The British Supermarine Spitfire was one of the most outstanding 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

One key factor which influenced the development of the Spitfire was the improvement in its engine, firstly the Rolls-Royce Merlin and then the 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 (770 kW) using the 87 octane aviation fuel which was generally available in 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 (FS gear) (1,387 kW).

Single stage and two-stage superchargers

Depending on the supercharger fitted engines were rated as low altitude (eg; Merlin 66, Griffon III), where the engine produced its maximum power below about , medium altitude (Merlin 45), where the engine produced its maximum power up to about , and high altitude (Merlin 70), 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 (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 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 (770 kW) at +6 pounds 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 is 1/3rd of that at sea level, 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.

One of the most fundamental changes 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. As a result of this, in respect to the Merlin 61 series, used from the Mk IX on, and the Griffon 61 series of the Mk XIV on, two sets of power ratings can be quoted because these engines were fitted with two-stage, two-speed superchargers. As an example, the maximum power generated by the Merlin 61 was 1,565 hp (1,167 kW) at (critical altitude) in the first stage of the supercharger or Medium Speed (Better known as M.S, referring to the speed at which the impellor was driven) stage, using + 15 pounds "boost". Once the aircraft reached and climbed through the critical altitude the power would start to drop as the atmospheric pressure, hence the weight of air dropped; a second stage compressor was needed to allow the engine to continue to perform at higher altitudes. When the aircraft reached an altitude of for the Merlin 61 the second Fast Speed (F.S) stage of the supercharger was started; the air/fuel mixture coming through the M.S stage then flowed through an intercooler and then into the second stage to be compressed again. An intercooler was required to stop the compressed mixture from becoming too hot and either igniting before reaching the cylinders (preignition knocking), or creating a condition known as knocking or detonation. This second stage blower 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 , using + 15 in of boost.

The Merlin 66 used in the L.F Mk IXs produced slightly more power but, because of the use of slightly different gear ratios for the impellers, the critical altitude ratings of the supercharger stages were lower, and respectively. This allowed the L.F Mk IX to produce its optimum performance at altitudes where experience showed most air-to-air combat over Europe was taking place.

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 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 a female engineer, Beatrice "Tilly" Shilling, was to fit a metal diaphragm with a hole in it was fitted across the float chambers. It partly cured the problem of fuel starvation in a dive.

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 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 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 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 Mercury (" Hg). One pound of boost equals of Mercury (Hg).

Inches of Mercury (" Hg)	Pounds of Boost
80" of mercury=	+25 lb boost
67" of mercury=	+18 lb boost
61" of mercury=	+15 lb boost
46" of mercury=	+8 lb boost
44.5" of mercury=	+6 lb boost

Dimensions, performance and armament

Due to the many differences in production Spitfires, performance could vary widely, even between aircraft with the same Mark number. Factors such as weight, external fittings, airframe 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 Aeroplane & Armament Experimental Establishment (A&AEE) based at Boscombe Down.

Early Merlin engines

Supermarine Spitfire- Merlin engine variants
	Mk 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	Rolls-Royce Merlin 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/ft² boost	1,135 hp (846 kW) at 12,250 ft (3,734 m) 100 Octane fuel, +9 pounds lb/ft² 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)
Maximum speed	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)
Rate of climb	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
Service ceiling	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)
Wing loading	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)
Power/mass	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 × 0.303" Browning machine guns; 350 rpg	8 × 0.303" Browning machine guns; 350 rpg	2 × 20 mm (0.79 in) Hispano II 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 × with 1 × bomb

