Supermarine Spitfire (late Merlin-powered variants)
The British Supermarine Spitfire was facing several challenges by mid-1942. The debut of the formidable Focke-Wulf Fw 190 in late 1941 had caused problems for RAF fighter squadrons flying the latest Spitfire Mk Vb. Rolls-Royce engineers were already working on a new version of the Merlin incorporating a two-stage supercharger; the combination of the improved Merlin and the Spitfire Mk Vc airframe in a "stop-gap" design allowed the RAF to combat the Fw 190 on equal terms.
In a second stream of development Supermarine was working on an improved, reinforced, Spitfire airframe which incorporated several new features and was designed for the Merlin 60 and 70 series engines. This new airframe later formed the basis for the Rolls-Royce Griffon powered Spitfires. This article presents a history of the Spitfire powered by two-stage engine variants and also describes some of the "drawing board" projects and experimental Spitfires. The Griffon powered variants are described in a separate article.
Wing types
The majority of the Spitfires from the Mk VIII on, used three basic wing types; C, D and EC type
Also known as the "universal wing" the new design was standard on the majority of Spitfires built from mid 1942. The design of the wing was altered to reduce labour and manufacturing time and carry various armaments: A type, B type and 2 x 20mm Hispano cannons, or four 20 mm Hispano cannon.The undercarriage mountings were redesigned and the undercarriage doors were bowed in cross-section allowing the legs to sit lower in the wells, eliminating the upper-wing blisters over the wheel wells and landing gear pivot points. Stronger undercarriage legs were raked 2 inches forward, making the Spitfire more stable on the ground and reducing the likelihood of the aircraft tipping onto its nose. During production of the Mk VIII and Mk IX a new undercarriage leg was introduced which had external v-shaped "scissor-links" fitted to the front of the leg; this also led to small changes in the shape of the undercarriage bay and leg fairings. Because the Spitfire was no longer to be used as a night fighter, the retractable landing lights were no longer fitted.
The Hispano Mk II adopted the French Châtellerault belt-feed system, using disintegrating-link belts stored in internal ammunition boxes, providing up to 120 rounds per gun. The fairings over the Hispano barrels were shorter and there was usually a short rubber stub covering the outer cannon port. New upper wing gun-bay doors incorporated "teardrop" shaped blisters to clear the cannon feed motors and the lower wings no longer had the gun-bay heating vents outboard of the gun-bays. The first series of Spitfire Mk IXs retained the bay doors first used on Spitfire VCs; these incorporated large blisters to clear the feed motors of two Hispano cannons. All later Spitfires had smaller, more streamlined blisters. To provide room for the belt feed system of the cannon, the inner machine gun bays were moved outboard to between ribs 13 and 14.
Several versions of the Spitfire, starting with the HF Mk VIIs had extra 13 gallon integral fuel tanks added to the wing leading edges between the wing-root and the inboard cannon bay. Although many earlier Spitfires had been modified to carry bomb racks capable of carrying bombs the first Spitfires to be specifically modified for the fighter-bomber role were those of the Second Tactical Air Force. The first Mk IX Spitfires started being used in the role in June 1944.
D Type
These were made for photo-reconnaissance Spitfires, including the PR Mk X and XI; no armament was fitted and the D-shaped leading edges of the wings, ahead of the main spar, were converted into integral fuel tanks, each carrying 66 gallons. To avoid the expansion of fuel in hot weather damaging the wing, pressure relief valves, with small external vent pipes, were fitted near the wing tips.E type
Structurally identical to the C wing. The outer machine gun ports were eliminated; although the outer machine-gun bays were retained, their access doors were devoid of empty cartridge case ports and cartridge deflectors. The new wing allowed for a four cannon armament and the inner bays could carry heavy machine guns. There were thus two possible weapon fits:either
or
The.303 machine guns mounted in the outer wings were no longer fitted, because rifle calibre bullets were ineffective against heavily armoured aircraft.. The 20 mm Hispano cannon were moved outboard and the.50 calibre Browning M2/AN, with 250 rpg were added to the inner gun-bays. The first trial installation was made in BS118 in November 1943; by mid-March 1944 the first service Spitfires to be modified were from 485, 222 and 349 Squadrons. Spitfires with this armament were at first referred to as Spifire LF.IX.5 and the E suffix was not officially introduced until early 1945. This armament was standard for all Spitfire Mk IXs and XVIs used by the 2nd Tactical Air Force as fighters and fighter-bombers from shortly after D-Day. It proved more effective for both air-to-air engagements and air-to-ground attacks.
Many Spitfires had their elliptically "pointed" wing tips replaced by shorter, squared off fairings. This slightly improved maximum speed at low altitude and enhanced the roll rate. While many "LF" Spitfires had the "clipped" wings, a number did not. The true distinguishing feature of "LF" versions was the fitting of low-altitude versions of the Rolls-Royce Merlin engine.
