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Paul K. Guillow, Inc. Balsa Wood Airplanes SUPERMARINE SPITFIRE

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Item: 403
Wing Span: 24 5/8" Plan Blown To: 75"
Scale: 1/16

[IMAGE] No fighter of World War 2 is more deserving of fame than the Supermarine Spitfire. During the Battle of Britain, the Spitfires blunted the attack of the German Luftwaffe and buoyed British hopes in their darkest hour. It also flew in every front between 1939 and 1945 and was engaged in every major action fought by the R.A.F. in that time

The Supermarine Spitfire was a British single-seat fighter aircraft, used by the Royal Air Force and many other Allied countries during the Second World War, and into the 1950s. It was produced in greater numbers than any other Allied design. The Spitfire was the only Allied fighter in production at the outbreak of the Second World War that was still in production at the end of the war.

The elliptical wing was able to reach a safe Mach number of 0.83 and maximum of 0.86 without encountering the problem of Mach-induced aileron flutter, a phenomenon which continued to blight many newer designs.

At first the complexity of the wing design, especially the precision required to manufacture the vital spar and leading edge structures, caused some major hold-ups in the production of the Spitfire. This was amplified when the work was put out to sub-contractors, most of whom had never dealt with metal-structured, high-speed aircraft. Over time, however, these problems were overcome and thousands of these wings, of six basic types, were built.[24]

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Produced by the Supermarine subsidiary of Vickers-Armstrongs, the Spitfire was designed by the company's Chief Designer R. J. Mitchell, who continued to refine the design until his death from cancer in 1937; the position of chief designer was then filled by his colleague, Joseph Smith.[4] Its elliptical wing had a thin cross-section, allowing a higher top speed than the Hawker Hurricane and many other contemporary designs.

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The distinctive silhouette imparted by the wing planform helped the Spitfire to achieve legendary status during the Battle of Britain. There was, and still is, a public perception that it was the RAF fighter of the Battle, in spite of the fact that the more numerous Hurricane shouldered a great deal of the burden against the potent Luftwaffe. Much loved by its pilots, the Spitfire saw service throughout the whole of the Second World War, in most theatres of war, in several roles and in many different variants. The Spitfire was to continue to serve as a front line fighter and in secondary roles for several air forces well into the 1950s.

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The Spitfire will always be compared to its main adversary, the Messerschmitt Bf 109: both were among the finest fighters of their day and followed similar design philosophies of marrying a small, streamlined airframe to a powerful liquid-cooled inline engine.

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R. J. Mitchell's 1931 design to meet Air Ministry specification F7/30 for a new and modern fighter capable of 250 mph, the Supermarine Type 224, resulted in an open-cockpit monoplane with bulky gull-wings and a large fixed, spatted undercarriage powered by the 600 horsepower evaporative-cooled Rolls-Royce Goshawk engine.[5] This made its first flight in February 1934.[6][7] This aircraft was a big disappointment to Mitchell and his design team, who immediately embarked on a series of "cleaned up" designs, using their experience with the Schneider trophy seaplanes as a starting point. The F7/30 design accepted was the biplane Gloster Gladiator.[8]

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Mitchell had already begun working on a new aircraft, designated Type 300, based on the Type 224. With a retractable gear and 6 ft shorter wingspan, the aircraft was submitted to the Air Ministry in July 1934, but again was not accepted.[9] The design evolved through a number of changes, including an enclosed cockpit, oxygen breathing-apparatus, even smaller and thinner wings, and the newly-developed and much more powerful Rolls-Royce PV-XII engine, later named the Merlin. In November 1934, Mitchell, with the backing of Supermarine's owner, Vickers-Armstrongs, started detailed design work on the Type 300.[10] The Air Ministry issued a contract AM 361140/34 on 1 December 1934, providing £10,000 for the construction of Mitchell's "improved F.7/30 design".[11] On 3 January 1935, the Air Ministry formalised the contract and a new Specification F.10/35 was written around the aircraft.[12] Just 15 months later, after several major design changes and refinements, on 6 March 1936[f] the sleek new prototype (K5054) took off on its first flight from Eastleigh Aerodrome (later Southampton Airport), just four months after the maiden flight of the contemporary Hawker Hurricane.[13]

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Testing continued until 26 May 1936, when Captain J. "Mutt" Summers, (Chief Test Pilot for Vickers (Aviation) Ltd.) flew K5054 to RAF Martlesham Heath and handed the aircraft over to Squadron Leader Anderson of the Aeroplane & Armament Experimental Establishment (A&AEE).

