The Boulton Paul Defiant is a British interceptor aircraft that served with the Royal Air Force (RAF) during World War II. The Defiant was designed and built by Boulton Paul Aircraft as a "turret fighter", without any forward-firing guns, also found in the Blackburn Roc of the Royal Navy.

In combat, the Defiant was found to be reasonably effective at destroying bombers but was vulnerable to the Luftwaffe's more manoeuvrable, single-seat Messerschmitt Bf 109 fighters. The lack of forward-firing armament proved to be a great weakness in daylight combat and its potential was realised only when it was converted to night fighting.[2] It was supplanted as a night fighter by the Bristol Beaufighter and de Havilland Mosquito. The Defiant found use in gunnery training, target towing, electronic countermeasures and air-sea rescue. Among RAF pilots it had the nickname "Daffy".

Development[edit | edit source]

Origins[edit | edit source]

During the 1930s, the increasing speed of military aircraft posed a particular challenge to anti-aircraft defences.[4] Advances in aircraft design achieved during the 1920s and 1930s had resulted in a generation of multi-engined monoplane bombers that were substantially faster than their contemporary single-engined biplane fighters then in service. The RAF came to believe that its new generation of turret-armed bombers, such as the Vickers Wellington, would be capable of readily penetrating enemy airspace and of defending themselves without any accompanying fighter escort, but also recognised that the bombers of other European air forces, such as the Luftwaffe, would similarly be able to penetrate British airspace with impunity.

During 1935, the concept of a turret-armed defensive fighter to counter the bomber threat emerged during a time in which the RAF anticipated having to defend Great Britain against massed formations of unescorted enemy bombers.[5][6] In theory, turret-armed fighters would approach an enemy bomber from below or from the side and coordinate their fire. The separation of the tasks of flying the aircraft and firing the guns would allow the pilot to concentrate on putting the fighter into the best position for the gunner to engage the enemy. However, manually-traversed turrets were viewed as having becoming more problematic and increasingly inadequate to effectively respond to ever-faster hostile aircraft, thus there was considerable interest in using a power-augmented turret.[4]

The earlier Hawker Demon biplane had tested the concept with 59 of the fighters, which had been manufactured by Boulton Paul under a sub-contract, having been equipped with a hydraulically-powered rear turret, while a number of aircraft already built were also converted as such.[7] Boulton Paul and its managing director John Dudley North had gained considerable experience with defensive turrets from producing several earlier aircraft, including the Boulton Paul Overstrand bomber, and had devised a four-gun power-operated turret, the concept and development work of which would later be a core part of the Defiant design.[4]

In April 1935, the Air Ministry released Specification F.9/35, which required a two-seater day and night "turret fighter" capable of 290 miles per hour (470 km/h) at 15,000 feet (4,600 m). The aircraft was to feature a clean design, concentrating its armament within a power-operated turret, and the accepted performance was to be only slightly beneath that of other emergent fighter designs of the period, along with a sufficient fuel capacity to allow it to perform standing patrols.[4] In particular, the powered turret was to offer considerable flexibility, possessing both a 360-degree upper hemisphere field of fire and be able to engage enemy bombers from a range of quarters, including below the aircraft itself.[4] Specification F.9/35 had followed the earlier Specification F.5/33, which had sought a pusher design combined with a forward-set turret; F.5/33 had been unceremoniously abandoned as the proposals had offered little in terms of performance gains over existing fighters, and the corresponding Armstrong Whitworth AW.34 design which had been ordered was not completed.

P.82[edit | edit source]

Boulton Paul, having been focused on turret-equipped aircraft for some time, decided to make a submission for Specification F.9/35; their design was given the company name of P.82.[4] The proposed fighter was similar in size and appearance to the more conventional Hawker Hurricane, differing in weight primarily due to the use of turret-based armaments. The central feature of the P.82 was its four-gun turret, based on a design by French aviation company Societe d'Applications des Machines Motrices (SAMM), which had been licensed by Boulton Paul for use in the earlier Boulton Paul Sidestrand bomber, eventually installed in the "follow-up" design, the Boulton Paul Overstrand and in the Blackburn Roc naval fighter.[8] The 'Type A' turret was an electro-hydraulically powered "drop-in" unit, with a crank-operated mechanical backup. Small bombs could be housed in recesses in the outer wing.[8] Some of the development work from the company's earlier B.1/35 tender was carried over into the P.82.

