Editing Rotary engine (section)

==Description==
===Distinction between "rotary" and "radial" engines===
A rotary engine is essentially a standard [[Otto cycle]] engine, with cylinders arranged radially around a central crankshaft just like a conventional [[radial engine]], but instead of having a fixed [[cylinder block]] with rotating [[crankshaft]], the crankshaft remains stationary and the entire cylinder block rotates around it. In the most common form, the crankshaft was fixed solidly to the airframe, and the [[propeller]] was simply bolted to the front of the [[crankcase]].

[[File:Rotary engine - animation slower.gif|thumb|Animation of a seven-cylinder rotary engine with every-other-piston firing order.]]

This difference also has much impact on design (lubrication, ignition, fuel admission, cooling, etc.) and functioning (see below).

The [[Musée de l'Air et de l'Espace]] in Paris has on display a special, "sectioned" working model of an engine with seven radially disposed cylinders. It alternates between rotary and radial modes to demonstrate the difference between the internal motions of the two types of engine.<ref>{{Cite web |url=https://vimeo.com/41546699 |title=Vimeo video of Musee de l'Air "rotary/radial" alternating aviation cross-sectional kinetic model display |access-date=2016-11-07 |archive-date=2019-07-02 |archive-url=https://web.archive.org/web/20190702212318/https://vimeo.com/41546699 |url-status=live }}</ref>

===Arrangement===
Like "fixed" radial engines, rotaries were generally built with an odd number of cylinders (usually 5, 7 or 9), so that a consistent every-other-piston firing order could be maintained, to provide smooth running. Rotary engines with an even number of cylinders were mostly of the "two row" type.

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 engine]]s<ref name = "Barry"/> and even [[single-cylinder engine|one-cylinder]] rotaries.

===Advantages and drawbacks===
Three key factors contributed to the rotary engine's success at the time:<ref>''Air Board Technical Notes'', RAF Air Board, 1917. Reprinted by Camden Miniature Steam Services, 1997</ref>
* Smooth running: Rotaries delivered power very smoothly because (relative to the engine mounting point) there are no reciprocating parts, and the relatively large rotating mass of the crankcase/cylinders (as a unit) acted as a [[flywheel]].
* Improved cooling: when the engine was running, the rotating crankcase/cylinder assembly created its own fast-moving cooling [[Aerodynamics|airflow]], even with the aircraft at rest.
* Weight advantage: rotaries shared with other radial configuration engines the advantage of a small, flat crankcase. The superior air-cooling imparted by the moving engine also meant that cylinders could be made with thinner walls and shallower cooling fins. Their [[power-to-weight ratio]] was further enhanced in comparison with engines that required an added flywheel for smooth running.

Engine designers had always been aware of the many limitations of the rotary engine, so when static style engines became more reliable and gave better specific weights and fuel consumption, the days of the rotary engine were numbered.
* Rotary engines had a fundamentally inefficient [[total-loss oiling system]]. In order to reach the whole engine, the lubricating medium needed to enter the crankcase through the hollow crankshaft; but the centrifugal force of the revolving crankcase was directly opposed to any re-circulation. The only practical solution was for the lubricant to be aspirated with the fuel/air mixture, as in most [[two-stroke engines]]. 
* Power increase also came with mass and size increases,<ref>for instance, compare [[Gnome Monosoupape]] to [[Bentley BR2]]</ref> multiplying [[gyroscopic precession]] from the rotating mass of the engine. Thus aircraft with these engines had stability and control problems, especially for inexperienced pilots. 
* Power output increasingly went into overcoming the air resistance of the spinning engine. 
* Engine controls were tricky (see below), and resulted in fuel waste.

The late [[World War I]] [[Bentley BR2]] was the largest and most powerful rotary engine; it reached a point beyond which this type of engine could not be further developed,<ref>{{cite book |last= Gunston |first= Bill |title=World Encyclopedia of Aero Engines |year=1986 |publisher=Patrick Stephens |location= Wellingborough |pages=22–26}}</ref> and it was the last of its kind to be adopted into RAF service.