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The Air Ministry placed an order for 310 aircraft on 3 June 1936,[14] before any formal report had been issued by the A&AEE, interim reports being issued on a piecemeal basis. Full-scale production began immediately. The British public first saw the Spitfire at the RAF Hendon air-display on Saturday 27 June 1936. The first production Spitfire rolled off the Woolston, Southampton assembly line in mid-1938.[1]

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The Air Ministry submitted a list of possible names to Vickers-Armstrongs for the new aircraft, now known as the Type 300. One of these was the improbable Shrew. The name Spitfire was suggested by Sir Robert MacLean, director of Vickers-Armstrongs at the time, who called his daughter Ann "a little spitfire."

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The word dates from Elizabethan times and refers to a particularly fiery, ferocious type of person.[15] The name had previously been used unofficially for Mitchell's earlier F.7/30 Type 224 design. Mitchell is reported to have said that it was "just the sort of bloody silly name they would choose",[16] possibly an oblique reference to the Type 224.

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In the mid-1930s, aviation design teams worldwide started developing a new generation of all-metal, low wing fighter aircraft. Aircraft such as the French Dewoitine D.520, and Germany's Messerschmitt Bf 109 were designed to take advantage of new techniques of monocoque construction, and new high powered, liquid cooled, in-line aero engines. They also featured refinements such as retractable undercarriages, fully enclosed cockpits and low-drag, all metal wings (all introduced on civil airliners years before, but slow to be adopted by the military who favoured the simplicity and maneouverability of the biplane).

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Mitchell's design aims were to create a well balanced high performance fighter aircraft which would be able to fully utilise the power of the Merlin engine and, at the same time would be relatively easy to fly. To that end his design team developed an airframe which, for its day, was complex.

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The exceptionally well streamlined semi-monocoque duralumin fuselage featured a large number of compound curves and was built up from a skeleton of 19 frames, starting from the main engine bulkhead, or frame number one. Aft of the engine bulkhead were five half frames to accommodate the fuel tanks and cockpit. From the seventh, which was the frame to which the pilot's seat and (later) armour plating was attached, to the 15th, which was mounted at a forward angle just forward of the tailfin, the frames were oval in shape, each reducing slightly in size, and had numerous holes drilled through them to lighten the structural weight as much as possible without weakening them. Frame 16 formed a double bulkhead with frame 17, which was extended to form the main spar of the vertical fin; frame 18 formed the secondary spar. Just aft of this the 19th frame formed the rudder post. A combination of 14 longitudinal stringers and two main longerons helped form a light but rigid structure to which sheets of alclad stressed skinning were attached. There was plenty of room to later fit camera equipment and fuel tanks.[17][18]

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The skin of the fuselage, wings and tailplane was attached with rivets, and in critical areas, such as the wing forward of the main spar where an uninterrupted airflow was required, flush rivets were used. In some areas, such as the rear of the wing, the top was riveted and the bottom fixed by woodscrews into sections of spruce; later pop-riveting would be used for these areas.[19] From 1943 on, flush riveting was used throughout the entire airframe; the first version of the Spitfire to change to flush riveting was the Mk XII closely followed by all Castle Bromwich built Mk IXs.[20] At first the ailerons, elevators and rudder were fabric covered. However, once combat experience showed that the fabric covered ailerons became impossible to use at high speeds the fabric was replaced with a light-alloy, enhancing control throughout the speed range.

The exceptionally well streamlined semi-monocoque duralumin fuselage featured a large number of compound curves and was built up from a skeleton of 19 frames, starting from the main engine bulkhead, or frame number one. Aft of the engine bulkhead were five half frames to accommodate the fuel tanks and cockpit. From the seventh, which was the frame to which the pilot's seat and (later) armour plating was attached, to the 15th, which was mounted at a forward angle just forward of the tailfin, the frames were oval in shape, each reducing slightly in size, and had numerous holes drilled through them to lighten the structural weight as much as possible without weakening them. Frame 16 formed a double bulkhead with frame 17, which was extended to form the main spar of the vertical fin; frame 18 formed the secondary spar. Just aft of this the 19th frame formed the rudder post. A combination of 14 longitudinal stringers and two main longerons helped form a light but rigid structure to which sheets of alclad stressed skinning were attached. There was plenty of room to later fit camera equipment and fuel tanks.[17][18]

