page_title: Wankel engine
section_title: Aircraft engines
hierarchical_section_title: Wankel engine / Applications / Aircraft engines
caption_attribution_description: Cypher UAV(Unmanned Aerial Vehicle), In the late 1980s, Sikorsky Aircraft flew a small UAV named "Cypher", with coaxial rotors inside a torus-shaped airframe. The torus shroud improved handling safety and helped increase lift. The first proof-of-concept Cypher was 1.75 meters (5.75 feet) in diameter and 55 centimeters (1.8 feet) tall, weighed 20 kilograms (43 pounds), and was first flown in the summer of 1988. This design was powered by a four-stroke, 2.85 kW (3.8 hp) engine and was mounted on a truck for forward-flight tests. It led to a true flight prototype Cypher that weighed 110 kilograms (240 pounds), had a diameter of 1.9 meters (6.2 feet) and was powered by a compact, 40 kW (53 hp) rotary engine. After an initial free flight in 1993, the Cypher prototype was used in flight tests and demonstrations through most of the 1990s, ultimately leading to a next-generation design, the Cypher II, which was a competitor in the United States Navy VT-UAV competition. The single prototype first flew in April 1992 and flew untethered in 1993. Since then, over 550 demonstration flights have been made for the US government. The Cypher can carry a sensor package on struts above its hull, or can transport loads weighing up to 50 lb (23 kg).
context_page_description: The Wankel engine is a type of internal combustion engine using an eccentric rotary design to convert pressure into rotating motion.
  Compared to the reciprocating piston engine, the Wankel engine has more uniform torque and less vibration and, for a given power, is more compact and weighs less.
  The rotor, which creates the turning motion, is similar in shape to a Reuleaux triangle, except the sides have less curvature. Wankel engines deliver three power pulses per revolution of the rotor using the Otto cycle. However, the output shaft uses toothed gearing to turn three times faster giving one power pulse per revolution. This can be seen in the animation below. In one revolution, the rotor experiences power pulses and exhausts gas simultaneously, while the four stages of the Otto cycle occur at separate times. For comparison, in a two-stroke piston engine there is one power pulse for each crankshaft revolution and, in a four-stroke piston engine, one power pulse for every two revolutions.
context_section_description: In principle, Wankel engines are ideal for light aircraft, being light, compact, almost vibrationless, and with a high power-to-weight ratio. Further aviation benefits of a Wankel engine include:
  Rotors cannot seize, since rotor casings expand greater than rotors;
  The engine is less prone to the serious condition known as "engine-knock", which can destroy a plane's piston engines in mid-flight.
  The engine is not susceptible to "shock-cooling" during descent;
  The engine does not require an enriched mixture for cooling at high power;
  Having no reciprocating parts, there is less vulnerability to damage when the engine revolves at a higher rate than the designed maximum. The limit to the revolutions is the strength of the main bearings.
  Unlike cars and motorcycles, a Wankel aero-engine will be sufficiently warm before full power is asked of it because of the time taken for pre-flight checks. A Wankel aero-engine spends most of its operational time at high power outputs, with little idling. This makes ideal the use of peripheral ports. An advantage is that modular engines with more than two rotors are feasible. If icing of any intake tracts is an issue, there is plenty of waste engine heat available to prevent icing.
  The first Wankel rotary-engine aircraft was in the late 1960s being the experimental Lockheed Q-Star civilian version of the United States Army's reconnaissance QT-2, essentially a powered Schweizer sailplane. The plane was powered by a 185 hp (138 kW) Curtiss-Wright RC2-60 Wankel rotary engine. The same engine model was also used in a Cessna Cardinal and a helicopter, as well as other airplanes. In Germany in the mid-1970s, a pusher ducted fan airplane powered by a modified NSU multi-rotor Wankel engine was developed in both civilian and military versions, Fanliner and Fantrainer.
  At roughly the same time as the first experiments with full-scale aircraft powered with Wankel engines, model aircraft-sized versions were pioneered by a combine of the well-known Japanese O.S. Engines firm and the then-extant German Graupner aeromodeling products firm, under license from NSU/Auto-Union. By 1968, the first prototype air-cooled, single-rotor glow plug-ignition, methanol-fueled 4.9 cm³ displacement OS/Graupner model Wankel engine was running, and was produced in at least two differing versions from 1970 to the present day, solely by the O.S. firm after Graupner's demise in 2012.
  Aircraft Wankel engines are increasingly being found in roles where the compact size, high power-to-weight ratio and quiet operation are important, notably in drones and unmanned aerial vehicles. Many companies and hobbyists adapt Mazda rotary engines, taken from cars, to aircraft use. Others, including Wankel GmbH itself, manufacture Wankel rotary engines dedicated for that purpose. One such use is the "Rotapower" engines in the Moller Skycar M400. Another example of purpose-built aircraft rotaries are Austro Engine's 55 hp (41 kW) AE50R (certified) and 75 hp (56 kW) AE75R (under development) both appr. 2 hp/kg.
  Wankel engines are also becoming increasingly popular in homebuilt experimental aircraft, such as the ARV Super2 which can be re-engined with the British MidWest AE series aero-engine. Most are Mazda 12A and 13B automobile engines, converted to aviation use. This is a very cost-effective alternative to certified aircraft engines, providing engines ranging from 100 to 300 horsepower (220 kW) at a fraction of the cost of traditional piston engines. These conversions were initially in the early 1970s. With a number of these engines mounted on aircraft, as of 10 December 2006 the National Transportation Safety Board has only seven reports of incidents involving aircraft with Mazda engines, and none of these were a failure due to design or manufacturing flaws.
  Peter Garrison, contributing editor for Flying magazine, has said that "in my opinion ... the most promising engine for aviation use is the Mazda rotary." Mazda rotaries have worked well when converted for use in homebuilt aircraft. However, the real challenge in aviation is to produce 
caption: Sikorsky Cypher Unmanned aerial vehicle (UAV) powered with a UEL AR801 Wankel engine
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