The Supermarine Spitfire is nearly synonymous with Britain in World War II. It was a superb fighter plane, beloved of its pilots for its speed and agility, and by the British public as a bona fide national icon and war winner. That status was in no small part thanks to its elegant elliptical wings and the evocative roar of its Merlin engine.

But it was also fundamentally broken. Its engine would often cut out just when the dogfights of the Battle of Britain got interesting: in diving, a staple defensive and attacking maneuver. Unfortunately for the British, the Luftwaffe’s Messerschmitt 109s suffered no such shortcoming, giving the 109s a decisive edge in combat against British fighters.

Britain won the Battle of Britain by the narrowest of margins, and so it was imperative that a solution was found if British fighters were to contribute to the liberation of Europe. Found one was, and from the most unlikely of sources: a female engineer named Beatrice Shilling. That a solution came from a woman circa 1940 was improbable enough, but perhaps as improbable was the simplicity of her solution.

Miss Shilling’s orifice, as it came to be known, arrived just in time for the bitterly fought European offensive that paved the way for the D-Day invasion of Normandy. It without question saved the lives of British pilots. Arguably, it helped turn the tide of the war.

The Supermarine Spitfire
The Supermarine Spitfire

The problem with the Spitfire’s Rolls-Royce Merlin engine was first spotted in 1938 when the first production models of Spitfire rolled off the factory lines at Woolston, Southampton. At the time, it wasn’t seen as much of a problem: diving simply wasn’t something pilots did much before World War II. But by the height of the Battle of Britain in 1940, it was a matter of basic survival.

Also typically glossed over in post-war recollection is the Spitfire’s troubled beginnings. It arrived years later than planned, and at great expense, due to administrative bungling that originated at the top levels of the British government, cascaded down through incompetent captains of industry, before crashing onto factory floors and their chaotic organization of labor. If and when there was any organization of labor, that is.

Despite a prototype Spitfire having first flown in March 1936⁠—the same year that work began on Castle Bromwich for the sole purpose of Spitfire production⁠—things only improved in May 1940 when, upon becoming Prime Minister, Winston Churchill appointed Lord Beaverbrook as Minister for Air Production. Beaverbrook immediately transferred ultimate responsibility of Spitfire production to the engineering firm Vickers. Under Vickers, production would eventually ramp up to 300 planes a month.

In July 1940, Alex Dunbar, the newly appointed manager at the Castle Bromwich Spitfire factory, wrote, “Incidentally, we are sacking 60 jig and tool draughtsmen next week. We have tried to find out what they are doing but the answer’s not a lemon. In the meantime, we build the odd Spitfire or two.”

But the problems with the Spitfire were only beginning. The Battle of Britain was a success despite a serious flaw in the Rolls-Royce Merlin engine used in both the Spitfire and the Hurricane. The Hurricane was still the workhorse fighter of the Royal Air Force (RAF), thanks in part to production issues with the Spitfire. It was a more than capable machine except for the engine issues it shared with its glamorous sibling.

Hurricane Pilots responding to an air raid during the Battle of Britain
Hurricane Pilots responding to an air raid during the Battle of Britain

That flaw was a tendency for the engine to falter under so-called negative gravity, when powering into a dive. Negative gravity is a confusing term because the problem wasn’t one of gravity, or even force, but rather of acceleration and inertia. When an aircraft accelerates downward faster than it would in free fall, anything not bolted down, like the blood surging through the pilot’s vessels, will accelerate more slowly due to its own inertia. In relative terms, so far as life aboard the aircraft is concerned, those things are moving upward rather than downward.

Inertia’s a tricky thing, particularly when it comes to liquid fuel. Imagine the skin of a pilot’s face going into a dive, engines at full power. Now imagine the same thing happening to the liquid fuel in the engine, and specifically the chamber of the carburetor designed to regulate the flow of fuel: fuel effectively flows upward. Unfortunately, the fuel outlet holes in Merlin engines were at the bottom of the chamber while, when accelerating into a dive, the fuel was forced to the top, unable to reach the engine. The engines were being starved of fuel, causing what was known as a “weak cut” and resulting in a tendency to stutter, or even to cut out completely, under negative gravity. Either way, the result was loss in power. It was just a question of how long that loss would last.

The trouble could also strike when hitting turbulence. “I opened fire at about three hundred yards, and seemed to hit the one I was aiming at because he pulled up sharply,” Wing Commander Bob Doe later recalled. “At that moment (I must have been down to about a hundred yards), I hit his slipstream and my engine cut⁠—stone dead.” Dogfights are presumably made all the more memorable with the added frisson of engine trouble.

