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| A Polikarpov U-2 in a Museum in Dresden, Germany |
Introduction
The Polikarpov U-2/Po-2 was a general purpose Soviet biplane. The Soviets nicknamed the craft Kukuruznik – maize duster or corn cutter. The NATO reporting name was Mule. It was a simple, reliable, and low-cost aircraft, and used in the military for ground attack, reconnaissance, psychological warfare, and liaison. It is the second most produced aircraft, and the most produced biplane, in the history of aviation. Over 40,000 of them were built between 1928 and 1953 – being in production for a longer period than any other Soviet made aircraft.
Development
The U-2 was designed by Nikolai Polikarpov in 1927 to replace the U-1 trainer aircraft, which was a license copy of the Avro 504. Its name was changed to Po-2 in 1944 after the death of Polikarpov, according the new Soviet system of naming with the designers initials.
The prototype, designated U-2PK, was powered by a 99hp Shvetsov M-11 air-cooled, five cylinder radial engine, flying on June 24, 1927 with test pilot M. M. Gromov at the controls. The prototype had been built for easy repair and maintenance, with the wings made of four identical rectangular panels with square wingtips, and common control surfaces. It showed very poor flight characteristics. After a series of modifications, the second prototype flew on January 7, 1928. It was an immediate success. Pre-production aircraft were tested at the end of the year, with production starting in 1929 at Factory Nr 23 in Leningrad. Production in the Soviet Union ended in 1953, but Polish license produced aircraft were still in production until 1959.
Design
The U-2 had a composite skeleton covered with fabric. Most variants did not have an engine cowling. Both the pilot and passenger had a windscreen, and a few models had enclosed cabins. All of the wings had ailerons connected by cables, and the tips of the lower wings had skids for protection should the undercarriage fail. The wings were staggered in layout, of single bay construction with rounded tips, and had a single set of bracing struts. The undercarriage was a conventional cross-axle gear. The rudder was tall and with a broad chord, and linked to the cockpit via control cables.
Operational History
From its beginnings, the U-2 became the basic Soviet civil and military trainer, mass produced in a “Red Flyer” factory near Moscow. It was also used as a light transport, and for liaison due to its short take-off and landing. It was produced as an agricultural aircraft, which earned it its nickname.
Though outclassed by its contemporaries, the U-2 served extensively on the Eastern Front during WWII, mainly as a liaison, medical evacuation, and general supply. It was very useful for supplying Soviet partisans behind the front lines. Due to its low cost and easy maintenance, it had a production run of over 40,000. Even after its production in the USSR, many were assembled in Aeroflot repair workshops.
The U-2/Po-2 saw operation in Albania, Bulgaria, the Peoples Republic of China, Czechoslovakia, Finland (captured aircraft), France, Germany (captured aircraft), East Germany, Hungary, Mongolia, North Korea, Poland, Romania, the Soviet Union, Turkey, and Yugoslavia.
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| A U-2 which was forced down in Ukraine and captured by the Germans. |
Armed Use
The first trials of the U-2 with bombs took place in 1941. During the defense of Odessa in September 1941, U-2’s were used as a reconnaissance aircraft, and a light bomber. From 1942, it was adapted as a light night ground attack aircraft. Nikolai Polikarpov was supportive of the project, and, under his leadership, a variant specifically adapted for that purpose was produced – the U-2VS (VS stood for Voyoskovaya Seriya – Military Series). It was a light bomber, holding bombs in bomb carriers beneath the lower wings, being able to hold a total of 350 kg. It was also armed with ShKAS or DA machine guns in the observer’s cockpit.
German troops nicknamed the U-2 Nähmaschine, or sewing machine, for its rattling sound. Finnish troops called it the Hermosaha, or nerve saw, because of the effects of night raids. As the axis became aware of the threat, they gave special instructions to their night fighter pilots for engaging the U-2’s, giving it the derogatory term of Rusfaner, or Russian Plywood.
Though the material effects of the U-2’s night raids were insignificant, the psychological effects on the German troops were significant. They usually attacked in the dead of night to prevent the Germans from sleeping, and to further the high stress level on the Eastern Front. The U-2’s would fly a few feet from the ground on their way to the target, and then climb up on their final approach. The engine would be cut off on the attack run, so that the targeted troops would only hear an eerie whistle of the bracing wires.
It was found that the U-2 was extremely hard to shoot down with night fighters for three main reasons: first, it could take enormous amounts of damage and keep flying; second, the Russian pilots flew at treetop level in the dead of night – a very dangerous position; third, the stall speed of the German aircraft were about the same as the U-2’s maximum cruise speed, only allowing a short time to target.
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| A U-2 LNB in the Polish Aviation Museum |
The most famous user of the U-2 was the 588th Night Bomber Regiment, composed entirely of women – including the ground crews. They were the best known operators, flying low-altitude night raids many times a night – one crew flying 18 such missions one night. What helped with the psychological effects was the fact that the Germans knew that their oppressors were female. This led to the name of Night Witches (German, Nachtexen; Russian, Nočnye Ved’my).
The Germans did attempt to quell the fearsome raids by bringing many searchlights and anti-aircraft guns around probable targets. To counter this, the Russians would fly in formations of three; two of the aircraft would fly in first to draw off the searchlights and flak while the third aircraft would attack. Then the formation would reform, and they would do it again.
The 588th earned several Hero of the Soviet Union citations, and many Order of the Red Banner medals. By the end of the war, most of the surviving pilots had flown almost 1,000 combat missions.
