Classic US fighters – The Boeing P-12

A Boeing P-12E at the National Museum of the United States Air Force in Hawaii

In June and August 1928 Boeing flew its Model 83 and Model 89 private-venture fighter prototypes, and with these machines’ first forays into the air it can truly be said that the last chapter in the history of US biplane fighters had begun, for with this basic type started an incredibly successful series of fighters for the US Army (P-12) and US Navy (F4B) together with a moderately large number of export variants that saw out the day of the biplane. The development of the type reflected Boeing’s concern that its position as the primary supplier of land-based and carrierborne fighters was being eroded by new generations of Curtiss fighters, and it was this threat that persuaded the company that it represented a worthwhile financial risk to design and build a prototype that could succeed Boeing’s current fighters, namely the PW-9 and F2B/F3B serving with the US Army and US Navy respectively.

Genesis
Given the aeronautical state of the art in the late 1920s and the innate technical conservatism of the US forces, Boeing sensibly decided to avoid anything radical in the prototype of its new fighter, and in its design and structure the new fighter was therefore essentially the consolidation of all the developments that the company had steadily built into its earlier fighters and other aircraft, combined in this instance with a high-powered radial engine and a number of detail design refinements that helped to boost performance by an appreciable margin over those attainable with current fighters.

The core of the design was the fuselage, which was based on a rectangular-section structure of light alloy tubes: the section between the nose and a point in line with the rear of the cockpit was of round-section Duralumin tubes welded together, and the section aft of this point was of square-section aluminium alloy tubes bolted together through Duralumin gussets. This core was then faired out to the required aerodynamic shape for minimisation of drag, and covered in light alloy panels forward and along the upper surface, and fabric aft. From front to rear, this fuselage carried the powerplant, fuel and oil tanks, pilot’s open cockpit, and tail unit. This last was of light alloy semi-monocoque construction with corrugated Duralumin skinning and comprised single horizontal and vertical surfaces, the former including an adjustable tailplane that was braced to the lower longeron on each side by a single strut and carried plain elevators, and the latter including a small wire-braced fin carrying a large horn-balanced rudder that was hinged at its lower end to the vertical knife-edge in which the fuselage terminated.

The staggered and unequal-span biplane wing cellule was of fabric-covered wooden construction with the exception of the ailerons, which were of light alloy semi-monocoque construction with corrugated Duralumin skinning and installed on the outboard ends of only the upper wing’s trailing edge for operation by a system of bellcranks and pushrods in the lower wing. The wings were a further step in Boeing’s shift away from tapered surfaces, as first introduced in the Model 77 (F3B) series, although in this instance of the straight type on the upper surface as well as the lower surface. The lower wing was built in halves which were bolted together before attachment to the lower fuselage as a unit whose flat outer panels extended from the lower longerons, while the upper wing was built as a single flat unit whose centre section was supported over the fuselage by two sets of outward-canted N-type cabane struts. The upper and lower wings were separated on each side by a single set of N-type interplane struts that angled out from bottom to top, and the whole wing cellule was braced with the normal arrangement of flying and landing wires.

The airframe was completed by the landing gear, which in each type was of the fixed tailskid type but differed from each other mainly in the arrangement of their main units: the Model 83 had two V-type units with a spreader bar braced to the lower longerons by diagonal struts extending from its midpoint and was also fitted with arrester gear, while the Model 89 had divided main units based on two tripod legs, and had provision under the fuselage centreline for a 500-lb (227-kg) bomb.

The Model 83 and Model 89 were each powered by one Pratt & Whitney R-1340-B Wasp air-cooled nine-cylinder radial engine rated at 450 hp (335.5 kW) in a nose installation that was uncowled but had fairings behind the cylinders, and drove a two-blade metal Hamilton Standard propeller whose pitch was ground-adjustable between two angles.

Both aircraft were evaluated by the US Navy with the unofficial designation XF4B-1, and the Model 89 was also tested by the US Army. Each of the services was highly enthusiastic about the performance and handling of the new fighter, and in November 1928 the US Army Air Corps placed an initial order for 10 examples of the P-12 fighter, which was essentially identical to the US Navy’s F4B-1 (a hybrid type with the arrester gear of the Model 83 and the landing gear and centreline bomb capability of the Model 89) apart from its lack of carrierborne equipment and its use of US Army rather than US Navy equipment items. The first of these Model 102 machines was delivered in February 1929 in non-standard form for use on a goodwill flight into Central America, and was then returned to Boeing for revision to full P-12 standard and markings.

