PZL P.11c

PZL P.11c

If you were to ask about the symbolic aircraft of the desperate defence of Polish sky during the tragic September ’39, without any doubt the answer should be one – the PZL P.11c.

Although older machines (PZL P.7a and P.11a) fought next to it, the mentioned variant was a true backbone of the defence force. Pilots liked P.11c but in all publications it is described by an adjective “obsolete”. It was the penultimate stage in the evolution of a long line of fighter planes, no longer having the disadvantages of the “infant period”, but also actually constituting a closed alley in the evolution of this type of design. The PZL P.11g modification showed that the already mature construction can only be improved to a small extent. Being an export alternative, faster, equipped with a much more powerful engine and better armed PZL P.24 were actually the end of the possibilities of high wing strut aircraft. This was understood in Poland, where design offices were no longer involved in the further development of such structures (P.11g was just an emergency attempt to obtain not the most modern, but an efficient fighter). Unfortunately, it was not possible to produce or buy a successor abroad on time. That is why Poland entered the conflict equipped with fighter planes, once belonging to the world leaders, but giving way to the latest enemy machines.

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It is worth remembering, however, that at the same time other countries, which were the object of German aggression too, did not have equipment equivalent to German contructions. Czechoslovakia had a comparable fighter – a slightly faster Avia B-534 biplane. Denmark – had completely outdated British Gauntlets and Nimrods (Nimroderne in the Danish version). Norway at the last moment before the invasion (and this took place six months after the attack on Poland), only got slightly better Gladiators (it is worth knowing that this biplane was equipped with an engine like the P.11g) and the American Hawks 75. Belgium did not have its own modern designs, also equipping itself with Gladiators and Fiats CR.42 Falco at the last minute. The Netherlands introduced low-wing planes eventually (and it didn’t help either). In every case (maybe except for Norway), we are talking about countries with a greater industrial potential in the field of aviation than Poland. Great powers such as France and Great Britain also did not have large amounts of equipment that had an advantage over German. The Supermarine Spitfire was born in pain and there was very little of it yet. The slightly more numerous Hurricane was only just entering the units in greater numbers. The Gladiator, which was not very promising, was still being developed, in case if more modern designs turned out to be a failure. In France, the MS.406 C1 was not an equal opponent for the Germans, but a really modern D.520 C1 and VG. 33/36/39 only timidly appeared in small numbers in 1940. Even Hitler’s allies – Italians (Fiats CR.42 were just entering service, and Fiat G.50 and Macchi MC.200 in 1939 were still in the trial phase) – did not yet have sufficiently modern machines. It was similar with the Soviets (I-16 and I-15 were slightly faster and better armed, but comparable to the P.11c). Thus, frequent complaints to Polish designers, industry and decision-makers about the failure to provide modern weapons to the Polish aviation are rather poorly motivated.

 

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Because of the surprising appearance of a new generation of the previously known Messerschmitt Bf 109, the equipment situation of the Polish aviation, considered not the best in January 1939, became alarming in April same year. It is worth remembering that when Bf 109 E was introduced to the units, it was equipped with a completely new DB 601 engine with direct fuel injection. What is worse, immediately in large numbers – due to problems with the engine, the Germans initially produced only airframes (from December 1938). Suddenly, in March and April, new machines quickly began to displace the older variants from the units. The C and partly D versions were 100 km/h faster than the P.11c (and only 50 km/h faster than the P.24), but less agile. Version E, though, had a difference of 185 km/h and a huge advantage in terms of climbing speed. Polish planes found themselves in the position of not only old, but also very obsolete machines. Just like the equipment of almost all of Europe. In fact, only the British had a machine comparable to the Bf 109 E, but not in big numbers (Spitfire Mk. I). There was simply not enough time to replace the equipment in the Polish aviation.

