Supermarine Spitfire

A total of 81 Mk 24s were completed, 27 of which were conversions from Mk 22s.

The last Mk 24 to be built was delivered in February 1948.

They were used by only one RAF squadron, [Vous devez être inscrit et connecté pour voir ce lien], until 1952. Some of the squadron's aircraft went to the Hong
Kong Auxiliary Air Force where they were operated until 1955.

Introduced into service in 1946, the F. Mk 24 differed greatly from the original Spitfire Mk I in many respects and undoubtedly brought the design to the peak of perfection, being twice as heavy, more than twice as powerful and exhibiting an increase in climb rate of 80% over the prototype aircraft, 'K5054'.

These remarkable increases in performance arose chiefly from the introduction of the Rolls-Royce Griffon engine in
place of the famous Merlin of earlier variants.

Rated at 2,050 hp (1,530 kW), the twelve-cylinder Vee liquid cooled Griffon 61 engine featured a two-stage supercharger, giving the Spitfire the exceptional performance at high altitude that had been sometimes lacking in early
marks.

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Supermarine Spitfire variants : specifications, performance and armament


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The Rolls-Royce Merlin and Griffon engines


One key factor which influenced the development of the Spitfire was the improvement in its engine, firstly the [Vous devez être inscrit et connecté pour voir ce lien] and then the more powerful [Vous devez être inscrit et connecté pour voir ce lien].

The evolution of high [Vous devez être inscrit et connecté pour voir ce lien] and improved [Vous devez être inscrit et connecté pour voir ce lien] designs enabled [Vous devez être inscrit et connecté pour voir ce lien] 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 [Vous devez être inscrit et connecté pour voir ce lien] (770 kW) using the 87 [Vous devez être inscrit et connecté pour voir ce lien] aviation fuel which was generally available in 1940; from early 1940 increasing supplies of 100 octane fuel allowed the maximum power to be increased to 1,310 hp (977 kW).

100/150 grade fuels enabled the Merlin 66 to produce 1,860 hp (1,387 kW) in F.S gear.

Single stage and two-stage superchargers


Depending on the [Vous devez être inscrit et connecté pour voir ce lien] fitted engines were rated as low altitude (eg; Merlin 66, Griffon III), where the engine produced its maximum power below about 10,000 feet (3,000 m), medium altitude (Merlin 45), where the engine produced its maximum power up to about 20,000 feet (6,100 m), and high altitude (Merlin 70), where the engine
produced its maximum power above about 25,000 feet (7,600 m).


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 (9.9 m) (this could vary slightly), or the "pointed" tips which increased the wingspan to 40 ft 2 in (12.29 m).

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 [Vous devez être inscrit et connecté pour voir ce lien].

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 [Vous devez être inscrit et connecté pour voir ce lien] before being fed into the engine's [Vous devez être inscrit et connecté pour voir ce lien].

The Merlin III produced 1,030 [Vous devez être inscrit et connecté pour voir ce lien] (770 kW) at +6¼lb/in² (43 [Vous devez être inscrit et connecté pour voir ce lien]) of "boost" (the "boost" is the pressure to which the air/fuel mixture is compressed before being fed to the cylinders).

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 30,000 feet (9,100 m) is 1/3rd of that at [Vous devez être inscrit et connecté pour voir ce lien], thus only 1/3rd of the amount of air can be drawn into the cylinder and only 1/3rd of the fuel can be burnt.

A supercharger can be thought of either as artificially increasing the density of the air by compressing it - or as forcing more air than normal into the cylinder every time the piston moves down.

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 available power, especially at higher altitudes.

Two-stage refers to the use of two impellers on a common driveshaft in effect 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 impellor compressed the air/fuel mixture and was then fed to the smaller second-stage impellor which further compressed the mixture.

The impellors were driven by a hydraulically operated two-speed gearbox.

At low to medium altitudes the supercharger was in Moderate Supercharger or M.S gear
(this referred to the speed at which the impellers were operating).

Once the aircraft reached and climbed through a set critical altitude (20,000 feet (6,100 m) for the Merlin 61
and 70 series) the power would start to drop as the atmospheric pressure, hence the weight of air dropped.

As the critical altitude was passed a pressure-operated [Vous devez être inscrit et connecté pour voir ce lien] capsule operated the gearbox which changed up to Full Supercharger (F.S) driving the impellers faster, thus compressing a greater volume of the air-fuel mixture.

