The Le Rhone 9J nine-cylinder rotary aircraft engine

By Theo Truter



Produced in France by Gnome et Rhône the Le Rhône 9J nine-cylinder rotary aircraft engine is also known as the Le Rhône 110 hp in a reference to its nominal power rating. The engine was fitted to a number of military aircraft types of the First World War. Le Rhône 9J engines were also produced under license in Great Britain by W.H. Allen Son & Company of Bedford, and in Germany by Motorenfabrik Oberursel.



In common with other Le Rhône series engines, the 9J featured highly visible copper induction pipes and used a single push-pull rod to operate its two overhead valves. The main visual difference between the 9J and the earlier, less powerful Le Rhône 9C engine is that the copper intake manifold tubing (with round section lower ends) on the 110 hp 9J is attached to the crankcase behind the cylinders, whereas on the 9C (80 hp) the intake manifolds (with rectangular lower ends) are fully visible from the front.

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 110 hp 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 110 hp 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 engine. The master rod was of a split-type, which permitted assembly of the connecting rods. It also employed three concentric grooves, designed to accept slipper bearings from the other cylinders. The other connecting rods used inner-end bronze shoes, which were shaped to fit in the grooves. The master rod was numbered as number one and the shoes of numbers two, five and eight rode in the outer groove, the shoes of three, six and nine in the middle groove and four and seven in the inner groove. Although this system was complex, the Le Rhône engines worked very well.



The Le Rhône engines used an unconventional valve actuation system, with a single centrally-pivoting rocker arm moving the exhaust valve and the intake valve. When the arm moved down it opened the intake valve and when it moved up it opened the exhaust value. To make this system work a two-way push-pull rod was fitted, instead of the more conventional one-way pushrod. This feature required the cam followers to incorporate a positive action, a function designed in by using a combination of links and levers. This design functioned but it did prevent the incorporation of valve overlap which limits power output. Due to the structural and cooling limitations of the overall engine design the Le Rhône engines produced as much power as they were capable of, regardless.

Any form of four-stroke engine with its cylinders arranged radially must have an odd number of cylinders; otherwise the four-stroke cycle cannot be repeated with each cylinder firing once in every two revolutions.

There have been many radial engines, both fixed and rotary, which have had even numbers of cylinders, but those have been arranged in even-numbered multiple rows of odd-number discs. The major difference between the rotary and the static radial engines is that the components which are normally expected to be stationary, such as crankcase and cylinders, are the ones which rotate.

In a rotary engine the crankshaft is stationary and usually serves as one of the key points for fixing the engine to the airframe. The crankcase and cylinders rotate on bearings around the crankshaft, dragging the conrods and pistons around with them. Because the centre of rotation of the crankcase is the main axis of the crankshaft, and the centre of rotation of the conrods is the offset main bearing journal of the crankshaft, the motion of the conrods and pistons relative to the cylinder heads provides the necessary induction and compression actions to generate the four-stroke cycle.

The long end of the crankshaft which is bolted to the airframe is hollow and the carburettor is mounted at the airframe end, opposite from the propeller. Petrol vapour is thus sucked into the crankcase and then admitted to the space above the piston crown by a variety of means, depending upon the particular design of rotary engine. There was also some variety in the details of the valve gear but every type utilised some form of drum-cam driven from the front of the crankcase. Ignition was provided from magnetos mounted on a stationary plate on the crankshaft's long end and driven by a ring gear fixed to the rotating crankcase. The same gear usually drove the oil pump, delivering castor oil to the main bearing journal at least, with the rest of the engine having to rely on splash in many cases.

The Le Rhone had two valves in the cylinder head, operated by a single push-pull tappet rod, again operated by central cams. Its cylinders differed from the Gnome, in that a cast iron liner, with better lubrication and heat conducting properties than steel, was shrunk into the steel cylinder. The mixture was transferred from the crankcase to the cylinders via copper induction pipes. It used 14 main bearings used in a shoe-in-groove arrangement, with conrods of three different lengths riding in three concentric grooves in a bronze-lined dinner plate of a central bearing. It had a proper throttle and was the favourite engine of many WW1 pilots.

The Clerget was much like a Gnome, with steel cylinders with one master and a set of slave connecting rods, but it had two valves operated by individual pushrods. French-built engines were of reasonable quality, but the Clergets licence-built by the British pump making company, Gwynne's, soon developed a reputation for unreliability.


Le Rhone rotary motors on display at Soesterburg in the Netherlands


Close up



• Type: Nine-cylinder, single-row rotary engine
• Bore: 105 mm (4.13 in)
• Stroke: 140 mm (5.51 in)
• Displacement: 10.89 L (664.47 cu in)
• Diameter: 94.5 cm (37.2 in)
• Dry weight: 121.5 kg (268 lb)
• Power output: 92 hp (68.6 kW) at 1,300 rpm (maximum)
• Compression ratio: 4.5:1

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