The P.Z.L. P.11 was the most important Polish fighter aircraft when the Germans invaded in 1939, but although it had been an advanced design when it was first introduced, by then it was outdated and outclassed.
The P.11 was a direct descendent of the P.Z.L. P.1, the first fighter to be designed by Zygmunt Pulawski. This aircraft had a gull wing, mounted just above the top of the fuselage, but with the normal central section of a parasol wing removed. Instead the inner sections of each wing sloped down to join the fuselage on either side of the fuselage, giving the pilot a much better upwards view than in a normal parasol wing aircraft, but keeping the excellent downwards view of the type. The P.1 was powered by a Hispano Suiza inline engine, and made its maiden flight in September 1929. At the time it was one of the most advanced designs in the world – most of its contemporaries were biplanes such as the Bristol Bulldog or the later Heinkel He 51.
The P.1 impressed the Polish military, but they didn’t want to use the inline Hispano engine, as the Polish Skoda works was already involved in licence built production of radial engines. As a result two radial powered versions of the aircraft, the P.Z.L. P.6 and P.Z.L. P.7 were developed, and the P.7 became the first to enter full scale production. The P.7 was powered by a 585hp Bristol Jupiter VII F radial engine produced under licence in Poland, and a total 150 aircraft were delivered to the Polish Air Force, some of which were still in front line service in 1939. Pulawski also continued work on the inline powered versions, producing the P.Z.L. P.8, but this was rejected by the Polish military.
Work on the P.11 began in late 1930, at about the same time as permission was given to produce two prototypes of the inline powered P.8. The aim was to modify the existing radial powered P.Z.L. P.7, which had made its maiden flight in 1929, to take the Bristol Mercury radial engine in place of the older Bristol Jupiter. The Mercury produced more power from a smaller frontal area, so it was a logical choice to install in the P.11.
Work on the P.11 began under the leadership of Zygmunt Pulawski, the designer of the original gull winged P.1. However he was killed in crash of his own P.Z.L. P.12 amphibian on 21 March 1931 and leadership passed to his assistant Wsiewolod Jakimiuk.
P.Z.L was given a contract to produce three prototypes and a structural test aircraft, modified to carry radial engines with a diameter of 1.31m/ 51.6in or less, with a power range of 500-700hp. This allowed a wide range of radial engines to be used on the P.11, and the closely related export version, the P.Z.L. P.24.
The P.11 kept the basic layout of the P.1 family, which was famous for its gull wing design. The wing was mounted just above the level of the top of the fuselage, but without the central section of a parasol wing. Instead the inner section of each wing angled down to join the fuselage on either side of the cockpit. This greatly improved the pilot’s view above the aircraft, while keeping the good downwards view of the parasol type. It had a fixed undercarriage, with part of the suspension carried inside the fuselage. The original P.1 had been powered by an inline engine, and had a flat sided fuselage. The first production version, the P.7, used a Polish Skoda licence built Jupiter radial engine, and had a fuselage with a circular cross section. The P.11 used the same circular fuselage as the P.7.
The first prototype, P.II/1, was completed with a Gnome-Rhone built 515hp Jupiter IX.ASb radial engine, as the first of the Bristol produced Mercury engines had yet to appear. This aircraft made its maiden flight in August 1931, and reached a top speed of 186.4mph. This was slightly slower than the production P.7.
This prototype was then used to try and win export orders for the type. In December 1931 it was sent to Romania, where it impressed the Romanian Air Force enough for a possible order for 60 aircraft to be placed. It was then sent to Turkey, where it won an International Fighter Contest, but for the moment no order followed. In 1932 the prototype was sold to Portugal. Greece, Sweden, Yugoslavia, Czechoslovakia and Japan all expressed an interest in the P.11, although most of these orders didn’t come through until the appearance of the P.Z.L. P.24.
