First-generation ‘modern’ monoplane fighters – The Soviet Polikarpov I-16

In the late 1920s the state of the aeronautical art had reached the point at which the monoplane fighter, albeit with a wire- or strut-braced wing, was becoming a fully practical warplane offering somewhat better outright performance than the biplane, whose strong point continued to be its greater agility. At this time the Soviet NII VVS (Scientific Test Institute of the Air Force) started to consider the real utility of the monoplane fighter, and by the beginning of the 1930s had decided that the threat of the monoplane bomber, already in Soviet existence in the forms of the Tupolev TB-1 and TB-3, was so great in terms of its payload and performance that the rapid design and development of monoplane fighters was essential. The NII VVS fully recognised that the first-generation monoplane fighter might offer higher performance than its biplane counterpart, but also appreciated that the agility of such monoplanes would be decidedly inferior to that of the dimensionally smaller and more lightly loaded biplanes, and therefore recommended that the Soviet air force adopt a mix of fast (and therefore almost inevitably monoplane) fighters and agile (and therefore almost inevitably biplane) fighters to cover the full spectrum of air warfare roles between interception and air combat.

At this time the mainstays of the Soviet fighter arm were the Tupolev I-4 sesquiplane and Polikarpov I-5 biplane types, each of which was powered by the M-22 version of the Bristol Jupiter air-cooled nine-cylinder radial engine, whose Soviet manufacture had been sub-licensed from Gnome-Rhône, Bristol’s French licensee. The I-4 and I-5 were capable aircraft by the standards of the day, and the Soviet authorities decided early in 1932 to order the development of a monoplane fighter making full use of the most advanced features currently envisaged. This would allow the USSR to miss the evolutionary step represented by aircraft such as the French Dewoitine D.500 series and American Boeing P-26, which were low-wing monoplanes with braced wings and fixed tailskid landing gear, and progress straight to the ‘modern’ monoplane fighter of largely metal construction with a low-set cantilever wing and retractable main landing gear units.

The task of designing such a fighter was entrusted to the KOSOS (Department of Experimental Aircraft Construction) within the TsAGI (Central Aerodynamics and Hydrodynamics Institute), headed by Andrei Nikolayevich Tupolev. This extremely talented designer and administrator was burdened with a host of programmes at this time, and entrusted the task of designing the required fighter to the design brigade led by Pavel Osipovich Sukhoi. The result was the ANT-31 design that became the I-14 prototype which first flew in May 1933 as an extremely advanced low-wing cantilever monoplane of all-metal construction with retractable main landing gear units, an enclosed cockpit, and a low-drag engine installation that located the air-cooled radial engine inside a long-chord cowling.

Embittered reaction

The fact that design of the new fighter had been entrusted to Tupolev was a bitter personal and technical blow to Nikolai Nikolayevich Polikarpov, whose design teams had been largely responsible for the USSR’s smaller warplanes up to this time. Polikarpov and his primary design team were currently incarcerated in the VT (Vnutrenniya Tyurma, or internal prison) at GAZ-39 for suspected dissidents, but the success of his I-5 biplane fighter persuaded the authorities to release him in January 1933. The Polikarpov design team developed the I-5 into the I-15, I-15bis (I-152) and I-153 (I-15ter) series of biplane fighters to meet the agile biplane half of the Soviet air force’s fighter requirement, and at the same time secured official permission to start work on an advanced monoplane to rival the I-14 and so capture the fast monoplane half of the requirement.

The design team started work toward the beginning of March 1933 and worked with enormous dedication to complete the design of the TsKB-12 as an extremely dumpy aeroplane with close-coupled flying surfaces and a fuselage tailored to the diameter of the selected engine, namely the M-25 unit that was the Soviet licence-built version of a US engine, the Wright R-1820 Cyclone air-cooled nine-cylinder radial unit. Despite its appearance of great modernity, the TsKB-12 was in fact of mixed rather than all-metal construction, a fact that reflected the USSR’s lack of any substantial capability for the manufacture of light alloys and the fabrication of major structures from such alloys, and also the nation’s readily available quantities of high-grade timber and the skills to work it effectively.

