The New Mid West Rotaries

Exhaust Port Path Taken By Tip Of Rotor

The compact Wankel’s advantages include a less fluctuating power output allowing use of a small, light gearbox. With fewer parts than a conventional engine, construction is also simpler and therefore less expensive. Several homebuilders have modified Mazda rotary car engines for their aircraft, while both the Norton motorcycle company and TCM spent time developing the concept to power pilotless drones. The Wankel has just three main components, its casing, rotor and shaft. Only the latter two parts actually move. Machined into the casing is a broad, near oval, figure of eight shaped space (the trochoid). The triangular rotor rotates eccentrically within this, its convex sides forming three revolving chambers between itself and the inside of the trochoid. On one side are spark plugs so that, as the rotating chambers change size and pass the plugs, they perform the normal four-stroke induction, compression, ignition and exhaust cycles. The driveshaft revolves concentrically inside the rotor, turned by its internal planetary gearing at a three to one ratio. Since the rotor forms three revolving chambers, there is one power stroke for each shaft revolution. The rotor’s tips alternately cover and uncover the inlet and exhaust ports, so there is no need for all the power absorbing paraphernalia of camshafts and valve gear. Rotor tips and sidewalls are sealed by cast metal strips, retaining the hot gases like a piston’s rings. Two rotors in parallel double the power from a single shaft.

Gear

Norton engineers plated their combustion chamber and introduced an oil spray into the induction tract to lubricate the tip seals and extend engine life, but still found it hard to optimise cooling at aviation’s normal continuous high powers. Their eventual solution was to duct cooling air through the engine’s core from a belt driven blower at its rear. Injecting an oil mist into this airflow improved lubrication, but the consequent total loss system led to a high consumption and a trailing oil slick. Having acquired the manned flight rights to Norton’s rotaries, Mid-West now buys in just the central casting, rotor and gearbox. They have perfected exhaust and induction systems, drastically reduced oil loss, improved lubrication, and cooling, fitted dual CD ignition, spark plugs and an optional belt driven vacuum pump. The MWAE muffler not only kills noise, but reduces back pressure, preventing gas leakage past the rotors’ sidewalls into the crankcase. They also use an effective oil separator to reduce lubricant loss to a fine mist (just 1% of fuel flow). The resulting warm, dry air can be used for carburettor and cabin heat. MWAE’s 50hp (37kW), 34kg (75lb), single rotor AE50R is certified under JAR 22 for powered gliders and sailplanes. The larger 100hp (75kW) twin rotor AE100R is undergoing UK CAA approved test and certification involving both extended ground running in an instrumented test cell and airborne trials in the company’s ARV Super 2. With 25hp (19kW) more for the same weight as its original engine, this airplane is transformed from a nice handling but slightly gutless flyabout into a real get up and go sportsplane.

The installation is neat and compact, with quick removal, recontoured cowlings incorporating a scoop for the cooling system’s small radiator. Just behind this are the effective twin mufflers, while a black ABS plastic air inlet manifold covers the top. The prop is geared at 2.964 to one, giving a conventional 1000 prop rpm at the engine’s 3000rpm idle. The box has helically cut gears for quietness and a torsional vibration damper, making everything nice and smooth. Run-up is at 5000rpm and, at the max static of 6400, the big, broad chord wooden prop dragged us forward, brakes locked, across the grass. Throttle response is immediate and, with sparkling acceleration, this little black bomb fairly zipped into the sky at 70kt (130kmh), climbing eagerly nose high at 950ft/min all the way up to 3000 feet. The maximum continuous rpm of 6800 gave 98kt (182kmh) at 3500 feet and 10°C OAT. 6000rpm gave a quieter and more economical 84kt (156kmh) cruise. The twin Tillotson diaphragm carburettors have no mixture control, but average fuel consumption is 17 litres per hour at between 80 and 95kt (148 and 176kmh). The engine uses around 0.15 of a litre of oil per hour, so the two litre oil tank permits thirteen hours of flight. Mid-West spent much time developing a fuel injection system which, although slightly heavier, should further improve fuel consumption and get rid of the current need for careful carb balancing.

Throughout the flight this engine was a delight to operate, always responding immediately to every power change, even when these were quite abrupt. Using the whole engine as a heat source, its carburettor heat will provide ice clearing warmth for far longer than conventional exhaust muffler systems which cool down once the engine stops running properly. After our flight the aircraft’s belly was clean and dry, unlike those powered by the original total loss Norton rotaries (and many conventional engines). The AE100R’s projected 1500 hour TBO will of course depend on service experience. When the airplane later departed, I was able to hear for myself the extremely low noise footprint as it flew over at 1000ft. Mid-West should soon make friends with airfield neighbours as well as among users. I also sampled MWAE’s single rotor AE50R in AMF Microflight’s Super Chevvron ultralight. This is a long winged, high efficiency lightweight normally powered by a 33hp (25kW) Konig which has a TBO of only 300 hours, and the aircraft could certainly use more power. The half size AE50R has a 1000 hour TBO, and should be more reliable than the Konig for the same weight. It drives a four bladed propeller through a wide, three to one toothed belt reduction unit. The AE50R engined Chevvron effortlessly out performed its predecessor in every respect. Maximum cruise speed was increased from 65 to 70kt (120 to 130kmh) using just nine litres of fuel per hour, and climb rate nearly doubled.

The ARV’s flight test AE100R now has well over 150 hours of airborne time. Ground based engines (both single and twin rotor) are now well into Mid-West’s 150 hour certification schedule. These tests are aimed at JAR E (general category, public transport) and are conducted at various power settings in continuous 20 hour chunks, including prolonged periods of full throttle operation. Afterwards, both ground and flying versions will be stripped and inspected for wear before further use. The AE100R also powers the South African Hummingbird observation aircraft, the Russian Aviatika 890K family of aerobatic biplane ultralights, the revolutionary pivoting winged Freewing, a Wisconsin Kitfox, a French Twin-Eze, and Dragonflies both in the US and Australia. Slightly modified versions have also been fitted into a Cessna 150, an Italian Elisport Angel helicopter, an Avid Flyer and a Pelican two seat cabin ultralight. It is also planned to go into four big German Dieter Schmidt Kibbitz biplanes, AMF’s Euron, an Australian Skylark and Aviation Scotland’s first Highlander (or Opus 280) kit. Single rotor versions should power 20 Schleicher ASH 26s and a Cessna-like German Remos Gemini two seat ultralight. So far, sales have been split 50:50 between homebuilders and commercial manufacturers. Not surprisingly, Mid-West’s engines are selling faster than they can make them and an order backlog is building up, so they are increasing production to more than the earlier one unit per week. Current orders stand at over 350, with nearly 130 delivered to date. Licensed to build aero engines up to 400hp (300kW), Mid-West also make other two strokes and has long term plans for bigger rotaries, but right now their primary ambition is “To sell lots of engines”. In doing so, they should bring light aviation its much needed powerplant revolution.