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

54 compression, the porting, the timing, the specification of the reed valve, the size of the carburettor; everything is based on operating at around 6000 rpm. “Our typical operating range is idle at around 1200 rpm and running to peak power a little over 6000 rpm. For model aerobatics they use the throttle all over the place, it is like a trials bike versus a race bike. At some points during an aerobatic routine the plane will be vertical, hanging off its prop like a helicopter. If the DA150 had kart engine characteristics the plane would soon crash! “I couldn’t go to the two-stroke textbooks, as they are applicable to the likes of Grand Prix bikes and go-karts. A lot of the theory didn’t really apply to what we wanted to do. We are toning the engine down, we wanted good peak power but we had to be careful not to make it cantankerous; we wanted to make it a workhorse. “I use that analogy a lot as some of our engines now are more like a thoroughbred, while some are more of a workhorse, and each have their place. That has carried over into the UAV world where a workhorse is needed, not a high-strung engine.” Why didn’t you go to a rotary valve? “There are pros and cons with both. Mechanically the rotary valve introduces complexity that is significant from the manufacturing and maintenance standpoints, so we shied away from that. The reed is simple and works really well. Also, the dynamic of the flow through the engine changes in operation so maybe you don’t want the fixed timing window of the rotary valve all of the time. “You can have ignition that advances mechanically as you open the throttle, but you don’t necessarily want that. Our electronic ignition has curves that aren’t linear progressions – sometimes the curve will go up and down. I look at rotary versus reed as the same thing. The reed valve will react to the instantaneous demand in a way that the rotary cannot, with its fixed window.” August/September 2017 | Unmanned Systems Technology Specification DA150 EFI UAV engine Horizontally opposed twin 49 mm (1.9291 in) x 40 mm (1.5748 in) = 150.86 cc (9.2 cu in) Naturally aspirated Gasoline fuel All-aluminium structure Linerless Three main bearings, ball Steel crankshaft, two pins Steel con rods Light alloy pistons; one ring Two plugs per cylinder option Reed valve Electronic ignition Fuel injection Engine management system 7.5:1 compression ratio Maximum rpm, 8500 The DA150 has an all-aluminium structure. The crankcase is billet 7075 T6 and is split transversally midway between the rear and centre bearings with a simple gasket between the two halves. The crankcase is held together from the rear by four 6 mm through bolts that pass into the front half of the case and sandwich the rear mounting plate, which supports the rear bearing. Each cylinder has a flange attached to the crankcase by four short 5 mm bolts with sealing by a Garlock gasket. The pressed-together steel crankshaft provides two crankpins, and runs in three bearings. Its front section includes the forward web, which is a counterbalance design. Then there are the two pins, the rear counterbalance web and integral stub plus the centre web as the fifth element. Forged steel con rods are driven by single-ring light alloy pistons operating in a linerless cylinder. There is a single or optional twin plug head. The spark plug is an NGK CM6, having a 10 mm thread and operated by a CDI system. That and the single injector EFI system are under the command of the ECU. HFE supplies the EFI and the air filter that surrounds the intake assembly. The throttle body is billet aluminium. It houses a servo-operated butterfly throttle. The servo is mounted on the throttle body, which also carries the single injector. The throttle body mounting base is a plastic moulded part, which is also the reed valve housing. It attaches directly to the crankcase (into which the reed valve projects just slightly). It and the reed valve attachment flange are sandwiched by 5 mm bolts running from the throttle body into the crankcase. Produced by Slimline Manufacturing, each exhaust header is attached to its cylinder by a pair of 5 mm bolts through its flange, with a gasket. The DA150 is cantilevered with a four-bolt rear plate attachment to the airframe. To strip and rebuild one takes no more than an hour. Some key suppliers to this engine Bore coating: ChemResearch Bearings: FAG Ignition: Desert Aircraft Australia Spark plugs: NGK Fuel injection system: HFE/ HFE-Currawong Exhaust: Slimline Manufacturing Air filter: HFE Oil: Red Line Machine tools: Haas Machine tools: DMG Mori CNC: Helmel Engineering Anatomy