Rotary Engine - World War I

World War I

The favourable power-to-weight ratio of the rotaries was their greatest advantage. While larger, heavier aircraft relied almost exclusively on conventional in-line engines, many fighter aircraft designers preferred rotaries right up to the end of the war.

Rotaries 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. Oil consumption was also very high. Due to the primitive nature of the carburetion and the absence of a true sump the lubricating oil had to be added to the fuel/air mixture. This meant that the engine fumes were heavy with smoke caused by partially burnt oil. Castor oil was the lubricant of choice, as its lubrictaion properties were not affected by the presence of the fuel, 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. Flying clothing worn by rotary engine pilots was routinely soaked with oil.

The rotating mass of the engine also made it, in effect, a large gyroscope. During level flight the effect was not especially apparent, however when turning the gyroscopic precession became noticeable. Due to the direction of the engine's rotation the left-turns required some degree of effort and happened relatively slowly, combined with a tendency to nose-up, while right-turns were almost instantaneous, with a tendency for the nose to drop. 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 1,000 ft (300 m). Predictably, the Camel's most famous German foe, the Fokker Dr.I triplane, also used a rotary engine, usually the Oberursel Ur.II clone of the French-built Le Rhone 9J 110 hp powerplant.

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 negative effects. This proved too complicated for reliable operation and Redrup changed the design to a static radial engine which was later tried in the experimental Vickers F.B.12b and F.B.16 aircraft, unfortunately without success.

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 rotating cylinders through the air. For instance, if an early-war model of 1,200 rpm increased its revs to only 1,400, the drag on the cylinders increased 36%, as air drag increases with the square of velocity. At lower rpm, drag could simply be ignored, but as the rev count rose, the rotary was putting more and more power into spinning the engine, with less remaining to provide useful thrust through the propeller.

One clever attempt to rescue the design, in a similar manner to Redrup's British "reactionless" engine concept, 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, so the set was effectively running at 1800 rpm. This was achieved by the use of bevel gearing at the rear of the crankcase, resulting in the eleven-cylindered Siemens-Halske Sh.III, with less drag and less net torque. Used on several late war types, notably the Siemens-Schuckert D.IV fighter, the new engine’s low running speed, coupled with large, coarse pitched propellers, gave types powered by it outstanding rates of climb.

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 otherwise redundant 110 hp (82 kW) Ur.II engines, themselves clones of the Le Rhône 9J rotary.

During 1918, the Germans were increasingly unable to obtain supplies of the castor oil necessary to properly lubricate their rotary engines. Substitutes were never entirely satisfactory - causing increased running temperatures and reduced engine life.