The first true P-12 recorded its maiden flight in April 1929 with the powerplant of one R-1340-7 engine rated at 450 hp (335.5 kW) at 5,000 ft (1525 m) and supplied with fuel from an internal capacity of 52 US gal (43.3 Imp gal; 196.8 litres) that could be supplemented by 47 US gal (39.1 Imp gal; 177.9 litres) of auxiliary fuel carried in the ventral tank. Other details included a span of 30 ft 0 in (9.14 m) with area of 227.50 sq ft (21.13 m²), length of 20 ft 1 in (6.12 m), height of 9 ft 7 in (2.92 m), empty weight of 1,758 lb (797 kg), maximum take-off weight of 2,536 lb (1150 kg), maximum speed 148.5 kt (171 mph; 275 km/h) at 5,000 ft (1525 m), cruising speed of 117 kt (135 mph; 217 km/h), initial climb rate of 2,080 ft (634 m) per minute, service ceiling of 28,200 ft (8595 m), and typical range of 452 nm (520 miles; 837 km).

The last of nine aircraft completed to this standard had been delivered by the end of the same month and these initial aircraft were regarded as service test machines.

Further development
The tenth aeroplane was completed to the Model 101 standard as the sole XP-12A that was in effect the true prototype for the USAAC series. This machine introduced Frise-type ailerons with their hinge lines parallel to the spar rather than at an angle to it as had been the case on the P-12, a shorter main landing gear arrangement, an engine cowling that replaced the P-12’s arrangement of a fairing behind each exposed cylinder, redesigned elevators, and a castoring tailskid. The XP-12A made its first flight in April 1929 and was lost after only four hours of flight as a result of a mid-air collision with a P-12.

The value of some of the changes effected in the XP-12A were recognised and adopted for the Model 102B which the USAAC placed in service as the P-12B. This variant was ordered to the extent of 90 aircraft, which represented the USAAC’s largest fighter order since 1921, and was the first genuine production variant. Features of the XP-12A that were carried over to the P-12B included the ailerons and revised elevators, but in most other respects the P-12B was based on the P-12 although larger wheels were fitted, the cylinder fairings were omitted and a narrow-chord ring cowling later adopted. The other details of the P-12B included the powerplant of one R-1340-7 engine rated at 450 hp (335.5 kW) at 5,000 ft (1525 m) and supplied with fuel from an internal capacity of 50 US gal (41.6 Imp gal; 189.25 litres) that could be supplemented by 49 US gal (40.8 Imp gal; 185.5 litres) of auxiliary fuel carried in the jettisonable ventral tank, span of 30 ft 0 in (9.14 m) with area of 227.50 sq ft (21.13 m²), length of 20 ft 3 in (6.17 m), height of 8 ft 10 in (2.69 m), empty weight of 1,945 lb (822 kg), maximum take-off weight of 2,638 lb (1197 kg), maximum speed 152 kt (175 mph; 282 km/h) at 5,000 ft (1525 m), cruising speed of 119 kt (137 mph; 220.5 km/h), initial climb rate of 2,040 ft (622 m) per minute, climb to 16,500 ft (5030 m) in 10 minutes 0 seconds, service ceiling of 27,450 ft (8365 m), and typical range of 468 nm (540 miles; 869 km).

The first P-12B was handed over in February 1930, and deliveries of all 90 aircraft had been completed by May of the same year.

A privately funded development at this time was the Model 218 prototype which made its first flight in September 1930 as a development of the P-12B with a semi-monocoque metal fuselage structure based on type pioneered in the Model 96 (XP-9), Model 202 (XP-15) and Model 205 (XF5B-1), a main landing gear unit of the spreader-bar type and, soon after the start of the trials programme, a vertical tail surface of revised shape as pioneered in the XP-15 and XF4B-1. This prototype was evaluated by the US Navy and USAAC, the latter according it the prototype designation XP-925 with the R-1340-D engine that was later replaced by an R-1340-E unit to produce the revised designation XP-925A. With the R-1340-D engine, the XP-925 had an empty weight of 1,954 lb (886 kg) and maximum take-off weight of 2,694 lb (1222 kg), and recorded a maximum speed of 169 kt (195 mph; 314 km/h) at 8,000 ft (2440 m). At the end of its trials programme the Model 218 was sold to China, where it was flown by Robert Short, an American volunteer with the Chinese forces, for a 1932 engagement in which the Model 218 shot down one Japanese fighter before succumbing to two others.

