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Clerget 9B
Clerget Assembly Movie

Clerget Assembly Movie

CLERGET  9B Specifications


9-cylinder air-cooled rotary engine.


120 mm (4.72 in)


160 mm (6.30 in)


16.29 L (993.8 cu in)

Dry weight:

175 kg (385 lb)

Fuel system:

Cooling system:


Ignition system:


Power output:

130 hp (96 kW) at 1,250 rpm

Compression ratio:


Clerget-Blin - Société Clerget-Blin et Cie - was a French precision engineering company formed in 1913 by the engineer and inventor Pierre Clerget and industrialist Eugène Blin.

Location   :

37, Rue Cavé in LEVALLOIS-PERRET (Seine).

Region :

Île de la Jatte, France.

Terminal  :

Tramway Madeleine-Levallois

Arrondissement :



WAGRAM 62-83   -    WAGRAM 84-17

Telegraph :


Levallois-Perret became an important center of the early French automotive industry.

The Clerget-Blin company mainly produced aircraft engines, see Clerget aircraft engines.
Their successful rotary engine designs were additionally built in Britain by companies such as Gwynnes Ltd, Ruston Proctor and Gordon Watney, to increase the output in the times of WW-I.

If any engine could be said to be the standard power unit of the Camel then it was the 130 h.p. Clerget 9B or later the 9BF Up until September 1918.
A Lang LP2850 propeller had been a standard fitting for 130 h.p. Clerget-Engined Camels. Lang propellers Ltd. was based at Hamm Moor lane, Weybridge (UK) and supplied wooden propellers to nearly every airplane company in England during WW1.

Each cylinder has a piston and valves to control the inlet of a fuel/air mixture and for outlet of exhaust after compression and combustion. Spark plugs ignite the fuel and air mixture.

Pistons are of aluminum alloy with three cast-iron rings to ensure full compression, sealing and heat dissipation. The top ring is a set of two (inner- and outer) obturator rings, which are made of a copper/silver alloy.

Each cylinder has two spark plugs and the ignition is provided by a magneto instead of a battery. This design improves reliability and this is an important consideration in aviation.

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.

The weight of the spinning engine mass created a gyroscopic effect, which would impact the flying characteristics of the airplane. The induced torque effect into the airframe 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. Some early engines had no throttle (the engines ran at full throttle, with the ignition being "blipped" to reduce power when necessary).

Clerget engines where 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 and was not straightforward. It became a common practice during landing to "blip" the engine as well.

Another interesting fact was that these engines had no exhaust system (the burnt gasses were simply released from the tops of the cylinders). They used a total loss oiling system, where the oil was exhausted with the burnt fuel, coating the aircraft with a heavy sheen of castor oil.

The fabled white scarf worn by these pilots had less to do with fashion, and more to do with wiping the oil off their faces and goggles.

The long “grass-hopper” spring on the exhaust valve was used on the Clerget 9B 130 hp Type "A" engines. The later type Clerget 9B 130 hp Type "B" used a helix type spring. Various experiments were carried out because the excessive heat to which these parts were subjected caused softening of the materials.

With the British-made 130hp Clergets, the performance of the Camel deteriorated rapidly. These engines were less satisfactory than the French-made engines. The RFC was gravely concerned over this shortcoming, but had to accept that the six squadrons due in France by end December 1917 would have Clerget Camels.

Another known feature of Rotary engines was its use of Castor oil. The reason is lost until we examine the lubrication system in detail.
With the fuel and oil mixed together in the crankcase it was important that the fuel not dissolve the oil and ruin its lubricating qualities.

The perfect choice was pharmaceutical-quality castor oil-it would stand the heat and centrifugal force, 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, leading to persistent diarrhea.
As already indicated, this also accounts for the pilot's use of a flowing white scarf-not only for a dashing image, but to wipe goggles clear of the persistent oil mist flowing past the cockpit.

Both the inlet and the exhaust valves are mechanically operated by push-rods and rockers. Induction pipes are carried from the crank case to the inlet valve casings to convey the mixture to the cylinders.

A Tampier Bloc-tube carburetor of the central needle type has been used.

The Master rod carries eight wrist pins, to which the other rods are attached, and the main rod being rigid between the crank pin and piston gudgeon pin determines the position of the pistons. The large end (banjo) of the Master rod embraces the crank pin and the pressure is taken on two ball-bearings housed in the big end of the master rod.