The second prototype, P.11/II, was completed in the autumn of 1931, with a 520hp Mercury IV.A radial engine. In its original form the engine was mounted within a streamlined nose, merging smoothly into a large propeller spinner, and with the cylinder heads outside the fuselage surface. However this version suffered from serious overheating problems, and it had to be modified before it could make its first flight. A long-chord Townend ring was installed, along with a new propeller spinner with built in cooling louvers. In this configuration the aircraft made its maiden flight in December 1931. For its first flight it had a long head-rest, but this was replaced early in 1932 by a short head rest that was used in the P.11a and P11b. This aircraft was also used to test a Gnome-Rhone built Mercury IV.S engine, and with a series of different air-screws. It achieved its best speed, 215mph at 13,120ft, using a Chauviere propeller.
In June 1932 the P.11/II came second in the International Aviation Meeting at Warsaw, coming second to the inline powered P.Z.L. P.8. In July it competed in the single-seat fighter class at the 1932 Zurich Aviation Meeting, where it came second behind the Hawker Yugoslav Fury. It was then entered into the Salon International de l’Aeronautique at Paris, where it was powered by a Gnome-Rhone 9K Mistral that had been provided free of cost by Gnome-Rhone. The Mistral powered version became the prototype for the P.11b, which was being developed for Romania.
The third prototype, the P.11/III, was also powered by the Bristol Mercury, and was simplified to make it easier to produce. It was built with a shorter-chord Townend Ring, which emphasised the pointed nose. In June 1932 it was shipped to the USA to take part in the National Air Race at Cleveland, where it might have won if it hadn’t flown across the finishing line from the wrong direction. The aircraft then returned to Poland, where it was given a collector ring in front of the cowling ring, and became the prototype of the P.11a.
The P.11 was followed into production by the modified P.24, which became the export version of the design. Many of the changes introduced on the P.24 were then used on the P.11c, which was the best version of the type in Polish service in 1939.
In the spring of 1933 the P.11 completed its Service acceptance trials, and it was ordered into production. Thirty P.11a fighters were ordered, and they followed the Romanian P.11bs onto the production line. They had the short head-rest introduced on the second prototype, a Townend ring, with a collector ring in front, so most of the pointed nose was hidden. It was powered by a Polish Skoda Works Mercury IV.S2, producing 497-517hp, and was armed with two machine guns carried in the side of the fuselage. The first P.11a aircraft entered service in the autumn of 1934, and were used to fully equip No.111 Eskadra Kosciuskowska. The remaining aircraft were scattered around other fighter squadrons.
The negotiations with Romania had been interrupted by a change of government in 1932, but in February 1933 an order was placed for 50 P.11s, powered by a 525hp Gnome-Rhone 9K Mistral engine. The Romanians also purchased a licence to build more aircraft themselves. The Romanian order was considered to be more important than re-equipping the Polish air force, so the P.11b followed the P.7a into production. The first production aircraft were delivered in October 1933 and the order had been completed by the summer of 1934.
The P.11c was the second version produced for the Polish Air Force and was similar to the P.24 export version. In both cases the engine position was lowered by 10cm while the pilot’s seat was moved up by 5cm and backwards by 30cm, in an attempt to improve the view forward, which wasn’t as good in the radial powered versions as in the original inline P.1. The wing inner section was also modified. to improve the view. The fuselage guns were moved further up the side of the fuselage to improve the pilot’s view of the bullet trajectory. It could also carry two more guns mounted in the wings. It was given the long head rest used early in the development of the second prototype. The tail was also modified.
The P.11c production prototype was completed early in 1934. It was used to test a 600hp Gnome-Rhone 9 Krse and 600hp Skoda Mercury IV.S2 engine and was used as an experimental aircraft. The Gnome-Rhone powered version was used as a development aircraft for the licence built Romanian P.11f.
The prototype was followed by fifty aircraft powered by a Mercury V.S2 providing 600hp at 14,765ft and 125 aircraft powered by a 645hp Mercury VI.S2, providing 645hp at 15,500ft. The first aircraft were delivered in the second half of 1935 and production was completed within a year.
