Hawker Tempest

The Hawker Tempest is a British fighter aircraft that was primarily used by the Royal Air Force (RAF) in the Second World War. The Tempest, originally known as the ''Typhoon II'', was an improved derivative of the Hawker Typhoon, intended to address the Typhoon's unexpected deterioration in performance at high altitude by replacing its wing with a thinner laminar flow design. Since it had diverged considerably from the Typhoon, it was renamed ''Tempest''. The Tempest emerged as one of the most powerful fighters of World War II and at low altitude was the fastest single-engine propeller-driven aircraft of the war.

Upon entering service in 1944, the Tempest performed low-level interception, particularly against the V-1 flying bomb threat, and ground attack supporting major invasions like Operation Market Garden. Later, it successfully targeted the rail infrastructure in Germany and ''Luftwaffe'' aircraft on the ground, as well as countering similar attacks by German fighters. The Tempest was effective in the low-level interception role, including against newly developed jet-propelled aircraft like the Messerschmitt Me 262.

The further-developed Tempest II did not enter service until after the end of hostilities. It had several improvements, including being tropicalised for combat against Japan in South-East Asia as part of the Commonwealth Tiger Force.

Design and development

Origins

During development of the earlier Hawker Typhoon, the design team, under the leadership of Sydney Camm, had already planned out a series of design improvements; these improvements cumulated in the ''Hawker P. 1012'', otherwise known as the ''Typhoon II'' or ''Thin-Wing Typhoon''. Although the Typhoon was generally considered to be a good design, Camm and his design team were disappointed with the performance of its wing, which had proved to be too thick in its cross section, and thus created airflow problems which inhibited flight performance, especially at higher altitudes and speeds where it was affected by compressibility. The Typhoon's wing, which used a NACA 4 digit series wing section, had a maximum thickness-to-chord ratio of 19.5 per cent (root) to 12 per cent (tip), in comparison to the Supermarine Spitfire's 13.2 per cent tapering to 6 per cent at the tip, the thinner design being deliberately chosen to reduce drag.Both the Spitfire and the Typhoon were designed when little was known about the behaviour of air at high subsonic Mach numbers, and of what would later become known as the Critical Mach number. The importance of this to high speed, high altitude flight would be discovered during combat in the Battle of Britain, where the Spitfire's thinner wing gave it an advantage at higher altitudes over the thicker wing-sectioned Hurricane, which was affected to a greater extent by compressibility. Fortuitously, the Spitfire had been designed with a thin wing that was subsequently discovered by the RAE to possess a high Critical Mach No. In addition, there had been other issues experienced with the Typhoon, such as engine unreliability, insufficient structural integrity, and the inability to perform high altitude interception duties.Mason 1967, p. 3.

In March 1940, engineers were assigned to investigate the new low–drag laminar flow wing developed by NACA in the United States, which was later used in the North American P-51 Mustang. A laminar flow wing adopted for the Tempest series had a maximum thickness-to-chord ratio of 14.5 per cent at the root, tapering to 10 per cent at the tip.Thomas and Shores 1988, p. 105. The maximum thickness of the Tempest wing was set further back at 37.5 per cent of the chord versus 30 per cent for the Typhoon's wing, reducing the thickness of the wing root by five inches on the new design. The wingspan was originally greater than that of the Typhoon at 43 ft (13.1 m), but the wingtips were later "clipped" and the wing became shorter; 41 ft (12.5 m) versus 41 ft 7 in (12.7 m).

The wing planform was changed to a near-elliptical shape to accommodate the 800 rounds of ammunition for the four 20 mm Hispano cannons, which were moved back further into the wing. The new wing had greater area than the Typhoon's, however, the new wing design sacrificed the leading edge fuel tanks of the Typhoon: to make up for this loss in capacity, Hawker engineers added a new 21 in (53 cm) fuel bay in front of the cockpit, with a 76 I gal (345 L) fuel tank.Mason 1967, pp. 3–4. In addition, two inter-spar wing tanks, each of 28 Igal (127 L), were fitted on either side of the centre section and, starting with late model Tempest Vs, a 30 Igal (136 L) tank was carried in the leading edge of the port wingroot, giving the Tempest a total internal fuel capacity of 162 Igal (736 L).

