From Polish Aircraft 1893-1939 Jerzy B Cynk
PZL P.50 Jastrząb
Shrouded in a heavy veil of official secrecy, not allowed to be photographed or even mentioned in the Polish pre-war press, the P.50 Jastrząb is even today something of a mystery. Its short career was rather confusing. In spite of tight security surrounding the project, the fighter was shown to a German ally, the Italian Foreign Minister, Count Galeazzo Ciano the P.50 Jastrząb who visited Warsaw at the end of February 1939; it was ordered into large-scale production off the drawing-board, only to be cancelled a few months before the outbreak of war ; and assessments of its capabilities, put forward during investigations into the reasons for the Lotnictwo Wojskowe's weakness in September 1939, conducted by the Polish Government in exile, were contradictory and biased. When the war ended it became evident that most official P.Z.L. documents had been lost, and only provisional general arrangement drawings of the Jastrząb and a few poor photographs showing small fragments of the aircraft appear to have survived.
The following objective account of the Jastrząb story is based on a critical study of all the available evidence and exclusive interviews with former P.Z.L. and Lotnictwo Wojskowe personnel.
In the second half of 1936, when far-reaching decisions regarding the future of the Lotnictwo Wojskowe were being taken by the Government and Aviation Command, the earlier plan to replace completely the P.11 with the P.39 twin-engined two-seat strike fighter was abandoned and the view was endorsed that the Polish fighter force should be armed with a highly manoeuvrable single-engined single-seat 'hunter' fighter, designed for dog-fighting and intended for the close defence of specific targets, and that the fast long-range twin-engined machine (eventually known as the P.38 Wilk), which could chase enemy raiders over the whole country, would be used only in the complementary capacity.
The P.24 was briefly considered for the former role, but as this model could offer only a marginal improvement over the P.11, the Aviation Command favoured a more progressive design, and in the autumn of 1936 Wsiewołod Jakimiuk, head of the P.Z.L. fighter team, put forward proposals for an advanced low-wing monoplane which offered improved all-round performance and great scope for future development. Rather than develop the fighter as a further projection of the P.11/P.24 line with a low cantilever wing (as the Rumanians did with the LA.R.80j81, which was broadly a low-wing derivative of the P.Z.L. P.24), Jakimiuk, profoundly influenced by American design philosophy and, in particular, the Seversky monoplanes, created a design easily adaptable to take bigger engines, which bore some resemblance to the Seversky machines. This aircraft, number 50 in the P.Z.L. designation sequence, and Korsak's competitive PA5 study for a light-weight interceptor, became the final contenders to the hunter fighter requirement, and the former was approved in preference to the latter for immediate development.
The P.50 was named Jastrząb (Hawk) and a full interceptor specification was written around the study by the Aviation Command working in collaboration with the LT.L. In view of the lack of time (due to the earlier official interest in the P.39 two years had already been lost in the fighter development programme), it was decided to use a lower-powered but well proven and established engine, and the 840 hp Mercury VIII was selected to power the first production variant, the P.50A Jastrząb A, the estimated top speed of 500 kmh (310•6 mph) at 4,300 m (14,107 ft) for the Mercury VIII powered model being considered adequate for target-defence purposes, and four wing-mounted 7•7 mm machine-guns were specified as the basic armament. The detailed design and a mock-up were approved in the autumn of 1937, and construction of a static-test airframe and two prototypes-the P.50jI Jastrząb I, designed to take radials of up to 1,200 hp and generally corresponding to the Jastrząb A, and the P.50jII Jastrząb II, stressed for radial engines of up to 1,600 hp and much more in line with Jakimiuk's original intentions-began at the W.P.l plant.