The TsKB-12 was based on a short fuselage whose circular-section forward section gradually transformed into oval-section central and rear sections, and its flying surfaces comprised a low/mid-set cantilever wing and a cantilever tail unit with single horizontal and vertical surfaces. The low-aspect ratio wing was tapered in thickness and chord to its rounded tips, and had its roots faired by substantial fillets. The wing’s centre section was based on two widely separated steel spars of built into strong truss, with ribs of truss type built up from riveted light alloy box sections and covered with a flush-riveted alloy skin. The steel and light alloy outer wing panels, which continued the dihedral of the centre-section halves, were based on spars whose sheet webs were riveted to tube booms, and these carried truss ribs riveted from box and angle sections under a skin of flush-riveted alloy as far to the rear as the main spar and fabric to the rear of this. Virtually the full span of the outer panels’ trailing edges was occupied by long-span slotted ailerons of fabric-covered light alloy construction: these ailerons had differential push/pull rod actuation, and could be drooped for landing.

Wooden fuselage

The fuselage was of traditional shpon (multi-ply wood) built up with casein glue on carcasses made in port and starboard halves each with four pine longerons and 11 half-frames. The whole of the tail unit was fabricated from light alloy (mainly pressed from sheet with lightening holes) under a covering of fabric. The moving surfaces were unbalanced and moved by cables in the case of the rudder or by rods and bell-cranks in the case of the elevators. The landing gear was of the tailskid type with wide-track divided main units each comprising a leg (incorporating an oleo-pneumatic shock absorber) braced by a rear strut and pivoted to retract inward into a bay in the underside of the wing and centre section under pull of a cable from a manually operated screw-jack in the cockpit: this system was adopted for its lightness and reliability only after prolonged investigation of pneumatic and hydraulic actuation. Each leg carried a single wheel with a manually (later pneumatically) operated brake, and the bay into which each unit retracted was closed by a hinged cover attached to the outside of each landing gear leg.

Although the intended engine was the M-25, the licence-production negotiations and the launch of Soviet production occupied some four years in the period between 1932 and October 1936. The only viable alternative for the TsKB-12 prototype was therefore the venerable M-22 already in service in two earlier Polikarpov fighters, the I-5 and I-15, but for the first time in Soviet practice this engine was installed under a long-chord cowling characterised by an optimised cooling-air slot at the rear, but in combination with the standard perforated crankcase fairing. The Duralumin propeller had two ground-adjustable blades, a diameter of  9 ft 2.25 in (2.80 m) and no spinner.

Tight-fitting cockpit

The pilot was accommodated in a cockpit which was comparatively narrow and accessed by means of a port-side folding sidewall panel and, as in the I-15, the seat was adjustable only in height. The cockpit was covered by a glazed aluminium alloy and glass canopy with an integral V-type windscreen, and this unit was arranged to slide forward with bungee cord assistance to overcome drag in flight. The pilot had basic instrumentation as well as a CP optical sight integral with the windscreen, and this CP sight was the only aiming aid for the fixed forward-firing armament of two 0.3 in (7.62 mm) ShKAS machine guns, each with 900 rounds, located in the leading edges of the centre section outside the disc swept by the propeller.

The airframe weighed 981 lb (445 kg), the engine and equipment added 992 lb (450 kg), and the take-off weight of the TsKB-12 was 2,890 lb (1311 kg). Despite the low power of its interim engine, the TsKB-12 returned moderately good performance figures and received considerable praise for this fact. Far less commendable, however, were the type’s handling characteristics, which can be described only as nasty: perhaps influenced by the success of the Granville Gee Bee racers, one of which had captured the world air speed record in September 1932, Polikarpov had kept the fuselage of the TsKB-12 as short as possible (shorter, in fact, than than of virtually all biplane fighters), and this resulted in longitudinal instability that was almost matched by the TsKB-12’s directional and lateral instability. This instability made for considerable agility, including rolls and loops of great rapidity that were nonetheless characterised by a propensity to cause the aeroplane to stall and enter a spin. It also made life very taxing on the pilot, who had to fly the aeroplane with great attention without any lapses of concentration, made it difficult to hold the guns on a target for all but the shortest of moments, and made engine-out flying all but impossible as the aeroplane had a severe stalling tendency in this regime unless the nose was held right down, resulting in an extremely rapid descent to the ground.

The TsKB-12bis second prototype recorded its maiden flight on 18 February 1934, and differed from the TsKB-12 primarily in its more powerful engine, which was an imported R-1820-F3 Cyclone rated at 710 hp (529 kW) at 7,000 ft (2135 m) and driving a two-blade Hamilton Standard metal propeller with a diameter of 9 ft 2 in (2.794 m) and a pair of two-position blades. The TsKB-12bis offered considerably higher performance without any further worsening of the type’s handling qualities, and in May 1934 the fighter was selected for production in preference to the ANT-31.

