The PZL P.11 is a Polish fighter aircraft, designed and produced in the early 1930s by Państwowe Zakłady Lotnicze. Possessing an all-metal structure, metal-covering, and high-mounted gull wing, the type held the distinction of being widely considered the most advanced fighter aircraft in the world at the time of its introduction into service.
Work on the PZL P.11 began in the 1930, based on experience with previous aircraft from the family of fighters derived from the PZL P.1. The primary individual responsible for their development was Polish aeronautical engineer Zygmunt Puławski, who has been attributed as having designed many of their innovative features. While most of the world's air forces still operated biplanes in the early 1930s, the P.11, like previous aircraft in the Puławski family, used a high-mounted and aerodynamically clean gull wing that produced less drag and provided the pilot with a superior field of view. In August 1931, the first prototype conducted its maiden flight. The design quickly drew international attention; the general layout became commonly known as the "Polish wing" or "Puławski wing".
The PZL P.11 served as the basic fighter of the Polish Military Aviation in the second half of the 1930s, including during the German-Soviet invasion of Poland. However, as a consequence of the rapid advances in aircraft development during the late 1930s (seen in such fighters as the Messerschmitt Bf 109), it was outclassed by its rivals at the onset of the war. Many fighter squadrons took up an unequal fight, achieving some successes. It is estimated that as many as 36 P.11s were evacuated to Romania and were subsequently taken over by the Royal Romanian Air Force.
In the early 1930s, the PZL P.11 won a contract for a new fighter aircraft for the Royal Romanian Air Force. During October 1933, deliveries of Polish-built P.11b to Romania commenced. From 1936, Romanian aircraft manufacturer Industria Aeronautică Română (IAR) license-built a further 95 aircraft under the designation IAR P.11f, powered by the Romanian-built IAR 9Krse engines. A more advanced development of the PZL P.11, which achieved much greater export success, was the PZL P.24. In the mid-1930s, several countries expressed their willingness to purchase the PZL P.11, of which Turkey, Bulgaria and Greece decided to acquire the new PZL P.24 instead.
Development Origins The history of the PZL P.11 started in 1929, when Polish aeronautical engineer Zygmunt Puławski began work on the design of an all-metal, monoplane fighter aircraft. Before this, the Polish aviation industry had concentrated on the licensed production of foreign-designed aircraft, particularly French; these typically had wooden structures, so Puławski's all-metal approach represented a considerable advance for Polish industry.Liss 1970, p. 3.
The initial design produced by Puławski, designated the P.1, had many of the features of contemporary French aircraft of the era; these similarities can be attributed to Puławski himself, who had received a considerable portion of his technical education in France only a few years before. Nevertheless, the P.1 also had a number of original features, including its characteristic monoplane wing design. While the majority of the world's forces were still using biplanes, the P.1 had a high-mounted and aerodynamically clean gull wing, which provided the pilot with a superior field of view.
During September 1929, the first prototype made its maiden flight.Liss 1970, p. 4. It demonstrated its outstanding performance compared to contemporary aircraft, such as the British Bristol Bulldog and the French Dewoitine D.27, when it achieved first place at an international air competition held in Bucharest, Romania. The design generated interest around the world; attention was such that the general layout became commonly known as the "Polish wing" or "Puławski wing". This wing had a two-spar duralumin structure, with rivetted ribs to both the spars and skin; the exterior of the wing was covered by finely corrugated duralumin sheet, while the slotted ailerons had a fabric covering.Liss 1970, pp. 3-4.
The Polish War Ministry objected to the aircraft's use of the licence-built Hispano-Suiza engine, citing insufficient practicality and poor economic grounds. Accordingly, a second prototype, which was designated as the P.6, was completed the next year. This revision featured several deviations from Puławski's original vision, including the adoption of a radial engine to power the type in place of the original in-line counterpart; during testing, the P.6's engine was plagued by overheating issues.
Refinement and production Following the disappointing performance of the P.6, a further improved design, designated as the PZL P.7, was developed. According to Liss, this revision represented the most significant of the P.11's forerunners. The P.7 was placed into series production, having been ordered for the Polish Air Force, who ultimately opted to procure 150 of the type.
After designing the P.7, Puławski started further variants with larger engines, eventually cumulating in the P.11. During August 1931, the first P.11/I prototype conducted its maiden flight, powered by the British Bristol Jupiter radial engine; the first flight had occurred shortly after Puławski's death in an air crash. It was followed by a pair of refined prototypes, the P.11/II and the P.11/III, which used the Bristol Mercury engine instead. They were later joined by several more pre-production aircraft to test out various configurations of engines, propellers, and other features; these test examples led to the P.11/VI, a production-representative version of the design.Liss 1970, pp. 4-5.
