Issue 15 - Hurricane: Refining the breed - The Mk.II, IV and V
By: Tim Callaway and Julian Humphries
Service experience with the Hurricane Mk.I highlighted two operational requirements which would need to be addressed if the fighter was to continue in front line service. The first was for more firepower, the second for greater speed. Hawker began addressing these requirements as early as 1939, developing re-engined and re-armed variants that began entering service towards the end of the Battle of Britain. The increased firepower helped the Hurricane to fill new roles, and was to keep the aircraft on the front line until the end of the Second World War.
It was quickly realised that the need to improve the Hurricane’s performance could not be met with the existing Merlin III engine. Two aircraft, L1856 and L2026, were therefore used for trials with two different versions of the Merlin during 1939, but it was not until mid-1940 that the fully developed Merlin XX was available.
The new engine initially produced 1185hp and very few changes were required to fit this to the Hurricane I airframe, designed as it was for maximum compatibility with earlier variants. The basic structure of the early Mk.II was virtually identical to the Mk.I; however, the opportunity was taken for detail design changes to solve issues revealed by service use. The machines that appeared from October 1940 onwards, known at this stage as Series 2, introduced a slightly longer, re-profiled nose and a semi circular oil splash guard behind the propeller.
On June 11, 1940, Hawker test pilot Philip Lucas took Hurricane P3269 on its first flight from the company’s Langley airfield with the new engine installed. A Rotol constant speed propeller was fitted and the aircraft was fully armed with eight machine guns. The overall weight was just less than 6700lb (3040kg) and this machine was the fastest armed Hurricane ever built, capable of 348mph (560kmh) in level flight.
Hurricanes with the new engine were designated as Mk.IIs from December 1940, but production had begun in August 1940. A few were issued to front line squadrons, including 111, during the Battle of Britain. The increase in available power raised the Hurricane’s top speed to a more respectable 342mph (550kmh); however, this was still some 15 to 20mph (24 to 32kmh) slower than the contemporary Spitfire I and Bf 109E.
A strange phenomenon was encountered with the Mk.II at this stage. It became uncontrollable at altitudes previously unobtainable by earlier versions of the Hurricane. The cause was found to be the lubricating grease used in the controls. This was unsuitable for high altitudes and froze, locking the flying surfaces. Early Mk.IIs were armed with eight .303 Browning machine guns and designated Mk.IIAs. The Mk.I remained in squadron service well into 1941, but was gradually replaced as the new aircraft rolled off the production lines. Many Mk.Is found their way into training units, where even a few very early machines with fabric covered wings continued to give good service.
Increasing the firepower
As early as 1939, consideration had been given to improving the firepower of British fighters. The eight Browning machine guns had a high rate and density of fire, but it was apparent that the rifle calibre bullets were becoming increasingly ineffective. As an interim measure, the number of machine guns was increased to 12 by fitting two additional pairs of .303 Brownings towards the wing tips, outboard of the wing leading edge landing lights.
To accommodate these guns it was necessary to mount them further forward than the main battery, so the muzzles projected beyond the wing’s leading edge. The inner gun of each extra pair was mounted slightly higher than the other guns and was fed from a magazine inboard of it. The outboard guns were fed from the opposite side. The Hurricane’s wing was so thick and spacious, even at this distance from the root, no blisters or other contour changes were necessary. Hurricanes so armed were referred to as Mk.IIBs.
Aircraft cannons had been investigated as early as the mid 1930s. Despite having a lower rate of fire than a machine gun, cannons were able to fire a wider variety of explosive and armour piercing ammunition. Several old airframes were tested on ranges and the effects of cannon fire were impressive, as it took only a few hits to cause lethal damage. The Air Ministry was in the process of negotiating manufacturing licenses from Hispano and Oerlikon for their 20mm cannons, but at this stage it was thought they would be too heavy for use in single engined fighters.
As a trial of the Swiss Oerlikons, two were fitted to Hurricane L1750 in 1939, carried below the wings. This installation was less than satisfactory and the aircraft was not used operationally. The French Hispano cannon was considered most suitable for the next generation of British fighters so arrangements were made for the parent company to establish a factory in Grantham, under the convoluted title the British Manufacturing and Research Company.
Several pairs of unserviceable wings were returned to Hawker to experiment with the installation of four Oerlikon cannon into the existing gun bays. The breeches could be accommodated in the vast bays, but the barrels projected well ahead of the wing. Small aerodynamic blisters on the gun access panels were necessary to clear the breech of each gun. Hurricane Mk.I V7360 was fitted with cannon wings and flew the full installation, but with drum fed guns, for the first time on June 7, 1940.
The first production version of the Mk.IIC, V2461, had belt fed guns. The additional weight of the guns and ammunition reduced the aircraft’s speed considerably and it was unable to achieve even 300mph (482.8kmh). The potential to create a heavily armed fighter was recognised, but the limiting factor at the time was the relatively low power of the Merlin III engine.