Late Merlin and Griffon

Supermarine Spitfire- Late Merlin and Griffon engine variants.
	L.F Mk IXe	Mk XII	Mk XIVe	F. Mk 24
Wingspan	32 ft 6 in (9.9 m)	32 ft 6 in (9.9 m)	36 ft 10 in (11.23 m)	36 ft 11 in (11.25 m)
Wing area	231 ft² (21.46 m²)	231 ft² (21.46 m²)	242.1 ft² (22.5 m²)	243.6 ft² (23.6 m²)
Length	31 ft 1 in (9.47 m)	31 ft 7 in (9.62 m)	32 ft 8 in (9.96 m)	32 ft 11 in (10.03 m)
Height	12 ft 8 in (3.86 m)	12 ft 8 in (3.86 m)	12 ft 9 in (3.89 m)	13 ft 6 in (4.11 m)
Empty weight	5,090 lb (2,309 kg)	5,564 lb (2,524 kg)	6,653 lb (3,034 kg)	6,160 lb (3,247 kg)
Loaded weight	7,400 lb (3,354 kg)	7,415 lb (3,363 kg)	8,574 lb (3,889 kg)	9,900 lb (4,490 kg)
Engine	Rolls-Royce Merlin 66: 150 Octane fuel, +25 lb/ft² boost.	Rolls-Royce Griffon III or IV	Rolls-Royce Griffon 65	Rolls-Royce Griffon 85
Power	1,720 hp (1,283 kW) at 11,000 ft (3,353 m)	1,735 hp (1,293 kW) at 1,000 ft (300 m)	2,050 hp (1,530 kW) at 9,800 ft (2,987 m)	2,120 hp (1,771 kW) at 12,250 ft (3,734 m)
Maximum speed	404 mph (650 km/h) at 21,000 ft (6,400 m)	397 mph (639 km/h) at 17,800 ft (5,425 m)	448 mph (717 km/h) at 25,900 ft (7,894 m)	454 mph (731 km/h) at 26,000 ft (7,802 m)
Rate of climb	4,745 ft/min (24.1 m/s) at 10,000 ft (3,048 m)	3,760 ft/min (19.1 m/s) at 2,600 ft (792 m)	4,580 ft/min (25.2 m/s) at sea level (0 m)	4,100 ft/min (21.0 m/s) at 17,000 ft (5,182 m)
Service ceiling	42,500 ft (12,954 m)	39,000 ft (11,887 m)	43,000 ft (13,560 m)	43,000 ft (13,560 m)
Wing loading	30.6 lb/ft² (149 kg/m²)	32.0 lb/ft² (155 kg/m²)	35.0 lb/ft² (171 kg/m²)	40.6 lb/ft² (198 kg/m²)
Power/mass	0.23 hp/lb (0.39 kW/kg)	0.23 hp/lb (0.39 kW/kg)	0.20 hp/lb (0.33 kW/kg)	0.23 hp/lb (0.39 kW/kg)
Combat range	434 mi (698 km) on internal fuel	493 mi (793 km) on internal fuel	460 mi (740 km) on internal fuel	390 mi (627 km) on internal fuel
Ferry range	980 mi (1,577 km)	791 mi (1,272 km)	855 mi (1,375 km)	965 mi (1,553 km) 90 gal drop tank
Armament	2 × 20 mm Hispano II cannon; 120 rpg 2 × 0.50 cal Browning M2 machine guns; 250 rpg Up to 2 × bombs (wing racks), plus 1 × bomb (centre-section rack).	2 × 20 mm Hispano II cannon; 120 rpg 4 × .303" Browning machine guns; 350 rpg Up to 2 × bombs (wing racks), plus 1 × bomb (centre-section rack).	2 × 20 mm Hispano II cannon; 120 rpg 2 × 0.50 cal Browning M2 machine guns; 250 rpg	4 × 20 mm Hispano V cannon; 175 rpg inboard, 150 rpg outboard Up to 2 × bombs (wing racks), plus 1 × bomb (centre-section rack).

