During the early 1920s, the tactical philosophy of the Royal Air Force for the interception of bomber forces stressed the value of standing fighter patrols: this was seen as the only practical method, before the advent of radar and the possibility of a scrambled take-off and rapid climb to the right location, of ensuring the interception and destruction of the bomber forces before they could reach their targets. This philosophy placed emphasis on fighters with long endurance and the ability to carry radio equipment, so that any pilot who spotted a bomber formation could report the fact to his ground controller and neighbouring fighters, and resulted in machines such as the Armstrong Whitworth Siskin and Gloster Gamecock. By 1924 the advent of the Fairey Fox high-speed day bomber, with its powerplant of one Curtiss D-12 water-cooled V-12 engine, was starting to persuade the RAF that this tactical philosophy was outdated: with its powerful engine and trim lines, the Fox could both outpace and outclimb current fighters. The RAF was also coming to an appreciation of the fact that standing patrols were wasteful of fuel, airframe and engine hours, and pilot endurance. The new tactical philosophy adopted by the RAF emphasised the use of fighters that were fast and could climb rapidly for the timely engagement of targets that had been spotted by ground observers.
This new philosophy was embodied in the Air Ministry’s F.17/24 requirement for a fast-climbing fighter to be powered by the Rolls-Royce Falcon X (later Kestrel) water-cooled V-12 engine. Yet current water-cooled engines such as the Falcon were heavy as well as powerful, and Captain Frank Barnwell and Roy Fedden, chief designers of Bristol’s aircraft and engine divisions respectively, therefore suggested replacement of the water-cooled V-type engine by an air-cooled radial of the type that was the particular speciality of the Bristol aero engine division. The Air Ministry appreciated the sound nature of Bristol’s criticism and revised the F.17/24 requirement into the F.9/26 requirement for a standard day/night fighter with a powerplant based on one engine of either the water-cooled V-type or air-cooled radial variety. The Fleet Air Arm had a requirement for a fighter at this time, and Barnwell therefore prepared the Type 102 design for a fighter based of the Type 99 Badminton racing biplane of 1925.
This Type 102 was to be powered by the new and extremely promising Bristol Mercury radial piston engine, and was offered to the RAF as the Type 102A with wheeled landing gear and to the FAA as the Type 102B with alternative twin-wheel landing gear for carrierborne use or twin-float alighting gear for seaplane use. At this time the Air Ministry revived its demand for a fast-climbing day interceptor, and Barnwell thus worked in parallel on two types: the day/night fighter to the F.9/26 requirement was the Type 105 that was an improved version of the Type 102 with the powerplant of one Bristol Jupiter VII air-cooled nine-cylinder radial engine rated at 440 hp (328 kW), and the interceptor to the F.20/27 requirement was the Type 107 Bullpup with the powerplant of one Mercury IIA air-cooled nine-cylinder radial engine rated at 480 hp (358 kW). Bristol thought that the Type 107 was the more important of its two designs, and it was clear that this would face stiff competition from four other prototypes ordered from Gloster, Hawker, Vickers and Westland. It should also be noted that the Air Ministry had expressed a preference for the successful F.9/26 contender to be powered by the Rolls-Royce F.XI water-cooled V-12 engine, so Bristol decided to build its Type 105 prototype as a private venture outside the main F.9/26 competition. The two Bristol designs had much in common, and in their original forms were each planned with an equal-span biplane wing cellule: this was later revised to a virtually sesquiplane layout with a lower wing of less span and chord than the upper wing.
The Type 107, it is worth noting, first flew in April 1928 with the powerplant of one Jupiter VI engine as the Mercury IIA was not yet ready, and was evaluated in 1929 after the Mercury IIA had been installed. In its definitive form, the Type 107 spanned 30 ft 0 in (9.14 m) and had a maximum take-off weight of 2,850 lb (1293 kg): the combination of this smaller size and lesser weight made the aeroplane both faster and handier than the Bulldog, but the Air Ministry sensibly decided that its superiority was not sufficient to warrant a change in production.
The Type 105 was of bolted high-tensile steel construction covered with fabric except on the forward fuselage, which was completed with light alloy panels. The fuselage was of basically circular section over its forward section becoming oval in section farther aft, and carried the flying surfaces, fixed tailskid landing gear including a main unit of the through-axle type, and the powerplant. This last was installed in the tapered nose with the cylinder heads protruding into the airflow, and drove a two-blade wooden propeller of the fixed-pitch type with a spinner that continued the lines of the forward fuselage for a good nose entry.
