Editing Rotary engine (section)

===World War I===
[[File:Siemens-Halske Sh.III 07.jpg|thumb|right|A [[Siemens-Halske Sh.III]] preserved at the ''[[Technisches Museum Wien]]'' (Vienna Museum of Technology). This engine powered a number of German fighter aircraft types towards the end of 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 primitive carburetion and absence of a true [[sump]], the lubricating oil was added to the fuel/air mixture. This made engine fumes heavy with smoke from partially burnt oil. [[Castor oil]] was the lubricant of choice, as its lubrication properties were unaffected by the presence of the fuel, and its gum-forming tendency was 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]].<ref>{{cite book| author = Arthur Gould Lee| title = Open Cockpit: A Pilot of the Royal Flying Corps| year = 2012| publisher = Grub Street| isbn = 978-1-908117-25-0 }}</ref> 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, but when turning the [[gyroscopic precession]] became noticeable. Due to the direction of the engine's rotation, left turns required 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.<ref name="AEHS">{{cite web |url=http://www.enginehistory.org/Gnome%20Monosoupape.pdf |title=Gnome Monosoupape Type N Rotary |access-date=2008-05-01 |last=McCutcheon |first=Kimble D. |publisher=Aircraft Engine Historical Society |url-status=dead |archive-url=https://web.archive.org/web/20080706041104/http://www.enginehistory.org/Gnome%20Monosoupape.pdf |archive-date=2008-07-06 }}</ref>  In some aircraft, this could be advantageous in situations such as dogfights. 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 the pilot applied full power at the top of a loop at low airspeeds. Trainee Camel pilots were warned to attempt their first hard right turns only at altitudes above {{convert|1000|ft|abbr=on}}.<ref>{{cite book |author2=E. Eugene Larrabee |last=Abzug |first=Malcolm J. | title = Airplane Stability and Control|url=https://archive.org/details/airplanestabilit00abzu |url-access=limited | year = 2002| publisher = Cambridge University Press| isbn = 0-521-80992-4| pages = [https://archive.org/details/airplanestabilit00abzu/page/n30 9] }}</ref> 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&nbsp;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.12]]b and [[Vickers F.B.16|F.B.16]] aircraft,<ref>{{cite book| last = Fairney| first = William| title = The Knife and Fork Man - The Life and Works of Charles Benjamin Redrup| year = 2007| publisher = Diesel Publishing| isbn = 978-0-9554455-0-7 }}</ref> 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&nbsp;rpm to 2,000 rpm. 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&nbsp;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.
[[File:Gegenläufer Umlaufmotor.gif|thumb|right|Animation of the Siemens-Halske Sh.III's internal operation]]

====Siemens-Halske bi-rotary designs====
One clever attempt to rescue the design, in a similar manner to Redrup's British "reactionless" engine concept, was made by [[Siemens]]. The crankcase (with the propeller still fastened directly to the front of it) and cylinders spun counterclockwise at 900&nbsp;rpm, as seen externally from a "nose on" viewpoint, while the crankshaft (which unlike other designs, never "emerged" from the crankcase) and other internal parts spun clockwise at the same speed, so the set was effectively running at 1800&nbsp;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.<ref name="Gray_Profile">{{cite book |last=Gray |first=Peter L. |title=Aircraft in Profile No.86 — The Siemens Schuckert D.III & IV |year=1966 |publisher=Profile Publications |location=Leatherhead, Surrey, England }}</ref>{{rp|4–5}} 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 that sometimes had four blades (as the SSW D.IV used), gave types powered by it outstanding rates of climb, with some examples of the late production Sh.IIIa powerplant even said to be delivering as much as 240 hp.<ref name="Gray_Profile" />{{rp|12}}

One new rotary powered aircraft, Fokker's own [[Fokker D.VIII|D.VIII]], was designed at least in part to provide some use for the Oberursel factory's backlog of otherwise redundant {{convert|110|hp|abbr=on}} [[Oberursel Ur.II|Ur.II]] engines, themselves clones of the [[Le Rhône 9J]] rotary.

Because of the Allied blockade of shipping, the Germans were increasingly unable to obtain the castor oil necessary to properly lubricate their rotary engines. Substitutes were never entirely satisfactory - causing increased running temperatures and reduced engine life.<ref>{{cite book| last = Guilmartin| first = John F. Jr.| title = Two Historians in Technology and War| year = 1994| publisher = [[United States Army War College]], [[Strategic Studies Institute]]| isbn = 1428915222| page = 10| chapter = Technology and Strategy: What Are the Limits? }}</ref><ref>{{cite book| last = Fisher| first = Suzanne Hayes| title = The European Powers in the First World War: An Encyclopedia| year = 1999| publisher = [[Taylor & Francis]]| isbn = 081533351X| page = 10| chapter = Aircraft, production during the war| editor = Spencer C. Tucker |editor2=Laura Matysek Wood |editor3=Justin D. Murphy }}</ref><ref>{{cite book |title=Tariff Information Surveys on the Articles in Paragraphs 44 and 45 of the Tariff Act of 1913 |year=1921 |author=[[United States International Trade Commission]]|page=40 |publisher=[[United States Government Publishing Office]]|location=Washington, D.C. }}</ref>