The first variant of the P.11 to be ordered by the Polish Air Force was the P.11a. From the onset, this was considered to be only an interim model of the type; accordingly, a series of 50 such fighters were constructed.Morgała 1997, p.60-61 Otherwise similar to the P.7, the P.11a mounted the 575 hp (429 kW) Bristol Mercury IV S2 radial engine, which was produced in Poland under licence. Upon the completion of the P.11a order, PZL immediately set about establishing the production of an improved model of the aircraft, which was designated as the P.11c.
The P.11c would be the principal (and final) variant of the type to be inducted into the Polish Air Force. First reaching fighter squadrons during late 1935, it featured the adoption of a new, refined fuselage, a major change of which being the relocation of the engine to be 13 cm lower down in the aircraft's nose, which had the advantage of providing the pilot with an improved exterior view. Besides, a new stronger engine Mercury V S2 had smaller diameter. Forward framework was longer, covering an enlarged and more comfortable cockpit, with pilot's seat moved 30 cm rearwards and raised by 5 cm. The central part of the wings was also modified. During 1934, production of the P.11c commenced, by 1936, the type was being produced at a rate of 25 fighters per month; in total, 150 aircraft were produced by the end of production in 1936 (older sources quoted a number of 175, which is not confirmed in any documents). The first series of approximately 50 P.11c aircraft were fitted with Mercury V S2 of 600 hp (447 kW), the rest with Mercury VI S2 of 630 hp (470 kW).
Apart from Poland, Romania showed interest in the new design. Even before the P.11a entered service with the Polish Air Force, 50 aircraft, designated as the P.11b, were ordered for the Romanian Air Force, while an agreement for licence production was agreed.Liss 1970, p. 5. Deliveries of Polish-built P.11bs to Romania commenced in July 1934.Mazur 2013, p.13-14 They were fitted with Gnome-Rhone 9Krsd Mistral engines, otherwise they were similar to the P.11a.Green and Swanborough 1985, p. 43. 49 were finally made, the 50th aircraft being P.11f prototype. After the P.11c had been developed, the Romanians decided to switch the licence production to the new model. As a result, from 1936 IAR built 95 aircraft as the IAR P.11f, powered by the Romanian-built IAR 9Krse engine, which was a licensed version of the Gnome-Rhone 9Krse giving .Morgała 1997, p. 63, 69 The Romanians then produced another Polish fighter, the PZL P.24, developed from the P.11 exclusively for export. Greece, Portugal, Yugoslavia, Turkey and Republican Spain were interested in buying the P.11, but finally Bulgaria, Greece and Turkey bought the P.24 instead.
Further development and successors During 1934, the year in which the P.11 entered operational service, it was a contemporary of the British Gloster Gauntlet and German Heinkel He 51 and was arguably the most advanced fighter then in service in the world.Winchester 2004, p. 219. However, due to a series of rapid advances and technological development in the field of aviation, the P.11 was considered to have been rendered obsolete by 1939. It had been overtaken in terms of performance by a new generation of fighter aircraft that commonly benefitted from features such as cantilever wings and retractable landing gear; such fighters included the British Supermarine Spitfire and German Messerschmitt Bf 109.Symanowski and Hoffmann 2006, p. 1.
Together with the older P.7, both remained the only Polish fighters in service, however, with about 185 P.11s available, distributed within six air regiments and the aviation school in Dęblin. Being aware that the P.11 was now outdated, the Polish Air Force had pinned their hopes on the in-development PZL.50 Jastrząb, which suffered from several delays. When it became apparent that the PZL.50 would not be in widespread service in time for a major conflict that was clearly looming, consideration was given to producing an updated version of the P.11; this was to have been powered by the 840 hp (626 kW) Mercury VIII and have been furnished with an enclosed cockpit, known as the P.11g Kobuz (hobby). Only the prototype of the P.11g with a maximum speed increase to a still-slow 390 km/h (~240 mph) was flown before the war, in August 1939.Liss 1970, p. 6.
In light of the unavailability of PZL.50, the only hope of replacing the obsolete P.11 lay in acquiring modern fighters from abroad. In 1939, after receiving the necessary credits, Poland ordered from France 120 Morane-Saulnier M.S.406s, and from Britain, 14 Hawker Hurricane Is (the P.11's chosen replacement), plus a single Supermarine Spitfire I for testing, in addition to 100 Fairey Battle light bombers."Polish Air Force." century-of-flight.net, 2003. Retrieved: 7 July 2011. However, none of these aircraft were delivered to Poland before September 1939.
Design The PZL P.11 was an innovative fighter aircraft for the early 1930s, with a high-mounted gull wing and an all-metal, metal-covered structure. It also had several relatively conventional features for the era, such as a fixed undercarriage. The P.11 was equipped with a pair of 7.92 mm machine guns mounted on the sides of the fuselage; reportedly, around a third of all P.11c-model fighters were fitted with two additional machine guns fixed onto the wings. The fuselage-mounted guns were synchronised, but all others were not; a gun camera could also be installed.The P.11c could carry up to four small 12.5 kg bombs, while the earlier P.11a model was not equipped to carry any.Liss 1970, pp. 6-7.