while being tested by the
A&AEE, July 1939. Note the overall aluminium paint scheme, with polished spinners. By the mid-1930s, aircraft designers around the world perceived that increased attack speeds were imposing shorter firing times on fighter pilots. This implied less ammunition hitting the target and ensuring destruction. Instead of two rifle-calibre machine guns, six or eight were required; studies had shown that eight machine guns could deliver 256 rounds per second. The eight machine guns installed in the Hurricane fired rifle-calibre rounds, which did not deliver enough damage to quickly knock out an opponent, and were dispersed at ranges other than that at which they were
harmonised. Cannon, such as the French 20 mm
Hispano-Suiza HS.404, which could fire explosive ammunition, offered more firepower and attention turned to aircraft designs which could carry four cannon. While the most agile fighter aircraft were generally small and light, their meagre fuel capacity limited their range and tended to restrict them to defensive and interception roles. The larger airframes and bigger fuel loads of twin-engined designs were favoured for long-range, offensive roles. The first British specification for a high-performance machine-gun monoplane was
Air Ministry specification F.5/34 for a radial-engined fighter for use in the tropics which led to four aircraft designs but the aircraft produced were overtaken by the development of the new Hawker and Supermarine fighters. The RAF
Air Staff thought that an experimental aircraft armed with the 20 mm cannon was needed urgently and specification F.37/35 was issued to British aircraft companies in 1935. The specification called for a single-seat day and night fighter armed with four cannon. The top speed had to be at least greater than that of contemporary bombers – at least at . Eight aircraft designs from five companies were submitted in response to the specification.
Boulton Paul offered the P.88A and P.88B (two related single engine designs differing in engine:
Bristol Hercules radial or
Rolls-Royce Vulture in-line respectively), Bristol the single-engined Type 153 with cannon in wings and the twin-engined Type 153A with cannon in nose. Hawker offered a variant of the Hurricane, the
Supermarine Type 312 was a variant of the company's
Spitfire and the Supermarine Type 313 a twin-engined (
Rolls-Royce Goshawk or
Hispano 12Y) design with four guns in the nose and potentially a further two firing through the propeller hubs if the 12T was used, the Westland P.9 had two
Rolls-Royce Kestrel K.26 engines and a twin tail. When the designs were considered in May 1936, there was concern that on the one hand a two engine design would be less manoeuvrable than a single-engined design and on the other that uneven recoil from cannon set in the wings would give less accurate fire. The conference favoured two engines with the cannon set in the nose and recommended the Supermarine 313. Although Supermarine's efforts were favoured due to their success with fast aircraft and the promise of the Spitfire which was undergoing trials, neither they nor Hawker were in a position to deliver a modified version of their single-engined designs quickly enough - over two years for Supermarine.
Handley Page slats were fitted to the outer wings and to the leading edge of the radiator openings; these were interconnected by
duraluminium torque tubes. In June 1941, the slats were wired shut on the recommendation of the Chief Investigator of the Accident Investigation Branch, after two Whirlwinds crashed when the outer slats failed during vigorous manoeuvres; tests by the
Aeroplane and Armament Experimental Establishment (A&AEE) confirmed that the Whirlwind's take-off and landing was largely unaffected with the slats locked shut, while the flight characteristics improved under the conditions in which the slats normally deployed. The engines were developments of the Rolls-Royce Kestrel K.26, later renamed
Peregrine. The first prototype,
L6844, used long exhaust ducts that were channelled through the wings and fuel tanks, exiting at the wing's trailing edge. This configuration was quickly changed to more conventional, external exhausts after Westland's chief test pilot
Harald Penrose nearly lost control when an exhaust duct broke and heat-fractured an aileron control rod. The engines were cooled by ducted radiators, which were set into the leading edges of the wing centre-sections to reduce drag. With the pilot sitting high under one of the world's first full
bubble canopies and the low and forward location of the wing, all round visibility was good (except for directly over the nose). Four 20 mm cannon were mounted in the nose; the 600 round/minute fire rate made it the most heavily armed fighter aircraft of its era. The clustering of the weapons also meant that there were no convergence problems as with wing-mounted guns. Hopes were so high for the design that it remained top secret for much of its development, although it had already been mentioned in the French press.
L6844 first flew on 11 October 1938, construction having been delayed chiefly due to the new features and also because of the late delivery of the engines and undercarriage. The Whirlwind exhibited excellent handling characteristics and proved to be very easy to fly at all speeds. The only exception was the inadequate directional control during take-off which necessitated an increased rudder area above the tailplane. This careful attention to streamlining and two Peregrine engines powered it to over , the same speed as the latest single-engine fighters. Earlier, due to the lower expected production at Westland, there had been suggestions that production should be by other firms (
Fairey or Hawker) and an early 1939 plan to build 800 of them at the
Castle Bromwich shadow factory was dropped in favour of Spitfire production; instead a further 200 would be built by Westland. Despite the Whirlwind's promise, production ended in January 1942, after the completion of just 112 production aircraft (plus the two prototypes). Rolls-Royce needed to concentrate on the development and production of the
Merlin engine, and the troubled
Vulture engines, rather than the Peregrine. Westland was aware that its design – which had been built around the Peregrine – was incapable of using anything larger without an extensive redesign. After the cancellation of the Whirlwind, Petter campaigned for the development of a Whirlwind
Mk II, which was to have been powered by an improved Peregrine, with a better, higher-altitude supercharger, also using 100 octane fuel, with an increased boost rating. This proposal was aborted when Rolls-Royce cancelled work on the Peregrine. Building a Whirlwind consumed three times as much alloy as a Spitfire. ==Operational history==