Supermarine Type 224

The Supermarine Type 224 was a British gull-wing monoplane fighter aircraft designed by R.J. Mitchell at Supermarine in response to Air Ministry Specification F.7/30, which sought to introduce a new fighter to succeed the Gloster Gauntlet. The Type 224 was powered by a Rolls-Royce Goshawk engine, which used an experimental evaporative cooling system.

Problems with the cooling system, combined with its disappointing performance during trials, led to the Type 224 being rejected by the Air Ministry, a contract for production aircraft eventually going to the Gloster Gladiator. The type is nevertheless notable because Mitchell learnt lessons from its design that were to contribute to his success with the Supermarine Spitfire.

Air Ministry Specification F.7/30
Air Ministry Specification F.7/30, which developed from O.R.1, was formally issued to the British aircraft industry in October 1931, called for an all-metal day and night fighter aircraft armed with four machine guns, a high maximum speed and rate of climb, and a landing speed of less than 60 mph. The importance of good visibility from the cockpit was specified. Although the use of any suitable engine was permitted, the Air Ministry expressed a preference for the Rolls-Royce Goshawk, then still being developed. Designs tendered to the specification came from Bristol, Hawker, Westland, Blackburn and Supermarine

Of the proposals submitted by British aircraft manufacturers, three were officially selected to produce prototypes for the Air MInistry, Supermarine (the Type 224), Blackburn (Blackburn F.3), and Westland (Westland F.7/30). Hawker had both its biplane and monoplane designs rejected but built the Hawker P.V.3, as a private venture, similarly Bristol had three rejected and built both the Bristol Type 123 biplane and Bristol Type 133 (a monoplane) with its own money. Gloster had not tendered a submission as they were engaged on the Gloster Gauntlet, but later built their private venture Gloster SS.37 using Hawker construction techniques to improve their Gauntlet design.

As a result of the success of Supermarine during the Schneider Trophy contests, Supermarine's chief designer R. J. Mitchell was confident that he could design a highspeed fighter. Less than four months after receiving the specification, Supermarine tendered a monoplane design, the Type 224.

Mitchell's design
Mitchell's design for the Type 224 included an inverted gull-wing, chosen in order to shorten the undercarriage legs and so reduce drag. He chose a thick airfoil for the wing, in contrast to the type of airfoil for the Schneider Trophy racers.

The fixed landing gear were encased in large fairings. The fuselage was of monocoque construction, with one pair of guns mounted either side of the cockpit and the other pair in the 'trouser' fairings of the undercarriage, which was fixed.

The wing had a single main spar. The engine cooling system's condensers formed the wing's entire leading edge, the combination of the two producing a 'D-box' spar that had a high torsional stiffness. Behind the main spar the wing was fabric-covered.

The Type 224 was powered by the 600 hp Goshawk II. The evaporative cooling system used by the Goshawk involved allowing the cooling water to reach a temperature greater than 100 °C without boiling by keeping it under pressure while circulating through the engine: this superheated water was then allowed to boil off by releasing the pressure, the resulting steam then being cooled in a condenser, collected as water and then recirculated through the engine. The system had been experimentally flown in other aircraft, but these were all biplanes, and the condensers and collector tank for the condensed water were all mounted in the upper wing. In the Type 224 the collector tanks were in the undercarriage fairings, and, as the condensed water was nearly at boiling point, it was liable to turn to steam under any slight change of pressure; this frequently occurred in the water pumps and would cause them to stop working.

Tests and subsequent development
Mitchell was not a proponent of the use of wind tunnel tests, but the Vickers wind tunnel was used to test aspects of the design of the Type 224, and the results obtained indicated the superiority of a gull-wing design. Wind-tunnel tests were carried out on a full-size model of the open cockpit area to ensure that the pilot would not be subjected to undue buffeting due to the design of the windscreen.

Supermarine submitted information on the predicted performance of the Type 224 to the Air Ministry based on the wind-tunnel data, which suggested that the aircraft would perform well, and in response, the ministry issued a production contact for a prototype. The Type 224 first flew on 19 February 1934, piloted by "Mutt" Summers. Its performance was disappointing—the maximum speed was 228 mph and it took 9.5 minutes to climb to 15000 ft, well below the predicted performance of a 245 mph speed and climb to 15000 ft in 6.6 minutes. The wing design created issues with lateral stability, and further wind-tunnel tests were carried out before the design was finalised, when Mitchell enlarged the fin.

Fate
Mitchell was in discussions about a number of improvements to the Type 224—a new wing, tailplane, and engine arrangements—which would give it a top speed of 265 mph, but the Air Ministry decided that a new aircraft, rather than a modification of the Type 224, was called for. In 1933. In January 1935, further construction was cancelled. Mitchell was disappointed in the Type 224’s under-performance, and came to doubt the reliability of the data produced by wind-tunnel tests.

Gloster's private venture was ordered in mid-1935 as the Gladiator to equip the expanding RAF.

On 25 May 1937 the aircraft was transferred to the Aeroplane and Armament Experimental Establishment at RAF Martlesham Heath. It ended its career during the summer of 1937, when it was used as for target practice at Orford Ness, Suffolk.

Legacy
The failure of the Type 224, coming soon after the Schneider racer successes, did not cause Mitchell to lose faith in his ability to design a fighter. He completely redesigned the type, producing a new design with straight wings and a thinner airfoil shape.

The Air Ministry quickly realized that Mitchell’s new design was a new airplane. It wanted a fighter designed by Mitchell and his team, and in January 1935 it issued Specification F.37/34, for the sole purpose of covering a new design by Supermarine. The specification, essentially an addendum to specification F.7/30 mentioning Supermarines specification 425a and drawing, led on to the design of the Supermarine Spitfire.

Supermarine had asked the Air Ministry for the name Spitfire to be reserved for the type. The name was adopted, but Mitchell disliked it, and the use of it for the Type 224 was later dropped.