However, would the replacement of equipment, even with a much more modern types, change something? In order to successfully resist the Germans (not counting the Soviets), at least 700–800 fighter planes were needed, and Poland had (with reserves) 279 machines (including 173 PZL P.11). There were 160 fighters in the line (including 130 P.11), 9 more were added from reserves and renovations. Even if the machines were to be replaced with modern ones and their number doubled, it would not be enough. That is why I admire the fact of the strong resistance and, despite the surprise, the good organization of this handful of heroes who have “elevens” at their disposal. In this situation, a dispute over the number of destroyed German aircraft (whether it was 146 enemy aircraft according to J. Pawlak, 125 according to the Bajan Commission, 97 according to J. Cynk or only 49 according to M. Emmerling) is completely secondary. M. Emmerling wrote: “Apart from outdated and defective equipment, these were also very small forces. Without a well-functioning alarm system and targeting the enemy did not pose a real threat to the Luftwaffe bomb formation. Therefore, the enthusiastic descriptions of historians in which the effectiveness of PZL is praised (!) are surprising1”. Contrary to the opinion of the German researcher, Polish historians praise the pilots rather than the machines, although they sometimes emphasize some of the advantages of this design, which allowed this outdated aircraft to engage in any fight with modern Luftwaffe equipment (maneuverability, strong structure allowing for sudden maneuvers). It is worth looking at PZL pilots against a wider European background. If you are interested, I refer to the history of the fighter aviation activities of Belgium, the Netherlands, Norway (not to mention Denmark at all, because it had exceptionally outdated aviation) in 1940, which had slightly better equipment (Gloster Gladiator, Fiat CR.42, Hurricane, Fokker D-XXI) and British-French support. They will not collect as many successes as the “eleven” (even in the lowest number given by M. Emmerling). Therefore, it is worth taking a closer look at this construction, which, despite its genuine obsolescence at the outbreak of the war, carried out by Polish airmen, was enlisted as the first Allied fighter in the war and put up a decisive but ineffective resistance to the invaders.

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Airplane development
PZL P.11c was the result of the development of the airframe line initiated by engineer Zygmunt Puławski in an all-metal PZL P.1 fighter from year 1928. However, only the shape of the wing and the method of its attachment actually connected it with the ancestral line. It is worth remembering that although the PZL P.1 was supposed to have an aerofoil covered with a fine grooved sheet of the Wibault patent, the prototype had a different sheet (similar to the Junkers patent), and it was not produced. In the meantime (and the pace was lightning), Puławski developed the PZL P.6, which had a completely different fuselage structure from the P.1 (oval half-shell, truss in the engine part) and a completely different radial engine. The plane, admired at rallies and air shows, was never produced, and it was replaced by the PZL P.7a, equipped with an engine with a compressor (Jupiter VII F). Even before the production of the new aircraft began, a decision was made to build its successor – the P.11 with the Bristol Mercury IV engine2. The originator of the wing design and the designer of the first series of fighters died two months later (March 12, 1931), therefore the new design was developed under the direction of enginner Vsevolod Yakimiuk.

The machine was supposed to be a “new P.7 with a different engine”, but in fact it became a completely new design. The strength calculations were carried out by engineer Piotr Bielkowicz. He designed a strong airframe, withstanding practically any load occurring in flight – the destructive coefficient for the wing was 19.1, and for the fuselage and tail 15, with a maximum of 13.5 in flight. It is worth remembering that these values were applied to the prototype, the PZL P.11c version had an even stronger airframe (especially the fuselage). Even the bombing attempts from a diving flight carried out before the war, with an additional (unforeseen in the design) load, caused only damage to the ailerons. It was great news for the Polish aviation after fresh experiences with SPAD 61C1s fighters, which had airfoils below the design strength and much below that in flight, and a significant improvement over the PZL P.1, whose poor airfoil sheeting almost eliminated the design from development (the accident of Major B. Orliński in August, 1928). The first prototype, with a smooth sheet metal wing, tail and fuselage borrowed from the PZL P.6, flew in August, 1931, with a weaker than expected Gnome et Rhône Jupiter VII 9Asb engine (French version of the Bristol engine). It had no cylinder covers, only individual fairings, later replaced by a narrow Townend ring. This copy appeared at various fairs and shows in Europe, arousing interest (and even willingness to buy, expressed by Romania, Yugoslavia, Portugal and Turkey). The second prototype – the P.11/II – received the expected Bristol Mercury IVA engine, as well as a wide engine cover, initially with exhaust pipes protruding halfway (from each cylinder). The ground tests were initially unsuccessful, the machine was vibrating strongly. Therefore, flight tests began late in December, 1931. Various propellers (Chaviere, Ratier, Letov) and fairings behind the cabin were tested, only the replacement of the engine with a new one and the propellers for the Bristol product (the Szomański propeller turned out to be better) eliminated most of the vibrations and the plane began to fly correctly. The third prototype powered by the Bristol Mercury IVS2 engine (475/555 hp) was intended for the US tour and participation in the National Air Races in Cleveland. It was the first (apart from the one for static tests) P.11 with a fine-grained surface and tail cover. The performance was unsuccessful, though, and the prototype was damaged.