Seafire

Supermarine Seafire
	Seafire Mk IIc	Seafire L. Mk III	Seafire F. Mk XV	Seafire F. Mk 47
Wingspan	36 ft 10 in (11.23 m)	32 ft 2 in (9.8 m)/13 ft 4 in (4.0 m) (wings folded)	36 ft 10 in (11.23 m)/13 ft 4 in (4.0 m) (wings folded)	36 ft 11 in (11.25 m)/19 ft 1 in (5.82 m) (wings folded)
Wing area	242.1 ft² (22.5 m²)	231 ft² (21.5 m²)	242.1 ft² (22.5 m²)	243.6 ft² (23.6 m²)
Length	29 ft 11 in (9.12 m)	29 ft 11 in (9.12 m)	31 ft 10 in (9.62 m)/32 ft 3 in (9.83 m)(late production larger fin and rudder)	34 ft 4 in (10.46 m)
Height (Over propeller, tail down)	11 ft 5 in (3.48 m)	11 ft 5 in (3.48 m)	12 ft 8 in (3.86 m)	12 ft 9 in (3.88 m)
Empty weight	5,300 lb (2,404 kg)	5,450 lb (2,472 kg)	6,300 lb (2,857 kg)	8,680 lb (3,937 kg)
Loaded weight	7,145 lb (3,240 kg)	7,220 lb (3,275 kg)	7,995 lb (3,626 kg)	10,700 lb (4,853 kg) loaded, clean;12,530 lb (5,683 kg) with 50 gal drop tank and two bombs
Engine	Rolls-Royce Merlin 46	Rolls-Royce Merlin 55M	Rolls-Royce Griffon VI	Rolls-Royce Griffon 88
Power	1,415 hp (1,155 kW) at 14,000 ft (4,267 m)	1,585 hp (1,226 kW) at 2,750 ft (838 m)	1,850 hp (1,379 kW) at 2,000 ft (609 m)	2,350 hp (1,752 kW) at 1,250 ft (380 m)
Maximum speed	342 mph (297 knots), (550 km/h) at 20,700 ft (6,309 m)	359 mph (312 knots), (578 km/h) at 5,100 ft (1,514 m)	392 mph (341 knots), (631 km/h) at 12,800 ft (3,901 m)	452 mph (393 knots), (727 km/h) at 20,500 ft (6,250 m)
Rate of climb	2,380 ft/min (12.0 m/s) at 16,000 ft (4,876 m)	3,460 ft/min (17.5 m/s) at 4,000 ft (1,219 m)	4,600 ft/min (23.4 m/s) at 4,000 ft (1,219 m)	4,800 ft/min (24.4m/s) at sea level (0 m)
Service ceiling	37,600 ft (11,460 m)	32,000 ft (9,753 m)	37,000 ft (11,277 m)	43,100 ft (13,135 m)
Wing loading	29.5 lb/ft² (133 kg/m²)	30.3 lb/ft² (137 kg/m²)	33.0 lb/ft² (161 kg/m²)	43.9 lb/ft² (205 kg/m²)or 51.4 lb/ft² (240 kg/m²)
Power/mass	0.20 hp/lb (0.33 kW/kg)	0.22 hp/lb (0.36 kW/kg)	0.23 hp/lb (0.39 kW/kg)	0.22 hp/lb (0.36 kW/kg)0.18 hp/lb (0.30 kW/kg)
Combat range	434 mi (698 km) on internal fuel	510 mi (821 km) on internal fuel	376 mi (605 km) on internal fuel	405 mi (652 km) on internal fuel
Ferry range	750 mi (1,207 km) with 60 gal drop tank	770 mi (1,239 km) with 60 gal drop tank	903 mi (1,453 km) with 90 gal drop tank	1,475 mi (2,374 km) with 90 gal drop tank
Armament	2 × 20 mm Hispano II cannon; 120 rpg 4 × .303" Browning machine guns; 350 rpg 2 × with 1 × bomb	2 × 20 mm Hispano II: late Seafire IIIs Hispano V cannon; 120 rpg 4 × .303" Browning machine guns; 350 rpg 2 × with 1 × bomb	2 × 20 mm Hispano II cannon; 120 rpg 4 × .303" Browning machine guns; 350 rpg 2 × with 1 × bomb	4 × 20 mm Hispano V cannon; 175 rpg inboard, 150 rpg outboard up to 8 × "60 lb" RP-3 rockets on zero-length launchers up to 3 × bombs

References

Bibliography

• Air Ministry. A.P 1565 Spifire IA and IB Aeroplanes: Merlin II and III engines, Pilot's Notes. London: Air Ministry, 1940.
• Air Ministry. A.P 1565B Spifire IIA and IIB Aeroplanes: Merlin XII Engine, Pilot's Notes (July 1940). London: Air Data Publications, 1972 (reprint). ISBN 0-85979-043-6.
• Chapel, Charles Eward; Bent, Ralph D; McKinley, James L. Aircraft Power Plants: by the technical development staff of Northrop Aeronautical Institute. New York: McGraw-Hill Book Company Inc., 1955.
• Gruenhagen, Robert W. Mustang: The Story of the P-51 Fighter (rev. ed.). New York: Arco Publishing Company, Inc., 1980. ISBN 0-6680-4884-0.
• Harvey-Bailey, A. The Merlin in Perspective: The Combat Years . Derby, UK: Rolls-Royce Heritage Trust, 1995 (4th edition). ISBN 1-872922-06-6.
• Morgan, Eric B and Edward Shacklady. Spitfire: The History. Stamford: Key Books Ltd, 2000. ISBN 0-946219-48-6.
• Price, Alfred. Late Marque Spitfire Aces 1942 - 1945. Oxford, UK: Osprey Publishing, 1995. ISBN 1-85532-575-6.
• Price, Alfred. Spitfire Mark I/II Aces 1939-41. London: Osprey Aerospace, 1996. ISBN 1-85532-627-2.
• Price, Alfred. The Spitfire Story: New edited edition. London: Weidenfeld Military, 1999. ISBN 1-85409-514-5.
• Robertson, Bruce. Spitfire: The Story of a Famous Fighter. Hemel Hempstead, Hertfordshire, UK: Model & Allied Publications Ltd., 1960. Third revised edition 1973. ISBN 0-900435-11-9.
• Smallwood, Hugh. Spitfire in Blue. London: Osprey Aerospace, 1996. ISBN 1-85532-615-9
• Thomas, Andrew.Griffon Spitfire Aces: Aircraft of the Aces 81. London: Osprey Aerospace, 2008. ISBN 1-84603-289-1.