The PZL P.24 is a Polish fighter aircraft, built in the late 1930s by Państwowe Zakłady Lotnicze. It was a much more advanced development of the PZL P.11, a Polish (gull) wing all-metal fighter designed by Zygmunt Puławski.
While the PZL P.11 had been powered with a licence-built Bristol Mercury radial engine, the terms of this licence did not permit PZL to export the engine as well as placing restrictions upon any aircraft that were powered by it. The French engine manufacturer Gnome-Rhône proposed the adoption of their 14K engine to PZL and offered to partially finance the development of a fighter using the engine, which would have no such export restrictions. Accordingly, during early 1932, PZL commenced work on a new derivative of the P.11, which became known as the PZL P.24. The prototypes soon demonstrated favourable performance during testing; notably, the second P.24/II prototype, often referred to as the "Super P.24", established a new world speed record for a radial engine-powered fighter of 414 km/h. The initial production aircraft closely resembled the P.24/II configuration, albeit with some changes such as the adoption of an enclosed cockpit.
In 1936, PZL received orders for the new fighter from several countries. The PZL P.24 entered service with the Turkish Air Force at the beginning of 1937, and with the Bulgarian Air Force in the summer of that year. It was produced under licence by Romanian state manufacturer Industria Aeronautică Română (IAR), who had also licence-produced the earlier PZL P.11. Many elements of the PZL P.24, such as the fuselage, engine cowling, cockpit and tail section, were incorporated into a Romanian-designed low-wing fighter, the IAR 80. In Greece, the Royal Hellenic Air Force was the only air service in Second World War to operate the PZL P.24 as its primary fighter. However, wartime experience soon showed that, as a result of the rapid advances in aircraft design made during the late 1930s, the P.24 had become outdated as early as 1940. Despite a relatively powerful engine and satisfactory armament, it could not stand up against the latest Axis fighters, such as the Italian Macchi MC.200 and the German Messerschmitt Bf 109. Just like the Greek fighters, by 1942, it had become clear that Romania's P.24s were unable to effectively challenge the latest fighters of the Soviet VVS as well, and were relegated to training tasks.
Development Background During the early 1930s, PZL had developed a series of innovative gull wing all-metal fighters, headed by Polish aeronautical engineer Zygmunt Puławski. The P.1, the P.7, and the P.11, each more refined than its preceding aircraft, were capable of outstanding performance for their time and were widely displayed at international exhibitions and competitive fly-offs, demonstrating their capabilities to the world. The Polish monoplanes were subject to considerable international interest, leading to PZL's offices receiving a deluge of enquiries on the topic of prospective export sales. According to aviation author J.B. Cynk, by 1933, outward interest had been expressed by Bulgaria, Czechoslovakia, France, Greece, Hungary, Japan, Portugal, Romania, Sweden, Turkey and Yugoslavia.Cynk 1967, p. 3.
In order for any export sales of the type to be successfully completed, PZL had to address two major obstacles. Firstly, the company was only capable of producing so many aircraft in the space of a year; PZL did not have the internal resources to substantially expand its production capacity, nor was financing from the Polish government or other bodies viewed as being particularly forthcoming during this time. According to Cynk, tentative customers for the fighter were typically adverse to making large currency exchanges as part of their payments, instead, they would usually prefer to pay in terms of goods (such as raw resources and industrial products) or to pay in numerous instalments. Furthermore, Cynk claims that several deals which had been formed with PZL had fallen through or had been otherwise postponed due to difficulties encountered in agreeing acceptable terms of payment between the customer and the manufacturer.
Another issue was posed by the engine used by the P.11. It was powered by a single Bristol Mercury radial engine, which was produced in Poland under a licensing agreement with the British Bristol Aeroplane Company; however, the terms of the 10-year agreement had stipulated that none of the Polish-built engines could be exported. In addition, the licensing agreement had placed restrictions upon the sale of those aircraft which were powered by the Bristol engine; these export restrictions were present even if the engines were to be supplied by the customers rather than by PZL themselves. Naturally, PZL was keen to eliminate or otherwise minimise any restrictions that were deterring export sales of their aircraft.
According to Cynk, the initiative to develop a dedicated export-focused derivative of the P.11 received a considerable boost during the early 1930s by a French Air Force initiative to furnish itself with a large force of modern first-rate fighters. While the majority of projects considered by French officials were indigenously developed, the French Air Force was also evaluating various foreign fighters as potential candidates for being licence-produced in France; Cynk claims that the P.11 was at the top of their list. This attention may have contributed to the decision by French engine manufacturer Gnome-Rhône to offer PZL a financial contribution of 150,000 francs towards the development of an export-suitable P.11 derivative, as well as to provide the first prototype's engine for free and to sponsor the fighter for the French competition, all of which was conditional to the selection of their engine to power the type.