Dumpy and tricky

Intensive factory and NII (scientific test institute) trials quickly established the fact that the TsKB-12 was fit for production and service as the I-16, even though the aeroplane was not easy to fly. It was only marginally stable about all three axes, lacked trim tabs, and in flight tended to stall from the glide and to stall and spin in hard manoeuvres. Other limitations included severe vibration from the engine, great difficulty in hand-cranking up the landing gear via 44 progressively harder turns, the tendency of the main landing gear units to jam when only partially retracted (a ‘solution’ was later found in the provision of cable shears as standard cockpit equipment), undamped shock struts so that landing was dangerous even on the USSR’s rare smooth fields, a high landing speed, a tendency to suffer wing drop if speed was allowed to bleed off, and the need to fly with the M-22’s throttle well open whenever the main landing gear units were extended.

Despite these and other failings, the TsKB-12 was regarded as the best available fighter and therefore ordered into production, in preference to the ANT-31, during May 1934. Subsequent models were allocated tip (type) numbers as it was believed, rightly as matters proved, that many variants would later appear.

The I-16 Tip 1, often designated as the I-16/M-22, was the original pre-production version, which was first flown late in 1934 and displayed in two flights of five aircraft during the course of the May Day parade of 1935. So great an emphasis was placed on the introduction of variants with more powerful engine that only few examples (perhaps 30) of the I-16 Tip 1 were built. The I-16 Tip 1 was based on the TsKB-12 first prototype with the M-22 engine, the armament of two 0.3 in (7.62 mm) ShKAS fixed forward-firing machine guns with 900 rounds per gun in the leading edges of the wing outboard of the disc swept by the propeller, and a length of 19 ft 8.25 in (6.00 m).

The I-16 Tip 1 was therefore the world’s first single-seat low-wing cantilever monoplane fighter with retractable main landing gear units to enter service.

The I-16 Tip 4 was the first definitive production model, and was based on the TsKB-12bis second prototype. Many hundreds of the aircraft were delivered in 1934/35 with a powerplant of one imported R-1820-F3 engine rated at 710 hp (529 kW) at 7,000 ft (2135 m) and driving an imported Hamilton Standard propeller. This engine was installed under a longer-chord cowling with a flat front (incorporating nine adjustable cooling air inlets) and a further optimised cooling air slot at the rear. The only other major changes were the introduction of hinged lower portions on the landing gear fairings to cover the wheels when retracted and, on the last aircraft, a 0.315 in (8 mm) armoured back to the pilot’s seat.

Replacing the I-16 Tip 4 in production during July 1935, the I-16 Tip 5 was a development of the I-16 Tip 4 with the licence-made M-25 rated at 700 hp (522 kW) at optimum altitude and driving a AV-1 two-blade propeller, and with hardpoints under the wing for two 220 lb (100 kg) FAB-100 free-fall bombs. Seeing combat over Spain from November 1936, the I-16 Tip 5 was dimensionally identical to its predecessors, but otherwise differed in details such as its empty weight of 2,646 lb (1200 kg), maximum take-off weight of 3,219 lb (1460 kg), maximum speed of 245.5 kt (283 mph; 455 km/h) at 13,125 ft (4000 m) declining to 213 kt (245.5 mph; 395 km/h) at sea level, climb to 3,280 ft (1000 m) in 1 minute 18 seconds, service ceiling of 29,855 ft (9100 m), and range of 442.5 nm (509.5 miles (820 km).

The I-16 Tip 6 was the standard 1936 production model with an airframe incorporating a measure of local strengthening for a weight penalty of 100 lb (50 kg), the uprated powerplant of one M-25A engine rated at 730 hp (544 kW), an empty weight of 2,778 lb (1260 kg) and a maximum take-off weight of 3,660 lb (1660 kg).

