The rotary engine was an early type of internal combustion aircraft engine in which the crankshaft remains stationary and the entire cylinder block revolves around it. The design was used mostly in the years shortly before and during World War I to power aircraft, and also saw use in a few early motorcycles and cars.
By the early 1920s the rotary aircraft engine was becoming obsolete, mainly because of an upper ceiling to its possible output torque, which was a fundamental consequence of the way the engine worked. It was also limited by its inherent restriction on breathing capacity due to the need for the fuel/air mixture to be aspirated through the hollow crankshaft and crankcase, which directly affected its volumetric efficiency. However, at the time it was a very efficient solution to the problems of power output, weight, and reliability.
A rotary engine is a standard Otto cycle engine, but instead of having a 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 the bulk of the engine's mass was an inherent large gyroscopic flywheel effect, which smoothed out the power delivery and reduced vibration. Vibration had been such a serious problem on 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 pointing 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.
Like radial engines, rotaries were generally built with an odd number of cylinders (usually either 7 or 9), so that a consistent every-other-piston firing order could be maintained, to provide smooth running.
Due to the Gnôme's large inertia, it was possible to adjust the appropriate fuel/air mixture by trial and error without stalling it. After starting the engine with a known setting that allowed it to idle, the air valve was opened until maximum engine speed was obtained. Since the reverse process was difficult, "throttling" was accomplished by temporarily cutting the ignition using the blip switch.
By the middle stages of World War I some throttling capability was found necessary to allow pilots to fly in formation, and the improved carburettors which entered use allowed a power reduction of up to 25%. The pilot would close off the air valve to the required position, then re-adjust the fuel/air mixture to suit. Experienced pilots would gently back off the fuel lever at frequent intervals to make sure that the mixture was not too rich: a too-lean mixture was preferable, since power recovery would be instant when the fuel supply was increased, whereas a too-rich mixture could take up to 7 seconds to recover and could also cause fouling of spark plugs and the cylinders to cut out.
The Gnôme Monosoupape was an exception to this, since most of its air supply was taken in through the exhaust valve, and so could not be controlled via the crankcase intake. Monosoupapes therefore had a single petrol regulating control used for a limited degree of speed regulation. Early models also featured variable valve timing to give greater control but this was abandoned due to causing burning of the valves.
Later rotaries still used blipping the ignition for landing, and some engines were equipped with a switch that cut out only some rather than all of the cylinders to ensure that the engine kept running and did not oil up. A few 9 cylinder rotaries had this capability, typically allowing 1, 3, or 6 cylinders to be kept running. Some 9 cylinder Monosoupapes had a selector switch which allowed the pilot to cut out six cylinders so that each cylinder fired only once per three engine revolutions but the engine remained in perfect balance. Some documentation regarding the Fokker Eindecker shows a rotary selector switch to cut out a selected number of cylinders suggesting that German rotaries did as well.
By 1918 a Clerget handbook advised that all necessary control was to be effected using the throttle, and the engine was to be stopped and started by turning the fuel on and off. Pilots were advised to avoid use of the cut out switch as it would eventually damage the engine.
The blip switch is, however, still recommended for use during landing rotary-engined aircraft in modern times as it allows pilots a more reliable, quick source of power that lends itself to modern airfields. The landing procedure using a blip switch involved shutting off the fuel using the fuel lever, while leaving the blip switch on. The windmilling propeller allowed the engine to continue to spin without delivering any power as the aircraft descended. It was important to leave the blip switch on while the fuel was shut off to allow the spark plugs to continue to spark and keep them from oiling up, while the engine could easily be restarted simply by re-opening the fuel valve. If a pilot shut the engine off by holding the blip switch down without cutting off the fuel, fuel would continue to pass through the engine without combusting and raw fuel/air mix would collect in the cowling. This could cause a serious fire when the switch was released, or alternatively could cause the spark plugs to oil up and prevent the engine restarting.
Felix Millet showed a 5 cylinder rotary engine built into a bicycle wheel at the Exposition Universelle in Paris in 1889. Millet had patented the engine in 1888, so must be considered the pioneer of the internal combustion rotary engine. A machine powered by his engine took part in the Paris-Bordeaux-Paris race of 1895 and the system was put into production by Darracq in 1900.
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.
Stephen Balzer of New York, a former watchmaker, constructed rotary engines in the 1890s. He was interested in the rotary layout for two main reasons:
Balzer produced a 3 cylinder, rotary engined car in 1894, then later became involved in Langley's Aerodrome attempts, which bankrupted him while he tried to make much larger versions of his engines. Balzer's rotary engines were later converted to static radial operation by Langley's assistant, Charles Manly.
The famous De Dion-Bouton company produced an experimental 4 cylinder rotary engine in 1899. Though intended for aviation use, it was not fitted to any aircraft.
The Adams-Farwell was another early US rotary engine which was being manufactured for use in automobiles by 1901. Emil Berliner sponsored its development as a lightweight power unit for his unsuccessful helicopter experiments. Adams-Farwell engines later powered fixed-wing aircraft in the US after 1910. It has also been asserted that the Gnôme design was derived from the Adams-Farwell, since an Adams-Farwell car is reported to have been demonstrated to the French Army in 1904. In contrast to the later Gnôme engines, the Adams-Farwell rotaries had conventional exhaust and inlet valves mounted in the cylinder heads.