Project launch In accordance with these converging interests, PZL decided to embark upon preliminary design work into the prospective development of a further evolved form of the P.11 that would be aimed specifically at the export market. The project, and the resulting design produced by it, soon received its own designation as the P.24. The Gnome-Rhône 14K, a powerful French radial engine, had quickly emerged as a strong candidate to power the tentative fighter; thus the engine company's offer of support was quickly accepted.Cynk 1967, pp. 3-4. The project was headed by Wsiewolod Jakimiuk, who had assumed the position early on after the untimely death of Puławski on 21 March 1931. During February 1932, the P.24 proposal was submitted for internal approval; this was granted two months later.
During May 1933, the first P.24/I prototype, which was largely based on the P.11a and powered by a Gnome-Rhône 14Kds engine, conducted its maiden flight, flown by Bolesław Orliński. This flight had been delayed since January 1933 due to a failure on the part of Gnome-Rhône to promptly deliver either the engine or its three-bladed metal propeller. Even following the installation of the engine and an alternative Szomanski-built propeller, issues were encountered during pre-flight powered runs, such as seizing fuel pumps. The maiden flight had not gone smoothly either; it had been cut short after severe vibrations had shattered the hub fairing, further damage to the propeller blades, engine bearing structure, and fuel tank attachments was also incurred, leading to the forward section of the prototype being reconstructed and reinforced. During October 1933, flight tests resumed.Cynk 1967, p. 4.
The experiences with the first prototype had determined that in excess of 150 modifications to various aspects of the design, including the adoption of a long-chord NACA cowling, the reinforcement of a cannon mounting structure and the revision of the exhaust and cooling systems, were required, leading to the construction of a second prototype, known as the P.24/II. Performing its first flight during March 1934, this prototype, also named the "Super P.24", quickly demonstrated its excellent performance, such as its ability to easily exceed 400 km/h in level flight. On 28 June 1934, P.24/II established a Fédération Aéronautique Internationale-recognised world speed record for radial engine-powered fighters of 414 km/h. Following Gnome-Rhône's announcement of the existence of a more powerful 14Kfs engine, a third P.24/III prototype, otherwise known as the "Super P.24bis", was called for, powered by this new engine. However, this engine was not immediately available due to a prior order placed by Finland having taken precedence; as such, the first 14Kfs engine did not arrive until July 1934.Cynk 1967, pp. 4-5.
During August 1934, the third prototype conducted its first flight; two months later, it performed a demonstration outside Warsaw attended by various Polish officials and several members of foreign missions. During late 1934, the newly completed P.24/III was publicly displayed at the Paris Air Show, where it reportedly attracted a high level of interest from visitors and other participants alike. According to Cynk, at the time of the Paris Air Show, the P.24 was commonly considered to have been both the quickest and the best-armed interceptor in the world. While the French Air Force were still expressing interest in the P.24 at this point, there was considerable political resistance to the procurement of it due to its non-indigenous origins and that French manufacturers were making their own advances, including some of those that were featured upon the P.24. As such, a French order was not forthcoming for the type.Cynk 1967, p. 5.
Production During late 1934 and early 1935, the P.24/III was involved in several promotional flights, including evaluation flights held near Warsaw on behalf of visiting Turkish and Romanian military officials, as well as a demonstration tour which saw it visit neighbouring Hungary and Bulgaria. In the spring of 1935, having judged the likelihood of sales to have been high at this point, PZL decided to commence work on a batch of six pre-production P.24s at their Warsaw plant. Unlike the prototypes, these aircraft had enclosed cockpits and the armaments were entirely carried within the wings rather than the forward fuselage, as well as the adoption of wing and tail surfaces reminiscent of the P.11c, alongside minor refinements and alterations. Different armament arrangements were installed across the pre-production aircraft, representative to the configuration that was intended for several separate production models. One pre-production aircraft was shown at the 1936 Paris Air Show, after which it conducted several demonstration flights on behalf of the French Air Force.Cynk 1967, pp. 5-6.
Gradually, the pre-production P.24s were modified to closely represent successive production variants of the type; this work included the installation of alternative engines, including the Gnome-Rhône 14N01, the Gnome-Rhône 14N21, the Renault 14T, and the Fiat A.80, which were freely provided by their respective manufacturers. During early 1936, Turkey became the first customer to place a firm order for the P.24, completing a deal to not only by 40 Polish-built fighters, but also a licensing arrangement and the materials to domestically produce a further 20 P.24As. These early fighters, which were furnished with a range of different armaments to conform with the specified Turkish requirements, were otherwise nearly identical to the pre-production aircraft. Production commenced almost immediately; the first deliveries took place in late 1936.Cynk 1967, p. 6.