During the Battle of Britain, V7360 was delivered to 46 Squadron at North Weald, still fitted with four cannons and used during the intense fighting in September. The gun feeds often failed to function correctly or jammed but at least one German bomber was claimed by Flight Lieutenant Alexander Rabagliati. The problems with the feed mechanism were overcome by the new Chatellerault feed, and with the introduction of more powerful versions of the Merlin, paved the way for full scale production of the cannon armed Hurricane Mk.IIC, which first flew on February 6, 1941. This mark would become the most widely used version of the Hurricane, with over 4700 built, mostly by Hawker at Langley. The wing was largely unchanged, except for the gun bays, which now had additional access panels on the undersurface to allow access to the cannon cocking levers and spent ammunition chutes. The cannons exerted a much greater recoil force than the machine guns which were dampened by prominent recoil springs around each barrel. Hurricane Mk.IICs entered RAF service in May 1941 and were among the world’s most powerfully armed fighters at the time.
The wing of the Hurricane Mk.II could also accept external stores. Either two 44 gallon (200 litre) external fuel tanks or a pair of 250lb (113.4kg) bombs could be carried, one under each wing. When bombs were carried it was usual practice to reduce the number of machine guns by two, the Brownings directly above the pylon attachment point being removed for better access.
The range of the ‘Hurribomber’, as they were unofficially known, was not greatly affected by the extra weight, but the aircraft’s speed was considerably reduced. The first attack took place on October 30, 1941, when a pair of Mk.IIBs attacked an electrical distribution installation near Tingery. At about this time other modifications to the Hurricane were considered, a new Spitfire sliding canopy devoid of the heavy metal framework was tested on at least one aircraft and a four bladed Rotol propeller was flow as early as December 1940. Neither modification was adopted as the Hurricane was seen as belonging to a previous generation and Hawker was preparing to produce the Typhoon.
When operations over northern France began, it was quickly found that the Hurricanes could not operate without fighter escort. Even with the greater power of the Merlin XX, Hurricanes were outclassed by new versions of the Luftwaffe’s Bf 109. Even more dramatic was the advantage enjoyed by the superb Focke-Wulf 190A, which outclassed even the latest incarnation of the Spitfire until the introduction of the Mk.IX. In a reversal of the previous year’s fighting, 1941 saw the RAF go on to the offensive, using fighter sweeps often mixed with a few Blenheims or ‘Hurribombers’ to force the Luftwaffe to engage in combat. Spitfires were the obvious choice for the escorts, on occasion outnumbering their charges by as many as three to one in an attempt to draw enemy fighters into combat.
Targets included forward Luftwaffe bases and the railway network, where the destructive power of the four cannon was proved beyond doubt. These operations confirmed the fighter-bomber concept; Hurricanes, once free of their bombs, were restored to being moderately useful fighters, whereas the slower Blenheim was a liability for the escorting fighters both to and from the target. By the latter half of 1941, it was apparent that the Hurricane was drawing to the end of its operational usefulness over Europe by day, despite the fact that more than 50 squadrons were equipped with the type.
German night raids on Britain continued throughout 1941. As a stop gap measure, Hurricanes were used to patrol high value target areas, often working in conjunction with Boulton Paul Defiants. Night fighter and night intruder operations will be covered later in this magazine. Hurricane IICs of 43 Squadron were among the first aircraft involved in the abortive raid on Dieppe on August 19, 1942, known as Operation Jubilee, which ultimately failed to achieve any of its objectives.
The RAF lost 106 aircraft in the action and nearly two thirds of the landing force of 6000 troops were either killed or taken prisoner. Despite this, crucial lessons were learned that would pay great dividends when the Allies returned to mainland Europe in 1944. This was the last occasion that Hurricanes were used in great numbers in an offensive capacity over Europe. The Hurricane Mk.IID was a ground attack aircraft fitted with a pair of podded Vickers 40mm ‘S’ guns for anti-tank missions. These are described later in this magazine.
The next and perhaps the most important development in British aircraft armament was the rocket projectile (RP). Hurricane Mk.IIA Z2415 was selected as the test bed for the new weapon, as it had already been strengthened for previous aerodynamic trials. The aircraft was first flown on February 23, 1943, with six rockets under each wing. It was later passed to the Aeroplane and Armament Experimental Establishment (A&AEE) at Boscombe Down for further trials and development. The production RP had a 60lb (27.2kg) warhead and four stabilising fins at the rear of the tube containing the propellant.
As the weapon was being developed the number of rockets carried by an individual aircraft was increased to eight. The rocket had many advantages over bombs and cannon. With practice, ground attack pilots could deliver them with great accuracy and they were effective against almost every type of battlefield or surface target. There was no recoil as the rockets left the rails, although pilots experienced a brief period of turbulence as the launch aircraft passed through their wake.