Produced in larger numbers than any other model, to the extent of 2,600 or more aircraft, the I-16 Tip 10 was the 1937 production model with a further measure of local strengthening to the airframe, the powerplant of one M-25V engine rated at 750 hp (559 kW), the fixed forward-firing armament increased to four 0.3 in (7.62 mm) ShKAS machine guns by the addition of two guns with 650 rounds each in the upper part of the forward fuselage with synchronisation equipment to fire through the propeller disc, a revised cockpit enclosure with a fixed windscreen but no canopy, a simple reflector gun sight that was later replaced by the more capable PBP-1 type, and provision for retractable ski landing gear. Earlier models of the I-16 had been adapted for winter use with skis in place of the otherwise standard main wheels, but in these earlier types this change required the main units to be left in the extended position, resulting in a speed loss of some 43 kt (50 mph; 80 km/h). On the I-16 Tip 10 the ski-fitted main landing gear units were retractable, and there was thus only the slightest of performance penalties by comparison with the wheeled landing gear.

The I-16 Tip 10 was dimensionally identical to its predecessors except in its length of 19 ft 7.5 in (5.98 m), but otherwise differed in details such as its empty weight of 2,976 lb (1350 kg), maximum take-off weight of 3,781 lb (1715 kg), and maximum speed of 237.5 kt (273 mph; 440 km/h) at 16,405 ft (5000 m) declining to about 208 kt (239 mph; 385 km/h) at sea level.

The I-16 Tip 17 was the standard production model introduced in 1938, and was basically the I-16 Tip 10 with the fixed forward-firing armament of the TsKB-12P (two 20 mm ShVAK cannon in the leading edges of the wing and two 0.3 in/7.62 mm ShKAS machine guns in the upper part of the forward fuselage), an increase in the area and thickness of the pilot’s protective armour, the latter from 0.315 to 0.35 in (8 to 9 mm), a further improved structure including KhGSA stainless steel wing spars, a tailwheel in place of the original tailskid and, because of this model’s weight increase to 4,244 lb (1925 kg), two-stage shock absorbers in the main landing gear units. Provision was also made for the ventral carriage of a pod accommodating a single 0.5 in (12.7 mm) Beresin BS machine gun, and the underwing carriage of two 220 lb (100 kg) FAB-100 free-fall bombs or a maximum of weight by more typically six 3.2 in (82 mm) RS-82 unguided rockets for air-to-air and air-to-surface use.

The I-16 Tip 17 entered service early in 1939, and otherwise differed from the I-16 Tip 10 in its empty weight of 3,296 lb (1495 kg) and maximum take-off weight of 3,990 lb (1810 kg).

Final pre-war model

The I-16 Tip 18 was the standard production model introduced in 1939, and was in essence the I-16 Tip 10 revised with the powerplant of one Shvetsov M-62 engine (a development of the M-25 with a two-speed supercharger) rated at 920 hp (686 kW) and installed under a long-chord cowling with a modified exhaust arrangement in which alternating single and twin pipes emerged through three holes on each side, an AV-1 or VISh-6A two-blade propeller, the internal fuel capacity reduced to 56.1 Imp gal (67.4 US gal; 255 litres), provision for two 44 Imp gal (52.8 US gal; 200 litre) drop tanks under the wing, and the fixed forward-firing armament of four 0.3 in (7.62 mm) ShKAS machine guns with 3,100 rounds. The I-16 Tip 18 had a length of 19 ft 9.75 in (6.04 m), and otherwise differed from its predecessors in its empty weight of 3,148 lb (1428 kg), maximum take-off weight of 4,034 lb (1830 kg), and maximum speed of 250 kt (288 mph; 464 km/h) at 16,405 ft (5000 m) declining to 222 kt (255 mph; 411 km/h) at sea level.

Introduced in the later part of 1939, the I-16 Tip 24 was the final definitive model and was in essence a development of the I-16 Tip 10 and I-16 Tip 18 with a stiffer wing (larger number of ribs, thicker leading-edge metal skinning, and plywood skinning between the main and rear spars on both the upper and lower surfaces), provision for larger drop tanks, and increasingly standard provision for a reflector sight, radio and oxygen. The first aircraft were delivered with the M-62 engine rated at 920 hp (686 kW), but the fully definitive standard was based on the M-63 uprated version of the M-62 driving a VV-1 or VISh-6 propeller, and had provision for several different fixed armament fits.

Production of the I-16 Tip 24 ended in the spring of 1940, terminating planned I-16 production after the delivery of some 6,555 single-seat fighters. In the autumn of 1940, however, the eastward progress of the German invasion deep into the USSR had resulted in such catastrophic losses by the Soviet air force that the I-16 was restored to production as an emergency measure. An additional 450 aircraft were completed to the slightly differing I-16 Tip 28 and I-16 Tip 30 standards that were each generally similar to the I-16 Tip 24 with the direct-drive M-63 engine, thereby increasing production of the single-seat I-16 to 7,005 aircraft.

