Miss Shilling's Orifice

This year is the 80th anniversary of the Battle of Britain, one of truly the seminal moments in British history, where the outcome of the battle could so easily have been different. The bravery and skill of all those involved has been quite rightly lauded, but there is one whose role in the Battle has been largely overlooked. This was the wonderfully named Beatrice "Tilly" Shilling, whose contribution was physically small, but enormous in its consequences.

The iconic Supermarine Spitfire, powered by the Rolls-Royce Merlin engine

Photo: courtesy of the public domain

There are few people who have not heard of the iconic Spitfire and Hurricane fighter aircraft, both powered by the Rolls Royce Merlin engine, one of the most successful aircraft engines of the World War era. The early versions of the Merlin, as used during the Battle, had one serious drawback. Equipped with a gravity fed SU carburettor, when a Merlin-powered Spitfire performed a negative G force manoeuvre (pitching the nose hard down), as would happen during a dogfight, fuel was forced up to the top of the carburettor's float chamber rather than down into the engine, leading to loss of power. If the negative G continued, fuel collecting in the float chamber would force the float to the floor of the chamber. Since this float controlled the needle valve that regulated fuel intake, the carburettor would flood and drown the supercharger with an over-rich mixture. The consequent rich mixture cut-out would shut down the engine completely. The German fighters had fuel injected engines and therefore did not suffer from this problem as the injection pumps kept the fuel at a constant pressure. The German pilots could exploit this by pitching steeply forward while opening the throttle, a manoeuvre that the pursuing British would be unable to emulate. The British countermeasure, a half roll so the aircraft would only be subjected to positive G, as it followed German aircraft into a dive, could take enough time to let the enemy escape.

The Rolls-Royce Merlin 66 aircraft engine

Photo: Courtesy of public domain

Beatrice 'Tilly' Shilling

Photo: Courtesy of public domain

It was Beatrice 'Tilly' Shilling, an engineer working at the Royal Aircraft Establishment (RAE) at Farnborough, who came up with a simple solution which would not involve taking the aircraft out of service, at a time when all aircraft were desperately needed. She designed a simple technical device (essentially a flat washer) with precisely calculated dimensions that allowed just enough fuel flow for maximum engine power and countered the problem of engine cut-out. Beatrice toured the RAF fighter stations with a small engineering team (largely female) fitting the Merlin engines with her simple invention. While it was officially called the R.A.E. restrictor, the device was immensely popular with pilots, who affectionately named it 'Miss Shilling's orifice' or simply the 'Tilly orifice'. While it was a brilliant solution, this simple measure was only (and literally) a stopgap: it did not allow inverted flight for any length of time. The problems were not finally overcome until the introduction of Bendix and later Rolls-Royce pressure carburettors in 1943.

'Tilly' with one of her first motorbikes

Photo: Courtesy of public domain

Beatrice (Tilly) Shilling OBE PhD MSc CEng (1909 – 1990) born in Hampshire, the daughter of a butcher. At the age of 14 she bought herself a motorbike and was already determined to become an engineer. After completing secondary school, she worked for an electrical engineering company for three years, installing wiring and generators. Her employer, Margaret Partridge (an electrical engineer, contractor and founder member of the Women's Engineering Society and the Electrical Association for Women), encouraged her to study electrical engineering at the University of Manchester where she received a bachelor's degree in 1932. The following year she was awarded a Master of Science degree in mechanical engineering. She was also a keen amateur racing driver and in the 1930s raced motorbikes at Brooklands, one of only three women awarded a BMCRC (British Motorcycle Racing Club) Gold Star for lapping the circuit at over 100 miles per hour (160 km/h). Beatrice married George Naylor in September 1938, who also worked at the RAE. Apparently, she refused to marry him until he also had been awarded the Brooklands Gold Star for lapping the circuit at over 100 mph. During the Second World War he was a bomber pilot with No. 625 Squadron RAF and was awarded the Distinguished Flying Cross (DFC). After World War II, Beatrice and George turned to racing cars, which were tuned and modified extensively in their home workshop. Post war, Beatrice worked on a variety of engineering projects including the Blue Streak missile and the effect of a wet runway upon braking. Shilling was once described by a fellow scientist as "a flaming pathfinder of Women's Lib", and always rejected any suggestion that as a woman she might be inferior to a man in technical and scientific fields. She worked at Royal Aircraft Establishment until her retirement in 1969.

The Tilly Shilling pub in Farnborough

Photo: Courtesy of public domain

In 2019, Royal Holloway University opened the Beatrice Shilling Building, home to its new department of Electronic Engineering, and a blue commemoration plaque was unveiled on the house in Farnborough where she lived during the wartime period. Also, in Farnborough, is The Tilly Shilling, a pub named in her honour and her wonderful invention.