The P-12C , which was known to the manufacturer as its Model 222, was ordered in June 1930 when the USAAC issued a contract for 131 examples of an improved P-12B with a number of detail changes and a later engine. The most obvious external changes in this model were the reversion to the Model 83’s type of through-axle main landing gear unit, and the adoption of a ring cowling for the R-1340-9 (R-1340-D) engine rated at 450 hp (335.5 kW) at 8,000 ft (2440 m) and supplied with fuel from an internal capacity of 50 US gal (41.6 Imp gal; 189.25 litres) that could be supplemented by 60 US gal (50 Imp gal; 227.1 litres) of auxiliary fuel carried in the ventral tank. The P-12C’s other details included a span of 30 ft 0 in (9.14 m) with area of 227.50 sq ft (21.13 m²), length of 20 ft 1 in (6.12 m), height of 8 ft 8 in (2.64 m), empty weight of 1,938 lb (879 kg), maximum take-off weight of 2,630 lb (1193 kg), maximum speed 155 kt (178 mph; 286 km/h) at 8,000 ft (2440 m), cruising speed of 122 kt (141 mph; 227 km/h), initial climb rate of 1,410 ft (430 m) per minute, service ceiling of 26,200 ft (7985 m), and typical range of 504 nm (580 miles; 933 km).

The USAAC received the disassembled aircraft in August 1930, and the type made its first flight in January 1931. In the event only 96 aircraft were delivered up to February 1931, the last 35 aircraft being completed to the improved P-12D standard known to Boeing as the Model 227. This was externally indistinguishable from the P-12C except by the relocation of the wiring harness from the back to the front of the engine and the removal of the P-12C’s cowling struts. The initial 35 aircraft were diverted from the P-12C order, and were delivered in disassembled form between February and April 1931, with a first flight made in March of the same year. Another 16 aircraft of this variant were later created as P-12C conversions, and the entire P-12C and P-12D force was later retrofitted with the vertical tail surface of the P-12E.

Impressed with the stronger fuselage structure, revised vertical tail unit and improved performance of the Model 218 prototype that it had tested as the XP-925 and XP-925A with different version of the R-1340 engine, the USAAC ordered a production derivative as the P-12E to which the manufacturer accorded the designation Model 234. In other respects this P-12E was basically similar to the P-12D with the exception of its powerplant and the details of its weights and performance. The order for the variant was placed in March 1931 and covered 135 aircraft, of which 110 were delivered to P-12E standard between September and October 1931, with a first flight recorded in October. All members of the P-12 series up to and including this variant were delivered with flotation equipment in the upper wing, but on the P-12E this provision was soon deleted and replaced by an inflatable rubber life raft stored in an enlarged pilot’s headrest.

Revised colour scheme
Up to this time, the standard USAAC colour scheme for fighters comprised an olive drab fuselage and chrome yellow flying surfaces superimposed by national, service and unit markings, and it was in this scheme that all the aircraft were delivered. The P-12E fighters were later revised with a USAAC blue fuselage, and in 1940 the surviving aircraft of the P-12E and P-12F models were revised in an all-over scheme of silver.

During the course of their service lives, the aircraft of the P-12E variant underwent a number of designation changes to indicate alterations from the baseline standard. The XP-12E was the first P-12E withdrawn from operational service immediately after its first flight for use in test work and, after passing through a number of other designation changes, the aeroplane became a standard P-12E once more.

The last production variant of the P-12 family was that known to Boeing as the Model 251, and comprised the last 25 aircraft of the P-12E order. These P-12F aircraft were delivered between March and May 1932 to a standard that differed from that of the P-12E primarily in the powerplant of one SR-1340-G engine rated at 500 hp (373 kW) at 11,000 ft (3355 m). In a change that was later retrofitted to the earlier aircraft and also to all of the P-12E aircraft, the last 10 machines were also delivered with a tailwheel in place of a tailskid. The very last machine, which was the final aeroplane of the P-12 and F4B family, was also delivered with a fully enclosed cockpit with a rearward-sliding section for access. The surviving aircraft were finally retired for use as instructional airframes during 1941.