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From early on Mitchell and the design staff were contemplating an elliptical wing shape to solve the conflicting requirements of having the lowest possible thickness-to-chord ratio to reduce drag, and having room to install a retractable undercarriage, as well as the projected armament and ammunition which, in April 1935, was changed from two .303 Vickers machine guns in each wing to four .303 Brownings.[21]

The exceptionally well streamlined semi-monocoque duralumin fuselage featured a large number of compound curves and was built up from a skeleton of 19 frames, starting from the main engine bulkhead, or frame number one. Aft of the engine bulkhead were five half frames to accommodate the fuel tanks and cockpit. From the seventh, which was the frame to which the pilot's seat and (later) armour plating was attached, to the 15th, which was mounted at a forward angle just forward of the tailfin, the frames were oval in shape, each reducing slightly in size, and had numerous holes drilled through them to lighten the structural weight as much as possible without weakening them. Frame 16 formed a double bulkhead with frame 17, which was extended to form the main spar of the vertical fin; frame 18 formed the secondary spar. Just aft of this the 19th frame formed the rudder post. A combination of 14 longitudinal stringers and two main longerons helped form a light but rigid structure to which sheets of alclad stressed skinning were attached. There was plenty of room to later fit camera equipment and fuel tanks.[17][18]

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It has been suggested that Mitchell copied the wing shape of the Heinkel He 70. Mitchell's aerodynamicist, Beverley Shenstone, however, has pointed out that the He 70 was designed to fulfill a completely different role and that other aircraft also had elliptical wings. The Spitfire wing was much thinner with a completely different section.[22] As a practical engineer Mitchell was fully aware of the efficiency of the elliptical wing, as were Siegfried and Walter Günther, who designed the Heinkel.[a]

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A design aspect of the wing which contributed greatly to its success was an innovative spar boom design, made up of five square concentric tubes which fitted into each other. Two of these booms were linked together by an alloy web creating a lightweight and very strong main spar. The undercarriage legs were attached to pivot points built into the inner, rear of the main spar and retracted outwards and slightly backwards into wells in the non-load carrying wing structure. The narrow undercarriage track was considered to be an acceptable compromise as it allowed the landing impact loads to be transmitted to the strongest parts of the wing structure.

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Ahead of the spar, the thick-skinned leading edge of the wing formed a strong and very rigid D-shaped box, which took most of the wing loads. At the time the wing was designed this D-shaped leading edge was intended to house steam condensers for the evaporative cooling system intended for the PV XII. The constant problems with the evaporative system in the Goshawk led to the adoption of a 100% glycol cooling system,[b] together with a new radiator duct design, devised by a Fredrick Meredith of the RAE at Farnborough, which used to cooling air to generate thrust, greatly reducing the drag produced by the radiators.[23] This meant that the leading edge structure lost its function as an evaporator, but it was later to become very useful as it was able to be adapted to house integral fuel tanks of various sizes.[24]

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The wing section used was a NACA 2200 series which had been adapted to create a thickness to chord ratio of 13% at the root reducing to 6% at the tip.[25] A dihedral of six degrees was adopted to give increased lateral stability.

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Another feature of the wing was its washout. The trailing edge of the wing twisted slightly upward along its span, the angle of incidence decreased from +2 degrees at its root to -1/2 degree at its tip.[25] This caused the wing roots to stall before the tips, reducing tip stall that may have resulted in a spin. This washout was first featured in the wing of the Type 224 and became a consistent feature in subsequent designs leading to the Spitfire.[26] In a tight turn the disturbance of the slipstream near the wing-root caused a distinctive "juddering" through the control column and fuselage skin, warning the pilot that the Spitfire was nearing a stall. This was described in the pilot's manual:

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(AP 1565B; Section 11: STALLING; Paragraph (ii):This aeroplane has sensitive elevators and if the control column is brought back too rapidly in a manoeuvre, such as a loop or steep turn stalling incidence may be reached and a high speed stall induced. When this occurs there is a violent shudder and clattering noise throughout the aeroplane which tends to flick over laterally and unless the control column is put forward instantly a rapid roll and spin will result.[27]

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If the stick was pulled back too far on the Spitfire in a tight turn, the aircraft could stall rather violently, flick over on to its back, and spin. Knowledge of this undoubtedly deterred Spitfire pilots from tightening their turns when being chased, particularly if they were inexperienced.[28]

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The elliptical wing was able to reach a safe Mach number of 0.83 and maximum of 0.86 without encountering the problem of Mach-induced aileron flutter, a phenomenon which continued to blight many newer designs.

