crack of thunder! “We can use this testing to try to decipher what the user has by way of vibration in the airplane. From that we can also assess what they have by way of component wear. “This tool really gave us a jump on identifying problems. We have had other engine models subject to failures we could never figure out until we started getting a true picture of what was going on inside the engine,” Johnson says. Gas exchange Naturally aspirated, the DA150 has a joint induction system for both cylinders, the single carburettor or EFI throttle body feeding into the centre of the crankcase. An air filter encloses the entire intake, the throttle body being topped by a base for its housing. The feed of air to the filter depends on the application. Johnson says, “We don’t like to force air, especially not into a carburettor. That form of supercharge of a two-stroke can lead you into problems. We prefer a neutral inflow, especially into a carburettor.” Unmanned Systems Technology | August/September 2017 A DA150 piston pin and small-end bearing We are toning the engine down, we wanted good peak power but had to be careful not to make it cantankerous; we wanted to make it a workhorse