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Slide show

Le Rhône 9C Gallery

Le Rhóne 9C

Engine Animations
Timing Gears Animation
Ignition Animation
Clerget Assembly Movie

Clerget Assembly Movie

Le Rhóne  9C Specifications


9-cylinder air-cooled rotary engine.


105 mm (4.13 in)


140 mm (5.51 in)


10.98 L (664.47 cu in)

Dry weight:

121.5 kg (268 lb)

Fuel system:

Cooling system:


Ignition system:


Power output:

80 hp (60 kW) at 1,300 rpm (maximum)

Compression ratio:


The Le Rhóne 9C
The manufacturer - Société des Moteurs Le Rhóne / Gnome et Rhóne - began manufacturing air cooled rotary engines in 1910.
Their rotary engine was sufficiently different than the first developed engines by the successful Seguin brothers of the Société des Moteurs Gnome, Paris. Their engine design circumvented the relevant Gnome patents, but they were taken over by Gnome in 1914.

Between 1914 and 1918, Société des Moteurs Le Rhóne produced 25,000 of their 9-cylinder Delta and Le Rhóne 110 hp (81 kW) rotary engines, while another 75,000 were produced by various licensees.
These engines powered the majority of aircraft in the first half of WW1, both Allied designs as well as German examples produced by Motorenfabrik Oberursel.

The Gnome et Rhóne company mainly produced aircraft engines, see Gnome et Rhóne aircraft engines.

There were several French built versions of the Le Rhóne (including the 9J, 9Ja, 9Jb, 9R) with outputs ranging from around 125-145 hp, although only the 9C and British licensed built versions of the 9J by W.H. Allen Son & Company of Bedford were built for the Admiralty.

Their successful rotary engine designs were additionally built in Germany, Sweden, Britain, the United States and Russia, to increase the output in the times of World War I. The 9-series was the primary engine for most early-war designs both in French and British service as well as in Germany where, perhaps somewhat ironically, Oberursel had taken out a license just before the war.

The World War I rotary engines had a unique operating characteristic in which the engine crankcase and cylinders would rotate, while the crankshaft was stationary. Making it self-cooling and eliminating the need for a flywheel simplifies the engine.
The direct radiation of heat from cylinders to the air does away with pumps, pipes, water jackets and the often troublesome radiator. Rotary engines are less sensitive to carburetor adjustments or changes in atmospheric pressure. Rifle and machine-gun bullets are deflected by the revolving engine, which makes a good shield for the pilot when flying head-on at the enemy.

However, the induced torque effect into the airframe also created a limitation for the rotary engines as aircraft grew in size requiring rpm above 1,400 and ever greater horsepower.

The gyroscopic action of such a large metal mass spinning at the front of a fairly light wood and fabric airframe must have been extremely powerful, especially in turns. However, due to the compact construction, the engine weight is concentrated near the center of gravity. The propeller produces a large part of the total gyroscopic effect

Some early engines had no throttle (the engines ran at full throttle, with the ignition being "blipped" to reduce power when necessary).

The Le Rhóne was fitted with the Tampier Bloc-tube carburetor and throttle-mixture control levers, but reducing power when landing involved simultaneously adjusting the throttle and mixture controls which was not straightforward. It became a common practice during landing to "blip" the engine as well.

The Rhóne engine used an unconventional valve actuation system. It had two sperate cam gears with cam followers and push-pull rod, which connected to a rocker and actuated both the inlet- and exhaust valves. To make this design work a two-way push-pull rod was fitted, instead of the more conventional one-way push rod.

This special concept required the cam followers to incorporate a positive action, a function designed in by using a combination of rocker arms and levers. This design prevented any inlet- or exhaust valve overlap and so somewhat limited power output.

To understand the valve action of the Le Rhóne engine it is necessary to keep in mind that the valve rod is constantly pulled outward by centrifugal force and that the cam movement - relative to the cylinders - is what actuates the valves.

A known feature of Rotary engines is its use of Castor oil. The reason is that the fuel and oil mixed together in the crankcase would not dissolve the oil and ruin its lubricating qualities. The perfect choice was pharmaceutical-quality castor oil because it would stand the heat and its gum-forming tendency were irrelevant in a total-loss lubrication system.

An unfortunate side effect was that pilots inhaled and swallowed a considerable amount of the oil during flight. The persistent oil mist flowing past the cockpit was leading to persistent diarrhea.

The Crank Shaft is of chrome nickel steel. It is hollow, acts as an induction pipe, and consists of two parts; the main crankshaft and the crankshaft Maneton, which are united by a coned joined.

The crankshaft provides a means of attaching the engine to the aeroplane and conveys oil to all rotating parts.
The crank pin is the fixed point against which the force of the explosion exerts itself in turning the engine.

