The Spitfire’s Fatal Flaw

There are few planes in history that are as admired as the Supermarine Spitfire, it played a pivotal role in the battle for air supremacy during World War 2. It’s iconic thin elliptical wings minimised drag, making it incredibly agile. But, it’s German counterpart the Messerschmitt was a worthy adversary. Both planes could reach speeds of up to 560 km/h, powered by huge V-12 piston engines. They were remarkably similar planes, featuring low-slung wings, which increased their manoeuvrability. A rapid departure from the biplane era from which their pilots came.

Both planes featured retractable landing gears, which many pilots were unfamiliar with causing some to forget to lower their wheels for landings. But the early versions of the Spitfire had an alarming flaw, when performing negative g manoeuvres the engine would cut-out. I believe you can actually see this in the opening shot of the 1969 movie Battle of Britain, listen for the sound of the engine cutting out, followed by black smoke, a sign of a surplus fuel entering the engine. To understand what is happening here we need to learn what a carburettor does and how it works. A carburettor is a device that blends air and fuel for an internal combustion engine.

The carburettor used in the Merlin engine used a float to control the flow of fuel into the carburettor tank, similar to how your toilet cistern works. The level here is important as it effects the flow rate into the mixing chamber. A higher level will result in a higher pressure at the bottom of the fuel chamber. There is a low pressure zone created at the nozzle with the venturi effect, which is the reduction in fluid pressure as it passes through a constriction. This pressure difference draws fuel from the float chamber, a larger pressure difference will result in a richer fuel mixture. This air/fuel mixture continues on and enters the piston cylinder to power the engine.

The air/fuel mixture percentage can be altered by opening and closing these two valves. This is the choke valve which closes when starting the engine, this increases the drop in pressure even more and draws more fuel from the float chamber, resulting in a richer air fuel mixture, which is needed when starting the engine, but an over-rich fuel mixture will not cause an increase in power it will actually stall the engine, remember this for later. The second valve is the throttle valve, which is controlled by the accelerator. When the accelerator is fully depressed this valve will be open to allow the maximum amount of air and fuel to enter the engine. Now this type of carburettor was okay for early cars, but for a plane that can turn upside and enter deep dives, it struggles. If a plane fitted with this system enters a negative g dive fuel is forced to the top of the float chamber, this results in the first loss of power as fuel can no longer enter the engine, but the float is now forced down, which opens this needle valve allowing fuel to enter the float chamber, if this manoeuvre is held too long too much fuel will enter the float chamber resulting in an over rich air fuel mixture will flood the engine and stall it, possibly making it impossible to start again. Now that clip makes a lot more sense, as the plane turns over the engine is starved of fuel and shuts off, then as it returns to level flight an over-rich fuel mixture enters the engine resulting in the black smoke. German pilots quickly realised this flaw and it gave them an edge in dog fights as their planes used a fuel injection system, which didn’t suffer from this problem. With the war raging Britain needed a quick fix for this problem and this is where Beatrice Shilling came in.

Beatrice was a young female engineer working for the Royal Aircraft Establishment. She came up with a beautifully simple stopgap solution for the problem. She installed a simple brass ring between the end of the fuel intake pipe and the entrance to the carburettor chamber. This restricted fuel flow into the carburettor to the maximum it needed during a dog fight. This did not solve the initial fuel starvation problem, but it did delay the more serious problem of flooding the engine. This allowed the spitfire pilots to be far more competitive until the new pressure carburettors were installed in 1942. Thanks for watching! At the start of this year I wrote down my ambition for the year and that was to reach 100,000 subscribers and it looks like I will be reaching that goal quicker than I ever expected. I want to thank you all and celebrate with a message that is important to me. One of the reasons I created this channel was to highlight some of the remarkable engineers, who’s work transformed this world. But out of the last 10 videos I have not mentioned a single female engineer, which is reflective of the actual numbers of women entering the engineering field and that’s a shame. I’m not sure the reasons for this, but I truly believe we need to bring attention to some of the great female role models in engineering. Beatrice Shilling is just one of them. Together I hope we can inspire a new generation of children that want to use their talents to make this world a better place. If you have a younger sister or a daughter, please share this video with them and let them see the impact they can have on the world.

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