Another important feature of the new wing was Camm's proposal that the radiators for cooling the engine be fitted into the leading edge of the wing inboard of the undercarriage. This eliminated the distinctive " chin" radiator of the Typhoon and improved aerodynamics. A further improvement of the Tempest wing over that of the Typhoon was the exceptional, flush-riveted surface finish, essential on a high performance laminar flow airfoil. The new wing and airfoil, and the use of a four-bladed propeller, acted to eliminate the high frequency vibrations that had plagued the Typhoon.Thomas and Shores 1988, p. 106. The design team also chose to adopt the new Napier Sabre IV engine for the Tempest, drawings of which had become available to Hawker in early 1941.Mason 1967, p. 4.

In February 1941, Camm commenced a series of discussions with officials within the Ministry of Aircraft Production on the topic of the P.1012. In March 1941 of that year, clearance to proceed with development of the design, referred to at this point as the Typhoon II, was granted. At this point, work was undertaken by a team of 45 draughtsmen at Hawker's wartime experimental design office at Claremont, Esher to convert the proposal into technical schematics from which to commence manufacture. In March 1941, the Air Ministry issued specification F.10/41 that had been written to fit the aircraft. By October 1941, development of the proposal had advanced to the point where the new design was finalised.

Prototypes

On 18 November 1941, a contract was issued by the Air Ministry for a pair of prototypes of the "Typhoon Mark II"; the new fighter was renamed "Tempest" on 28 February 1942. Complications were added to the Tempest program by external factors in the form of engine issues: the Rolls-Royce Vulture engine and corresponding Hawker Tornado aircraft which was being developed in parallel were both terminated. Due to this previous experience on other programs, the Air Ministry was sufficiently motivated to request that a total of six Tempest prototypes be built using different engines so that, if a delay hit one engine, an alternative powerplant would already be available. This measure turned out to be prudent, as engine development was not trouble-free on some of the variants of the Tempest.Mason 1967, pp. 4-6.

The six prototypes built were as follows:Mason 1991, p. 331.Mason 1967, pp. 4–5.
*One Tempest Mk.I (serial number ''HM599''), equipped with the Napier Sabre Mk.IV engine
*Two Tempest Mk.II (serial numbers ''LA602'' and ''LA607''), equipped with the Bristol Centaurus Mk.IV engine (LA607 later receiving a Centaurus Mk.V)
*One Tempest Mk.III (serial number ''LA610''), equipped with the Rolls-Royce Griffon 85 engine (originally planned for the Griffon IIB)
*One Tempest Mk.IV (serial number ''LA614''), which was never completed but planned to be equipped with a Griffon 61 engine
*One Tempest Mk.V (serial number ''HM595''), equipped with the Napier Sabre Mk.II engine

The Tempest Mk.I featured other new features, such as a clean single-piece sliding canopy in place of the car-door framed canopy, and it used wing radiators instead of the "chin" radiator. Due to development difficulties with the Sabre IV engine and its wing radiators, the completion of the Mk.I prototype, ''HM599'', was delayed, and thus it was the Mk.V prototype, ''HM595'', that would fly first.

On 2 September 1942, the Tempest Mk.V prototype, ''HM595'', conducted its maiden flight, flown by Philip Lucas from Langley, Berkshire, England. ''HM595'', which was powered by a Sabre II engine, retained the Typhoon's framed canopy and car-style door, and was fitted with the "chin" radiator, similar to that of the Typhoon. It was quickly fitted with the same bubble canopy fitted to Typhoons, and a modified fin that almost doubled the vertical tail surface area, made necessary because the directional stability with the original Typhoon fin had been reduced over that of the Typhoon by the longer nose incurred by the new fuel tank. The horizontal tailplanes and elevators were also increased in span and chord; these were also fitted to late production Typhoons. Test pilots found the Tempest a great improvement over the Typhoon in performance; in February 1943 the pilots from the A&AEE at Boscombe Down reported that they were impressed by "a manoeuvrable and pleasant aircraft to fly with no major handling faults".