Romanian had purchased a licence to build the P.11b, but this was switched to a version based on the more advanced P.11c before production had begun. The resulting P.11f was built by Industria Aeronautica Romana (I.A.R.) at Brasov, and was similar to the P.11c, but with a Romanian built 9K Mistral engine and had a modified engine cowling. About 80 were probably built in 1936-38, before production switched to the P.24E.
P.11g Kobuz (Hobby/ Small falcon)
The P.11g was developed after earlier attempts to replace the P.11c failed. The first plan had been to replace the P.11 with the P.39 two-seat twin engined fighter/ dive bomber, but this plan was cancelled in 1936. Work then moved onto the P.Z.L. P.50 Jastrzab, a more modern single seat fighter, but this programme was delayed then cut back. In an attempt to fill the gap it was decided to produce a modernised version of the P.11c. The new P.11g was to be powered by the 840hp P.Z.L. Mercury VIII radial engine. One P.11c was used as a prototype, but it didn’t make its maiden flight until 15 August 1939. The production version was to be similar to late production P.24s, and be armed with four KM Wz 36 machine guns, all carried in the wings. It had an estimated top speed of 260mph. The first aircraft were expected to be ready by the start of 1940.
By the outbreak of war in 1939 twelve Polish fighter squadrons had been equipped with the P.11c, while three were still using the P.7a. In service the P.11c was armed with two 7.7mm KM Wz 33 machine guns (Vickers machine guns) and four Swiatecki racks for 27.5lb bombs. Plans were in place to add the wing guns, but by the time the extra guns were available in the summer of 1938 it was no longer seen as worth installing them, so only about 20% of the P.11cs had all four guns. At the start of 1939 the Polish Air Force had 185 P.11s on strength.
The fighter squadrons were split between the Pursuit Brigade, which had the task of defending Warsaw, and the Armies’ Air Forces, which were to cooperate with individual armies. The Pursuit Brigade contained the 1st Air Regiment, which had four P.11 Squadrons (No.111, No.112, No.113 and No.114) and one P.7a equipped squadron, No.123. None of the P.11 squadrons were fully equipped with a single type, and most had a mix of four gun and two gun P.11cs and P.11as. Eight squadrons (Nos 121, 122, 131, 132, 141, 142, 152 and 161) were with the Army Air Force.
By 1939 the P.11 was too poorly armed and too slow to deal with the German Bf 109, although its manoeuvrability in the hands of skilled Polish pilots made it quite hard to hit. However the only reason the P.11 was able to score its victories was because the Germans were largely willing to engage in combat – if they’d wanted to avoid it all but their slower machines could easily outpace the P.11.
After the German invasion the first Polish victory was scored by a P.11c of No.121 Squadron, which shot down a Junkers Ju 87 near Olkusz at 5.30am on 1 September. During the campaign the Pursuit Brigade claimed 42 victories at the coast of 37 of its own aircraft. The entire Polish fighter force claimed 126 victories, while the Germans acknowledged the loss of 285 aircraft during the campaign. The Poles lost 116 or their 166 front line fighters in combat, 11 to ground fire and the rest in the air. Very few were destroyed on the ground. Surprisingly only 12 pilots were killed, another 15 wounded and 7 missing, demonstrating the advantages of fighting on home ground.
On 17 September, when the Soviets attacked Poland from the east, the surviving 38 P.7s and P.11s were evacuated to Romania.
Engine: Mercury VI (P.11c)
Span: 35ft 2.5in (P.11a)
Length: 23ft 9.5in (P.11a)
Height: 9ft 10.25in (P.11a)
Empty Weight: 2.461lb (P.11a), 2,529lb (P.11c)
Loaded Weight: 3,483lb (P.11a), 3,594lb (P.11c)
Maximum Speed: 242.3mph at 18,044ft, 184.6mph at sea level (P.11c)
Climb rate: 6 min to 16,404ft, 13min to 22,965ft (P.11c)
Ceiling: 26,246 (Service), 36,080ft (Absolute) (P.11c)
Range: 435 miles (P.11c)
Guns: two or four machine guns