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The Westland Whirlwind helicopter was a British licence-built version of the U.S. Sikorsky S-55/H-19 Chickasaw. It primarily served with the Royal Navy's Fleet Air Arm in anti-submarine and search and rescue roles. In 1950, Westland Aircraft, already building the American Sikorsky S-51 under licence as the Westland Dragonfly, purchased the rights to manufacture and sell Sikorsky's larger Sikorsky S-55 helicopter. While a Sikorsky-built pattern aircraft was flown by Westland in June 1951, converting the design to meet British standards (including the provision of a revised main-rotor gearbox), was time-consuming, and the first prototype British aircraft, registered G-AMJT, powered by the 600 hp Pratt & Whitney R-1340-40 Wasp did not fly until August 1953. This was followed by ten Whirlwind HAR.1s, which entered service shortly afterwards. They served in non-combat roles, including search and rescue and communications functions. The HAR.3 had a larger 700 hp Wright R-1300-3 Cyclone 7 engine. The performance of early versions was limited by the power of the American Wasp or Cyclone engines, and in 1955, the HAR.5, powered by an uprated engine, the Alvis Leonides Major, flew for the first time. This was followed by the similarly powered HAS.7, which became the first British helicopter designed for anti-submarine warfare in the front-line when it entered service in 1957. It could either be equipped with a dipping Sonar for submarine detection or carry a torpedo, but could not carry both simultaneously, so sonar equipped "Hunters" were used to direct torpedo armed "Killers". The HAS.7 was powered by a 750 hp (560 kW) Alvis Leonides Major 755/1 radial engine. It had a hovering ceiling at 9,400 ft (2,900 m) and a range of 334 miles at 86 mph. In 1960 Westland introduced a Whirlwind powered by the 1,000 hp Bristol Siddeley Gnome turboshaft, the greater power giving much improved performance over the earlier piston-engined variants; helicopters receiving this modification were redesignated as the HAR.9. The Gnome featured an early computer controlled fuel system that removed variations in engine power and made for much easier handling by the pilot. More than 400 Whirlwinds were built, of which nearly 100 were exported to foreign customers. 848 Naval Air Squadron of the Royal Navy's Fleet Air Arm was the first squadron to receive HAR.1s, which replaced Sikorsky-built HAR.21 versions of the Whirlwind, for utility and search-and-rescue service from July 1954. After entering service with the Royal Navy, the Whirlwind also entered service with the Royal Air Force and French Navy, which received 37 Whirlwind HAR.2 between 1954 and 1957. The Royal Air Force Search and Rescue Force used Whirlwinds painted in overall yellow for rescuing people in distress around the coast of the United Kingdom. Westland Wessex, and eventually Westland Sea King, helicopters later supplemented and eventually replaced Whirlwinds in this role. For details of the 19 variants, click here.

The Westland Whirlwind was a British twin-engined fighter developed by Westland Aircraft. A contemporary of the Supermarine Spitfire and Hawker Hurricane, it was the first single-seat, twin-engined, cannon-armed fighter of the Royal Air Force. When it first flew in 1938, the Whirlwind was one of the fastest combat aircraft in the world and with four 20 mm Hispano-Suiza HS.404 autocannon in its nose, the most heavily armed. Protracted development problems with its Rolls-Royce Peregrine engines delayed the project and only 114 Whirlwinds were built. During the Second World War, only three RAF squadrons were equipped with the aircraft and, despite its success as a fighter and ground attack aircraft, it was withdrawn from service in 1943. 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 40 mph (64 km/h) greater than that of contemporary bombers – at least 330 mph (530 km/h) at 15,000 ft (4,600 m). 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 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. Westland, which had less work and was further advanced in their project, was chosen along with the P.88 and the Type 313 for construction. A contract for two P.9s was placed in February 1937 which were expected to be flying in mid-1938. The P.88s were ordered in December along with a Supermarine design to F37/35 but both were cancelled in January. The Westland design team, under the new leadership of W. E. W. "Teddy" Petter designed an aircraft that employed state-of-the-art technology. The monocoque fuselage was tubular, with a T-tail at the end, although as originally conceived, the design featured a twin tail, which was discarded when large Fowler flaps were added that caused large areas of turbulence over the tail unit. By the employment of the T-tail, the elevator was moved up out of the way of the disturbed airflow caused when the flaps were down. 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. The airframe was built mainly of stressed-skin duralumin, with the exception of the rear-fuselage, which used a magnesium alloy stressed skin. 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 lb/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. Many pilots who flew the Whirlwind praised its performance. Sergeant G. L. Buckwell of 263 Squadron, who was shot down in a Whirlwind over Cherbourg, later commented that the Whirlwind was "great to fly – we were a privileged few... In retrospect the lesson of the Whirlwind is clear... A radical aircraft requires either prolonged development or widespread service to exploit its concept and eliminate its weaknesses. Too often in World War II, such aircraft suffered accelerated development or limited service, with the result that teething difficulties came to be regarded as permanent limitations". Another 263 Squadron pilot said "It was regarded with absolute confidence and affection". By comparison the test pilot Eric Brown described the aircraft as "under-powered" and "a great disappointment". For more deetails about development, operational history and variants, click here.