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. Its iconic thin elliptical wings minimised drag, making it incredibly agile. But, its 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.
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 a 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 two valves in the carburettor. One is called 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 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.
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.
Read more "The one error that Spitfire Designers made"