Le Rhone 9J
Le Rhône 9J Rotary Aircraft Engine

The Le Rhône was a popular rotary aircraft engine produced in France by Société des Moteurs Le Rhône around 1916. It powered a number of military aircraft of the First World War.

Although not powerful, the largest wartime version gave 130 horsepower (97 kW), they were dependable rotary engines. The Le Rhône 9 was a development from the Le Rhône 7, a seven–cylinder design.

The Le Rhône had a conventional induction system and an unusual method of connecting the link rods to the master rod. In the Le Rhône, the fuel–air mixture went first to an annular chamber at the back of the crankcase and thence, via polished copper pipes, to conventional intake ports and cam–operated valves in the cylinder heads. The copper induction tubes had their crankcase ends located in different places on the 80 and 110 horsepower (60 and 82 kW) versions – the 80 hp versions had them entering the crankcase in a location forward of the vertical centerline of each cylinder, while the (82 kW) version had them located behind the cylinder's centerline. This resulted in the 80 hp version's intake plumbing being "fully visible" from the front, while the version had the lower ends of its intake tubes seemingly "hidden" behind the cylinders.

A complicated slipper bearing system was used in the Le Rhône. Its master rod was a split–type to allow assembly of the connecting rods, and had three concentric grooves to take slipper bearings from all the other cylinders. The remaining rods carried bronze shoes, shaped to fit in the grooves, at their inner ends. Counting the master rod as no. 1, the shoes of nos. 2, 5, and 8 rode in the outer groove, those of 3, 6, and 9 in the middle groove, and 4 and 7 in the innermost one. Although complex, Le Rhônes worked very well.

The Le Rhônes employed an unusual method of valve actuation. A single rocker arm, pivoted near its center, was made to operate both the exhaust valve and the intake valve. Pulled down, it opened the intake valve; pushed up, it opened the exhaust. To do this, the rocker had to be actuated by a push–pull rod instead of by the usual pushrod. This, in turn, meant that the cam followers had to have a positive action, which was accomplished by a system of links and levers. This system works well enough – some makers used it up to the late 'twenties – but its use makes overlap of valve openings impossible. In an engine designed for high– power and speed, the intake valve begins to open before the exhaust valve is quite closed, but on the Le Rhône, the rocker arm must clear the exhaust valve before it can contact that of the intake. While this put a limit on power output, most Le Rhône models produced all the power that their structural strength and cooling arrangements could cope with.

Le Rhône 9J
  • Date of Manufacture: 1915
  • Country: France
  • Type: Rotary
  • Diameter: 960 mm
  • Bore: 112 mm (4.41 in)
  • Stroke: 170 mm (6.69 in)
  • Displacement: 15,074 cc (919.9 cubic inches)
  • Dry weight: 149 kg (330 lb)
  • Components
  • Valvetrain: cam–operated single rocker for both inlet and exhaust
  • Oil system: castor oil, total loss
  • Cooling system: air–cooled
  • Power output: (82 kw)
  • Maximum rpm.: 1,200
  • Aircraft Example:
    • Airco DH.5
    • Avro 504
    • Bristol M.1
    • Bristol Scout
    • Caudron G.3
    • Caudron G.4
    • Hanriot HD.1
    • Macchi M.14
    • Morane–Saulnier N
    • Nieuport 11 "Bebe"
    • Nieuport 17
    • Nieuport 27
    • Sopwith Camel
    • Sopwith Pup
    • Standard E–1
    • Thomas–Morse S–4C
    • Fokker Dr.I
    • Mosca MB 2 bis
  • Oberursel UR.II, a clone of the Le Rhône 9J
  • 80 hp (60 kW) Le Rhône Type 9C
  • (82 kW) Le Rhône Type 9J

Oberursel copy

Oberursel made the (82 kW) model, supposedly without authorization in Germany. The Oberursel Ur.II was a straight copy of the Le Rhône, but the Le Rhône was preferred over the Oberursel due to the superior materials used in the French product. However, by July 1918 there was a shortage in Germany of castor oil, a plant–derived lubricant that the rotaries required as it could not be easily dissolved into the fuel, and because it possessed lubrication qualities superior to mineral oils of the day. A new Voltol–based lubricant, derived from mineral oil, was substituted and was blamed for a rash of engine failures on rotary–engined German fighters such as the Fokker E.V which used the Oberursel Ur.II. It has been suggested that without the proper lubricants, the Le Rhône rotary would have been equally failure prone.


As well as production by Société des Moteurs Gnome et Rhône, which had bought out Société des Moteurs Le Rhône, the Le Rhône was produced in Germany (by Motorenfabrik Oberursel), Austria, the United Kingdom, and Sweden. Le Rhône 80 hp (60 kW) engines were made under license in the United States by Union Switch and Signal of Pennsylvania, and the (82 kW) Oberursel UR.II rotary engine used by Germany in World War I, in such famous fighters such as the Fokker Dr.I triplane, was a close copy of the (82 kW) 9J version.