On a bright spring morning in 1940, the Royal Air Force pilot was in the fight of his life. Strapped into his brand new Supermarine Spitfire, he was locked in mortal combat with a Luftwaffe pilot over the English Channel in the opening days of the Battle of Britain. The Spitfire was behind the Messerschmitt and almost within range to unleash a deadly barrage of rounds from the
four eight Browning machine guns in the leading edges of the elliptical wings. With the German plane just below the centerline of the gunsight’s crosshairs, the British pilot pushed the Spit’s lollipop stick forward to dive slightly and rake his rounds across the Bf-109. He felt the tug of the harness on his shoulders keeping him in his seat as the nimble fighter pulled a negative-g dive, and he lined up the fatal shot.
But the powerful V-12 Merlin engine sputtered, black smoke trailing along the fuselage as the engine cut out. Without power, the young pilot watched in horror as the three-bladed propeller wound to a stop. With the cold Channel waters looming in his windscreen, there was no time to restart the engine. The pilot bailed out in the nick of time, watching his beautiful plane cartwheel into the water as he floated down to join it, wondering what had just happened.
Although the story is made up, the engineering problem facing the RAF was all too real. Early in the Battle of Britain, the now-legendary Rolls-Royce Merlin engine, powerplant for not only the Spitfire but also the Hawker Hurricane and the Lancaster bomber, was having a serious problem. RAF Spitfire pilots reported that the fighter would lose power during negative-g maneuvers, meaning that a simple jinking move to line up a shot on an enemy pilot or a quick dive to get out of the line of fire could stall the engine. Sometimes the power loss was momentary, but too often, the engine would just die in flight and fail to restart.
Like all good pilots, the young RAF flight officers quickly adapted to the shortcoming of their new fighters. They learned to do a half-roll before diving, to avoid a negative-g attitude and keep the Merlin running. It worked, but it was a stopgap at best, and a potentially deadly restriction on the ability to maneuver when it mattered most. What’s worse, the Luftwaffe pilots were quick to notice the problem — it was hard not to notice the black smoke and loss of power, even in the midst of a dogfight — and they capitalized on the enemy fighter’s weakness. Something had to be done, lest the tide of the Battle of Britain turn against the RAF.
In many ways, the Spitfire and the Hurricane were planes built around an engine. While the airframe of the Spitfire, with its beautifully elliptical wings and sleek lines, was certainly revolutionary, it was the mighty Merlin that made the plane what it was. The liquid-cooled, supercharged engine was powerful, simple, and reliable, but the choice of carburetors over fuel injection would come back to haunt the engine’s designers.
The carbs used in the Merlin were much the same as any carb found on a lawnmower or older car today: fuel was metered into a bowl by a simple float valve before being sucked into the intake airstream under suction provided by the Venturi effect. In straight and level flight, the carbs worked fine. But in a negative-g situation, fuel was forced to the top of the float chamber away from the jet, cutting off the flow of fuel and causing the engine to lose power. Returning to a positive-g attitude, fuel sloshed back into the float chamber and flooded into the jets, providing an over-rich fuel mixture to the cylinders. Raw fuel entered the exhaust manifold, where it burned and produced the sooty black exhaust. In the “right” situations, enough fuel would enter the supercharger to flood it, stalling the engine altogether and preventing it from restarting.
The obvious solution was to replace the float-bowl carbs with pressure carbs. But with the Battle of Britain raging, taking planes out of service for engine overhauls was not sensible. The RAF needed a quick fix until a more permanent solution could be fielded.
Miss Shilling Saves the Day
The fix that eventually saved the Merlin came in the unlikely personage of Miss Beatrice Shilling. In a time of strict social conventions and well-defined roles, Beatrice, who went by Tilly, broke all the rules. Fascinated by engineering since she was a teenage girl taking apart motorcycles and racing them, Tilly bucked convention, earning a degree in electrical engineering in 1932 as one of only two women in her class. She followed that up with an MSc in mechanical engineering the next year.
While racing motorcycles competitively — she won awards and set records in 1934 on a 500cc Norton bike to which she had added a supercharger, clocking a 106 mph lap — she started working at the Royal Aircraft Establishment (RAE) in Farnborough. There, in the opening days of World War II, she specialized in aircraft carburetors.
While Rolls-Royce worked on new carb designs for the Merlin, Tilly came up with a fix for the RAF’s woes, and like many such solutions, it was deceptively simple. She reasoned that restricting the fuel flow to the carburetor bowl would prevent flooding, so she designed a simple brass disc with a small hole in it. She calculated the dimensions of the disc to allow just enough fuel for maximum power. As a bonus, the device could be added to the planes quickly and easily, without removing the planes from service.
Officially, the device became known as the “RAE Restrictor,” but as Tilly Shilling toured RAF bases to oversee the installation of the device, the rough and ready aircrews had other ideas. “Miss Shilling’s Orifice” became the new name of the life-saving brass disc, which once installed in the fuel lines solved the problem. The Spitfires were back in the fight, and the RAF eventually pushed the Luftwaffe back across the Channel, thanks in no small part to Tilly Shilling.
Tilly did not rest on her laurels. With her workaround in place, she returned to the RAE and continued work on improved carburetor designs. She continued working for the RAE until she retired in 1969, making contributions to fields as diverse as rocket designs and braking aircraft on wet runways. Switching from motorcycles to cars of her own design, she also continued racing well into her 60s alongside her husband George Naylor, a fellow engineer and later RAF bomber pilot whom she married in 1938 on the condition that he first post a 100-MPH lap on a motorcycle.
Beatrice Shilling was a larger than life figure who deserved the many honors bestowed upon her, including the Order of the British Empire and a pub in Farnborough named after her. But being the engineer who fixed the Spitfire and turned the tide for the RAF was probably her proudest achievement.
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