The flying surfaces comprised a cantilever tail unit and a staggered single-bay wing cellule. The flat upper-wing centre section was of narrow chord and thin section, and supported above the fuselage by the normal arrangement of cabane struts: this centre section supported the dihedralled outer panels of the upper wing, which were of thicker section and greater chord, each carried a fuel tank and an outboard aileron, and separated from the smaller lower-wing panels by a pair of interplane struts on each side; the wing cellule was completed by the standard arrangement of flying and landing wires.
The Type 105 or Bulldog Mk I prototype recorded its maiden flight on 17 May 1927 and soon revealed a pleasing combination of good handling and adequate performance, the latter including a maximum speed of 150 kt (173 mph; 278 km/h) at optimum altitude. Though officially it was flown outside the F.9/26 competition, the Type 105 was clearly superior to three of the official contenders (the Armstrong Whitworth A.W.XVI, Boulton & Paul P.33 Partridge, and Gloster SS.19) and thus rivalled only by the Hawker Hawfinch, and the Air Ministry thereupon ordered a single Type 105A prototype for official evaluation as the Bulldog Mk II. This prototype recorded its maiden flight in January 1928, and differed from the Bulldog Mk I mainly in having an extended rear fuselage that increased overall length from 23 ft 0 in (7.01 m) to 25 ft 0 in (7.62 m), and a slightly revised wing cellule that reduced span from 34 ft 0 in (10.36 m) to 33 ft 10 in (10.31 m) and area from 307.00 sq ft (28.52 m²) to 306.50 sq ft (28.50 m²). The competition between the two types was particularly hard fought, but the Air Ministry in June 1928 selected the Bulldog Mk II as the winner largely on the grounds that it was of all-steel construction and therefore more durable than the Hawfinch, which was of steel and light alloy construction.
An initial order was placed for 25 aircraft, and these machines were delivered between May and October 1929, entering service with Nos 3 and 17 Squadrons as replacements for the Hawker Woodcock fighters that these two units had flown largely in the night-fighter role. The Air Ministry then ordered a second batch of aircraft, and these 23 machines were delivered in 1930 as the equipment of Nos 17 and 54 Squadrons. Other than those above, the details of the Bulldog Mk II included an empty weight of 2,200 lb (998 kg), maximum take-off weight of 3,490 lb (1583 kg), maximum speed of 155 kt (178 mph; 286.5 km/h) at 10,000 ft (3050 m), and service ceiling of 29,300 ft (8930 m). The Bulldog was already acquiring an excellent reputation for handling and performance, and this combined with the type’s reputation for structural reliability and the low cost of its well-proved Jupiter engine to attract export orders. Some 12 aircraft with an armament of two Oerlikon machine guns and the powerplant of one Gnome-Rhône (Bristol) Jupiter VI or Gnome-Rhône 9Asb engine (10 and two aircraft respectively) were delivered in 1929 and 1930 to Latvia, which later sold 11 of them to Spain. Another 12 were delivered to Estonia in 1930 with the powerplant of one Gnome-Rhône (Bristol) Jupiter VI engine, and smaller orders were placed by Australia (eight aircraft delivered in January 1930 with the powerplant of one Jupiter VIF engine), Sweden (three aircraft used for evaluation of the Bulldog), Siam (two aircraft delivered in January 1930 with the powerplant of one Jupiter VII engine), the USA (two aircraft delivered to the US Navy in 1929 and 1930 for trials purposes, in the course of which the first machine was lost as a result of aileron flutter and separation in a terminal-velocity dive), and Japan (one aeroplane with the powerplant of one Nakajima [Bristol] Jupiter VII engine for evaluation). In Japan the Bulldog was extrapolated with Bristol assistance into the Nakajima JSSF with the Jupiter VII engine, but this variant’s two prototypes did not pave the way for any production.
In May 1930 the Air Ministry ordered another 92 fighters to the improved Bulldog Mk IIA standard with the Jupiter VIIF engine rated at 440 hp (328 kW) for take-off and 520 hp (388 kW) at 10,000 ft (3050 m), revised wing spars, a measure of local strengthening to permit operation at higher weights, a redesigned oil system, and a wider-track main landing gear unit carrying larger wheels with Bendix tyres and wheel brakes; the aircraft were later revised with a modified fin and a tailwheel in place of the original tailskid. The other details of the Bulldog Mk IIA included the armament of two 0.303 in (7.7 mm) Vickers Mk II or Mk IIN fixed forward-firing machine guns with 600 rounds per gun together with four 20 lb (9.1 kg) bombs on a rack under the port lower wing, span of 33 ft 10 in (10.31 m) with area of 306.50 sq ft (24.47 m²), length of 25 ft 0 in (7.62 m), height of 9 ft 10 in (3.00 m), empty weight of 2,412 lb (1094 kg), maximum take-off weight of 3,530 lb (1601 kg) later increased to 3,660 lb (1660 kg), maximum speed of 154.5 kt (178 mph; 274 km/h) at 10,000 ft (3050 m) declining to 141 kt (162 mph; 261 km/h) at 20,000 ft (6095 m), climb to 20,000 ft (6095 m) in 14 minutes 30 seconds, service ceiling of 29,300 ft (8930 m), and range of 304 nm (350 miles; 563 km).
