In concept, a rotary engine is simple. It is a standard Otto cycle engine, but instead of having an orthodox fixed cylinder block with rotating crankshaft as with the radial engine, the crankshaft remains stationary and the entire cylinder block rotates around it. In the most common form, the crankshaft was fixed solidly to an aircraft frame, and the propeller simply bolted onto the front of the cylinder block.
The effect of rotating a very large mass was an inherent large gyroscopic flywheel effect, smoothing out the power and reducing vibration. Vibration had been such a serious problem on other conventional piston engine designs that heavy flywheels had to be added. Because the cylinders themselves functioned as a flywheel, rotary piston engines typically had a power-to-weight ratio advantage over more conventional engines.
Most rotary engines were arranged with the cylinders pointed outwards from a single crankshaft, in the same general form as a radial, but there were also rotary boxer engines and even one-cylinder rotaries.
History in aircraft
Lawrence Hargrave first developed a rotary engine in 1889 using compressed air, intending for it to be used in powered flight. Weight of materials and lack of quality machining prevented it becoming an effective power unit.
The first effective rotaries were built by Stephen Balzer, who was interested in the design for two main reasons:
- In order to generate 100 hp (75 kW) at the low rpm at which the engines of the day ran, the pulsation resulting from each combustion stroke was quite large. In order to damp out these pulses, engines needed to mount a large flywheel, which added weight. In the rotary design the engine itself doubled as its own flywheel, thus rotaries could be lighter than similarly sized engines of regular design.
- The cylinders had good airflow over them even when the aircraft in which they were mounted were sitting still, which was an important concern given the alloys they had to work with at the time. Balzer's early engines did not even use cooling-fins, a feature of every other air-cooled design, and one that is complex and expensive to manufacture.
Balzer's first designs were ready for use in 1899, at which time they were the most advanced in the world. Other aircraft engines would not catch up in performance for a decade. He then became involved in Langley's Aerodrome attempts, which bankrupted him while he tried to make much larger versions.
The next major advance in the design was Louis and Laurent Seguin's Gnôme series from 1908. This design was developed from a German single-cylinder stationary engine intended for light industrial use, the Gnom, which the brothers were producing under license from Motorenfabrik Oberursel. They essentially took several Gnom cylinders and combined them on a common shaft to produce a seven-cylinder rotary; this, the Gnôme Omega No.1, still exists in the collection of the Smithsonian's National Air and Space Museum. A production version soon reached the aviation market, a 7-cylinder of 50 hp (37 kW), quickly upgraded to 80 hp (60 kW) and eventually 110 hp (80 kW). The engine was at this 80 hp standard when World War I started, as the Gnôme Lambda, and the Gnome quickly found itself being used in a large number of aircraft designs. It was so good that it was licensed by a number of companies, including the German Oberursel firm who designed the original Gnom engine. Oberursel was later purchased by Fokker, whose 80 hp Gnôme Lambda copy was known as the Oberursel U.0. It was not at all uncommon for French Gnômes, as used in the earliest examples of the Bristol Scout biplane, to meet German versions, powering Fokker E.I Eindeckers, in combat, starting during the latter half of 1915.
The Gnôme (and its copies) had a number of features that made it unique, even among the rotaries. Notably, the fuel was mixed and sprayed into the center of the engine through a hollow crankshaft, and then into the cylinders through the piston itself, a single valve on the top of the piston let the mixture in when opened. The valves were counter balanced so that only a small force was needed to open them, and releasing the force closed the valve without any springs. The center of the engine is normally where the oil would be, and the fuel would wash it away. To fix this, the oil was mixed in liberal quantities with the fuel, and the engine spewed smoke due to burning oil. Castor oil was the lubricant of choice, its gum-forming tendency being irrelevant in a total-loss lubrication system. An unfortunate side-effect was that World War I pilots inhaled and swallowed a considerable amount of the oil during flight, leading to persistent diarrhoea. Finally, the Gnôme had no throttle or carburetor. Since the fuel was being sprayed into the spinning engine, the motion alone was enough to mix the fuel fairly well. Of course with no throttle, the engine was either on or off, so something as simple as reducing power for landing required the pilot to cut the ignition. "Blipping" the engine on and off gave the characteristic sputtering sound as though the engine was nearly stalling, though it did not stall as quickly as conventional engines due to its great rotational inertia.
Throughout the early period of the war, the power-to-weight ratio of the rotaries remained ahead of their competition. They were used almost universally in fighter aircraft, while traditional water cooled designs were used on larger aircraft. The engines had a number of disadvantages, notably very high fuel consumption, partially because the engine was always at full throttle, and also because the valve timing was often less than ideal. The rotating mass of the engine also made it, in effect, a large gyroscope, which resulted in tricky handling, with the aircraft turning at right angles to control inputs due to the gyroscopic effect. The Sopwith Camel, for example, was notoriously dangerous. Nevertheless, rotaries maintained their edge through a series of small upgrades, and many newer designs continued to use them.
