Wengkel Engine

Another promising design for aircraft use was the Wankel rotary engine. The Wankel engine is about one half the weight and size of a traditional four stroke cycle piston engine of equal power output, and much lower in complexity. In an aircraft application, the power to weight ratio is very important, making the Wankel engine a good choice. Because the engine is typically constructed with an aluminium housing and a steel rotor, and aluminium expands more than steel when heated, unlike a piston engine, a Wankel engine will not seize when overheated. This is an important safety factor for aeronautical use. Considerable development of these designs started after World War II, but at the time the aircraft industry favored the use of turbine engines. It was believed that turbojet or turboprop engines could power all aircraft, from the largest to smallest designs. The Wankel engine did not find many applications in aircraft, but was used by Mazda in a popular line of sports cars. Recently, the Wankel engine has been developed for use in motor gliders where the small size, light weight, and low vibration are especially important.

Wankel engines are becoming increasingly popular in homebuilt experimental aircraft, due to a number of factors. Most are Mazda 12A and 13B engines, removed from automobiles and 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 engines. These conversions first took place in the early 1970s, and with hundreds or even thousands 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 is of a failure due to design or manufacturing flaws. During the same time frame, they have reports of several thousand reports of broken crankshafts and connecting rods, failed pistons and incidents caused by other components which are not found in the Wankel engines. Rotary engine enthusiasts refer to piston aircraft engines as "Reciprosaurs," and point out that their designs are essentially unchanged since the 1930s, with only minor differences in manufacturing processes and variation in engine displacement.

Peter Garrison, contributing editor for Flying magazine, has said that "the most promising engine for aviation use is the Mazda rotary." Garrison lost an airplane which he had designed and built (and missed death literally by inches), when a piston-powered plane had engine failure and crashed into Garrison's plane, which was waiting to take off.