The I-16 was blooded in action during the Spanish Civil War (1936/39). Dubbed ‘Mosca’ (fly) by the Republicans and ‘Rata’ (rat) by the Nationalists, the I-16 proved generally superior to the Fiat CR.32 which was the Nationalists’ main fighter. It is also rumoured that the type was built in Spain near Alicante with imported Cyclone engines, and 32 of these fighters were later pressed into air force service after the Nationalist victory, remaining in service into the late 1940s. In 1937 the USSR supplied the Chinese government with more than 250 I-16 series aircraft for use in the 2nd Sino-Japanese War (1937/45). Combat showed the I-16 to be approximately equal with the Mitsubishi A5M and superior to all other Japanese fighters of the early part of this war, but the advent of the Mitsubishi A6M Reisen left the I-16 in a position of qualitative inferiority. From 1940, therefore, the type was used in declining numbers as a ground-attack machine, and finally disappeared from service in 1943.

The main operator of the type was of course the USSR, which first used the type in combat during the Khalkhin Gol fighting (May/September 1939) between the Soviets and the Japanese along the Outer Mongolian frontier. In this campaign the I-16 was mainly pitted against the more agile Nakajima Ki-27, and was therefore employed mainly in dive/climb attacks rather than dogfights. The I-16 was also used in the Russo-Finnish ‘Winter War’ (1939/40), proving moderately successful, but was completely outclassed by German fighters from the very beginning of the ‘Great Patriotic War’ of World War II. Large numbers were lost in the air and on the ground in the second half of 1941, but the survivors soldiered on into 1943 as fighters and increasingly as fighter-bombers before being relegated to secondary tasks. A small number of captured aircraft were operated by the air forces of Finland and Romania in fighting the USSR.


Polikarpov I-16 Tip 24

Type: fighter

Accommodation: pilot in the open cockpit

Fixed armament: two 0.3 in (7.62 mm) ShKAS fixed forward-firing machine guns with 650 rounds per gun in the upper part of the forward fuselage with synchronisation equipment to fire through the propeller disc and two 0.3 in (7.62 mm) ShKAS fixed forward-firing machine guns with 900 rounds per gun or 20 mm ShVAK fixed forward-firing cannon with 180 rounds per gun in the wing leading edges, or four 0.3 in (7.62 mm) ShKAS fixed forward-firing machine guns and one 0.5 in (12.7 mm) Beresin BS fixed forward-firing machine gun with 300 rounds in the upper part of the forward fuselage between the two fuselage-mounted ShKAS machine guns

Disposable armament: up to 1,102 lb (500 kg) of disposable stores carried on two hardpoints (both under the wing with each unit rated at 551 lb/250 kg), and generally comprising two 551 lb (250 kg) FAB-250 free-fall bombs or six 3.2 in (82 mm) RS-82 air-to-air or air-to-surface unguided rockets

Equipment: standard navigation equipment, plus a PBP-1 or PBP-1a reflector gun sight

Powerplant: one Shvetsov M-63 air-cooled 9-cylinder radial piston engine rated at 1,100 hp (820 kW) at optimum altitude

Internal fuel: 56.1 Imp gal (67.4 US gal; 255 litres)

External fuel: up to 111.75 Imp gal (134.2 US gal; 508 litres) in two 55.9 Imp gal (67.1 US gal; 254 litre) drop tanks

Dimensions: span 29 ft 6.33 in (9.00 m); area 156.51 sq ft (14.54 m²); length 20 ft 1.33 in (6.13 m); height 8 ft 5 in (2.57 m)

Weights: empty 3,285 lb (1490 kg); normal take-off 4,215 lb (1912 kg); maximum take-off 4,619 lb (2095 kg)

Performance: maximum level speed 264 kt (304 mph; 489 km/h) at 13,125 ft (4000 m) declining to 237.5 kt (273 mph; 440 km/h) at sea level; cruising speed 161 kt (185 mph; 298 km/h) at optimum altitude; climb to 16,405 ft (5000 m) in 4 minutes 0 seconds; service ceiling 29,530 ft (9000 m); range 378 nm (435 miles; 700 km) with standard fuel

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