A few of nine-cylinder rotaries managed to accomplish a partial "throttle" functionality by switching off either three or six cylinders (or other numbers of them), instead of all nine of them, when the "coupe switch" was depressed to cut the spark. It is believed both German and Allied WW I rotaries had this ability, as some surviving documentation regarding the Fokker Eindecker shows a rotary selector switch to cut out a selected number of cylinders on its rotary engine. The Gnôme Monosoupape series is known to have this feature, and long after WW I was demonstrated by a 160 hp Monosoupape-powered reproduction Camel at Old Rhinebeck Aerodrome in flight in the 1990s.
As the war progressed, aircraft designers demanded ever-increasing amounts of power. Inline engines were able to meet this demand by improving their upper rev limits, which meant more power. Improvements in valve timing, ignition systems, and lightweight materials made these higher revs possible, and by the end of the war the average engine had increased from 1,200 RPM to 2,000. However the rotary was not able to use the same "trick," due to the drag of the cylinders through the air as they spun. For instance, if an early-war model of 1,200 RPM increased to only 1,400, the drag on the cylinders increased 36%, as air drag increases with the square of velocity. At lower speeds, drag could simply be ignored, but as speeds increased, the rotary was putting more and more power into spinning the engine, and less into driving the propeller.
One clever attempt to rescue the design was made by Siemens AG. The crankcase (with the propeller still fastened directly to the front of it) and cylinders spun counterclockwise at 900 RPM, as seen externally from a "nose on" viewpoint, while the crankshaft and other internal parts spun clockwise at the same speed. This was achieved by the use of bevel gearing at the rear of the crankcase, resulting in the Siemens-Halske Sh.III, running at 1800 RPM with little net torque. It was also apparently the only rotary engine to use a normal carburetor, able to be controlled by a conventional throttle, just as in an in-line engine. Used on the Siemens-Schuckert D.IV fighter, the new engine created what is considered by many to be the best fighter aircraft design of the war.
One new rotary powered aircraft, Fokker's own D.VIII, was designed at least in part to provide some use for their Oberursel factory's backlog of now-useless 110 hp Ur.II engines, themselves clones of the Le Rhône 9J rotary. By the time the war ended, the rotary engine had become obsolete, and on the whole it disappeared from use quite quickly. The British Royal Air Force probably used rotary engines for longer than most other operators - RAF's standard post-war fighter, the Sopwith Snipe, used the Bentley BR2 rotary, and the standard trainer, the Avro 504K, had a universal mounting to allow several types of low powered rotary, of which there was a large surplus supply. The cheapness of war-surplus engines had to be balanced against their poor fuel economy and the expense of their total loss lubrication system.
By the mid-1920s, rotaries had been more or less completely displaced even in British service, largely by the new generation of air-cooled radials.
Other rotary engines
Besides the configuration described in this article with cylinders moving around a fixed crankshaft, several other very different engine designs can also be described as rotary engines. The most notable pistonless rotary engine, the Wankel rotary engine has also been used in cars (notably by NSU in the Ro80 and by Mazda in a variety of cars such as the RX-series which includes the popular RX-7 and RX-8), as well as in some experimental aviation applications.
Difference between rotary and radial engines
There has been some confusion when comparing rotary engines and radials. When looked at from the outside, both rotaries and radials look strikingly similar.
The difference between these two engines is that Radial engines have pistons that move in a reciprocating fashion that cause the crankshaft to rotate. In rotary engines however, the crankshaft does not rotate. Instead, the cylinders that accommodate the reciprocating pistons will rotate around the crankshaft.
In aviation, planes using radial engines have their propellers connected in one way or another to the crankshaft while the cylinders and crankcase are mounted on the airframe. Planes using rotary engines however, have their propellers connected to the cylinders and crankcase while the "crankshaft" is fixed to the airframe.
An external difference is, radial engine cylinders are usually finned for cooling, whereas rotary engine cylinders are often not finned, as the cooling is sufficient without the extra expense and complexity of construction.
Notes and references
- Hargrave, Lawrence (1850 – 1915). Australian Dictionary of Biography Online.
- Animation of Gnome Rotary in action
- Ray Williams' operable miniature rotary engine website
- A rotary engine that runs solely on compressed air
Lists relating to aviation
|General||Timeline of aviation · Aircraft · Aircraft manufacturers · Aircraft engines · Aircraft engine manufacturers · Airports · Airlines|
|Military||Air forces · Aircraft weapons · Missiles · Unmanned aerial vehicles (UAVs) · Experimental aircraft|
|Notable incidents |
|Military aviation · Airliners · General aviation · Famous aviation-related deaths|
|Records||Flight airspeed record · Flight distance record · Flight altitude record · Flight endurance record · Most produced aircraft|
|This article is licensed under the GNU Free Documentation License.|
It uses material from the Wikipedia article "Rotary engine".