The aircraft were delivered between October 1930 and May 1931, and by the end of 1931 some 10 of the RAF’s 13 home-based fighter squadrons were equipped with the Bulldog: these units were Nos 3, 17, 19, 23, 29, 32, 41, 54, 56 and 111 Squadrons. Orders were later placed for an additional 162 examples of the Bulldog Mk IIA, which was numerically the most important British fighter of the early 1930s. Yet the type was essentially an interim fighter whose performance was hampered by its use of a modestly rated radial engine in an uncowled installation: the advantages of this powerplant were its reliability and low purchase cost, but by the early 1930s its disadvantages were a basic lack of potential for development into significantly higher-rated forms and the emergence of powerful V-12 engines whose low-drag installations offered the possibility of much improved overall performance. Thus the replacement of the Bulldog began as early as 1933, when No. 23 Squadron converted to the Hawker Demon. The replacement process was slow, however, and it was June 1937 before the last unit, No. 3 Squadron, relinquished its Bulldogs.
The Bulldog Mk IIA also secured a limited number of export orders. Denmark received four Type 105D aircraft in March 1931 with an armament of two 0.3 in (7.62 mm) Madsen machine guns and a powerplant of one normally aspirated Jupiter VIFH engine: three of these aircraft were still in service as trainers at the time when Germany overran Denmark in April 1940 at the start of her second European campaign after the start of World War II. It is worth noting that negotiations were begun for a licence under which the Danish Naval Dockyard would produce the type in Denmark, but this plan did not come to fruition. In May 1931 Sweden received eight aircraft with a powerplant of one Jupiter VIIF engine: these aircraft were often operated on ski landing gear during the winter months, and during December 1939 Sweden gave three of the aircraft to Finland, which was fighting the bitter ‘Winter War’ (1939-40) against Soviet aggression.
Bristol was not content to let the Air Ministry dictate the whole of the Bulldog’s development, and thus pursued its own course of active development that served to improve the breed as a whole and also allow the evaluation of several types of powerplant. The last Bulldog Mk II was thus retained by Bristol as a development ‘hack’. This machine was first flown with a geared Mercury IV engine, and further development saw use of the Mercury III engine driving a four-blade propeller and then the Gnome-Rhône (Bristol) Jupiter VI engine. This latter powerplant was intended for an export version for countries in which the licence-built version of the Jupiter held sway, and it was while practising for a demonstration tour to these countries that a Bristol pilot was forced to bale out of the machine after it had suffered a damaged rudder during a flick roll. A replacement machine was built, and this was engined successively with the Gnome-Rhône 9Asb engine (a developed version of the French-built Jupiter), Mercury IVS2 engine, Bristol Perseus IA air-cooled nine-cylinder radial engine, and finally the Mercury VIS2 engine driving a three-blade Hamilton Standard propeller. Further evolution of the basic airframe resulted in the Bulldog Mk IIIA, which was produced as a private venture to compete with the Gloster SS.19B for an order as replacement for the Bulldog in RAF service.
The Bulldog Mk IIIA first flew in September 1931 with a powerplant of one Mercury IVA radial engine that was later replaced by a Mercury IVS2 radial engine rated at 560 hp (417.5 kW) and enclosed in a short-chord ring cowling of the Townend type. Other than its higher-rated powerplant, the Bulldog Mk IIIA could be distinguished from the Bulldog Mk IIA by its deeper wing sections, which were of the biconvex type and allowed the fuel tanks to be accommodated entirely within the wing section, its lower wing of reduced chord to provide the pilot with improved downward fields of vision, its single rather than double flying wires, its deeper rear fuselage that provided additional stiffness, and spatted main wheels. The Bulldog Mk IIIA was considerably faster than the Bulldog Mk IIA, recording a maximum speed of 181 kt (208 mph; 335 km/h) at 15,000 ft (4570 m), but was lost in a crash during March 1933. Bristol built a second Bulldog Mk IIIA prototype, but no order for the type was forthcoming as the Air Ministry preferred the SS.19B that was ordered into production as the Gauntlet.
The second Bulldog Mk IIIA was then converted in 1934 into the Bulldog Mk IV prototype. This was intended to provide Bristol with a contender for the F.7/30 requirement, which called for a four-gun day/night fighter. Bristol again found itself pitted against Gloster for a production contract, and as a result of the two types’ official evaluation the SS.37 was preferred to the Bulldog Mk IV and ordered into production as the Gladiator. This was hardly a surprising decision as the Bulldog Mk IV could achieve a maximum speed of only 194.5 kt (224 mph; 360.5 km/h) by comparison with the S.S.37’s figure of 217 kt (250 mph; 402 km/h).