This was problematic for RAF pilots for a number of reasons. First among them was that, by 1940, it was becoming increasingly evident that diving at full tilt was actually rather a good idea, tactically speaking, not to mention an intuitive reaction to being chased down by the fearsome Messerschmitt Bf 109E and Bf 110C fighters of the German Luftwaffe. Diving had the distinct advantage of making it harder to be shot down. Second was that Luftwaffe pilots being tailed by a British fighter did much the same thing to evade their pursuers, and there was really only one way to stay in hot pursuit of a diving fighter plane: dive straight after it. The third was that the Daimler-Benz engines of the Messerschmitts did not share the Merlin’s fuel-system flaw.

Luckily, the British pilots had home-court advantage. The proximity of their fuel supply meant that they could remain in combat for longer than the Luftwaffe, who had to travel some distance to theaters of war across the English Channel, fight, then fly home again.

Overall, then, the sixteen-week-long Battle of Britain was characterized by its numerous narrow margins. Though the battle was over by November 1940, the war in Europe was not. There was still the task of liberating large swathes of mainland Europe, and both the Spitfire and the Hurricane would play vital roles. Quite simply, an answer to the stalling problem needed to be found, and quickly.

The Rolls-Royce Merlin engine
The Rolls-Royce Merlin engine

Naturally, the war effort put its finest minds to the task, including Cyril Lovesey of Rolls-Royce and A.D. Fisher of the Royal Aircraft Establishment. Both devised ingenious technical solutions, both of which failed utterly in solving the problem. Their valves and diaphragms may have solved the weak cut, but this was only the start of the trouble. It was swiftly followed by a longer and more dangerous “rich cut”⁠—when the chamber floods with excess fuel, giving a ratio of fuel to air that is too high, or too rich, to actually burn.

It would take an intimate knowledge of engines to identify the more serious problem. So it was remarkable for any number of reasons that, in 1940s Britain, the ultimate solution would come from a woman.

•     •     •

It was around the time of the first production Spitfire test flights of 1938 that Beatrice “Tilly” Shilling rebuilt and tested her own beloved machine: a 490cc Norton motorcycle. The same machine saw her become the fastest woman ever to ride a motorcycle at Brooklands: the first purpose-built race track in the world, which had opened near Weybridge in Surrey in 1907. Shilling was an exceptional racer, but an even better engineer.

To say that women tended not to be engineers in the early part of the 20th century would be stretching the definition of understatement to its very limits. The same stretch could be made for politicians, doctors, land owners, or indeed any role of influence.

But, even in childhood, there were signs that Beatrice Shilling would not conform to societal norms. When Shilling was still small enough to be known as Baby among the family, she was reported to have had this short exchange with her mother:

Mother: “Baby, you mustn’t bite your sister.”
Beatrice: “Have.”
Mother: “Well, say that you’re sorry.”
Beatrice: “Shan’t.”

Around 1919, the 10-year-old Beatrice grew tired of being left behind by her sisters on cycling trips and began to save for a motorcycle. Aged 12, she built a working Meccano model spinning wheel and promptly won a national competition set by Meccano Magazine.

By the age of 14, she’d achieved her goal, her chosen bike a two-stroke Royal Enfield. When she wasn’t giving her sister Anne pillion rides, she was taking it to bits and putting it back together again, engine and all. Physically speaking, Beatrice was small, which would later prove advantageous.

At 15, she decided engineering was the career for her. The problem was that it was 1924. “The average woman does not possess the same engineering instinct as the average man,” was one opinion recorded in the Daily News at around that time. It belonged to the manager of the Education Research Department at British Westinghouse. “For a woman in the 1920s a career in lion-taming would have been more realistic,” observes Shilling’s biographer, Matthew Freudenberg.

But there was a glimmer of hope in the form of the Women’s Engineering Society. It was borne, in 1919, out of the need to protect the right to work of women who had been allowed to work in arms manufacture and other engineering trades during World War I.

In May 1926, the Society distributed a letter to girls schools throughout the country. Shilling’s mother convinced her to reply. By age 17, she became an apprentice electrical engineer learning the ropes, or more accurately cables, in the new electrical power plant at Bungay, Suffolk.

With support from the Women’s Engineering Society, Shilling joined the Department of Electrical Engineering at the Victoria University of Manchester in October 1929. She was one of only two women accepted that year; up from the none that had been accepted before. So her student record card didn’t allow for the possibility of female titles or honorifics.

Fortunately, her education afforded her the freedom to attend classes in thermodynamics and mechanical engineering, closer to her true interest: engines. Shilling received her Master of Science in 1933, and promptly joined a lecturer, Dr G.F. Mucklow, in researching fuel consumption, heat loss, and supercharger performance in Rolls-Royce and Napier single cylinder engines.

Young Beatrice Shilling
Young Beatrice Shilling

When Beatrice Shilling turned up at Brooklands Race Track on 24 August 1934, no one thought much of her new 490cc Norton motorcycle. Like the regular riders, Shilling had come for the Hutchinson 100 event and its attractive prize pot.

