Supermarine Spitfire variants: specifications, performance and armament
The British Supermarine Spitfire was one of the most popular fighter aircraft of the Second World War. The basic airframe proved to be extremely adaptable, capable of taking far more powerful engines and far greater loads than its original role as a short-range interceptor had allowed for. This would lead to 24 marks of Spitfire, and many sub-variants within the marks, being produced throughout the Second World War and beyond, in continuing efforts to fulfill Royal Air Force requirements and successfully combat ever-improving enemy aircraft.
The Spitfire was also adopted for service on aircraft carriers of the Royal Navy; in this role they were renamed Supermarine Seafire. Although the first version of the Seafire, the Seafire Ib, was a straight adaptation of the Spitfire Vb, successive variants incorporated much needed strengthening of the basic structure of the airframe and equipment changes in order to survive the demanding maritime environment. As a result, the later Seafire variants were usually heavier and, in the case of the Seafire XV/XVII and F. 47 series, they were very different aircraft to their land-based counterparts.
It is notable that throughout the entire development process, which took place over twelve years, from 1935 through to 1948, there were no outstanding failures of the basic design: this is a real testament to the original genius of Reginald J. Mitchell, his successor Joseph Smith, and the design teams they led.
The Rolls-Royce Merlin and Griffon engines
A key factor which allowed the continued development of the Spitfire was the development of progressively more powerful and improved engines, starting with the Rolls-Royce Merlin and progressing to the bigger and more powerful Rolls-Royce Griffon. The evolution of high octane aviation fuels and improved supercharger designs enabled Rolls-Royce to extract increasing amounts of power from the same basic designs. For example, the Merlin II and III which powered the Spitfire I produced a maximum of 1,030 hp using the 87 octane aviation fuel which was generally available from 1938 through to 1941; from early 1940 increasing supplies of 100 octane fuel allowed the maximum power to be increased to 1,310 hp with an increased supercharger boost pressure, albeit for a maximum time limit of 5 minutes. In 1944 100/150 grade fuels enabled the Merlin 66 to produce 1,860 hp at low altitudes in F.S gear.Single stage superchargers
Depending on the supercharger fitted, engines were rated as low altitude, where the engine produced its maximum power below about, medium altitude, where the engine produced its maximum power up to about, and high altitude, where the engine produced its maximum power above about. As a result, the prefixes which were used on most later Spitfire variants, L.F. Mark..., F. Mark.. and H.F Mark indicated whether the engines fitted were suited for low, medium or high altitude. The use of these prefixes did not change according to the wings, which could be fitted with "clipped" tips, reducing the wingspan to about 32 ft 6 in , or the "pointed" tips which increased the wingspan to 40 ft 2 in.The original Merlin and Griffon engine designs used single-stage superchargers. For engines equipped with a single-stage supercharger the air being forced through the supercharger air intake was compressed by the supercharger's impeller. In the case of the Merlin II/III, XII and 40 series as the air was being compressed it was mixed with fuel which was fed through an SU carburettor before being fed into the engine's cylinders. The Merlin III produced 1,030 hp at +6¼lb/in² of "boost". The limitation of the single stage supercharger was that the maximum power dropped quickly as higher altitudes were reached; because air pressure and air density decreases with altitude the efficiency of a piston engine drops because of the reduction in the weight of air that can be drawn into the engine; for example the air density, at is 1/3 of that at sea level, thus only 1/3 of the amount of air can be drawn into the cylinder and only 1/3 of the fuel can be burnt.
Two-Stage, Two-Speed superchargers
The most fundamental change made to the later Merlin and Griffon engines was the incorporation of a two-stage, two-speed supercharger, which provided a considerable increase in power, especially at higher altitudes. Two-stage refers to the use of two impellers on a common driveshaft, constituting two superchargers in series. As air was drawn through the air intake, fuel was pumped into the airstream by the carburettor. The first-stage impeller compressed the air-fuel mixture and this was then fed to the smaller second-stage impeller which further compressed the mixture.The impellers were driven by a hydraulically operated two-speed gearbox. At low to medium altitudes, the supercharger was in Moderate Supercharger or M.S. gear. Once the aircraft reached and climbed through a set critical altitude, the power would start to drop as the atmospheric pressure dropped. As the critical altitude was passed a pressure-operated aneroid capsule operated the gearbox, which changed speed to Full Supercharger gear, which drove the impellers faster, thus compressing a greater volume of the air-fuel mixture.
An intercooler was required to stop the compressed mixture from becoming too hot and either igniting before reaching the cylinders or creating a condition known as knocking or detonation. The intercooler, which was separate from the engine cooling system, with its own supply of glycol and water coolant, was mounted in the induction system, between the outlet of the second-stage supercharger and behind the cylinder blocks. The hot air-fuel mixture from the supercharger was circulated though and around the coolant tubes and was then passed on to the main induction manifold, through which it was fed into the cylinders. The intercooler also circulated coolant through passages in the supercharger casing and between the impellers. Finally, an extra radiator was used to dissipate the intercooler's excess charge heat.
