By: Norm Goyer
Immediately after the 2012 Sun ‘n Fun gang packed up their tents and went home, the mail started arriving. In essence, most of the mail concerned a display or forum many had attended at Lakeland and it left a few very confused. Not to worry, this subject has been confusing the aviation community for years; the culprit, rotary and Wankel rotaries vs. radial engines. The US Navy is often quoted as stating, “If it ain’t round, it ain’t sound.” In the Navy’s case they sure won the argument flying huge radial engines, made by Wright and Pratt & Whitney, for years from carriers in the South Pacific. Here is a composite letter that I assembled to better understand the misunderstanding of many pilots.
Hi Norm, I thought that only Wankel was allowed to make rotary engines and that they were the same ones in Mazda cars and English police motorcycles. At Sun ‘n Fun this year one forum I attended talked about rotary engines of World War One. Aren’t these engines early versions of the Wankel design? Is a radial engine also a Wankel design? Sun ‘n Fun Attendee.
Round engine: any engine with a circular configuration including Wankels and turbines but mostly used to describe radial engines.
Radial engine: Engines with a master cylinder which is attached to the crankshaft while the other cylinders and rods are attached to the master connecting rod.
Rotary engine WWI: crankshaft is attached to the firewall and cylinder block with attached propeller rotates
Rotary engine Wankel: the crankshaft is attached to a triangular piston which rotates, sealing off partitions for intake, power and exhaust strokes. Very smooth, but fuel hungry.
Let’s go back to square one, all of those engines are completely different and have zero relationships with each other. A WWI rotary engine is essentially a standard Otto cycle, engine, but instead of having a fixed cylinder block with rotating crankshaft as with a conventional radial engine, the crankshaft remains stationary and the entire cylinder block rotates around it.
Three key factors contributed to the rotary engines success at the time preceding World War I and during the entire war. It was the engine of choice for many English, American, French and German aircraft designers. All of these engines were essentially clones of each other. During the later days of the war, when improved conventional engines, such as the BMWs and Mercedes in some German aircraft were becoming more popular, many designers still opted for the light weight dependable rotary.
The Rotary engine was very smooth running and delivered power very smoothly because there were no reciprocating parts. The huge rotating mass of cylinders acted as a flywheel smoothing out the operation. Many early conventional engines had to have heavy flywheels added to smooth out power impulses and reduce vibration. Rotary engines gained a substantial power-to-weight ratio advantage by having no need for an added flywheel. Here are some little known facts about the rotary design. The engine was actually used in a few cars and motorcycles during the 1890s and its predecessor was a radial engine, which was redesigned as a rotary engine. At one point in its development the rotary engine had as many as 11 cylinders in a single bank. There were also twin row rotary engines which were used in some late-in-the-war aircraft.
The rotary design was discontinued after World War I with the advent of the much improved radial engine with its conventional rotating crankshaft and prop and carburetor which was able to control the rpm of the engine which the rotary could not. A rotary engine, was on or it was off plus the pilot had to treat the engine very carefully as it was possible to stall it, to run fuel into cowl and catch fire and it was difficult to turn against torque. The huge rotating mass of the engine also caused a gyroscopic effect which influenced all types of maneuvers. Ordinary lubricating oils did not work in the rotary. It was necessary to use castor oil, which does not burn away when heated. The rotary engine is a total loss system and the oil in the exhaust contains the castor oil which was in the exhaust gases. Pilots would sometimes wish they were towing a Porta Potty behind their aircraft. But, the rotary engine was the engine of World War I and proved to be reliable, powerful and available. On the negative side, it burned copious amounts of fuel and the castor oil was a full loss system.
The Wankel engine is also classified as a rotary, due to the principal of the rotating cylinder with wipers on the three sides. These wipers contain the mixture, firing and exhaust as it rotates. Wankels are very smooth running and can be banked together to obtain more power. The Norton English Police motorcycles has a two bank engine with about 90 hp. A modification of this engine was produced briefly for the experimental aviation market. I flew one years ago in an amphibian and it was super smooth. OS Motors have been marketing a Wankel in miniature size for model builders and it has been popular for many years. I have also flown a military target drone using an air cooled version of a single bank engine derived from 1/2 of the Norton rotary engine. The engine was too large and too heavy and was a poor choice when compared to the same drone with a Sundstrand turbine. To the many who wrote to me about rotary vs rotary engines I hope my explanation, with the help of Wikipedia, answers your question. A rotary engine such as the Mazda is a fine running unit, it does burn more fuel, but the engine has been proven and drivers seem to have accepted them.