Seafire Mk III
The Seafire Mk III was the first Seafire to feature folding wings, a capability that had been designed by Joseph Smith as early as 1939. However, political and operational circumstances, including Winston Churchill’s insistence that the Spitfire not be used on carriers, delayed the actual deployment of folding-wing Seafires until 1943.
The first folding-wing conversion was performed on MA970, a production Mk IIc that was flown to Worthy Down on October 2, 1942. The wing-folding mechanism involved a break point outside the undercarriage bays, with the wings folding inward and the tips folding outward. While powered wing folding was considered, it was rejected due to weight concerns. It took five men about 45 seconds to fold the wings manually. The Admiralty initially wanted the Seafire Mk III to carry four Hispano cannons, but the combined weight of the folding wings and the extra armament was too much, so the outer cannons were removed. The design also incorporated Martin-Baker feed mechanisms for the two remaining cannons, which eliminated the need for bulky upper-wing blisters.
The Seafire Mk III was powered by the Merlin 55 engine and was given the Supermarine Type Number 358. Westland led production, assisted by Cunliffe-Owen, though initial delays in producing wing jigs meant that the first 32 aircraft were delivered without folding wings and were subsequently redesignated as Mk IIcs.
Both the first production Mk III (LR765) and the second (LR766) had stability issues due to their center of gravity (CoG), which was initially measured at 8.1 inches from the datum, just shy of the maximum limit of 9 inches. Trials revealed that the problem was caused by airflow disruptions over the wing-fold joints. Additional modifications were implemented to correct this. Another issue was the overheating of the guns, which persisted despite several modifications.
Later Mk IIIs were equipped with lighter Hispano Mk V cannons, which had shorter barrels. The wing was modified to accommodate rockets (either four 25lb or 60lb 3-inch rockets) or bombs (two 250lb general-purpose bombs or one 500lb bomb on the centerline). After the 90th aircraft, the tropical filter was replaced by the Aero-Vee unit, streamlining the carburettor intake. From the 130th aircraft onwards, overload fuel tanks under the wings became a standard feature.
Undercarriage issues persisted, as side loads during landings could cause collapse. This was mitigated by adopting a link-style undercarriage and additional modifications to handle side loads. Damage to the tail caused by the arrestor hook bouncing off the deck during landings was fixed by installing a damper to prevent bouncing.
A photo-reconnaissance version of the Mk III was also developed, similar to the Mk IIc variant. The prototype, MB194, carried both vertical and oblique F.24 cameras, and the final 129 Mk IIIs built by Cunliffe-Owen were finished in this configuration. Additionally, a low-altitude version (the L Mk IIIc or LF Mk IIIc) was produced, featuring the Merlin 55M engine.
The first squadron to receive the Mk III was No. 894 Squadron, followed by Nos. 801, 887, 889, and 890 Squadrons. The Seafire Mk III remained in service until March 1946, when Nos. 887 and 894 Squadrons were the last to operate it before disbandment.