On 24 February 1943, the second prototype ''HM599'' first flew, representing the "Tempest Mk.I" equipped with the Napier Sabre IV engine; this flight had been principally delayed by protracted problems and slippages encountered in the development of the new Sabre IV engine. ''HM599'' was at first equipped with the older Typhoon cockpit structure and vertical tailplane. The elimination of the "chin" radiator did much to improve overall performance, leading to the Tempest Mk.I quickly becoming the fastest aircraft that Hawker had built at that time, having attained a speed of 466 mph (750 km/h) during test flights.

On 27 November 1944, the Tempest Mk.III prototype, ''LA610'', conducted its first flight; it was decided to discontinue development work on the Mk.III, this was due to priority for the Griffon engine having been assigned to the Spitfire instead. Work on the Tempest Mk.IV variant was abandoned without any prototype being flown at all. The Tempest Mk.II, which was subject to repeated delays due to its Centaurus powerplant, was persisted with, but would not reach production in time to see service during the Second World War. Continual problems with the Sabre IV meant that only the single Tempest Mk.I (''HM599'') was built; consequently, Hawker proceeded to take the Sabre II-equipped "Tempest V" into production instead.Thomas and Shores 1988, p. 107.

In August 1942, even before the first flight of the prototype Tempest V had been conducted, a production order for 400 Tempests was placed by the Air Ministry.Mason 1967, p. 5. This order was split, with the initial batch of 100 being Tempest V "Series I"s, powered by the 2,235 hp (1,491 kW) Sabre IIA series engine, which had the distinctive chin radiator, while the rest were to have been produced as the Tempest I, equipped with the Sabre IV and leading-edge radiators. These 300 Tempest Is were intended to replace an order for a similar quantity of Typhoons for the Gloster Aircraft Company. As it transpired, the difficulties with the Sabre IV and the wing radiators led to this version never reaching production, the corresponding order was switched to 300 Tempest V "Series 2"s instead.Ovčáčík and Susa 2000, p. 1.

Tempest Mk.V

During early 1943, a production line for the Tempest Mk.V was established in Hawker's Langley facility, alongside the existing manufacturing line for the Hawker Hurricane. Production was initially slow, claimed to be due to issues encountered with the rear spar. On 21 June 1943, the first production Tempest V, ''JN729'', rolled off the production line and its maiden flight was conducted by test pilot Bill Humble.

During production of the first batch of 100 Tempest V "Series Is", distinguishable by their serial number prefix ''JNxxx'', several improvements were progressively introduced and were used from the outset on all succeeding Tempest V "Series 2s", with serial number prefixes EJ, NV and SN. The fuselage/empennage joint originally featured 20 external reinforcing "fishplates", similar to those fitted to the Typhoon, but it was not long before the rear fuselage was strengthened and, with the fishplates no longer being needed, the rear fuselage became detachable.Bentley 1973, pp. 92–93. The first series of Tempest Vs used a built-up rear spar pick-up/bulkhead assembly (just behind the cockpit) which was adapted from the Typhoon. Small blisters on the upper rear wing root fairing covered the securing bolts. This was later changed to a new forged, lightweight assembly which connected to new spar booms: the upper wing root blisters were replaced by small "teardrop" fairings under the wings.