Merlin 61 RR

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An [Vous devez être inscrit et connecté pour voir ce lien], was required to stop the compressed mixture from becoming too hot and either igniting before reaching the cylinders (pre-ignition knocking), or creating a condition known as [Vous devez être inscrit et connecté pour voir ce lien].

The intercooler, which was entirely 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 (mounted in the starboard radiator duct under the wing of the Spitfire) was used to dissipate the intercooler's excess charge temperature.

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 (1,167 kW) at 12,250 feet (3,730 m) (critical altitude) at M.S speed, using + 15 lb/in² "boost".

The F.S gear required approximately 200 hp (149 kW) to drive it.

As a result the maximum power generated by the Merlin 61 in F.S was 1,390 hp (1,036 kW) at 25,900 feet (7,900 m) 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 [Vous devez être inscrit et connecté pour voir ce lien] driving smaller impellors, the critical altitude ratings of the supercharger stages were lower, 7,000 feet (2,100 m) and 18,000 feet (5,500 m) respectively.

By contrast the Merlin 70, which was optimised for high altitude flight, had critical altitudes of 14,000 feet (4,300 m) (M.S) and 25,400 feet (7,700 m) (F.S).

Unlike the Merlin engines the Griffons used superchargers which were designed to achieve maximum performance over a wider altitude band; as such there were no Griffon engined L.F or H.F Spitfire variants.

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.

However, during the air battles over Dunkirk and during the [Vous devez être inscrit et connecté pour voir ce lien] 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 [Vous devez être inscrit et connecté pour voir ce lien] 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 a female engineer, [Vous devez être inscrit et connecté pour voir ce lien], was to fit a metal [Vous devez être inscrit et connecté pour voir ce lien] with a hole in it was fitted across the float chambers.

It partly cured the problem of [Vous devez être inscrit et connecté pour voir ce lien] in a dive.

The full remedy was to use the [Vous devez être inscrit et connecté pour voir ce lien], which allowed more precise metering of the amount of fuel used by the engine and prevented the problem of fuel starvation which had affected RAF fighters during the Battle of Britain.

This new carburettor was used from the Merlin 66 series and on all Griffon engines.

In these engines the carburettor injected fuel at 5 [Vous devez être inscrit et connecté pour voir ce lien] 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 which 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 (or psi).

The normal atmospheric pressure at sea level is 14.5 psi, so a reading of +6 means that the air/fuel mix is being compressed by a supercharger blower to 20.5 psi before entering the engine; +25 means that the air/fuel mix is now being compressed to 39.5 psi.

The Americans measured their boost ratings using inches of [Vous devez être inscrit et connecté pour voir ce lien] (" Hg).

One pound of boost equals 2.04 inches (52 mm) of Mercury (Hg).

Dimensions, performance and armament

Due to the many differences in production Spitfires, performance could vary widely, even between aircraft with the same [Vous devez être inscrit et connecté pour voir ce lien].

Factors such as weight, external fittings, [Vous devez être inscrit et connecté pour voir ce lien] and engine condition, among others, influenced how an aircraft
performed.

For example, even relatively minor damage on the wing leading edges could drastically reduce top speed.

The most reliable performance figures and weight measurements came from the tests carried out throughout the Second World War by the [Vous devez être inscrit et connecté pour voir ce lien] (A&AEE) based at [Vous devez être inscrit et connecté pour voir ce lien].

Early Merlin engines


Supermarine Spitfire- Merlin engine variantsMk IA (K9793-early production, de Havilland two speed propeller)Mk IIA (P7280)Mk VB (W3134)Mk VI (AB200)L.F Mk VIII (JF880)

