Why Not Wankels?

By:    Norm Goyer

A Wankel rotary engine makes a dependable power plant for Experimental aircraft and drones.

“The Wankel rotary engine was invented by German engineer Felix Wankel. He began its development in the early 1950s at NSU before completing a working, running prototype in 1957. NSU then licensed the concept to companies around the world, which have continued to improve the design.” Wikipedia

This animation shows the workings of a Wankel engine, simple, light and powerful.

Wankel rotary engines were first used in NSU cars built in Germany during the late 1950s and early 1960s. They are still being used in certain Mazda vehicles and have proven themselves over many years of operation. When they first appeared, they did have a problem with the seals on the rotor wearing out prematurely, or not holding contact with the case, causing slight loss of power. Another problem was overheating. The overall size of the physical engine is very small and that restricts the amount of heat that can be transferred from the engine.  If for some reason a hose from the radiator sprung a leak, the engine temperature would rise immediately. If the engine continued to run, even for a short time, the heat would destroy the engine. Friend Walter Schomburg owned a high performance auto repair facility and maintained cars powered with Wankels including NSUs and Mazdas. Another fact of the inherent design of the Wankel was its need for more fuel than reciprocating engines. This has been reduced by new technology but it is still above average. That is one reason why Mazda installed them in their special purpose sports cars, whose owners normally don’t consider miles per gallon as the ultimate deciding factor for purchase. Keep this point in mind as it also affects their use in aircraft.

This Diamond DA-20 has been retrofitted with a Diamond Wankel engine for testing purposes. There are many homebuilts flying with Wankel power.

Wankel engines have a high power-to-weight ratio and a very small frontal profile. Their design is such that addition power can be achieved by banking several engines together, similar to the multi bank radials of World War II. These huge engines were called “Double Row”. P &
W even manufactured a quadruple engine , the four row Pratt and Whitney Corn cob engines which powered the last of the reciprocating powered airliners. I have always thought that Wankel engines should have been further developed for aviation but other than modified car Mazda rotary engines that are used in a large number of homebuilts, there has been no real movement in that direction. Several large companies including Curtiss Wright and Rolls did build experimental Wankels however.  Why haven’t Wankels been certified? General aviation has always favored air-cooled horizontally opposed engines. But, Wankel engines are very well suited for aircraft use, because they are almost immune to catastrophic failure. A Wankel engine which loses compression, cooling or oil pressure will lose a large amount of power, and will die over a short period of time; however, it will usually continue to produce some power during that time. Piston engines under the same circumstances are prone to seizing or breaking parts that almost certainly results in major internal damage of the engine and an instant loss of power. For this reason, Wankel engines make dependable engines for aircraft and to snowmobiles, which often take users into remote places where a failure could result in frostbite or death.

Pictured is the Mazda Wankel powered racing car which has won several 24 hour Le Mans races. The engine is very powerful and reliable.

Wankel engines also have been popular in homebuilt experimental aircraft, due to a number of factors. 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 at a fraction of the cost of traditional engines. These conversions first took place in the early 1970s. With a large number of these engines mounted in aircraft the NTSB has only seven reports of incidents involving aircraft with Mazda engines up to 2006, and none of these is of a failure due to design or manufacturing flaws. During the same period they have issued 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.

In my opinion, the main reason that these engines are popular with homebuilders is their initial cost, compared to a new Continental or Lycoming the resurrected car engines are cheap, real cheap, and they are an excellent dependable engine. Homebuilders usually don’t fly their aircraft on business or take long family trips, so the added cost for the fuel hungry rotaries is not a determining factor.  Pilots flying certified engines in their aircraft are extremely fuel conscious, they take pride in running their engines, looking for the smallest fuel consumption per hour. They are flying aircraft valued in the hundreds of thousands yet they strive to save a few dollars on gasoline, you figure it out, I have never been able to. (Yes, Mooney owners belong in this group)

Rotary engines make fascinating reading, I suggest you Google Mazda Rotary, Wankel Rotary engine and read the pages and pages of technical data on a great engine concept.

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