Hollow connecting-rods are used, and the lubricating oil for the pistons pins are passed from the crankshaft through the centers of the rods.

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

Two high tension magnetos of the A.D.S type are used. They are fitted with a pinion and crown wheel. The pinion is driven by the main driving wheel. The magnetos are set to give a spark in the cylinder at 22 - 26 degrees before the end of the compression stroke.

The Gun interrupter mechanism was designed by Harry Kauper, an Australian aviator and radio engineer. In his design, the interrupter gear had the bowden trigger normally enabled and the interrupter mechanism would disable the trigger when a propeller blade was in the way.
The firing of the gun was triggered by the depression of a cam , mounted onto the distribution disc, passing under the two tappet rollers.

For details, see Sopwith-Kauper No. 3   mechanical synchronizing gear.

 Further explained in the Kauper interrupter animation video.

There were several French built versions of the Clerget 9B (including the 9Ba, 9Bb, 9Bc, 9BF) with outputs ranging from around 125-145 hp, although only the 9B and British licence built versions of the 9B by Gwynnes for the Admiralty, and Ruston & Proctor for the RFC.

This image shows the French Clerget name plate of the Clerget Blin company. Other name plates used also show the full address;

Clerget Blin et Cie, 37 rue Cavé, Levallois-Perret.

Here is shown how the induction tubes are mounted onto the Rear drum by means of the tube flange bolts, by which the bronze clamps are held down by nuts onto the flanges of the induction tubes.

Two A.D.S. type magnetos, mounted on the Central support, where used for ignition. The magnetos are driven by the main driving wheel and are connected via the carbon brushes in de brush-holders, to the two rings of contacts on the distributor. The engine uses two spark plugs per cylinder, which are set to give a spark at the same time.

Petrol Air-pressure pump

This engine driven pump is driven by its pinion, and through worm gear.
This 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.

Petrol Air-pressure pump

The delivery valve is of the plate type and of ample size. The pump, in addition to supplying the air required to displace the petrol used, will deliver an ample margin to allow for reasonable leakage from the delivery pipe or petrol tank, and to deal with this excess air and adjustable relief valve is provided.

Petrol Air-pressure pump Cut-away 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.

Rotherham - Petrol 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.

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 standard (Clerget) engine driven air-pressure pump.
The pump in the middle has been seen on some, other than Clerget, engines (e.g. Gnome engines).
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 French “AVIA” Magneto (A.D.S. type).

Here the end cap is taken off to show the rotating contact-breaker assembly.
The contact-breaker assembly is attached to the rotating armature. The contact-breaker rocker is activated through the two "half moon" shaped shoes, which are screwed onto on the inside of the rear housing.

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


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.

    See Magnetos simply explained for additional details.

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 Operation

The oil pump is driven by its pinion through the worm gear. Two cams, formed on the worm shaft, acting against the internal springs, force the regulating plunger (R) and the valve plunger (L) to descend.
The oil flows into the inlet chamber, through the gauze strainer, and into the annular space around the reduced portion of the valve plunger; the valve plunger descends, and the regulator plunger ascends, drawing the oil after it through the opening between the two chambers; the valve plunger then ascends, and the regulating plunger descends, forcing the oil back through the opening into the space under the valve plunger, and from there into the oil supply pipe.

The Oil pump

This image shows the regulator screw by which the travel distance of the valve plunger can be adjusted and therefore the amount of oil being pumped by the oil pump.

Oil Pump

The engine driven Oil Pump and its parts.

Speedometer Drive / Reduction Gear Box.

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.

Speedometer Drive / Reduction Gear Box

This image shows the internal gears in detail.

The oil pump pinion (36 teeth) is driven by engine's main gear (63 teeth), making the oil pump rotating at 1 3/4 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.
The nut on the left is screwed onto the oil pump main shaft and drives the inner reduction gears through the spring coupling. The flexible cable is screwed onto the brass end of the gear box.

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

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.

Sopwith Camel and Clerget

This images shows how the Clerget is mounted in a Sopwith Camel.

The Central support drum is secured against the Front Engine plate (D2019-4) by 16 bolts, washers and nuts.
The Rear Support flange, screwed onto the crank shaft of the engine, is bolted onto the Back engine plate (D2029) by 8 bolts, washers and nuts.