Occupying a unique position in Soviet aviation history, the Polikarpov U-2 primary trainer biplane had an inauspicious start. The U-2TPK prototype, which appeared in early 1927, had been built to achieve economy in repair and maintenance, the wings comprising four identical thick-section interchangeable rectangular panels with square tips. Similarly, a common control surface was used for ailerons, elevators and rudder. The result was a biplane with very poor flight characteristics. It had thus to be re-designed, appearing as a neat, manoeuvrable biplane having staggered single-bay wings with rounded tips, conventional cross-axle landing gear, and tandem open cockpits for an instructor and pupil. Most noticeable in the new aircraft was a much larger rudder. Powered by a 100 hp (74.6 kW) radial engine, the new prototype made its first flight on 7 January 1928 piloted by M.M. Gromov. An immediate success, showing great longitudinal stability making it virtually impossible to spin out in flight. It was placed in quantity production, with deliveries starting in 1928, and by the time of the German invasion of the Soviet Union in June 1941 over 13,000 had been completed.

Built in large numbers in Poland as the CSS-13, the Polikarpov U-2 (known as the Po-2 after the designers death in 1944)  was used in a wide variety of roles both military and civil. Built in greater numbers and variants than any other aircraft in history.

Though its principal role was primary training, the U-2 was soon modified as a light passenger transport, air ambulance and agricultural aircraft. Production continued on a massive scale during World War II, and the U-2 took on an even wider range of duties, including liaison, light attack, night nuisance raider and propaganda aircraft complete with microphone and loud-speaker.

After Polikarpov's death, on 30 July 1944, the U-2 was redesignated Po-2 in his honour, and post-war it continued in production in the Soviet Union for several years. Trainer and ambulance variants were built on a large scale in Poland from 1948 to 1953. Po-2s served with many Soviet allies and a small number still remain in flying condition in the Soviet Union and several other countries. The total number of aircraft built is credibly reported to be in excess of 33,000, with a number possibly approaching near 40,000. Actual numbers built are almost impossible to calculate since many were built by aeroclubs, repair shops and even enthusiasts, with small scale production continuing until at least until 1959.

The Night Witches

In 1942 the Soviet Union formed three regiments of women combat pilots. The 586th Fighter (IAP) Regiment (Yak-1s and later Yak-7Bs), the 587th Day Bomber Regiment (Petylakov PE-2s) and the 588th Night Bomber Regiment (Polikarpov Po-2s). The 'Night Witches' were the women of the 588th Night Bomber Regiment. All of the mechanics and ordnance loaders of this regiment, as in the 586th Fighter (IAP) and the 587th Day Bomber Regiment, were also women.

Trained for six months at the town of Engels on the River Volga near Stalingrad, they flew obsolete Polikarpov Po-2 biplanes, that were otherwise used as trainers. Generally unarmed they carried a variety of droppable ordnance consisting of bombs/grenades (even Molotov Cocktails) thrown from the aircraft by the observer, and on later aircraft, underwing racks for up to 264 lbs (120 kg) of bombs or four RS-82 rockets. Like most Night Bomber Regiments, they were nuisance raiders with targets usually of little strategic importance, but the psychological effect of the terror raids had a major effect on the Germans who soon gave the women pilots the nickname of Nachthexen or Night Witches, presumably because of the fact that they were women and flew wooden aircraft at night.

Because of the aircraft's wooden frame and fabric covering construction combined with the low heat signature of the small 100 hp (74.6 kW) engine, and flying very low to the ground, they were difficult (if not impossible) for radar equipped night fighters to detect them. Flying close to the target and then cutting their engines and gliding in to attack, also added to their terrifying effect.

Harassment night bombing was very difficult to do, considering the low performance of the Po-2 biplanes was even less than most World War I fighters. Very slow, but it was also very manoeuvrable. When a German fighter tried to intercept it, the Russian plane could turn violently and nimbly at a speed much less than the pursuing fighters stalling speed, requiring that the German make a wide circle to come in for another pass. Then he was again met with the same evasive tactic, time after time, hoping that in frustration he would break off the attack altogether.

The Germans later devised a defence to their attacks, that the Russians later called a 'Flak Circus' wherein searchlights and flak guns encircled probable targets. The 'Night Witches' adopted a dangerous tactic in which, flying in formations of three aircraft, the first two aircraft would fly into the target drawing the searchlights and flak onto themselves and away from the third aircraft that would attack the target. After a successful attack they would regroup and switch places with each aircraft taking its turn as the bait.

The Soviet women bomber pilots earned in total 23 Hero of the Soviet Union medals and dozens of Orders of the Red Banner. Altogether, the Po-2 pilots flew more than 24,000 sorties and dropped 23,000 tons of bombs earning their place in history.

Variants

Polikarpov U-2A (U-2AP/U-2AO) "Aeropyl" - A single-seat agricultural duster aircraft, built in sub-types U-2AP and U-2AO from 1930, with 551 lbs (250 kg) chemical hopper in rear fuselage. Modified landing gear incorporated double wheels (usually a second rim welded to the original) to assist the aircraft when operating on unprepared and often wet fields. Post-1944 it continued in production and use as Po-2A powered by 115 hp (86 kW) M-11K engine (9000 aircraft).

Polikarpov U-2G - A one-off experimental model with all controls linked to the control column.

Polikarpov U-2KL - Two special aircraft with rear cabin having a bulged canopy appearing in 1932.

