Miss Shilling's Orifice
The SU carb is best known for its automotive applications. But it wasn't always this way. With the outbreak of the Second World War, SU halted road car applications, instead turning its attention to aircraft. SUs became standard fitment to the Rolls-Royce Merlin engine which powered the legendary Hurricane and Spitfire fighter planes.
​So far so good. Then came a 'hiccup'. In more ways than one.
In situations known as negative gravity, in other words, when Spitfire and Hurricane pilots pitched the nose of the aircraft hard down into a steep dive, fuel was forced up into the top of the SU's float chamber. This manoeuvre meant insufficient fuel reached the engine leading to a temporary, but critical loss of power.
During the pivotal cat and mouse battles over France and Britain, the enemy fighters exploited this weakness. The opposition aircraft had fuel injected engines and therefore did not suffer from this negative gravity phenomenon. Their fuel injection pumps kept the fuel to the engine at a constant pressure allowing German pilots to pitch steeply forward while simultaneously cracking open the throttle. As the spluttering British aircraft attempted to follow, precious seconds were lost and many enemy aircraft escaped unscathed from an encounter with the most feared fighters of WWII.
Animated protests from pilots concentrated the minds of the boffins to find a much-needed solution. The War Office put its finest minds to the task. Exclusively, these were middle-aged men, including Cyril Lovesey of Rolls-Royce and A.D. Fisher of the Royal Aircraft Establishment. To their credit these men devised ingenious and complex technical solutions. To their chagrin the solutions were abject failures. Their intricate valves and regulators solved the lack of fuel, but this was only the start of the trouble. Their modifications caused the greater and far more dangerous problem of over-fuelling. Too much fuel choked the engine with the rich 100% octane fuel used by the RAF and the marvellous Merlin suddenly fell silent as it stalled in mid air.
At the front line, the frustration of the pilots grew. In the absence of a solution by the Air Ministry, pilots devised their own workarounds that were not to be found in any technical manual, summed up nicely here by one pilot of the time:
"When an enemy fighter dived behind us from height and fired, one could not immediately dive without the engine temporarily cutting out. This could be avoided by rolling upside down, pulling back on the stick into a dive then rolling level. Similarly, on sighting a target below, one suffered momentarily if one pushed the nose down to attack. A grave disadvantage."
The fundamental problem continued to be an intractable one.
Up stepped Beatrice Shilling. She returned to basics and started by working out the precise volume and pressure of fuel needed for the Merlin engine in different scenarios. Once this had been established, she designed and engineered the wonderfully simple solution of an air & fuel mixture restrictor. This was, in, essence a flat shim, with a hole in its centre. Like all the best solutions in may have been simple, but it needed to be carefully manufactured to precisely allow the maximum flow of fuel and air into the Merlin, giving maximum power, at the same time as preventing the flooding of the engine.
Producing a brilliantly simple resolution was only half the battle. Many aircraft needing the fix were in service all over the country. Miss Shilling had an answer for that too. She set about organizing and leading a small group of trained engineers, she herself usually travelling solo on her Norton, to war torn airfields all over the country.
Crucially, Shilling's solution could be fitted without the removal of major components, consequently the fix could be made in-situ at operational airfields quickly and with the minimum of downtime for the aircraft. Her appearance on her motorcycle with a bag of tools and easy manner became something of a legend. With typical war time humour, an engineering wag dubbed the unassuming drilled washer, "Miss Shillings Orifice". The name stuck. As did her solution.
Beatrice Shilling, was a remarkably gifted woman. After the war ended, she went on to have a distinguished engineering career including working on the Blue Streak missile development programme and reducing the deadly risk of aircraft aquaplaning on wet runways. She held an honorary doctorate from the University of Surrey and was awarded an OBE in a New Year's Honours list in 1948.
As if that wasn't enough, she raced cars at Goodwood such as an Austin Healey Sebring and a Formula Junior single seater, with distinction. Perhaps her greatest accolade was to have the pub chain Weatherspoons name a pub after her in Farnborough in 2011.
We fancy she would have rather liked that.
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