The first 100 Tempest Vs were fitted with 20 mm (.79 in) Hispano Mk.IIs with long barrels which projected ahead of the wing leading edges and were covered by short fairings; later production Tempest Vs switched to the short-barrelled Hispano Mk.Vs, with muzzles flush with the leading edges. Early Tempest Vs used Typhoon-style 34 by 11 inch (83.4 by 28 cm) five-spoke wheels, but most had smaller 30 by 9 inch (76.2 by 22.9 cm) four-spoke wheels.Ovčáčík and Susa 2000, pp. 2, 4. The new spar structure of the Tempest V also allowed up to 2,000 lb (907 kg) of external stores to be carried underneath the wings. As a result, several early production Tempest V aircraft underwent extensive service trials at Boscombe Down for clearance to be fitted with external stores, such as one bomb or eight RP-3 rockets under each wing. On 8 April 1944, the Tempest Mk.V attained general clearance to carry such ordnance, but few Tempest Mk.V deployed bombs operationally during the war. Rockets were never used operationally during the war.

As in all mass-produced aircraft, there may have been some overlap of these features as new components became available. In mid-to-late 1944 other features were introduced to both the Typhoon and Tempest: A '' Rebecca'' transponder unit was fitted, with the associated aerial appearing under the portside centre section. A small, elongated oval static port appeared on the rear starboard fuselage, just above the red centre spot of the fuselage roundel. This was apparently used to measure the aircraft's altitude more accurately.

Unusually, in spite of the Tempest V being the RAF's best low- to medium-altitude fighter, it was not equipped with the new Mk.IIC gyroscopic gunsight (as fitted in RAF Spitfires and Mustangs from mid-1944), which would have considerably improved the chances of shooting down opposing aircraft. Tempest pilots continued to use either the Type I Mk.III reflector gunsight, which projected the sighting graticule directly onto the windscreen, or the Mk.IIL until just after the Second World War, when the gyro gunsight was introduced in Tempest IIs.

Two Tempest Vs, ''EJ518'' and ''NV768'', were fitted with Napier Sabre Vs and experimented with several different Napier-made annular radiators, with which they resembled Tempest IIs. This configuration proved to generate less drag than the standard "chin" radiator, contributing to an improvement in the maximum speed of some 11 to 14 mph. ''NV768'' was later fitted with a ducted spinner, similar to that fitted to the Fw 190 V1.Mason 1991, p. 332.

Another experimental Tempest V was ''SN354'', which was fitted with two experimental underwing Vickers 47 mm Class P anti-tank guns at the end of the war.Williams, A. G
"The RAF'S 47 mm Class P Gun Project."
''quarry.nildram.co.'' Retrieved: 16 March 2012.Mason 1967, p. 13. These guns were part of a project to develop a 47 mm airborne anti-tank gun, of which several company designs existed. The Vickers design was called ''PV'', for ''Class P, Vickers''. The PV guns were housed in long slim streamlined gun pods carried on the bombracks and had 38 rounds each. Surviving photographs suggests that the 20 mm wing guns were removed for this installation. Testing of the guns revealed that the weapon had potential, but no production was undertaken.

Tempest Mk.II

As a result of the termination of the Tornado project, Sydney Camm and his design team transferred the alternative engine proposals for the Tornado to the more advanced Tempest. Thus, it was designed from the outset to use the Bristol Centaurus 18 cylinder radial engine as an alternative to the liquid cooled engines which were also proposed. A pair of Centaurus-powered Tempest II prototypes were completed. Apart from the new engine and cowling, the Tempest II prototypes were similar to early series Tempest Vs. The Centaurus engine was closely cowled and the exhaust stacks grouped behind and to either side of the engine: to the rear were air outlets with automatic sliding "gills". The carburettor air intakes were in the inner leading edges of both wings, an oil cooler and air intake were present in the inner starboard wing. The engine installation owed much to examinations of a captured Focke-Wulf Fw 190, and was clean and effective.