Polikarpov U-2LSh (U-2VOM-1) - Light Shturmovik. A large number of pre-war aircraft converted for close-support military role from 1941 onwards, plus new production. Armed with one 7.62 mm (0.30 in) ShKAS machine-gun on a ring mounting over rear cockpit, and racks for 264 lbs (120 kg) of bombs, plus rails for four RS-82 rockets. It had a high reputation among Soviet troops and earned nickname Kukuruznik or corn-cutter, resulting from its successful low-level operations. An alternative designation was U-2VOM-1.

Polikarpov U-2LNB - A wartime production from 1941 onward with the offensive load raised to 441 lbs (200 kg). The designation translates as light night bomber, and the type often carried flares or a searchlight and usually had a silencer attachment for engine exhaust.

Polikarpov U-2LPL - A prone-pilot research development of 1935.

Polikarpov U-2M (MU-2) - Alternative designation MU-2 tested in 1931. The first of several floatplane versions with large central float and two wingtip stabilising floats. No version built in large numbers.

Polikarpov U-2NAK - Light artillery observation and reconnaissance variant with an observer and provided with an army radio.

Polikarpov U-2S (U-2S-1 and U-2SS) - Limited series built from 1934 with the rear cockpit being replaced by enclosed cubicle for medical attendant and compartment for stretcher case, with hinged top decking over patient. Later had windows installed. Other versions known as U-2S-1 and U-2SS (Sanitamyi Samolyet, or ambulance aircraft).

Polikarpov U-2SP - Third open cockpit installed with other cockpits relocated. Built between 1934 and 1939, mostly for Aeroflot for use as passenger and mail delivery aircraft. Survivors were impressed in wartime service for liaison duties (861 aircraft).

Polikarpov U-2SPL - A limousine model with rear cabin for two passengers in place of rear cockpit.

Polikarpov U-2UT - Built in limited numbers from 1941, mainly for training and powered by the 115 hp (86 kW) Shvetsov M-11D engine.

Polikarpov U-2VS - A basic Soviet air force aircraft with large numbers produced and used by senior personnel for wartime liaison flights. Over 9,000 were in service for liaison duties in 1945, by then redesignated Po-2VS.

Polikarpov U-3 - A development by N.G. Mikhelson and A.I. Morshchikhin in 1934 as a better flying training model with 200 hp (149 kW) M-48 engine. In no way superior to basic U-2.

Polikarpov U-4 - A cleaned-up model of U-2 with slimmer fuselage developed by N.G. Mikhelson but not built in quantity.

Polikarpov Po-2GN - A 'voice from the sky' propaganda aircraft with loudspeaker for addressing enemy ground troops, built from 1944.

Polikarpov Po-2L - A limousine version with enclosed passenger cabin and access door on port side.

Polikarpov Po-2P - A wartime floatplane, built in limited numbers only.

Polikarpov Po-2S (Po-2S-1/2/3) - A wartime ambulance aircraft, Po-2S-1 was similar to pre-war ambulance aircraft. The Po-2S-2 had the M-11D engine, and Po-2S-3 carried two underwing containers each with stretcher patient. Alternative designation for this last variant was Po-2SKF.

Polikarpov Po-2ShS - A staff liaison machine built from 1943 onwards with a new fuselage with an enclosed cabin for pilot and two or three passengers.

Polikarpov Po-2SP - A specialised post-war version, used for geographic survey and air photography.

Polikarpov RV-23 - An N.G. Mikhelson float derivative of the U-2 for seaplane altitude record attempts in 1937. Built in small numbers with the 710 hp (529 kW) Wright R-1820-F3 Cyclone radial engine.

Polikarpov CSS-13 - The designation for the Polish version, some with glazed cabin over crew cockpits.

Polikarpov CSS-S-13 - The Polish ambulance version with a canopy over the cockpits for pilot and medical attendant and an enclosed section for a stretcher patient to the rear.

Polikarpov E-23 - A 1935 research version for inverted flight testing.

Experimental - Many experimental versions were built including one to the Rudlicki 'butterfly' tail and another aircraft re-engined with a Siemems Sh 14 engine. 

Specifications (Polikarpov U-2VS/Po-2VS)

Type: Two Seat Multi-Purpose, Liaison, Night Nuisance Raider, Ground Support, Ambulance & Trainer

Accommodation/Crew: Pilot & Gunner/Observer

Design: Designer Nikolai Nikolayevich Polikarpov

Manufacturer: State Industries (Polikarpov Design Bureau)

Powerplant: One 100 hp (74.6 kW) Shvetsov M-11 5-cylinder air-cooled radial engine driving a two-bladed fixed pitch wooden propeller.

Performance: Maximum speed 97 mph (156 km/h) at sea level; service ceiling 13,125 ft (4000 m); landing speed 40.3 mph (65 km/h); flight endurance 3 hours 30 minutes.

Range: 249 miles (400 km) on internal fuel.

Weight: Empty equipped 1,400 lbs (635 kg) with a maximum take-off weight of 1,962 lbs (890 kg). Wing loading 5.3 lbs/sq ft (25.8 kg/sq m); power loading 18.9 lbs/hp (8.6 kg/hp)

Dimensions: Span 37 ft 4 3/4 in (11.40 m); length 26 ft 9 3/4 in (8.17 m); height 10 ft 2 in (3.10 m); wing area 356.86 sq ft
(33.15 sq m).

Armament: (Optional) One 7.62 mm (0.30 in) ShKAS machine-gun on ring mounting in rear cockpit.

Variants: See Below.

Equipment/Avionics: None.

History: First flight (U-2TPK) early 1927; first flight (redesigned prototype) 7 January 1928.