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The P.11/IV equipped with the French Gnome et Rhône Mistral 9Ker engine (500/550KM), a narrow Townend ring, a three-bladed propeller and elegant fairings on wheels were exhibited at the 13th International Air Show in Paris in 1932. It became the model for PZL P.11b, exported to Romania (1933–1934). These planes were the first mass-produced P.11s (later they had a two-bladed propeller and no wheels fairings). In turn, after minor modifications to the tail and the introduction of the exhaust manifold in the leading edge of the engine cover, the PZL P.11/III prototype became the model for the PZL P.11a, 50 of which were made and delivered in 1934. It was not a machine without flaws, some of them inherited from the P.7a, e.g. not the best visibility from a cramped cabin and difficulties with tracking series from low positioned machine guns. In addition, exhaust fumes penetrated the cockpit, causing health problems for the pilots. Therefore, at the turn of 1933 and 1934, the military authorities became interested in the P.11/V prototype prepared for export. It was actually a new plane, which was supposed to improve the competitiveness of the P.11 in the fight for orders, especially in terms of speed, stability and utility values. The differences from the P.11a were very serious: the engine was placed 13 cm lower. The truss part was extended behind the cockpit. It was not visible, because the sheets kept their round cross-section there. However, this facilitated access to most important installations – it was possible to remove whole sheet covers. The position of the pilot’s seat was raised (by 5 cm) and it was retracted 30 cm. This forced the use of new belts, which allowed the pilot to lean freely in a much more spacious cockpit – thus it was the first Polish plane with inertial pilot belts. This had a serious impact on safety – in the case of an emergency landing, facial injuries were much less frequent than, for example, in PZL P.7a (or in Armstrong-Whitworth Siskin, which was then withdrawn in Great Britain, which left the pilots with a broken nose after harder landings, the so-called “siskin nose”). The fairing behind the cabin was extended to the base of the rebuilt tail. There was a place for a radio station (not all copies had one). The new shape of the vertical tail, together with the fairing, improved stability. The wings were redesigned and now they could fit two additional machine guns. The Gnome et Rhône Mistral 9Krse engine (560/600 HP) was provided for the export version. It was produced under license in Romania under the designation of P.11f (according to various data, from 70 to 95 copies were made). The Polish version was to have a Skoda Bristol Mercury VS2 engine (565/600 hp). It was planned to use a reinforced Bristol Mercury VIS2 engine (605/645 hp), but it has been confirmed that this unit was mounted only on the plane with serial no. 8.1293. One hundred and seventy five aircrafts were completed to this standard and bearing the name PZL P.11c. They were sent to units at the turn of 1935 and 1936, when the trials for the production of its export version were already beginning… Well, was it a replacement or successor? The P.24 was treated from the beginning as an export option, i.e. a substitute for the P.11c in this market. On the other hand, in 1938–1939, when looking for an aircraft to replace the rapidly aging “eleven”, all machines were compared to the P.24 as an aircraft that “we could have anytime.” Therefore, temporary problems with P.50 Jastrząb resulted in the suspension of the project, which seemed to “not promise progress to P.24.”

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So we were left with “eleven” (which, even after a significant modernization which was the P.11g with the Bristol Mercury VIII engine, stubbornly did not want to exceed 400 km/h). Polish pilots set off to fight on machines that were quickly aging in four years, but they rapidly became obsolete with the entry into service of the Messerschmitt Bf 109E. An additional problem was the need to carry out repairs. Frequent cracking of engine beds in 1938 resulted in the suspension of training until the replacement (winter 1938/1939) of these elements in most of the units. This also explains why the September machines were darker than those from previous periods – almost all of them underwent airframe renovation. By the way, wing MGs were removed from some of the machines armed with four machine guns4. The reverse process was happening with the radio stations. Unfortunately, the same cannot be said for heavily worn engines.

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