It takes two things to race at Brooklands: a motor vehicle, and the intestinal fortitude to use it at tremendous velocity. Even in 1906, it was a track built for sheer speed. More or less egg-shaped in plan, it consists of little more than long straights and even longer, steeply banked bends. And by 1934, the track had lost its smooth concrete complexion, having had potholes gouged out of it by decades of hulking racing cars.

Brooklands’ resulting holes were patched, but they still made for a bumpy surface, which was disconcerting at those 100-mph speeds. Worse, a section of the track crosses the river Wey, creating undulations capable of throwing fast racers clear of the track. To complicate matters further, this was on one of the banked turns. Those chasing the fastest lap times would race near the steepest, highest edge of the bank⁠—circumstances under which staying in contact with the ground is, according to conventional wisdom, a good idea.

Having given the Norton the once over, course handicapper “Ebby” Ebblewhite sent Shilling off as one of the first riders in the three-lap race. Because the event saw great power discrepancies between the competing machines, it was Ebby’s job to let the slowest bikes out first, with the most powerful 1,000cc “scratch bikes” starting last. An extract from the contemporary publication Motor Cycling records what happened next:

“A feature of the first handicap was the brilliant riding of Miss B. Shilling on a very standard-looking 490cc Norton. After a slowish first lap, she made up for lost time with a second circuit of 101.02 mph, thus joining the select ranks of Gold Star holders, being the second woman motorcycle racer to do so.”

Evidently impressed, Ebby put her on scratch for the next race, making her the first woman to do so when racing against men. She promptly lapped at 101.85 mph. Shilling was not the first woman to claim a Brooklands Gold Star⁠—that honor was accorded to Fanny Blenkiron in April 1934⁠—but Shilling would go on to set the record for the fastest woman to lap the circuit at 106 mph. Her record stands today.

Shilling was fast for several reasons. Her size was one advantage. At five feet tall, she was short enough to lie more or less flat, reducing drag. And though history doesn’t record all the specifics of the tweaks and modifications Shilling made to her machine, they were numerous. Freudenberg indicates that, in later letters to friends, she experimented with the length of inlet tract. This is the part of the engine that supplies the potent mixture of air and fuel to the cylinders, its length affecting power, torque, and fuel efficiency. Though she wouldn’t have known it at the time, Shilling was gaining the unique understanding that would prove so pivotal in saving the Spitfire and Hurricane in the depths of World War II: the physics of providing fuel to engines.

The following years saw more tweaks, greater speeds, and more impressive wins, including against professional racers, and track record holders like Ben Bickell and Noel Pope. It was 1935 when she set her enduring speed record.

Shilling starting a race at Brooklands Race Track
Shilling starting a race at Brooklands Race Track

In 1938 and 1939, the Norton motorbike was rebuilt from scratch with a homemade supercharger, and a new fuel tank where the saddle once was. Unfortunately, Shilling was too short to ride it. By then she was married, and her husband George Naylor, who stood a whole foot taller, took to racing instead. He became an able rider with the help of Shilling’s coaching, which was certainly unique: when they married in 1938, it had been on the condition that Naylor first earn himself a Brooklands Gold Star. It came in the nick of time. Racing at Brooklands ended forever in 1939. World War II had begun.

•     •     •

By the outbreak of war, Shilling had been working at the Royal Aircraft Establishment for more than three years. She initially wrote technical documentation, work that the hands-on Shilling found tedious. But, Freudenberg writes:

“By then Beatrice’s professional gifts had been recognized. She could identify the physical laws involved in a new problem or requirement, quantify them and design a mechanical solution. She could also design the tests that ensured that the instrument or engine performed as intended, and if not, why not.”

By November 1939, after a series of promotions, she had reached the position of Technical Officer in charge of carburettor research and development work. In other words, she was perfectly positioned to tackle the skittish Merlin engines of the Hurricane and the Spitfire.
In lieu of a fix, pilots had devised their own workarounds: dramatic maneuvers deployed if and when the engine cut out. Peter Brothers, a flight lieutenant with No. 32 Squadron, later recalled his own novel solution:

“When an enemy fighter dived from behind, fired and carried on diving past, one could not immediately dive in pursuit without the engine temporarily cutting and causing one to be left far behind. This could be avoided by rolling upside down, pulling back on the stick into a dive (positive g) then rolling level in the dive. […] Similarly, on sighting a target below, one suffered momentarily if one pushed the nose down to attack, a grave disadvantage.”

Tests ordered and overseen by Shilling identified the true problem: the rich cut that followed the weak cut addressed by Lovesey and Fisher.