This was not quite the end of the road for the Bulldog, however, for the Finnish air force was highly impressed by the Type 105’s combination of great ruggedness, considerable agility and adequate performance. In December 1933, therefore, the Finnish government placed an order for 17 Bulldog Mk IVA fighters with the powerplant of one Mercury VIS2 engine enclosed in a long-chord cowling that transformed the looks of this purposeful aeroplane. The order was complicated by the fact that Finland lay in the franchise area for French-built versions of Bristol engines, and it was January 1935 before the 17 fighters were delivered. Some 10 of these aircraft were still operational with the Finnish air force in November 1939 as the equipment of HLeLv 26 (26th Fighter Squadron) of the Lentorykmentti 2 (2nd Aviation Regiment): the aircraft were credited with the destruction of five Soviet warplanes, but were retired from first-line service in January 1940, when they were replaced by Gladiators. The aircraft were then flown as trainers for a short time.
At the end of 1931, one of the 100 aircraft on order as the RAF’s second batch of Bulldog Mk IIA fighters was retained for conversion as the Type 124 tandem-seat trainer, and as such became the prototype Bulldog TM (Training Machine). This prototype was evaluated during 1932 and the Central Flying School found it acceptable. There followed an initial production batch of 17 Bulldog TM aircraft that served initially with the CFS and the Coastal Area at Leuchars in Scotland. Production of the Bulldog TM eventually reached 59, allowing the type to serve additionally with six RAF squadrons, where they served as conversion trainers, the RAF College at Cranwell in Lincolnshire, and a number of flying training schools including No. 4 FTS at Abu Sueir in Egypt, which was the only overseas unit to have Bulldogs as a part its its establishment.
Some of these aircraft remained operational to 1939, and the details of the Bulldog TM included the powerplant of one Jupiter VIFH engine, span of 34 ft 2 in (10.41 m) with area of 309.00 sq ft (28.71 m²), length of 25 ft 3 in (7.70 m), height of 8 ft 9 in (2.67 m), empty weight of 2,200 lb (998 kg), maximum take-off weight of 3,300 lb (1497 kg), maximum speed of 146 kt (168 mph; 270 km/h) at optimum altitude, and service ceiling of 28,000 ft (8535 m). The Bulldog TM was also used as the basis for a testbed in which air-cooled engines as diverse as the Alvis Leonides nine-cylinder radial, Armstrong Siddeley Cheetah seven-cylinder radial and Napier Rapier H-16 units were tested.
The operators of Bulldog aircraft were the Australia (Royal Australian Air Force in the form of Nos 1 and 2 Squadrons), Denmark, Estonia, Finland, Japan, Latvia, Siam, the Spanish Republic, Sweden and UK (RAF in the form of Nos 3, 17, 19, 23, 24, 29, 32, 41, 54, 56 and 111 Squadrons, Nos 3 and 5 Flying Training Schools, Central Flying School and RAF College, Cranwell).
Bristol Bulldog Mk IVA
Accommodation: pilot in the open cockpit
Powerplant: one Bristol Mercury IVS2 air-cooled 9-cylinder radial piston engine rated at 620 hp (462 kW) for take-off and 645 hp (481 kW) at 15,500 ft (4725 m)
Fuel capacity: 70 Imp gal (84.1 US gal; 318.2 litres)
Performance: maximum level speed ‘clean’ 194.5 kt (224 mph; 360 km/h) at 16,000 ft (4875 m) declining to 152 kt (175 mph; 282 km/h) at sea level; cruising speed not available; initial climb rate not available; climb to 10,000 ft (3050 m) in 4 minutes 41 seconds; service ceiling 33,400 ft (10180 m); maximum range about 260.5 nm (300 miles; 483 km); endurance about 2 hours 0 minutes
Weights: empty 2,960 lb (1220 kg); normal take-off 4,010 lb (1819 kg); maximum take-off 4,100 lb (1860 kg)
Dimensions: span 33 ft 8 in (10.27 m); area 293.60 sq ft (27.27 m²); length 25 ft 2.5 in (7.68 m); height 9 ft 10.75 in (3.01 m); wheel track 5 ft 6 in (1.67 m)
Armament: two 0.303 in (7.7 mm) Vickers Mk II* fixed forward-firing machine guns with 600 rounds per gun in the upper sides of the forward fuselage with synchronisation equipment to fire through the propeller disc, and up to 80 lb (36 kg) of disposable stores carried on one hardpoint (under the port lower wing and rated at 80 lb/36 kg), and generally comprising four 20 lb (9.1 kg) free-fall bombs