Wingspan	36 ft 10 in (11.23 m)	36 ft 10 in (11.23 m)	36 ft 10 in (11.23 m)	40 ft 2 in (12.29 m)	36 ft 10 in (11.23 m)
Wing area	242.1 ft² (22.5 m²)	242.1 ft² (22.5 m²)	242.1 ft² (22.5 m²)	248.5 ft² (23.1 m²)	242.1 ft² (22.5 m²)
Length	29 ft 11 in (9.12 m)	29 ft 11 in (9.12 m)	29 ft 11 in (9.12 m)	29 ft 11 in (9.12 m)	31 ft 6 in (9.58 m) (late with enlarged rudder)
Height	9 ft 10 in (3.02 m)	9 ft 10 in (3.02 m)	11 ft 5 in (3.48 m)	11 ft 5 in (3.48 m)	12 ft 8 in (3.86 m)
Empty weight	4,306 lb (1,953 kg)	4,541 lb (2,059 kg)	4,963 lb (2,251 kg)	5,075 lb (2,302 kg)	5,190 lb (2,354 kg)
Loaded weight	5,935 lb (2,692 kg)	6,172 lb (2,799 kg)	6,525 lb (3,071 kg)	6,740 lb (3,057 kg)	7,990 lb (3,624 kg)
Engine	[Vous devez être inscrit et connecté pour voir ce lien] III	Rolls Royce Merlin XII	Rolls-Royce Merlin 45	Rolls-Royce Merlin 47	Rolls-Royce Merlin 66
Power	1,030 hp (770 kW) at 16,000 ft (4,877 m) 87 Octane fuel, +6 lb/in² boost	1,135 hp (846 kW) at 12,250 ft (3,734 m) 100 Octane fuel, +9 pounds lb/in² boost	1,470 hp (1,096 kW) at 11,000 ft (3,353 m)	1,415 hp (1,054 kW) at 14,000 ft (3,353 m)	1,720 hp (1,283 kW) at 5,750 ft (1,752 m)
[Vous devez être inscrit et connecté pour voir ce lien]	367 mph (582 km/h) at 18,600 ft (5,669 m)	354 mph (570 km/h) at 17,550 ft (5,349 m)	371 mph (597 km/h) at 20,000 ft (6,096 m)	354 mph (570 km/h) at 17,400 ft (5,349 m)	404 mph (650 km/h) at 21,000 ft (6,401 m)
[Vous devez être inscrit et connecté pour voir ce lien]	2,175 ft/min (11.0 m/s) at 9,700 ft (2,956 m)	2,995 ft/min (15.3 m/s) at 10,000 ft (3,962 m)	3,250 ft/min (16.5 m/s) at 15,000 ft (4,572 m)	1,350 ft/min (13.5 m/s) at 28,000 ft (4,267 m)	4,660 ft/min (23.7 m/s) at sea level
[Vous devez être inscrit et connecté pour voir ce lien][nb 6]	34,400 ft (10,485 m)	37,600 ft (11,460 m)	35,000 ft (11,300 m)	39,200 ft (11,064 m)	41,500 ft (12,649 m)
[Vous devez être inscrit et connecté pour voir ce lien]	24.5 lb/ft² (117 kg/m²)	25.4 lb/ft² (122 kg/m²)	27.9 lb/ft² (137 kg/m²)	27.0 lb/ft² (137 kg/m²)	32.0 lb/ft² (155 kg/m²)
[Vous devez être inscrit et connecté pour voir ce lien]	0.17 hp/lb (0.28 kW/kg)	0.18 hp/lb (0.30 kW/kg)	0.22 hp/lb (0.36 kW/kg)	0.21 hp/lb (0.35 kW/kg)	0.21 hp/lb (0.35 kW/kg)
Combat range	425 mi (680 km) on internal fuel	405 mi (651 km) on internal fuel	470 mi (760 km) on internal fuel	428 mi (688 km) on internal fuel	680 mi (1,094 km) on internal fuel, 1,180 mi (1,899 km) 90 Imp gal (108 US gal) drop tank
Ferry range			1,135 mi (1,827 km)		1,530 mi (2,462 km) with 170 Imp gal (204 US gal) drop tank
Armament	8 × [Vous devez être inscrit et connecté pour voir ce lien] machine guns; 350 rpg	8 × 0.303" Browning machine guns; 350 rpg	2 × 20 mm (0.79 in) [Vous devez être inscrit et connecté pour voir ce lien] cannon; 60 round drum 4 × 0.303" Browning machine guns; 350 rpg 2 × 250 lb (113 kg) or 1 × 500 lb (227 kg) bombs	2 × 20 mm (0.79 in) Hispano II cannon; 60 round drum 4 × 0.303" Browning machine guns; 350 rpg	2 × 20 mm Hispano II cannon,; 120 rpg 4 × .303" Browning machine guns; 350 rpg 2 × 250 lb (110 kg) with 1 × 500 lb (230 kg) bomb