Shilling worked out the precise volume and pressure of fuel being pumped into the chamber by the Merlin engine and designed a brass restrictor with a hole precisely the diameter needed to allow maximum flow of fuel, and therefore maximum power, without flooding the engine. Crucially, Shilling’s solution could be fitted without the removal of the carburettor, so the fix could be made in situ at operational airfields. Though it didn’t eliminate engine cut-outs altogether, it did minimize it to an acceptable degree.

This left only the logistical problem of how to get the restrictors to Fighter Command airfields in good time. Shilling organized a small band of engineers to assist, though inevitably she travelled up and down the country solo, and by her preferred mode of travel: her trusty Norton motorcycle. By then she’d made the small concession, perhaps under duress, to detune the Norton somewhat to make it more suitable for public roads.

“Her appearance at airfields with a bag of tools and a brisk manner became something of a legend,” Freudenberg writes. “Thanks to Sir Stanley Hooker, the engineer who led supercharger development at Rolls-Royce at the time, the restrictor became known as Miss Shilling’s orifice to pilots and fitters of the RAF.”

Early versions of the restrictor have been described as cone-shaped, though in her 2018 book Women of Invention: Life-Changing Ideas by Remarkable Women, Charlotte Montague describes the original restrictors as brass thimbles. They were later refined to a flat washer design, though inevitably Shilling and team would go on to eliminate them, and the entire negative gravity problem, by redesigning the carburettor outright.

Her efforts were of inestimable value to Fighter Command. Though the Battle of Britain, which prevented a Nazi invasion of Britain, was doubtless its crowning achievement, the ever-improving Spitfire would go on to perform operations for the rest of the war.

But fighting over mainland Europe, it was the British fighters that suffered from their limited range, and in some operations, losses were catastrophically high. It was due to these ongoing narrow margins in battle that we can be sure Shilling’s innovation saved lives.

Seldom-seen (and barely legible) schematic of the Merlin carburettor
Seldom-seen (and barely legible) schematic of the Merlin carburettor

Years later, Keith Maddock, chief engineer at Hangar 42, an RAF base during the war, went so far as to describe the restrictor as a war-winning modification. “Beatrice Shilling helped us to win World War II⁠—of that there is no doubt,” he told the BBC in 2017. Her war efforts weren’t limited to improvements to the Merlin engine. She also contributed to a range of engines to improve starting in freezing conditions, and operation at higher altitudes.

Though she undoubtedly deserved to rise to the top of the Royal Aircraft Establishment, this was prevented by the organization’s leadership, which was entirely male. She disliked formality and bureaucracy, joking after the war that Britain had been on the winning side due to a shortage of paper. Yet Freudenberg records that she was as capable of dealing with senior members of government and industry as successfully as anyone. Her technical knowledge was very possibly unsurpassed. If nothing else, she was eventually awarded the Order of the British Empire for her wartime efforts.

Biographer Montague likewise argues that a case can be made that Shilling helped to win the war. Unfortunately, it’s impossible to quantify her contribution. But at least as significant is the trail Shilling blazed as a pioneering woman in British engineering. She later served on the committee of the Women’s Engineering Society, and actively encouraged young women into engineering careers. She is often described as “a flaming pathfinder of Women’s Lib,” including by her alma mater, now Manchester University. (Though in Spitfire: The Biography, writer Jonathan Glancey attributes the descriptor to a colleague of Shilling’s.)

After the war, Shilling was compelled to work in the burgeoning fields of rocketry, ramjets, and guided weaponry. The war may have been won, but with NATO powers vying with Russia, military and aeronautical supremacy remained high on Britain’s agenda. In 1955, she was promoted to be the Royal Aircraft Establishment’s Senior Principal Scientific Officer. By 1957, she was in charge of heat tests on scale models of the liquid oxygen tanks of what would become the Blue Streak missile: initially developed as part of Britain’s nuclear deterrent and later repurposed as the European Space Agency’s Europa satellite launch system.

Shilling died on 04 November 1990 from cancer of the spine. Her husband George Naylor, who himself had gone on to fly Lancaster bombers during World War II, died six years later. He had always been proud of Shilling, and especially her solution to the problem of negative gravity.

The question remains as to what Shilling herself thought of the dubious term “Miss Shilling’s orifice.” Freudenberg reckons that it “probably amused her, recognising the language of familiarity rather than disrespect.” By the time the words were being bandied about Britain’s airfields she was certainly held in high esteem, and Freudenberg records her dry sense of humor and “unrepentant brevity.”

He has a point. After the war, Shilling had taken to racing cars rather than bikes, but that came to a sudden end in a bad crash on 23 June 1962. Naylor noted at the time that Shilling “was virtually pushed off course by a clot who could not drive and who was determined not to be beaten by a woman.”

Shilling’s take? “He was an ex-RAF pilot, so he was too busy checking the instruments to look where he was going.”