With the two-stage, two-speed supercharger, two sets of power ratings can be quoted. As an example, the maximum power generated by the Merlin 61 was 1,565 hp at at M.S. speed, using + 15 lb/in² "boost". The F.S. gear required approximately 200 hp to drive it. As a result, the maximum power generated by the Merlin 61 in F.S. was 1,390 hp at using + 15 lb/in² of boost. The Merlin 66 used in the L.F. Mk IX produced slightly more power but because of the use of slightly different gear ratios driving smaller impellers, the critical altitude ratings of the supercharger stages were lower, and respectively. By contrast the Merlin 70, which was optimised for high-altitude flight, had critical altitudes of and .
Carburettors
The original production variants of the Merlin used an SU manufactured carburettor in which the fuel flow was metered through a float. In most circumstances this proved to be sufficient but during the air battles over Dunkirk and during the Battle of Britain it was found that whenever the Merlin was subjected to negative "g" forces, such as a quick "bunt" into a dive, the engine would briefly lose power through petrol starvation. This was because the petrol in the float was being thrown away from the feed pipe to the supercharger. The fuel injected Daimler-Benz DB 601 engine gave the Bf 109 especially an advantage over the carburettor-equipped engine; no Spitfire could simply "bunt" and dive away from an opponent as the 109 could.The remedy, invented by Beatrice "Tilly" Shilling, was to fit a metal diaphragm with a hole in it, across the float chambers. It partly cured the problem of fuel starvation in a dive. The device was commonly referred to as 'Miss Shilling's Orifice'.
The full remedy was to use the Bendix-Stromberg pressure carburettor, which allowed more precise metering of the amount of fuel used by the engine and prevented fuel starvation. This new carburettor was used from the Merlin 66 series and on all Griffon engines. In these engines the carburettor injected fuel at 5 psi through a nozzle direct into the supercharger and the compressed air-fuel mixture was then directed to the cylinders. The final development was the SU injection carburettor, that injected fuel into the supercharger using a fuel pump driven as a function of crankshaft speed and engine pressures; although this was fitted to the 100 series Merlins, which were not used in production Spitfires, it was used in the Griffon 60 and 80 series.
Boost pressure measurements
The British measured boost pressure as lbs./sq.inch above a nominal value of atmospheric pressure at sea level. A reading of +6 meant that the air/fuel mix was being compressed by a supercharger blower to 20.7 psi before entering the engine; +25 meant that the air/fuel mix was being compressed to 39.7 psi – 14.7 psi atmospheric pressure added to the "boost" pressure of 25 psi. Typically "absolute pressure" is indicated in inches of mercury because "absolute pressure" is dependent on multiple atmospheric influences and can be used to indicate and measure "vacuum", or pressure below atmospheric pressure, as well. Mercury-filled manometers that apply pressure directly to a column of liquid have much greater vacuum range when the fluid is much more dense than water.Subjecting water to pressures above or below atmospheric pressure raise or lowers its triple points and causes it to boil at "standard" temperature if subjected to near one atmosphere of "vacuum" Such vacuum levels are common in spark-ignition internal-combustion engine intake manifolds at "idle" speeds and loads rendering water useless as a liquid-column manometer test medium. Even at lower vacuum levels it evaporates to readily and quickly to provide consistent vacuum readings.
"Absolute pressure" also has an atmospheric pressure-independent alternative called "gauge pressure" and knowing which is in use in technical information is crucial to proper use of indicated pressure data. Since atmospheric pressure is a given and a "constant" a "boost gauge" or manometer calibrated to indicate 0 psi or in.-Hg. at sea level on a standard day allows "boost" OR "vacuum" to be directly displayed and "read" as "gauge pressure". Absolute pressure requires subtracting 14.7 psi from "boost pressure" or adding it to indicated "vacuum" to determine "gauge" or actual intake manifold pressure.
At idle speed and load with nearly closed throttle "butterflies" and during part-throttle, light-load operation, even supercharged spark-ignition engines have intake manifold "vacuum" due to the intake restriction of carburetor venturis and/or throttle "butterflies". And "vacuum" is used to properly tune carburetors or fuel injection systems and ignition timing as well as determine engine condition and diagnose engine malfunctions, mis-adjustments and failures. engine problems result in "vacuum leaks" or levels and the intensity and timing and duration of constant or intermittent vacuum level departures from normalare used to diagnose internal engine problems without engine disassembly and inspection. Such departures are generally far too small to make inflated "absolute" vacuum pressure use practical or even possible.
Therefore, "gauge pressures" independent of atmospheric pressure-influenced "absolute" pressures are preferred for monitoring, tuning and diagnosing engine operation and performance when mechanical fuel systems and gauges are in use. Only electronic engine control and monitoring systems used in conjunction with "manifold absolute pressure sensors" capable of replacing vacuum/boost gauges and turbocharging which is capable of providing increased "boost" as engine speed decreases under load have made "absolute pressure" a useful and important manifold pressure measurement. Outside of MAP sensor data "gauge pressure" is typically used for engine system vacuum and pressure measurement. Which is why "boost gauges" almost universally indicate 0 psi at idle speed and load.
| Inches of Mercury | Pounds of Boost |
| 80.9" of mercury= | +25 lb boost |
| 66.6" of mercury= | +18 lb boost |
| 60.5" of mercury= | +15 lb boost |
| 48.3" of mercury= | +9 lb boost |
| 42.2" of mercury= | +6 lb boost |