The Crankshaft contains the main crank shaft bearings and the trust bearing. Coupling nuts are screwed onto the crank shaft for fixing the central support and for the carburettor which is mounted on the rear end of the hollow crank shaft.

The Master rod & Connecting Rods
A quite complicated slipper bearing system was used in the Le Rhóne engine. The master rod was of a split-parts type, containing the 2 radial ball-races, which permitted assembly of the connecting rods. It employed three concentric grooves, designed to accept slipper bearings from the other cylinders connection rods.

The connecting rods used inner-end bronze bushes, which were shaped to fit in the grooves. The Master rod carries eight pins, to which the other rods are attached, and the main rod being rigid between the crank pin and gudgeon pins fixed the position of the pistons.

The Connecting Rods
Solid H section connecting-rods are used, and the lubricating oil for the pistons pins are passed from the crankshaft through the center drilling of the rods. The large end of one of the nine rods, the Master Rod, embraces the crank pin and the pressure is taken on two ball-races housed in the end of the rod.

Inlet and exhaust valves can be set independently of one another - a useful point since the correct timing of the opening of these valves is of importance. The inlet valve opens 4 degrees from top dead center (TDC) and closes after the bottom dead center (BDC) of the piston.

Actuated by a single push-pull rod, each cylinder has two valves to control the inlet of a fuel/air mixture and for outlet of exhaust after compression and combustion. Induction pipes are carried from the crankcase to the inlet valve casings to convey the mixture to the cylinders.

The inlet- and exhaust timing mechanism contains two gears. The outer gear (50 teeth), part of the cam carrier, is attached to two cam plates and rotates around the inner gear (45 teeth).

Each cam plate is shaped in the form of 5 cams and as the cam carrier is driven at nine-tenths of the engine speed, the engine overtakes the cam plates one every 10 revolutions, during which period the arms are operated 5 times.

Both the inlet and the exhaust valves are mechanically operated by single push-pull rods and rockers. The 9 push-pull rods are jointed at their lower ends to the cam roller rocker arms which are mounted within the extension of the crankcase. These rocker arms have steel rollers at either end, resting on the cam plates. The inlet cam plate carries the leading rollers, the trailing rollers are following the exhaust cam plate. The exhaust cam plate is at the front.

The exhaust valve opens 68 degrees before the bottom center (BDC) and closes 4 degrees after the top dead center (TDC) of the piston.

The magneto is set to give a spark in the cylinder at 18-25 degrees before TDC, the end of the compression stroke.
Most commonly Avia or Lavalette magnetos were used, although the engines built in the USA were often equipped with Dixie magnetos. The magneto is fitted with a pinion and crown wheel. The pinion is driven by the main driving wheel.

The Induction Tubes

The nine induction tubes are mounted onto the crankcase front extension by means of the tube flange bolts.

The Tampier Bloc-tube carburettor

Mounted on the end of the hollow crankshaft through which the mixture is passed to the crankcase and next via the induction tubes into the cylinders.

The Tampier Bloc-tube carburettor
The petrol was fed through the patented Tampier fine adjustment mixture device to the horizontal jet at the side of the body.

The carburettor body contains a throttle slide, from which a tapered needle enters the jet and reduces the petrol flow through the jet and the air through the carburettor. The cylinder at the back end contains a copper gauze filter. Two horizontal air intakes tubes, connected by rubber tube pieces, are leading to the sides of the fuselage.

Petrol Air-pressure pump.
This engine driven pump is driven by its pinion and provides the required air-pressure to the petrol tank(s).
The pump has no suction valve, the air being admitted through the 4 holes in the barrel when the piston is near the inward end of its stroke.

(*) This engine driven Petrol Air-pressure pump was used for aircraft that didn't have and external air-pressure pump, e.g. the Rotherham air driven pump, mounted to the rear right cabane wing strut. To avoid stress and damage to the cabane strut (due to vibration), the pump was later fitted to the under carriage. This however was not liked by the pilots because they could not see the pump working.

The Petrol Air-pressure pump - Exploded view
The two valve plates are visible in front of the valve bodies on the bottom left. Each valve body contains a spring that presses the valve plate into its closed position (closing the holes in the cylinder head) during the suction stroke.
The air is being admitted through the holes in the cylinder barrel when the piston is near the bottom stroke of its stroke. During the compression stroke, the valve plates are pushed against their springs, thus opening the holes in the cylinder head, allowing the air to flow out through the cut-away channels in both valve bodies. The relieve valve plate movement is adjustable as to leak the excess air.

The Petrol Rotherham Air-pressure pump
The Rotherham & Sons Ltd (Coventry) Patent Mechanical Air Pump was designed for putting Air Pressure in petrol tanks of Aeroplanes. The pump is adjustable and gives a range between one and ten pounds (psi). The pump is rotary with a stroke of 5/8 inch (15.875mm.) and is driven by a spindle and propeller.