On 28 June 1943, the first Tempest II, ''LA602'', flew powered by a Centaurus IV (2,520 hp/1,879 kW) driving a four-blade propeller. ''LA602'' initially flew with a Typhoon-type fin and rudder unit. This was followed by the second, ''LA607'', which was completed with the enlarged dorsal fin and first flew on 18 September 1943: ''LA607'' was assigned to engine development. The first major problem experienced during the first few flights was serious engine vibrations, which were cured by replacing the rigid, eight-point engine mountings with six-point rubber-packed shock mounts. In a further attempt to alleviate engine vibration, the four blade propeller was replaced with a five blade unit; eventually, a finely balanced four bladed unit was settled on.Mason 1967, p. 12. Problems were also experienced with engine overheating, poor crankshaft lubrication, exhaust malfunctions and reduction-gear seizures. Because of these problems, and because of the decision to "tropicalise" all Tempest IIs for service in the South-East Asian theatre, production was delayed.Ovčáčík and Susa 2000, p. 2.

Orders had been placed as early as September 1942 for 500 Tempest IIs to be built by Gloster but in 1943, because of priority being given to the Typhoon, a production contract of 330 Tempest IIs was allocated instead to Bristol, while Hawker were to build 1,800. This switch delayed production even more.Mason 1967, pp. 11-12. On 4 October 1944, the first Tempest II was rolled off the line; the first six production aircraft soon joined the two prototypes for extensive trials and tests. With the end of the Second World War in sight, orders for the Tempest II were trimmed or cancelled; after 50 Tempest IIs had been built at Bristol's Banwell facility, production was stopped and shifted back to Hawker, which built a total of 402, in two production batches: 100 were built as fighters, and 302 were built as fighter-bombers (FB II) with reinforced wings and wing racks capable of carrying bombs of up to 1,000 lb.Ovčáčík and Susa 2000, pp. 2, 7–8.

Physically, the Tempest II was longer than the Tempest Mk.V (34 ft 5 in/10.5 m versus 33 ft 8 in/10.3 m) and 3 in (76 mm) lower. The weight of the heavier Centaurus engine (2,695 lb/1,222 kg versus 2,360 lb/1,070 kg) was offset by the absence of a heavy radiator unit, so that the Tempest II was only some 20 lb (9 kg) heavier overall. Performance was improved; maximum speed was 442 mph (711 km/h) at 15,200 ft (4,633 m) and climb rate to the same altitude took four and a half minutes compared with five minutes for the Tempest Mk.V; the service ceiling was also increased to 37,500 ft (11,430 m).

Tropicalising measures included the installation of an air filter and intake in the upper forward fuselage, just behind the engine cowling, and the replacement of the L-shaped pitot head under the outer port wing by a straight rod projecting from the port outer wing leading edge. All production aircraft were powered by a (2,590 hp/1,932 kW) Centaurus V driving a 12 ft 9 inch (3.89 m) diameter Rotol propeller. Tempest IIs produced during the war were intended for combat against Japan and would have formed part of Tiger Force, a proposed British Empire long-range bomber force based on Okinawa.Mason 1991, p. 340. The Pacific War ended before they could be deployed.

Tempest Mk.VI

Various engineering refinements that had gone into the Tempest II were incorporated into the last Tempest variant, designated as the ''Tempest VI''. This variant was furnished with a Napier Sabre V engine with 2,340 hp (1,700 kW). The more powerful Sabre V required a bigger radiator which displaced the oil cooler and carburettor air intake from the radiator's centre; air for the carburettor was drawn through intakes on the leading edge of the inner wings, while the oil cooler was located behind the radiator. Most Tempest VIs were tropicalised, the main feature of this process being an air filter which was fitted in a fairing on the lower centre section. Other changes included the strengthening of the rear spar and the inclusion of spring tabs, which granted the variant superior handling performance.

