flywheel. Improved cooling: when the engine was running, the rotating crankcase/cylinder assembly created its own fast-moving cooling airflow, even with the aircraft at rest. Weight advantage: rotaries shared with other radial configuration engines the advantage of a small, flat crankcase. The superior air-cooling imparted by the moving engine also meant that cylinders could be made with thinner walls and shallower cooling fins. Their power-to-weight ratio was further enhanced in comparison with engines that required an added flywheel for smooth running. Engine designers had always been aware of the many limitations of the rotary engine so when the static style engines became more reliable and gave better specific weights and fuel consumption, the days of the rotary engine were numbered. Rotary engines had a fundamentally inefficient total-loss oiling system. In order to reach the whole engine, the lubricating medium needed to enter the crankcase through the hollow crankshaft; but the centrifugal force of the revolving crankcase was directly opposed to any re-circulation. The only practical solution was for the lubricant to be aspirated with the fuel/air mixture, as in most two-stroke engines. Power increase also came with mass and size increases, multiplying gyroscopic precession from the rotating mass of the engine. This produced stability and control problems in aircraft in which these engines were installed, especially for inexperienced pilots. Power output increasingly went into overcoming the air-resistance of the spinning engine. Engine controls were tricky (see below), and resulted in fuel waste. The late WWI Bentley BR2, as the largest and most powerful rotary engine, had reached a point beyond which this type of engine could not be further developed, and it was the last of its kind to be adopted into RAF service. Rotary engine control Monosoupape rotaries It is often asserted that rotary engines had no throttle and hence power could only be reduced by intermittently cutting the ignition using a "blip" switch. This was only true of the "Monosoupape" (single valve) type, which took most of the air into the cylinder through the exhaust valve, which remained open for a portion of the downstroke of the piston. Thus the mixture of fuel and air in the cylinder could not be controlled via the crankcase intake. The "throttle" (fuel valve) of a monosoupape provided only a limited degree of speed regulation, as opening it made the mixture too rich, while closing it made it too lean (in either case