Of the seven designs tendered, the Air Ministry ranked the P.82 as being the second-best submission, after the Hawker Hotspur but ahead of others such as Armstrong Whitworth's twin-engined design. The Air Ministry wanted several designs investigated and the production of two prototypes of each but the associated costs involved in this preference were in excess of the funding thus special permission from HM Treasury was sought.[4] The Treasury agreed to finance the completion of seven prototypes (two Hawker, two Boulton Paul, two Fairey and one Armstrong Whitworth) but only prototypes of the two most promising designs, the P.82 and the Hotspur, were ordered in late 1935.[4][9] In 1936, Boulton Paul commenced assembly on the first P.82 prototype, K8310, at their new Wolverhampton facility; an order for a second prototype, K8620, was received by the following year.[10]

In 1937, the first P.82 prototype, K8310, was rolled out. Furnished with a 1,030 hp (768 kW) Rolls-Royce Merlin I and initially lacking its turret, the aircraft bore a great resemblance to the contemporary Hawker Hurricane, although it was at least 1,500 pounds (680 kg) heavier. On 11 August 1937, K8310, which had recently received the name Defiant, conducted its maiden flight.[10] This initial flight, piloted by Boulton Paul's chief test pilot Cecil Feather, occurred nearly a year ahead of the rival Hotspur but still without the turret. Official acceptance trials did not commence until nine months later.[10] On 30 July 1939, the second prototype, K8620, equipped with a Merlin II engine and a full turret, conducted its first flight. K8620 had received various modifications over the first prototype, such as telescopic radio masts and revisions to the canopy and to the undercarriage fairing plates; implementing these improvements had incurred delays to the completion of the second prototype.[10]

Production orders had been prepared for the Hotspur, the initial front-running submission but Boulton Paul's turret design had gained the attention of the Air Ministry. Hawker's progress on the project had been delayed by their commitments on other aircraft programs including the more conventional Hurricane; thus the prototype Hotspur, K8309, did not conduct its maiden flight until 14 June 1938.[11] On 28 April 1937, an initial production order for 87 aircraft was received by Boulton Paul for the P.82; as this was prior to the first flight of the prototype, the aircraft had effectively been ordered 'off the drawing board'.[10] The order for the rival Hotspur was cancelled in 1938.

Completing its acceptance tests with the turret installed, the Defiant attained a top speed of 302 miles per hour (486 km/h) and subsequently was declared the victor of the turret fighter competition.[10] Flight trials had revealed the aircraft to possess positive flight characteristics and considerable stability, which was of particular value when using the turret. According to aviation author Michael Bowyers, the usefulness of the Defiant had suffered due to the overly long development time for the type, observing that the Defiant's service entry was delayed to such an extent that only three production aircraft had reached the RAF, and these were only for trial purposes, by the outbreak of the Second World War.[10] Due to delays with the type entering production, there were not enough available Defiants to begin standing patrols in 1940, by which point the introduction of not only more advanced fighters but bombers as well had allegedly undermined the usefulness of the type.

Production[edit | edit source]

On 30 July 1939, the first production Defiant, L6950, conducted its maiden flight; it commenced official trials with the Aeroplane and Armament Experimental Establishment (A&AEE) in September that year.[10] Apart from some detail changes, the production Defiant Mk I looked similar to the two Defiant prototypes. It was powered by the Rolls Royce Merlin III engine, which was capable of generating 1,030 hp/768 kW or 1,160 hp/865 kW.

By January 1940, over half of the original production batch had been completed.[10] Beyond the initial production order in April 1937, follow-on orders had been issued for the type; in February 1938, an additional 202 Defiant Mk I aircraft were ordered; three months later, another 161 aircraft were ordered.[10] In December 1939, yet another 150 aircraft were ordered, raising the overall total to 513. In 1940, this rose to 563 Defiant Mk Is on order, while a further 280 were ordered under a rearranged manufacturing plan issued in mid-1940.[10] However, the performance of the Defiant had been determined to be inadequate by this point, which led to manufacturing being sustained principally for economic reasons.[10] A total of 713 Defiant Mk I aircraft were completed.