Variants
Mark numbers, type numbers
The Mark numbers did not necessarily indicate a chronological order, the Mk IX was a stopgap brought into production before the Mks VII and VIII. Some Spitfires of one mark or variant may have been modified to another; several of the first Mk Vbs were converted from Mk Ibs; the first Mk IXs were Mk Vcs converted, in some instances, by Rolls-Royce at their Hucknall facility.Up until the end of 1942, the RAF names had Roman numerals for mark numbers. In the period 1943–1948 new aircraft entering service were given Arabic numerals for mark numbers but older aircraft retained Roman numerals. From 1948, Arabic numerals were used exclusively. This article adopts the convention of using Roman numerals for the Mks I–XVI and Arabic numerals for the Mks 17–24. Type numbers e.g.; are the drawing board design numbers allocated by Supermarine.
Features of late Merlin powered Spitfires
With the development of the Merlin 61/63/66 and 70 series engines, with a two-stage, two-speed supercharger requiring an intercooler, several important modifications were made to the basic airframe and applied to all aircraft powered by these engines. The longer Merlin 61 meant the nose forward of the engine/fuel tank bulkhead was 7 inches longer. An intercooler was mounted behind the engine, on the supercharger casing. A small rectangular air scoop for a "Heywood" air compressor was fitted on the starboard upper engine cowling.The carburettor air intake on early to mid-production Spitfire IXs was a different shape from those of single-stage engined aircraft; they were shorter and had a wider air inlet. From 1943, an "Aero-Vee" tropical filter in a long, streamlined fairing was introduced for the carburettor air intake. This was fitted as standard to all Mk VIIs, VIIIs, PR Mk Xs and Mk XVIs, on mid to late production Mk IXs, and to most PR Mk XIs.
A new 10 ft 9 in diameter four-bladed Rotol propeller was fitted, housed in a pointed spinner. The exhaust units were changed to six "fishtail" stacks per side. Under the nose, the three piece cowling was changed to a one piece layout. The oil tank was no longer a part of the cowling structure. Early Mk IXs had a teardrop shaped blister for a Coffman engine starteron the lower starboard side cowling, just behind the propeller. This was replaced by an improved electric starter on most two-stage Merlin powered Spitfires and, from late 1942 the blister like bulge was seen on only a few aircraft.
The Type C wing was fitted as standard. Some mid and most late production Spifire Mk IXs and all Mk XVI were fitted with the Type E wing. While Spitfire IXs converted from Mk Vcs had the large teardrop fairings on the upper surfaces, on the majority of Mk VIIs, VIIIs, IXs and XVIs the teardrop shaped blisters covering the Hispano feed motors were reduced in size and more streamlined than those on the Mk Vc. According to A.P 1565J P & L the red painted undercarriage indicator rods, which projected through the tops of the wings when the undercarriage was down, were used only on early production Mk IXs. When fitted these rods supplemented an "Electric visual indicator" mounted on the instrument panel. The indicator rods seemed to be fitted on all Mk VIIs and VIIIs, supplementing the electric visual indicator. A light for the retractable tailwheel was mounted on the instrument panel, just below the main visual indicator.
Because the intercooler required a radiator, the radiator under the starboard wing was halved in size and the intercooler radiator housed alongside. Under the port wing a new radiator fairing housed a square oil cooler alongside the other half-radiator unit. When the engine was running at low speed, one radiator section provided enough coolant; a thermostatic switch turned off the starboard radiator section until more power was called for and extra engine cooling was required.
Other structural changes included flush riveting for the fuselage, introduced in mid-1943. A streamlined round rear-view mirror, with a bullet-shaped fairing replaced the rectangular version. On new production Mk IXs the small, teardrop shaped identification light behind the radio mast was removed.
While early Mk IXs converted from Mk Vcs had the original elevator horn mass-balances, most had the enlarged version with the straightened leading edge. A new rudder of greater area, which could be identified by a "pointed" tip, was fitted to many Mk VIIs, Mk VIIIs, and mid to late production Mk IXs and Mk XVIs.
All Mk VII and Mk VIII Spitfires had the following changes:
The internal structure was strengthened and revised. On the wings the ailerons were reduced in span by outboard of the outer hinges. There had been some instances of earlier models breaking up in the air in steep high speed dives, it was thought, because of aileron flutter.
The main fuselage fuel tanks were increased in capacity; 47 gal for the upper tank and 49 gal for the lower. In combination with the wing tanks this gave a total internal capacity of 122 gal, a near 50% increase over the 85 gal carried by earlier Spitfire marks. In addition a 13 gal fuel tank was fitted in each wing leading edge between the wingroot and the inner gun-bay.
The main undercarriage legs, for the first time in the Spitfire's life, were fitted with forward-facing torque links. In addition, the leg doors were slightly concave, allowing the undercarriage to sit lower in the wheel wells when retracted: this meant the upper wing skinning was free of the small bulge which had hitherto been necessary to clear the wheels. The wheels themselves were a new strengthened Dunlop AH10019 "four spoke" pattern, replacing the "five spoke" pattern used since the first Mk Is. This revised undercarriage was also fitted on some mid to late Mk IXs and all Mk XVIs. A retractable tailwheel was fitted, covered by small doors when in flight. This also applied to the PR Mk Xs and most PR Mk XIs.