The original Tempest V prototype, ''HM595'', was extensively modified to serve as the Tempest VI prototype. On 9 May 1944, ''HM595'' made its first flight after its rebuild, flown by Bill Humble. In December 1944, ''HM595'' was dispatched to Khartoum, Sudan to conduct a series of tropical trials. During 1945, a further two Tempest V aircraft, ''EJ841'' and ''JN750'', were converted to the Tempest VI standard in order to participate in service trials at RAF Boscombe Down.

At one point, 250 Tempest VIs were on order for the RAF; however, the end of the war led to many aircraft programs being cut back intensively, leading to only 142 aircraft being completed. For a long time, it was thought there were Tempest VIs that had been converted for target towing purposes; however, none of the service histories of the aircraft show such conversions and no supporting photographic evidence has been found. The Tempest VI holds the distinction of being the last piston-engined fighter in operational service with the RAF, having been superseded by jet propelled aircraft.

Drawing board designs

In 1944, in response to the issuing of Air Ministry Specification F.13/44, which sought a long-range fighter equipped with the Rolls-Royce R.46 engine, Sydney Camm initiated work on a new design, designated as the ''P.1027''. This was essentially a slightly enlarged Tempest which was powered by the R.46 engine, which was projected to develop around 2,500–4,000 hp (1,864–2,983 kW). This engine would have driven a pair of four-bladed contra-rotating propellers. The radiator was relocated into a ventral bath set underneath the rear fuselage and wing centre section: the wingspan was 41 ft (12.5 m) and the length was 37 ft 3 in (11.4 m).

However, work upon the P.1027 design was soon dropped in favour of concentrating upon a further developed design, designated as the ''P.1030''. It featured wing leading edge radiators and had larger overall dimensions of 42 ft (12.8 m) wingspan and 39 ft 9 in (12.1 m) length. The top speed was expected to be in the region of 508 mph (817 km/h), with a rate of climb of 6,400 ft/min (1,951 m/min). Service ceiling was projected to be 42,000 ft (12,802 m).Buttler 2004, p. 30. Work on both tenders was ultimately dropped when Camm decided to focus design efforts upon the more promising jet engine designs he was working on instead.

Design

The Tempest was a single engine fighter aircraft that excelled at low-level flight. In service, its primary role soon developed into performing "armed reconnaissance" operations, often deep behind enemy lines. The Tempest was particularly well suited to the role because of its high speed at low to medium altitudes, its long range when equipped with two 45-gallon drop tanks, the good firepower of the four 20mm cannon and the good pilot visibility. The three-piece windscreen and side windows of the Tempest had directly benefited from examination of captured Focke-Wulf Fw 190s, improvements included the careful design and positioning of the frame structure, blind spots being reduced to an absolute minimum. It had a bullet-resistant centre panel made up of two layers, the outer 1.5 in (38 mm) thick and the inner 0.25 in (6.5 mm).

The majority of production Tempests were powered by a single high-powered Napier Sabre II 24-cylinder engine. All versions of the Sabre drove either a four-bladed, 14 ft (4.267 m) diameter de Havilland Hydromatic or Rotol propeller. Starting with EJxxx series, on the Tempest V, both the improved Sabre IIB and IIC were used, these engines were capable of producing over 2,400 hp (1,789 kW) on emergency boost for short periods of time."Tempest MK V Performance."
''wwiiaircraftperformance.org.'' Retrieved: 10 August 2010. Alternative engines were used on some production variants, such as the Tempest II, for which a single Bristol Centaurus 18 cylinder radial engine was adopted, or the final Tempest VI, upon which a Napier Sabre V was used. Early on in development, the adoption of several other engines was proposed, some of which were tested upon multiple prototypes.

The wing of the Tempest was one of its more significant design features, having been developed from the Typhoon to use a thinner laminar flow design approach. The wing planform was of a near-elliptical shape; the aircraft's armament of four 20 mm Hispano cannons with 800 rounds of ammunition was also embedded into the wing. The ailerons were fitted with spring-loaded tabs which lightened the aerodynamic loads, making them easier for the pilot to use and dramatically improving the roll rate