Design and developmentEdit
By 1942, Supermarine designers had realised that the aerodynamics of the Spitfire's wing at high Mach numbers might become a limiting factor in increasing the aircraft's high-speed performance. The main problem was the aeroelasticity of the Spitfire's wing; at high speeds the relatively light structure behind the strong leading edge torsion box would flex, changing the airflow and limiting the maximum safe diving speed to 480 mph (772 km/h) IAS. If the Spitfire were to be able to fly higher and faster, a radically new wing would be needed.
Joseph Smith and the design team were aware of a paper on compressibility, published by A D Young of the R.A.E, in which he described a new type of wing section; the maximum thickness and camber would be much nearer to the mid-chord than conventional airfoils and the nose section of this airfoil would be close to an ellipse. In November 1942 Supermarine issued Specification No 470 which (in part) stated:
A new wing has been designed for the Spitfire with the following objects: 1) To raise as much as possible the critical speed at which drag increases, due to compressibility, become serious. 2) To obtain a rate of roll faster than any existing fighter. 3) To reduce wing profile drag and thereby improve performance.
The wing area has been reduced to 210 sq ft (20 m2) and a thickness chord ratio of 13% has been used over the inner wing where the equipment is stored. Outboard the wing tapers to 8% thickness/chord at the tip.Specification 470 described how the wing had been designed with a simple straight-tapered planform to simplify production and to achieve a smooth and accurate contour. The wing skins were to be relatively thick, aiding torsional rigidity which was needed for good aileron control at high speeds. Although the prototype was to have a dihedral of 3° it was intended that this would be increased in subsequent aircraft. Another change, to improve the ground-handling, was replacing the Spitfire's narrow-track, outward-retracting undercarriage with a wider-track, inward-retracting system. The Air Ministry were impressed by the proposal and, in February 1943, issued Specification F.1/43 for a single seat fighter with a laminar flow wing; there was also to be provision made for a wing folding scheme to meet possible Fleet Air Arm requirements. The new fighter was to use a fuselage based on a Spitfire VIII.
The new wing was fitted to a modified Spitfire XIV NN660, in order to make a direct comparison with the earlier elliptical wing, and was first flown on 30 June 1944 by Jeffrey Quill. Although the new Spitfire's speed performance was comfortably in excess of an unmodified Spitfire XIV, the new wing displayed some undesirable behaviour at the stall which, although acceptable, did not come up to the high standards of Mitchell's earlier elliptical wing. NN660 crashed on 13 September 1944, killing pilot Frank Furlong. No reason for the loss was officially established.
In the meantime, the opportunity had been taken to redesign the Spitfire's fuselage, to improve the pilot's view over the nose and to eliminate gross directional instability by using a larger fin and rudder. This instability had been apparent since the introduction of the more powerful Griffon engine. The instability was exacerbated by the increase in propeller blade area due to the introduction of the four-bladed and subsequent five-bladed Rotol airscrews for the next aircraft, NN664 (for which Specification F.1/43 had been issued). The updated design incorporated the new fuselage (although lacking the enlarged fin/rudder) and, as it was now substantially different from a Spitfire, the aircraft was named "Spiteful" (although "Victor" had been originally proposed).