Figure 1. Gnome Monosoupape Rotary

Figure 2. Cylinder Fabrication Sequence

Gnome engines were beautifully constructed of polished steel, with many finely machined cooling fins and tiny fasteners. They were mostly built of forged steel machined to very thin sections for lightness. Each cylinder, for example, began life as a 97-pound nickelsteel bar. When it was completed, it was only 1.5 mm thick and weighed five and one-half pounds. Similarly, the crankcase started as a 106 pound forging and was reduced to a final weight of thirteen and one-half pounds. Although this fabrication technique is common in today’s aerospace industry, it was quite unusual at the time.

Induction

Induction and exhaust in the early Gnomes was interesting. There was no carburettor or intake manifold. Air entered the engine through the hollow crankshaft. A simple needle valve, in combination with an air valve, facilitated the pilot’s selection of the correct fuel/air mixture. This along with lubricating oil entered the crankcase, was thoroughly mixed by the thrashing of internal parts, and distributed by centrifugal force. On each intake stroke, automatic intake valves in the crown of each piston opened, admitting the mixture to the combustion chamber. These valves were counterbalanced to account for centrifugal force and piston acceleration, with just the right balance to allow the suction of the intake stroke to open them. Once in the combustion chamber, the mixture was compressed, ignited via a spark plug, burned to produce a standard power stroke, and finally exhausted directly to the atmosphere past exhaust valves in the top of each cylinder. The exhaust valves were operated by push rods and controlled by a central cam ring.

Figure 3. Early Cylinder and Valve Details

Since the crude fuel delivery system had no provisions for throttling, most rotaries ran at wide-open throttle all the time. Slight variations in power were possible by careful adjustment of the fuel/air mixture, and this allowed formation flight. The aircraft control stick was fitted with a "blip switch" which served to momentarily shut off engine ignition, making powered landings (and go-arounds) possible. The automatic intake valves of the early Gnomes turned out to be quite a headache for maintenance personnel. In addition to presenting a fundamental limit on engine speed and volumetric efficiency, they got out of balance easily, gummed up and got sticky, and worked poorly at higher altitudes. Thus, they presented constant maintenance problems, accounting in large part for the extremely short time between overhauls (TBO).

Figure 4. Monosoupape Induction Timing

Lubrication

Lubrication of the Gnome was accomplished by injecting castor oil into the fuel/air mix with a small pump. Castor oil was used because it could not be easily dissolved into the gasoline fuel, and because it possessed lubrication qualities superior to mineral oils of the day. The lubrication system was a total-loss type, with over two gallons of castor oil being sprayed into the air during each hour of engine operation. This explains why most rotaries were fitted with a three-quarters cowl ring, open at the bottom. The cowl directed the spray of castor oil, along with sparks from the exhaust, away from the flammable airplane structure. In spite of these attempts to deal with the excess lubricating oil, pilots were still subject to, and in many cases the victim of, the well-known laxative qualities of castor oil. Many unscheduled stops and off-airfield landings were credited to the call of nature. Some pilots reportedly kept a flask of blackberry brandy as an antidote to the effects of the oil.

The Gnome was, for its time, light and reliable. Compared to liquid-cooled engines of the day, it started easily, warmed up rapidly, and allowed its pilot to become quickly airborne to do his days bidding (We must not forget the Gnome was in large part the prime-mover of a weapons system). Due to the gyroscopic forces of this rapidly rotating mass, aircraft handling characteristics in a dogfight were strange. The craft turned left instantly, but right very reluctantly. In the hands of a skilled pilot, this could be quite an advantage in combat. Starting the engine was an adventure. It was turned over by hand in a nearly flooded condition, and would often catch fire due to the excess gasoline. The fire would typically be confined to the cowl ring and grass under the engine. The pilot would shut off the gas and allow the engine to continue to run on the excess as ground handlers hauled the aircraft backward out of the fire. All would wait for the fire to burn itself out. At just the right moment, the pilot would re-open the fuel valve and a successful start would have been achieved. When the engine warmed up, the air valve was opened wide and fuel re-adjusted to produce a correct mixture. The engine was then run at full throttle for the take-off, climb, and cruise, and landed through the use of the "Blip switch", which was used to temporarily ground out the ignition. Though the Gnome was widely used and had a number of advantages in its particular niche, it also had a number of problems. It had a very high fuel consumption so that the total weight of engine plus fuel and oil placed it at a distinct disadvantage for long flights. Its performance fell off rapidly with altitude. Though reliable if properly maintained, it was extremely temperamental, requiring skilled mechanics to overhaul it at typical intervals of 15-20 hours. It was expensive, with a 70hp motor costing $4000 in 1912! Finally, its rotating mass limited its size and consequently, its ultimate power. By the end of the war, when tactics favoured a single high-speed pass instead of the  dogfight, larger liquid-cooled engines prevailed and the rotary was rapidly becoming an anachronism.

Configuration: 9-cylinder air-cooled rotary radial

Output: 160 hp @1,300 RPM

Weight: 330 lb

Displacement: 970 in

Bore x Stroke: 4.53" x 6.69"

Compression Ratio: 5.45:1

Mean Effective Pressure: 100.5 psi

Specific Weight: 2.06 lb/hp

Specific Output: 0.165 hp/in

Fuel Consumption: 29.1 gal/hr @ full power

Specific Fuel Consumption: 1.09 lb/hp/hr @ full power

Oil Consumption: 2.56 gal/hr @ full power

Specific Oil Consumption: 0.12 lb/hp/hr @ full power

6 hr mission specific weight: 1.55 lb/hp/hr (engine + fuel + oil)