The launch rails were mounted on a thin steel plate to protect the wings from the blast effects as the rockets were launched, in either pairs or in a salvo of all eight. Once the Hurricane had launched its rockets, it reverted to its fighter role and therefore did not require escort in the traditional manner. Rockets were used over mainland Europe for the first time by 137 and 164 Squadrons on September 2, 1943, against the lock gates at the Hansweert Canal in Holland.
This was among the last offensive actions undertaken by Hurricanes in Western Europe, but in the Middle and Far East the RP was to keep the Hurricane at the forefront of the fighting. A single Mk.IV was employed to test the massive Long Tom rocket during 1945. The warhead weighed 500lb (226.8kg) and only two could be carried, but the weapon was never used operationally by RAF Hurricanes.
Hurricane Mk.IV and V
During 1942 the Hurricane was evolving into a specialist ground attack aircraft and as such would need an adaptable wing that could be reconfigured in the field for the varied missions the aircraft was expected to undertake. The universal wing was designed to accept bombs of 250 or 500lb (113.4 or 226.8kg), long range fuel tanks of 45 or 90 gallon (200 or 400 litre) capacity, or the Vickers 40mm cannon in pods. It was also wired to carry rockets and smoke generators to lay smoke screens. Just two Browning machine guns were retained for sighting purposes. This version was initially referred to as the Mk.IIE, but after production commenced a new designation was created.
Externally, the Mk.IV was identical to the standard Hurricane Mk.II, with the exception of the heavily armoured radiator, which now had an angular, flat sided appearance. Many liquid cooled aircraft were lost at low level through damage to the coolant system, so additional armour was fitted around this, the engine and cockpit. The Mk.IV weighed 6150lb (2790kg) empty, in comparison to the Mk.IIA’s 5150lb (2336kg), and was powered by a Merlin 24 or 27 engine producing 1620hp. Hawker test pilot Philip Lucas flew the first Hurricane Mk.IV KX405 on March 14, 1943.
More than 500 Mk.IVs entered service with 20 RAF Squadrons, all of which were built in the UK by Hawker Aircraft and Austin Motors. In service the type upheld the Hurricane’s reputation as a robust and reliable machine. The Mk.IV Hurricanes had serial numbers preceded by the following letters: HL, HM, HV, HW, KW, KX, KZ, LB, LD, LF, PG and PG. In Burma, Mk.IVs were frequently operated with an asymmetric payload of a single fuel tank and four rockets.
In early 1943, two Hurricane Mk.IVs were taken from the production line and converted to accept the Merlin 32 engine which was tailored to low level performance and produced 1700hp. To absorb this power, a Rotol four bladed propeller was installed. The intention was to produce an even more powerful low level attack aircraft coupled to the universal wing of the Mk.IV for use in the Far East. The new version was to be known as the Mark V.
The first, KZ193, was flown by Lucas shortly after he had tested the first MkIV, on April 3, 1943, and was fitted with Vickers ‘S’ guns. The second conversion, KX405, was completed with a bulged engine cowling to fully accommodate the Merlin 32. A single prototype Mk.V, NL255, was built, but no further aircraft were ordered as production of a new version so similar in performance was considered unnecessary. The Mk.V was predictably the heaviest of all the Hurricanes at 6405lb (2905kg) empty. It was almost a third heavier than the Mk.I. Fully laden it had a maximum takeoff weight of 9300lb (4218kg).
As one would expect for such a widely used aircraft, the Hurricane was subject to numerous tests and trials. Alternative power plants were considered such as the Napier Dagger and the Bristol Hercules. At one stage the 2000hp Griffon was considered, but the engineers at Rolls-Royce were able to extract ever more power from the Merlin, making such measures unnecessary.
Other Hurricanes were employed to gather high altitude weather data by the meteorological flights. This lonely and unglamorous role was diligently undertaken by single flights in the UK, Egypt and Iraq. Hurricanes modified for this role had sensors and rudimentary recording equipment mounted on the starboard wing. As a stopgap measure, several Hurricanes were converted to carry out photo reconnaissance (PR) missions.
To meet the urgent need for a suitable high level reconnaissance platform on Malta, Hurricane V7101 was stripped of all non essential items to save weight and improve performance. At some time the empty gun bays were used to house additional fuel tanks probably salvaged from other aircraft on the island so the aircraft could reach Sicily. In the hands of Flight Lieutenant George Burgess it provided valuable intelligence about enemy activity.
Burgess stated that the aircraft displayed some undesirable flying characteristics at very high altitude, probably as a result of the rearward shift of the centre of gravity when two F.24 cameras were installed behind the pilot’s seat.
The Hurricane was replaced by dedicated PR versions of the Spitfire as they became available. At least one Mk.II was also adapted for the role and was fitted with as many as four vertical cameras to give as wide a field of coverage as possible.