Meanwhile the XVIIth Session of the K.S.D.S., sitting in October 1936, approved the four-year Lotnictwo Wojskowe expansion programme, to be completed by 31 March, 1941, which called among other things for 15 squadrons of single-seat interceptors with ten aircraft per squadron and 100 per cent reserve. In view of this and the quickly deteriorating equipment situation in the fighter field, the Aviation Command ordered 300 Jastrząb As off the drawing-board, paying for the first 100 in advance in 1938, delivery of the first 50 being expected by September 1939. At the same time the P.Z.L. W.S.1 at Okęcie received production contracts for 450 Mercury VIII engines. To speed up the P.50 programme the British Dowty company was given an order for an inwardly-retracting undercarriage for the P.50jI prototype, while the P.Z.L.-designed Avia-manufactured gear was being evolved for the production model. Unfortunately the Dowty undercarriage equipment arrived over four months late, and the Jastrząb I, completed in September 1938 and fitted with the Bristol-built 810-840 hp Mercury VIII engine (No. M42l02, despatched to Poland on 28 July, 1938), could not fly until the following February.
The flight trials conducted by Maj Bolesław Orlinski, the factory's chief test pilot, were not entirely encouraging. The Jastrząb, flown with the Bristol and P.Z.L. Mercury VIIIs, proved too massive an aircraft for the specified engine; 'rate of climb was poor and the top speed in the fully loaded condition was only 442 kmjh (274-4 mph). The aircraft was unstable in low-speed turns and tail-flutter developed at the other end of the speed scale. Modifications were put in hand, but during the second phase of trials in April, Orlinski reported that further major improvements would be necessary. A puzzling aspect of the trials was the fact that none of the engines fitted to the P.50jI airframe developed full power, and it was not until May when a young engineer, looking at the prototype, was suddenly struck by the thought that the carburettor air intake was too small. He was soon proved to be right. As it turned out, the intake was taken from an engine of another type and its suitability for the Mercury installation was never examined. In the early summer the machine was provided with an enlarged air intake and this, combined with changes introduced to the tail unit and wing/fuselage fillets, resulted in considerably improved performance and handling characteristics. In August the P.50/I exceeded a speed of 500 km/h (310'6 mph) in level flight, but by the outbreak of war the LT.L.airworthiness tests had not been entirely completed and Service acceptance trials, conducted by the Experimental Wing attached to the LT.L., had not begun.
The much more potent P.50/II airframe, substantially differing from that of the Jastrząb I, had been completed in the spring of 1939 and was waiting for an engine. The machine, featuring an all-round-vision hood and slimmer rear fuselage, had provision for additional fuel tanks and for a 300 kg (661Ib) bomb under the fuselage for dive-bombing or attack duties. The pilot's seat was protected by heavy armour-plate, and the fixed armament comprised two 20 mm cannon in the wing roots in addition to the four wing-mounted machine-guns. The new indigenous P.Z.L. Waran (an African lizard) radial, rated at 1,200-1,400 hp, was to supply the power, and the fighter's guaranteed performance included a maximum speed of 560 km/h (347'9 mph). Unfortunately, development of the War an was behind schedule and in the summer of 1939 there was little prospect of the engine being available for installation before the middle of 1940. In the search for a solution to the powerplant problem a number of other engines were considered, and the 1,375 hp Bristol Hercules and the 1,400 hp Gnome Rhone 14N50 series radials were selected as suitable alternatives. It seems that the decision to test the P.50jII with the Hercules was taken, and in view of structural differences between the Jastrząb II and the Jastrząb A, a new factory designation is believed to have been allocated to the design in the final weeks before the outbreak of war.
In the second half of 1938, with Gen Zając's critical re-appraisal of the Air Force's re-equipment prospects and mounting opposition to Gen Rayski's unreserved faith in the concept of a radial-powered fighter, the possibility of purchasing the manufacturing rights for the powerful Hispano-Suiza liquid-cooled vee engines began to be explored. Anticipating events, Jakimiuk began initial work on a projection of the basic Jastrząb design adapted for such a powerplant. Proposals involving the use of the 1,000-1,200 hp Hispano-Suiza 12Y, and, later, the brand-new 1,400-1,600 hp 12Z engine, studied in the winter of 1938-39 and the following spring, were covered by the designation P.56 Kania (Kite). However, Dąbrowski's competitive design (believed to have been the P.62) became the subject of a development contract and the Kania project is thought to have been cancelled in the summer.