The only subvariant of the P-12B was an experimental development, the XP-12G with a turbocharged engine in the form of one Y1SR-1340-G/H unit inside the type of ring cowling later adopted for the P-12B: this aeroplane was returned to P-12B standard at the end of the engine trials. The sole subvariant of the P-12D was the XP-12H, which was the thirty-third aeroplane experimentally fitted with the GISR-1340-E geared engine. Trials revealed the unsatisfactory performance of this engine, and the aeroplane was converted back to P-12D standard. The P-12J was another modified P-12E, in this instance with a special bomb sight and the SR-1340-H engine rated at 575 hp (429 kW) at 2,500 ft (760 m). With the XP-12E and five P-12E aircraft, the P-12J later became one of the seven YP-12K test aircraft with the SR-1340-E fuel-injected engine; all these aircraft reverted to basic P-12E standard in June 1938. Before this time the machine that had been the XP-12E became the sole XP-12L between January 1934 and February 1937 after its engine had been revised with a Type F-7 turbocharger.

Private and export aircraft
The Model 100 designation was reserved for variants of the P-12 series delivered to private and export customers. The basic designation was applied to four aircraft operated in the USA (one by the Bureau of Air Commerce, one Pratt & Whitney and the last two by Boeing), while the designation Model 100A was used for a single example of a two-seat variant built to the order of Howard Hughes. The designation Model 100E was applied to two fighters sold to Siam (later Thailand): these aircraft were basically P-12E fighters that were delivered in disassembled state in November 1931. Finally, the Model 100F was a civil version of the P-12F sold to Pratt & Whitney for engine development work.

Specification

Boeing P-12E

Type: fighter and fighter-bomber

Accommodation: pilot in the open cockpit

Fixed armament: one 0.5-in (12.7-mm) Browning M2 fixed forward-firing machine gun with 200 rounds and one 0.3-in (7.62-mm) Browning fixed forward-firing machine gun with 600 rounds, or two 0.3-in (7.62-mm) Browning fixed forward-firing machine guns with 600 rounds per gun in the upper side of the forward fuselage with synchronisation equipment to fire through the propeller disc

Disposable armament: up to 700 lb (317.5 kg) of disposable stores carried on five hardpoints (one under the fuselage rated at 500 lb/227 kg and four under the wing with each unit rated at 50 lb/27 kg), and generally comprising four 25-lb (11.3-kg) bombs under the lower wing as the centreline hardpoint was normally used for the ventral tank although it was capable of lifting one 500- or 250-lb (227- or 113-kg) HE bomb, or two 122-lb (55-kg) demolition bombs

Equipment: standard navigation equipment, plus an optical gun sight

Powerplant: one Pratt & Whitney R-1340-17 air-cooled 9-cylinder radial piston engine rated at 500 hp (373 kW) for take-off and 525 hp (391 kW) at 6,000 ft (1830 m)

Internal fuel: 55 US gal (45.8 Imp gal; 208.2 litres)

External fuel: up to 55 US gal (45.8 Imp gal; 208.2 litres) of auxiliary fuel in one 55 US gal (45.8 Imp gal; 208.2 litre) jettisonable tank carried under the fuselage

Dimensions: span 30 ft 0 in (9.14 m); area 227.50 sq ft (21.13 m²); length 20 ft 3 in (6.17 m); height 9 ft 0 in (2.74 m)

Weights: empty 1,999 lb (907 kg); maximum take-off 2,690 lb (1220 kg)

Performance: maximum level speed ‘clean’ 164 kt (189 mph; 304 km/h) at 7,000 ft (2135 m) declining to 148.5 kt (171 mph; 275 km/h) at sea level; cruising speed, maximum 139 kt (160 mph; 257.5 km/h) at optimum altitude; initial climb rate 2,050 ft (625 m) per minute; climb to 10,000 ft (3050 m) in 5 minutes 48 seconds; service ceiling 26,300 ft (8015 m); typical range 504 nm (580 miles; 933 km)

Operators: China (1), Siam (2) & USA (366)

Variant totals:
P-12 USA (10)
P-12A: USA (90)
P-12C: USA (96)
P-12D: USA (35)
P-12E: USA (110)
P-12F: USA (25)
Model 100E: Siam (2)
Model 218: China (1)

US serial numbers:
P-12: 29-353/361
XP-12A: 29-362
P-12B: 29-329/341, 29-433/450 & 30-29/87
P-12C: 31-147/242
P-12D: 31-243/277
P-12E: 31-553/586 & 32-39/76
XP-12E: 31-553
P-12F: 32-77/101
XP-12G: 29-329
XP-12H: not available
P-12J: 32-42
YP-12K: 32-33, 32-36, 32-40. 32-46 & 32-49
XP-12L: 31-553

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