Korean War
During the Korean War, the North Koreans used the Po-2 in a similar fashion as the Russians did in World War II. A significant number of Po-2’s were used by the Korean Peoples Air Force, who caused much damage on night raids against the UN. UN soldiers named them “Bed-check Charlie”. The UN also had the same problems as the Germans, with great difficulty shooting down the Po-2’s. Even though night fighters of the 50’s had RADAR as a standard, the wood and fabric construction of the Po-2 provided a minimal radar echo, making it extremely difficult to track a target. On June 16, 1953, a USMC AD-4 Skyraider from VMC-1, piloted by Major George H. Linnemeier and CWO Vernon S. Kramer, shot down a Po-2, achieving the only documented Skyraider aerial kill of the Korean War. One F-94 Starfire was lost while trying to slow to intercept a Po-2.
Variants
U-2: Basic model, built in large numbers as two-seat primary trainer, but had many different civil and military versions including light transport, utility, recon, training. Powered by M-11 radial piston engine, 100hp. Later models had up-rated M-11’s boasting 150hp. Some had rear closed cabins, and others fitted with sledges or floats. Redesignated Po-2 after 1944.
U-2A: Two-seat agricultural crop duster, powered by 115hp M-11K radial. Redesignated Po-2A after 1944.
U-2AO: Two seat agricultural aircraft.
U-2AP: Agricultural aircraft with rear cab replaced with a container to carry 200-250kg of chemicals. 1,235 built 1930-40.
U-2G: Experimental aircraft with all controls linked to control column. 1 built.
U-2KL: Two aircraft fitted with bulged canopy over rear cabin.
U-2LSh: Two-seat ground-attack, close-support aircraft. Armed with single 7.62mm ShKAS gun in rear cockpit; could carry up to 120kg bombs and four RS-82 rockets. Also known as U-2VOM-1.
U-2LPL: Experimental prone-pilot research aircraft.
U-2M: Floatplane version fitted with large central float and two small stabilizing floats. Not built in large numbers. Also MU-2.
U-2P: Floatplane version, limited numbers, several variants with different designations.
U-2S: Air Ambulance version, built from 1934. Could take physician and injured on stretcher in rear fuselage, under cover. Variant U-2S-1, from 1939, had raised fuselage top upon stretcher. From 1941, also used two containers for stretchers which could be fitted over lower wings, or two containers for two seating injured each, fitted under lower wings.
U-2SS: Air Ambulance.
U-2ShS: Staff Liaison version, built from 1943. Had wider fuselage and closed 4-place rear cab.
U-2SP: Civil transport, could carry two passengers in open individual cabs, built from 1933. Other roles included aerial survey, aerial photography. 861 built between 1934 and 1939.
U-2SPL: Limousine version fitted with rear cabin for two passengers.
U-2UT: Two-seat trainer, powered by 115hp M-11D radial. Limited numbers.
U-2LNB: Soviet Air Force night attack version, built from 1942. Armed with one 7.62mm ShKAS, and up to 250kg of bombs under the wings. Earlier aircraft converted to improvised bombers from 1941.
U-2VS: Two-seat training and utility. Redesignated Po-2VS after 1944.
U-2NAK: Two seat night artillery observation and recon aircraft. Built from 1943.
U-3: Improved flying training model, fitted with 200hp M-48 engine.
U-4: Cleaned up version with slimmer fuselage. Not built in large numbers.
Po-2GN: “Voice from the sky” propaganda aircraft, fitted with loud speaker.
Po-2L: Limousine version with enclosed passenger cabin.
Po-2P: Post-war floatplane version. Small numbers.
Po-2S: Post-war Ambulance variant, closed rear cabin.
Po-2S-1: Post-war ambulance version, similar to pre-war U-2S.
Po-2S-2: post-war ambulance, powered by M-11D.
Po-2S-3: Post-war ambulance, two underwing containers, each designed to transport one stretcher patient. Also known as Po-2SKF.
Po-2ShS: Staff communications aircraft, fitted with enclosed cabin for pilot and two to three passengers.
Po-2SP: Post-war aerial photography, geographic survey aircraft.
RV-23: Floatplane version of U-2 built in 1937. Used in number of seaplane altitude record attempts. Powered by 710hp Wright R-1820-F3 Cyclone radial.
CSS-13: Polish license version, built in Poland in WSK-Okęcie and WSK-Mielec after WWII. About 500 built, 1948-1956.
CSS S-13: Polish ambulance version, closed rear cab and cockpit and Townend ring. 53 built in WSK-Okęcie, 1954-55, plus 38 converted.
E-23: Research version, built in Soviet Union in 1934 for research in inverted flight.
Specifications
Polikarpov U-2
Crew: 1 Pilot/Instructor, 1 Passenger/Student
Length: 26ft 10in
Height: 10ft 2in
Wingspan: 37ft 5in
Wing Area: 357 sq ft
Empty Weight: 1,698lb
Loaded Weight: 2,271lb
Useful Load: 573lb
Max Takeoff Weight: 2,976lb
Powerplant: 1x 125hp Shvetsov M-11D 5-cylinder radial engine
Maximum Speed: 94mph
Range: 391 miles
Ceiling: 9,843ft
Armament (U-2VS, U-2LNB): 1x 7.62mm ShKAS machine gun in rear cockpit; up to 350kg of bombs
Sources:
http://www.wwiivehicles.com/ussr/aircraft/bomber/polikarpov-u-2-po-2.asp
http://www.aviastar.org/air/russia/pol_po-2.php
http://www.aircraftaces.com/polikarpov-2.htm
http://www.century-of-flight.net/Aviation%20history/photo_albums/timeline/ww2/2/Polikarpov%20U%202%20Po%202.htm
http://en.wikipedia.org/wiki/Polikarpov_Po-2