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At first the complexity of the wing design, especially the precision required to manufacture the vital spar and leading edge structures, caused some major hold-ups in the production of the Spitfire. This was amplified when the work was put out to sub-contractors, most of whom had never dealt with metal-structured, high-speed aircraft. Over time, however, these problems were overcome and thousands of these wings, of six basic types, were built.[24]

One flaw in the thin-wing design of the Spitfire manifested itself when the aircraft was brought up to very high speeds. When the pilot attempted to roll the aircraft at these speeds, the aerodynamic forces on the ailerons were enough to twist the entire wingtip in the direction opposite of the aileron deflection (much like the way an aileron trim tab will deflect the aileron itself). This so-called aileron reversal resulted in the Spitfire rolling in the opposite direction to the control column input and occurred at much lower airspeeds than on other contemporary designs. In March 1943, R.A.E. noted that at 400 mph I.A.S., roughly 65% of aileron power was lost due to wing twist.[29] The new wing of the Spitfire F Mk 21 and its successors was designed to help alleviate this problem.[c] [30][31]

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The ellipse also served as the design basis for the Spitfire’s fin and tailplane assembly, once again exploiting the shape’s favourable aerodynamic characteristics. Both the elevators and rudder were shaped so that their centre of mass was shifted forward thus reducing control surface flutter. The longer noses and greater propeller wash resulting from larger engines in later models necessitated increasingly larger vertical and, later, horizontal tail surfaces to compensate for the altered aerodynamics, culminating in those of the Mk 22/24 series which were 25% larger in area than those of the Mk I.[32][33]

Early in its development, the Merlin engine's lack of direct fuel injection meant that both Spitfires and Hurricanes, unlike the Bf 109E, were unable to simply nose down into a steep dive. This meant a Luftwaffe fighter could simply "bunt" into a high-power dive to escape an attack, leaving the Spitfire sputtering behind, as its fuel was forced by negative "g" out of the carburettor. RAF fighter pilots soon learned to "half-roll" their aircraft before diving to pursue their opponents. The use of carburettors was calculated to give a higher specific power output, due to the lower temperature, and hence the greater density, of the fuel/air mixture fed into the motor, compared to injected systems. In March 1941, 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, and became known as "Miss Shilling's orifice" as it was invented by a female engineer, Beatrice "Tilly" Shilling. Further improvements were introduced throughout the Merlin series, with Bendix-manufactured pressure carburettors introduced in 1943.

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In February 1936 Vickers-Armstrongs director, Sir Robert MacLean, guaranteed production of 5 aircraft a week, beginning 15 months after an order is placed. On 3 June 1936, the Air Ministry placed an order for 310 aircraft, for a price of £1,395,000.[34] Full-scale production of the Spitfire began at Supermarine's facility in Woolston, Southampton, but it quickly became clear that the order could not be completed in the 15 months promised. Supermarine was a small company, already busy building the Walrus and Stranraer, and its parent company, Vickers, was busy building the Wellington. The initial solution was to subcontract the work out.[34]The first production Spitfire rolled of the assembly line in mid-1938, [1] and was flown on 15 May 1938, almost 24 months after the initial order.[35] The final cost of the first 310 aircraft, after delays and increased programme costs, came to ₤1,870,242 or ₤1,533 more per aircraft than originally estimated.[36]

With war becoming increasingly inevitable, and to help build the Spitfires in the numbers anticipated, a huge new facility was started on 12 July 1938 at Castle Bromwich, near Birmingham, as a "shadow" to Supermarine's original factories in Southampton: the most modern machine tools then available were being installed two months after work started on the site.[36] Although the project was at first managed and equipped by Morris Motors Ltd under Lord Nuffield, who was an expert in mass construction in the motor-vehicle industry, it was funded by government money. However, although the new factory had been completed in late 1939, continual problems were experienced in building a complete airframe. The Spitfire's stressed-skin construction required skills and techniques outside the experience of the local labour force and a continual stream of changes were demanded by the RAF. Finally, on 17 May 1940, with no sign of a single Spitfire being built, Lord Beaverbrook, Minister of Aircraft Production, outmanoeuvred Lord Nuffield and took over Castle Bromwich for the government.[37] Beaverbrook immediately sent in experienced management staff and experienced workers from Supermarine and Vickers-Armstrongs. In June 1940, 10 Mk IIs were built,[38] in July, 23 rolled out, in August they produced 37, and in September the count was 56.[39] These were the first of thousands of Spitfires to emerge from Castle Bromwich.

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