The Gnôme engine was the work of the three Seguin brothers, Louis, Laurent, and Augustin. They were gifted engineers and the grandsons of famous French engineer Marc Seguin. In 1906 the eldest brother, Louis, had formed the Société des Moteurs Gnôme to build stationary engines for industrial use, having licenced production of the Gnom single-cylinder stationary engine from Motorenfabrik Oberursel.
Louis was joined by his brother Laurent who designed a rotary engine specifically for aircraft use, using Gnom engine cylinders. The brothers' first experimental engine was a 5 cylinder model which developed , which was a radial rather than a rotary. They then turned to rotary engines in the interests of better cooling, and the first production engine, the 7 cylinder, "Omega" was shown at the 1908 Paris automobile show. The Gnôme Omega No.1 still exists and is in the collection of the Smithsonian's National Air and Space Museum. The Seguins used the highest strength material available - recently developed nickel steel alloy - and kept the weight down by machining components from solid metal; the cylinder wall of a 50 hp Gnôme was only 1.5 mm thick, while the connecting rods were milled with deep central channels to reduce weight. While somewhat low powered in terms of horsepower per litre, its power to weight ratio was an outstanding per kg.
The following year, 1909, the inventor Roger Ravaud fitted one to his Aéroscaphe, a combination hydrofoil/aircraft, which he entered in the motor boat and aviation contests at Monaco. However, it was Henry Farman's use of the Gnôme at the famous Rheims aircraft meet that year which brought it to prominence, when he won the Grand Prix for the greatest non-stop distance flown - - and also created a world record for endurance flight.
Production of the Gnôme rotaries increased rapidly, with some 4,000 being produced before World War I, and the Omega's power output was increased to , and eventually to . By the standards of other engines of the period, the Gnôme was considered not particularly temperamental, and was considered reliable, being credited as the first engine able to run for ten hours between overhauls.
In 1913 the Seguin brothers introduced the new Monosoupape ("single valve") series, which eliminated the cylinder inlet valves, and had a single exhaust valve in each cylinder head which doubled as an air intake. Each cylinder had transfer ports of the type used on two-stroke engines at its bottom which connected with the crankcase. The engine speed was controlled by varying the opening time and extent of the exhaust valves using levers acting on the valve tappet rollers, a system which was later abandoned due to causing burning of the valves. The weight of the Monosoupape was slightly less than the earlier two-valve engines and it used less lubricating oil. The 100 hp Monosoupape was built with 9 cylinders, and developed its rated power at 1,200 rpm.
Rotary engines produced by the Clerget and Le Rhône companies used conventional pushrod-operated valves in the cylinder head, but used the same principle of drawing the fuel mixture through the crankshaft.
The 80 hp (60 kW) Gnôme was the standard at the outbreak of World War I, as the Gnôme Lambda, and it 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 Motorenfabrik 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, from the latter half of 1915 on.
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 typically run 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. During level flight the effect wasn't especially apparent, however under turning it was far more pronounced. Due to the direction of the force right-turns required some degree of effort and happened slowly, while left-turns were almost instantaneous. In some aircraft this could be advantageous in situations such as dogfights while the Sopwith Camel suffered to such an extent that it required left rudder for both left and right turns and could be extremely hazardous if full power was used over the top of a loop at low airspeeds. Trainee Camel pilots were warned to attempt their first hard right turns only at altitudes above .
Even before the First World War attempts were made to overcome the inertia problem of rotary engines. As early as 1906 Charles Benjamin Redrup had demonstrated to the Royal Flying Corps at Hendon a 'Reactionless' engine in which the crankshaft rotated in one direction and the cylinder block in the opposite direction, each one driving a propeller. A later development of this was the 1914 reactionless 'Hart' engine designed by Redrup in which there was only one propeller connected to the crankshaft, but it rotated in the opposite direction to the cylinder block, thereby largely cancelling out rotational inertia. This proved too complicated for the Air Ministry and Redrup changed the design to a static radial engine which later flew in Vickers FB12b and FB16 aircraft.
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. The rotary was not able to do the same due to the drag of the cylinders through the air. 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, which could 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 the Oberursel factory's backlog of now-useless Ur.II engines, themselves clones of the Le Rhône 9J rotary.
By the time the war ended, the rotary engine had become obsolete, and it disappeared from use quite quickly. The British Royal Air Force probably used rotary engines for longer than most other operators - the 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 the use of several different types of low powered rotary, of which there was a large surplus supply. However, the cheapness of war-surplus engines had to be balanced against their poor fuel economy and the operating 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.
Although rotary engines were mostly used in aircraft, there were also a few cars and motorcycles with rotary engines. The most famous motorcycle (probably because of winning many races) is the Megola, which had a rotary engine inside the front wheel. Another motorcycle with a rotary engine was Charles Redrup's 1912 Redrup Radial, which was a three-cylinder 303cc rotary engine fitted to a number of motor-cycles by Redrup.
In 1904, the Barry engine, also designed by Charles Redrup was built in Wales, a rotating 2 cylinder boxer engine inside a motorcycle frame, weighing 6.5 kg. In the 1940s Cyril Pullin developed the Powerwheel, a wheel with a rotating one-cylinder engine, clutch and drum brake inside the hub, but it never entered serial production.
Besides the configuration described in this article with cylinders moving around a fixed crankshaft, several other very different engine designs are also called 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.