In response to a service request which sought greater performance, the Defiant Mk II, powered by the 1,260 hp Merlin XX engine, was promptly developed. On 20 July 1940, N1550, the first production Defiant Mk II performed its initial flight.[10] The Mk II featured a pressurised fuel system, additional fuel, an enlarged rudder, a deeper radiator, a modified engine mounting and elongated cowling. Once sufficient quantities of the Merlin XX engine were available, production of the improved variant commenced; in August 1941, the first production deliveries of the Defiant Mk II took place.[10]

The Defiant Mk II was soon paired with the newly developed airborne interception radar (AI) to become a night fighter.[10] While initial AI equipment was too heavy and bulky to be practical for equipping smaller aircraft, the improved AI Mk. IV radar was suitably sized for the Defiant; the first such furnished Defiants were introduced in late 1941.[12] Later versions of the AI radar were adopted over time, such as the AI Mk VI.[13] The need for both the Defiant and the Hurricane in the night fighter role petered out by 1942 as the larger Bristol Beaufighter became the RAF's primary night fighter type, freeing both aircraft for other duties.[13]

In the search for alternative uses for the Defiant, which included limited service with the RAF Search and Rescue Force and suitability trials for cooperative operations with the British Army, it was determined that Defiant production would continue in order to satisfy a pressing requirement for high speed gunnery targets.[13] A dedicated version of the aircraft, the Defiant TT Mk I, was developed for this purpose; modifications included the removal of the turret, the installation of target-towing equipment, including a target stowage box and a wind-driven winch, and the addition of a winch operator under an enclosed canopy.[13] In January 1942, the prototype Defiant TT Mk I, DR863, conducted its maiden flight; fighter production was phased out shortly thereafter.[14]

The last Defiant Mk IIs under construction were completed as TT Mk I aircraft. Dozens of existing Defiant Mk Is would be remanufactured to the similar Defiant TT Mk III standard; roughly 150 such conversion took place during 1943–1944.[14] So that the type could be used to meet the growing overseas demand for target-towing aircraft, the Defiant was tropicalized, a large portion of which was the installation of large filters underneath the aircraft's nose.

P.85[edit | edit source]

The P.85 was Boulton Paul's tender to Specification O.30/35 for the naval turret fighter. A version of the Defiant for Fleet Air Arm (FAA), it had leading edge slats and a deeper fuselage, for the lower landing speeds required of carrier aircraft. The engine would be either a Bristol Hercules radial or the Merlin. Despite the P.85's higher estimated top speed, the Blackburn Roc was selected. With Blackburn already busy producing other projects, the detail design and production of the Roc was given to Boulton Paul.[15] Ultimately, the only use of the Defiant within the FAA was its adoption of the target tug version.

P.94[edit | edit source]

The first Defiant prototype had not been fitted with a turret at first and had an impressive top speed. In 1940, Boulton Paul removed the turret from the prototype as a demonstrator for a fixed-gun fighter based on Defiant components. The armament offered was either 12 .303 inches (7.7 mm) Browning machine guns (six per wing) or four 20 millimetres (0.79 in) Hispano cannon in place of eight of the Brownings. The guns could be depressed for ground attack. By that time, the RAF had sufficient quantities of Hawker Hurricanes and Supermarine Spitfires and did not require a new single-seat fighter. With a calculated top speed of about 360 miles per hour (580 km/h) at 21,700 feet (6,600 m) the P.94 was almost as fast as a contemporary Spitfire although less manoeuvrable.

Design[edit | edit source]

The Defiant was a single-engine interceptor aircraft. It used a monoplane structure which was coupled with main landing gear which retracted into a broad mainplane section. The pilot's cockpit and rear turret were faired into a streamlined upper fuselage section. Tankage for up to 104 gallons of fuel was housed within the wing centre section along with a large ventral radiator that completed the resemblance to the Hawker fighter.[4] The center section employed a two-spar arrangement, and the wing itself had removable wingtips. The rear fuselage comprised two metal cones connected by a two-foot transitional section and a flat upper deck.[4] The Defiant employed an all-metal stressed skin monocoque structure, which was built in sections that were subsequently bolted together, a manufacturing method previously used on other Bolton Paul-designed aircraft. It was a relatively clean design and made use of a simple, weight-saving structure.[4]

The primary mission of the Defiant was the destruction of incoming enemy bombers. The principal armament of the aircraft is its powered dorsal turret, equipped with four 0.303 in (7.7 mm) Browning machine guns. The fuselage was fitted with aerodynamic fairings that helped mitigate the drag of the turret; they were pneumatically powered and could be lowered into the fuselage so that the turret could rotate freely. The Brownings were electrically fired and insulated cut-off points in the turret ring prevented the guns firing when they were pointing at the propeller disc or tailplane.[4] The gunner could rotate the turret directly forward and transfer firing control of the guns to the pilot, with the guns firing along each side of the cockpit canopy; this was rarely done as the turret's minimum forward elevation was 19° and the pilot did not have a gunsight, possibly because the Defiant was outfitted to perform zero deflection shooting, as were several contemporaneous designs arising from Air Ministry specifications

Operational history[edit | edit source]

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