The installation of a radial engine in the P.Z.L. P.7 diminished the excellent forward view for the pilot that was achieved in the P.Z.L. P.1 with its narrower V-12 engine, and it was proposed to improve this situation by the introduction of a Bristol Mercury radial engine, which was of smaller diameter than the Jupiter that powered the P.Z.L. P.7a. This version of the fighter was designated P.Z.L. P.11, but delay in delivery of a Mercury engine from Bristol resulted in the P.11/I prototype being flown initially, in August 1931, with a 515 hp (384 kW) Jupiter IX.ASb engine licence-built by Gnome-Rhône. It was not until December 1931 that the P.11/II was flown with a 530 hp (395 kW) Bristol Mercury IV.A enclosed in a long-chord Townend ring. This prototype was later re-engined with a 500 hp (373 kW) Gnome-Rhône 9K Mistral engine, with which powerplant it was exhibited at the 1932 Paris Salon de l'Aeronautique. A third aircraft with a Mercury engine, the P.11/III, served as a pre-production prototype and, following satisfactory official testing, was approved for production for the Polish air force as the P.11a.

A three sided diagram of a PZL P.11c showing the two extra guns in the wings 

However, it was preceded on the production line by 50 Mistral-powered P.11b aircraft for Romania, all of them delivered by the summer of 1934. Production of the P.11a began with a batch of 30, these being similar to late production P.11b aircraft, but differed by having the 517 hp (386 kW) Skoda built Mercury IV.S2 engine. The major production variant, however, was the P.11c which adopted more radical measures to improve the pilot's field of view, lowering the engine and resitting the pilot farther to the rear on a raised seat, and a number of other improvements were incorporated at the same time. Production of this version totalled 175, the first batch being powered by the 560 hp (418 kW) Skoda built Mercury V.S2, but the remainder by the P.Z.L. built Mercury VI.S2. A version of the P.11c, powered by a licence-built 9K Mistral engine, was built under licence in Romania by I.A.R. under the designation P.11f, about 80 being produced during 1936-38.

Deliveries of the P.11c to Polish fighter squadrons were completed by the end of 1936, and at the outbreak of World War II 12 squadrons were equipped with the type, claiming the destruction of 126 Luftwaffe aircraft for the loss of 114 of their own number. When, in early 1939, it became clear that the planned P.Z.L. P.50 Jastrzab fighter was unlikely to materialise, efforts were made to provide the P.llc with greater capability by the installation of a 840 hp (626 kW) licence-built Mercury VIIIa engine and four-gun armament. A prototype was flown as the P.11g Kobuz and quantity production was initiated, but the German invasion of Poland had started before any of these aircraft were delivered.

Variants

PZL P.11a - A third aircraft with a Mercury engine, the P.11/III, served as a pre-production prototype and, following satisfactory official testing, was approved for production for the Polish air force as the P.11a.

PZL P.11b - First on the production line was 50 Mistral-powered P.11b aircraft for Romania, all of them delivered by the summer of 1934.

PZL P.11c - The major production variant, however, was the P.11c which adopted more radical measures to improve the pilot's field of view, lowering the engine and re-sitting the pilot farther to the rear on a raised seat, and a number of other improvements were incorporated at the same time. Production of this version totalled 175, the first batch being powered by the 560 hp (418 kW) Skoda built Mercury VI-S2, but the remainder by the P.Z.L. built Mercury VI-S2. Four 7.9 mm (0.31 in) PWU Wz 33 machine-guns were also used as an alternative armament to the 7.7 mm (0.303 in) KM Wz 33 machine-guns.

PZL P.11f - A version of the P.11c, powered by a licence-built 9K Mistral engine, was built under licence in Romania by I.A.R. under the designation P.11f, about 80 being produced during 1936-38.

PZL P.11 Kobuz - In order to provide the P.llc with greater capability, the installation of a 840 hp (626 kW) licence-built Mercury VIIIa engine and four-gun armament was proposed. A prototype was flown as the P.11g Kobuz and quantity production was initiated, but the German invasion of Poland had started before any of these aircraft were delivered. 

Specifications (PZL P.11c)

Type: Single Seat Fighter

Design: Zygmund Pulaski (Wsiewolod Jakimiuk after Pulawkis died on 31 March 1931)

Manufacturer: Panstwowe Zaklady Lotnicze (PZL) in Warsaw, Poland

Powerplant: (P.11a) One 500 hp (373 kW) Skoda built Bristol Mercury IV-S2 radial engine. (P.11b) One 595 hp (444 kW) IAR Gnome-Rhône K9 Jupiter radial engine.(P.11c) One 645 hp (481 kW) P.Z.L. built Bristol Mercury VI-S2 radial engine driving a wooden two bladed Szomanski propeller.

Performance: Maximum speed 242 mph (390 km/h) at 18,045 ft (5500 m); service ceiling 26,245 ft (8000 m); initial climb rate of 2,441 ft/min (12.4 m/sec); stalling speed 74.5 mph (120 km/h); time to 6,562 ft (2000 m) 2 minutes and 29 seconds.

Range: 435 miles (700 km) on internal fuel. 503 miles (810 km) on economical cruise.

Weight: Empty equipped 2,529 lbs (1147 kg) with a maximum take-off weight of 3,594 lbs (1630 kg).

Dimensions: Span 35 ft 2 in (10.72 m); length 24 ft 9 1/4 in (7.55 m); height 9 ft 4 1/4 in (2.85 m); wing area 192.68 sq ft (17.90 sq m).

Armament: Two 7.7 mm (0.303 in) KM Wz 33 machine guns each with 500 rounds inside the fuselage and two more 7.7 mm (0.303 in) KM Wz 33 machine guns each with 300 rounds in the wings at the junction of the struts, plus underwing racks for two 27 lbs (12.25 kg) bombs. Shortage of the KM Wz 33 machine guns meant that only about one third of the 175 aircraft produced carried all four machine guns.

Variants: P.11a, P.11b (Romania), P.11c, P.11f, P.11g Kobuz.

Avionics: None.

History: First flight (P.11/I) August 1931; production (P.11a) June 1933; service delivery (P.11b) summer 1934.