La Rhone


The rotary engine was an early type of internal-combustion engine, in which the crankshaft remained stationary and the entire cylinder block rotated around it. In its most common form, the crankshaft was fixed solidly to the airframe, and the propeller was simply bolted onto the front of the crankcase. There were several advantages to such an arrangement, among them were that the rotaries delivered power very smoothly because there are no reciprocating parts and the relatively large rotating mass of the cylinders tended to smooth out power impulses, reduced vibration, and acted as a flywheel. The elimination of a heavy flywheel gave the rotary-equipped airplanes a substantial power-to-weight ratio advantage. Additionally, when the engine was running, the rotating cylinder block created its own fast-moving airflow, cooling the engine even with the aircraft at rest.

The lubrication system, as with all rotary engines, was a total-loss type in which castor oil was injected into the fuel–air mix with a small pump. Castor oil was used because it could not be easily dissolved into the fuel, and because it possessed lubrication qualities superior to mineral oils of the day. Over two gallons of castor oil were sprayed into the air during each hour of engine operation. This explains why many rotaries were fitted with a roughly 270º perimeter “horseshoe”-shape ring cowl, with the lowermost quarter of the cowl omitted to be open at the bottom. The cowl directed most of the spray of castor oil, along with sparks from the exhaust, away from the flammable aircraft structure. However, unburned castor oil from the engine still tended to be hurled into the pilot’s face, often having an unfortunate laxative effect on the pilot.

In 1918 Le Rhone conducted limited experiments with two-row rotary and even four-row rotary engines. Imagine four rows of seven cylinders each rotating around a single four-throw crankshaft!

Thousands of Le Rhone rotary engines were built by Gnome et Rhone in France and under license around the world, including in the U.S.. The Le Rhone worked well and was a very successful design in spite of its complexity. Rotary engines were used to power Sopwith, Nieuport, Vickers, Bristol, Caudron, Thomas-Morse, Morane-Saulnier and other aircraft. In Germany, a clone of the 110 hp model Le Rhone was developed and manufactured by Oberursel.   

The engine on display is an 80hp Le Rhone built in 1918 for the US Army under license by Union Switch and Signal of Pennsylvania. This engine was procured by Jim Smith as a non-working museum exhibit. However, it was effectively repaired and has been adapted to run on propane for demonstration purposes.