“Miss Shilling’s Orifice”: Simple solutions to technical issues can make all the difference

All too often air forces tend to associate solutions to capability problems with the use of high-tech, often expensive, fixes. In this post, Squadron Leader Michael Spencer uses the story of Beatrice Shilling to highlight that this should not be the case. Shilling’s simple yet effective solution to an engine design flaw that limited the performance of RAF Spitfires and Hurricanes during the early World War II air battles over France and Britain stands out as the exemplar of pragmatic problem solving enabled by technical mastery and creativity.

The Battle of Britain made the legend of the Spitfire and the Hurricane. But despite the rightfully earned reputation of these exceptional fighters, they were not without their design flaws. One flaw, in particular, placed the British fighter pilots at a significant disadvantage against their Luftwaffe foes; a design flaw in the Merlin engine that limited the aircraft’s’ ability to perform negative g manoeuvres, forcing pilots to adapt to operational limits in their air combat manoeuvres. A technical solution to this problem was found by a scientist at the Royal Aeronautical Establishment (RAE), Miss Beatrice “Tilly” Shilling. Her relatively simple solution, fondly nicknamed “Miss Shilling’s Orifice”, significantly improved the combat effectiveness of the RAF aircraft and their pilots, and helped make a legend of the aircraft and an engineer.

Beatrice “Tilly” Shilling [Image via University of Manchester]

The performance of a combustion engine is mainly due to the amount of air and gasoline that flows into the engine cylinders, controlled by a float-type carburettor, under the force of gravity. The pistons combust the fuel/air mixture and energy is released to drive the propeller driveshaft. A weakness of the Rolls-Royce Merlin engines, the engines powering the RAF Spitfire and Hurricane, was a sudden loss of power from fuel starvation occurring in the gravity-fed carburettor whenever the aircraft nose was suddenly pitched slightly downwards in a quick negative g manoeuvre. If the negative g was sustained, the engine would stop completely. The ability to rapidly transition into a steep dive to either pursue or escape an enemy fighter was an important attribute for fighter aircraft. The Merlin’s loss of power as it experienced negative g disrupted RAF pilots’ efforts to line-up or escape a pursuing enemy fighter when engaged in mortal combat. This created issues during the Battle of France and the Battle of Britain.

Their German foes did not face this same issue. Daimler-Benz engines installed in Luftwaffe Messerschmitt Bf-109s had been configured with fuel-injection systems since 1937. Mechanically or electrically controlled spray nozzles would pressurise the fuel and inject it directly into each of the engine piston cylinders. The pressurised fuel flow in the Messerschmitt engines were unaffected by negative g manoeuvres. This meant that a Luftwaffe pilot could simply “bunt” into a high-power dive to escape a Spitfire or Hurricane attacking them from behind. This provided the Luftwaffe fighter pilots with a critical manoeuvre advantage over the RAF fighter pilots.

This RAF engine problem was solved in 1941 by installing a diaphragm, a metal disc with a small hole in the middle, designed by “Tilly” Shilling specifically to address the Merlin engine carburettor problem and stop fuel being forced upwards, away from the piston chambers, during a negative g manoeuvre. This modification enabled RAF pilots to perform steep dive manoeuvres without experiencing a loss of power or the engine stopping. The diaphragm was fitted across the carburettor float chamber to prevent the fuel from draining and starving the engine during a negative g manoeuvre. The simplicity of the diaphragm design meant it could be fitted to an engine while it was still in the aircraft at an operational airfield without needing to remove the carburettor. In early 1941, Miss Shilling travelled around England with a small work team to retrofit the diaphragms, giving priority to front-line combat fighter units. By March 1941 the diaphragm had been installed, as standard, throughout the entire RAF Fighter Command.

Tilly became famous for her invention which was fondly nicknamed “Miss Shilling’s orifice.” “Miss Shilling’s orifice” was in service for two years, during which time Miss Shilling is recognised as having contributed to the RAF shooting down many enemy aircraft and also to having saved the lives of many RAF pilots.

Improvements to the Merlin carburettor continued until the design incorporating “Miss Shilling’s orifice” was finally superseded in 1942 by the Bendix-manufactured pressure carburettors, which used a pressure system to negate the fuel flow problem completely. Although the Merlin engines are mostly known for powering the RAF Spitfire and Hurricane fighters, they were also used in the de Havilland Mosquito and Avro Lancaster bombers, Bristol Beaufighter, Fairey Battle light bomber, Halifax and Wellington Bombers, Boulton Paul Defiant II, and in the upgraded USAAF P-51 Mustang. Merlin engine production was finally concluded in 1950 after a total of nearly 150,000 engines had been manufactured.

Tilly originally started work in 1936 as a technical writer at the Royal Aircraft Establishment (RAE) Farnborough, the research and development agency of the Royal Air Force. Six months later, she moved to the Carburettor Section of the Engine Experimental Department. In 1939, she was promoted to Technical Officer-in-charge of carburettor research and development. Apart from her professional interests in aerospace engineering, Tilly also had a pilot’s licence and raced motorcycles, including a Norton motorcycle that she had stripped down, rebuilt, and tuned herself. She continued to work at the RAE after WWII, working on projects including the investigations into the De Havilland Comet crashes, aircraft refrigeration, and providing scientific advice to the British bobsleigh team on their vehicle design. In 1948 she was awarded the OBE for her contributions to the war effort. Tilly retired from her post as Senior Principal Scientific Officer at RAE Farnborough in 1969 and received an honorary doctorate from the University of Surrey in the same year. Miss Shilling OBE PhD MSc CEng died in 1990.

Tilly’s story highlights that solutions to critical technical issues can be relatively simple. The realisation of innovation in tactics and system designs will assist to maintain the viability of capability systems as the future environment unfolds throughout its lifecycle. Even the suggestion of a small hole in a metal disc can improve in capability. Workforces should be trained and educated in a way that promotes and supports innovation and disruptive thinking to constantly evaluate operational capabilities to push them to the limits of viability until new capability change can be justified.

Squadron Leader Michael Spencer is currently serving at the RAAF Air Power Development Centre in Canberra, analysing potential risks and opportunities posed by technology change drivers and disruptions to future air power. His Air Force career has provided operational experiences in long-range maritime patrol, aircrew training, and weaponeering, and management experiences in international relations, project management, air and space concept development, air capability development, and joint force capability integration. The opinions expressed are his alone and do not reflect those of the Royal Australian Air Force, the Australian Defence Force, or the Australian Government.

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