The bottom nut, with a spring loaded brass plunger under the connecting rod, is an oiler. Oil collects in it and every time the connecting rod pushes it down, a jet of oil shoots up the inside of the connecting rod.

The various Petrol Air-pressure pumps.
This image shows the various air-pressure pumps that were used by Aero plane engine manufacturers.

The left version is the (Clerget) engine driven air-pressure pump.
The pump in the middle has been used by Gnome and other Aero engine manufacturers.
The pump on the right is the wind driven Rotherham air pump which was commonly installed on the Sopwith Camel, mounted on the wing strut or under carriage.

The DIXIE (USA) and the French “AVIA” Magnetos.

Most commonly, the French Avia or Lavalette magnetos were used. The Le Rhóne engines built in the USA were often equipped with Dixie magnetos.
The magneto is fitted with a pinion (16 teeth) and is driven by the engine's main driving wheel (36 teeth).

The French “AVIA” Magneto (A.D.S. type) with its two permanent magnets


See Magnetos simply explained for additional details.

Also shown is the provision for an earthing switch, used to prevent the magneto from producing sparks when not required by means of short-circuiting the primary coil. Therefore any inductive effect in the secondary coil is prohibited. This shut-down switch will be connected to the screw terminal, protruding through the rear magneto end.

Cross Section of the “AVIA” Magneto


This image shows the cross section of the magneto, its various parts and the coil with the Primary (inner) and Secondary winding (outer). To the right, inside the brass end of the armature, the condenser / condenser “plates stack” is visible.

See Magnetos simply explained for additional details.

Contact Breaker

Close up of the contact breaker assembly which is attached to the rotating armature.
The “L” shaped rocker is connected to the frame (earth) through the blade spring. One end of the primary winding of the armature coil is connected to the insulated central member (by the long central bolt), the other end is also connected to the frame (earth).
The fiber contact-breaker rocker heel will be pressed inwards when touched by either one of the two cam shoes inside the cam-ring. This causes the opening of the contact points, breaking the circuit (twice per revolution) and causes the induction of the H.T. (hight voltage) in the secondary winding.

See Magnetos simply explained for additional details.

Oil Pump

The oil is forced through the passages in the crankshaft by a single plunger, oscillating cylinder pump. The oil is delivered at various points along the main oil passage through the crankshaft.

The oil pump is driven by its pinion through the worm gear. It consists of a casing containing the oil and a piston in an oscillating cylinder which has a single port to the casing. During the suction stroke the port is open and the oil from the casing is drawn into the cylinder. The cylinder pivots by the action of the eccentric and in the exhaust position, the cylinder port is aligned with the delivery outlet connector.

Oil Pump

Pump with the cover removed to show the internal parts. The worm gear drives the eccentric and the pump plunger in the pump cylinder.

Since the oil is delivered into the crank case, which is a part of the induction system and consequently under suction, there is little pressure required on the oil line. The pump however, should have enough capacity if the passage is obstructed.

Oil pump with Speedometer Drive / Reduction Gear Box

The oil pump pinion (20 teeth) is driven by engine's main gear (36 teeth), making the oil pump rotating at 1.8 of the engine speed.
The reduction gear box reduces the speed to one-quarter of the engine speed to limit undue wear of the flexible cable.

A flexible cable, connected to the RPM indicator in the cockpit, is screwed onto the brass end of the gear box. The RPM indicator reverses the reduction and shows the correct RPM of the engine.

Reduction Gear Box.

This image shows the internal gears in detail.

The speedometer drive/reduction gear box is mounted on the oil pump and drives the cockpit RPM indicator, which is connected by means of a flexible cable.
The function of this reduction gear box is to reduce the driving flexible cable speed to prevent undue wear.

The Throttle Quadrant

This invention relates to the device for actuating the throttle and mixture controls on aircraft. The usual method of actuating the controls on aircraft; for example, the controls of the carburettor, is by means of levers arranged to move through an angle which does not exceed approximately 90 degrees.

The two control levers or handles are mounted in planes parallel with and in close proximity to each other so that the pilot may, if necessary move one or two together.

Tampier Filter valve and Operation

This images shows the Tampier mixture control and Throttle quadrant mounted in a Sopwith Camel. To the right is the petrol tanks selector switch. It allows the selection of the Main or Gravity petrol tanks or to completely switch off the petrol supply.

Mixture control is provided by the mixture lever which operates the regulator plunger through the control bell-crank. The regulator plunger contains the regulator needle, allowing for fine adjustment of petrol flow through the regulator.
The petrol passes through a fine #30 mesh filter at the bottom of the body. The tapered needle position determines the amount of petrol that will be supplied from the selected petrol tank, through the output connection pipe, to the carburetor.