{{short description|Fuel flow restrictor retro-fitted to Merlin engines}} {{Use dmy dates|date=April 2015}} {{Use British English|date=April 2015}} 300px|thumb|upright=1.14|The Rolls-Royce Merlin engine originally came with an updraught carburettor, prone to cut-out due to fuel flooding in negative G. [[file:Beatrice SHILLING (cropped).jpg|thumb|Beatrice Shilling]] '''Miss Shilling's orifice''' was a technical device created to counter engine cut-outs experienced during negative G manoeuvres in early Spitfire and Hurricane fighter aircraft during the Battle of Britain. Officially called the '''R.A.E. restrictor''', it was referred to under various names, such as '''Miss Tilly's diaphragm''' or the '''Tilly orifice''' in reference to its inventor, Beatrice "Tilly" Shilling.<ref>[https://www.youtube.com/watch?v=-InZ5B8EmNg How did She Save Britain’s Troubled WW2 Spitfire Warplane?] www.youtube.com</ref>
==Engine cut-out problems== Early versions of the Rolls-Royce Merlin engine came equipped with dual-choke updraught SU carburettor. When an aeroplane equipped with such an engine performed a negative g force manoeuvre (pitching the nose hard down), 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.<ref>{{harvnb|Price|1986|p=78}}</ref>
During the Battle of France and Battle of Britain, German fighters had gasoline direct injection engines and did not suffer from this problem as injection pumps kept fuel at constant pressure. German pilots could exploit this by pitching steeply forward while opening the throttle, a manoeuvre that pursuing British aircraft 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 RAE restrictor== Complaints from pilots over engine cut-out during dives and brief inverted flight led to concentrated search for a solution. Engine manufacturer Rolls-Royce produced an improved carburettor, but this failed in testing. Beatrice Shilling, an engineer working at the Royal Aircraft Establishment at Farnborough Airfield, invented a device which could be fitted without taking the aircraft out of service. She designed a thimble-shaped brass flow restrictor with precisely calculated dimensions to allow just enough fuel flow for maximum engine power. It came in two versions, one for {{convert|12|psi|kPa|link=on}} manifold pressure and another for the {{convert|15|psi|kPa|abbr=on}} achieved by supercharged units.<ref>{{harvnb|Lumsden|2003|p=32}}</ref> The design of the thimble-shaped flow restrictor was later refined to a simplified flat brass washer.
While not completely solving the problem, the restrictor, along with modifications to the needle valve, permitted pilots to perform quick negative g manoeuvres without loss of engine power. This improvement solved the drawback of the RAF's Rolls-Royce Merlin-powered fighters versus the German Messerschmitt Bf 109E machine. The Daimler-Benz DB 601 inverted V12 powerplant had utilised fuel injection since 1937. During early 1941, Shilling travelled with a small team to fit the restrictors in one RAF base after another, giving priority to front-line units. By March 1941 the device had been installed throughout RAF Fighter Command. Officially named the 'R.A.E. restrictor', the device was immensely popular with pilots, adopting the affectionate nickname 'Miss Shilling's orifice' (or simply the 'Tilly orifice'), given to the restrictor by Sir Stanley Hooker, the engineer who led supercharger development at Rolls-Royce.<ref>{{harvnb|Freudenberg|2003}}</ref>
This measure was only a stopgap: it did not allow inverted flight for any length of time. The problems were not finally overcome until introduction of the Bendix pressure carburettor in 1943.
==References==
{{reflist}}
===Sources=== {{refbegin}} * {{Cite book |last=Lumsden |first=Alec |title=British Piston Engines and their Aircraft |date=2003 |publisher=Airlife Publishing |isbn=1-85310-294-6 |location=Marlborough, Wiltshire}} * {{Cite book |last=Price |first=Alfred |title=The Spitfire Story: Second edition |date=1986 |publisher=Arms and Armour Press Ltd. |isbn=0-85368-861-3 |location=London}} * {{Cite book|last=Freudenberg|first=Matthew|title=Negative Gravity: A Life of Beatrice Shilling|publisher=Charlton Publications|year=2003|isbn=978-0954616502|location=England}} {{refend}}
== Further reading == {{refbegin}} * {{Cite book |last=Rearden |first=Jim |url={{google books|plainurl=yes|id=wn_xAAAAMAAJ}} |title=Koga's Zero: The Fighter That Changed World War II |date=1995 |publisher=Pictorial Histories Publishing Company |isbn=0-929521-56-0 |edition=second |location=Missoula, Montana}} **Originally published as {{Cite book |title=Cracking the Zero Mystery: How the U.S. Learned to Beat Japan's Vaunted WWII Fighter Plane |isbn=978-0-8117-2235-3 |last1=Rearden |first1=Jim |date=1990 |publisher=Stackpole Books }} *{{Cite web |last=Holloway |first=James |date=October 2020 |title=How Miss Shilling's Orifice Helped Win the War |url=https://www.damninteresting.com/how-miss-shillings-orifice-helped-win-the-war/ |access-date=2025-01-02 |website=Damn Interesting |language=en-US}} *{{Cite web |last=Spencer |first=Michael |date=2017-09-15 |title="Miss Shilling's Orifice": Simple solutions to technical issues can make all the difference |url=https://www.williamsfoundation.org.au/post/miss-shilling-s-orifice-simple-solutions-to-technical-issues-can-make-all-the-difference-em-mi |access-date=2025-01-02 |website=Williams Foundation |language=en}} {{refend}}
==External links== *[https://m.youtube.com/watch?v=YzRlga2-Hh Video] explaining the Merlins carburettor and the Tilly orifice
Category:Rolls-Royce aircraft piston engines Category:Engine technology