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"Everything else is credit" - New PZLs in Spain (Part 4)
The proposed 'P.24PL' variant had never been anything but an interim solution.
Spanish gold reserves had secured gull-winged PZL fighters for the Republic. Now that same gold - along with P.24 export profits - would help finance development of the next generation of Polish fighters. These more modern fighters would take part in the final defence of Catalonia ... and then they would help defend Warsaw from the Luftwaffe.
Replacing the 'P.24PL' in established Lotnictwo Wojskowe planning was the advanced, low-winged PZL.50 Jastrząb (Hawk). The P.11c was obsolescing quickly but, by contrast, progress on the Jastrząb was positively glacial. PZL's chief designer, inż Wsiewołod Jakimiuk, had been held up on completing the PZL.50 by oft-changing engine choices and delayed delivery of a retractable undercarriage sourced from Dowty in the UK. With no sign that a Jastrząb prototype would be completed before 1939, an 'interim-interim' solution was needed.
Thus, it was with something of a panic that PZL engineers Franciszek Misztal and Jerzy Dąbrowski were re-assigned to the task of producing a competitive low-winged monoplane fighter as rapidly as possible. Baked-in as a priority was the need to evolve this new fighter from the maximum possible P.11/P.24 components in order to speed both the design process and production. Like all new Polish military aircraft, this fighter was assigned a 'popular' name - Krogulec (or Sparrowhawk).
Price of Progress - A Partly New Iron in the Fire
Ultimately, the new fighter aircraft would be designated PZL P.51 - although, to be fair, that 'P' was a misnomer. Although the brief was to incorporate as many P.11c and P.24 components as was practical into the P.51, there was little or nothing left of Zygmunt Puławski's original 1931 PZL P.11/I design - either conceptually or materially. The rear fuselage was retained but, in reality, that refined P.11c structure had been designed by inż Jakimiuk after the death of Puławski.
In a division of labour, Franciszek Misztal was tasked with creating the wing for what became the P.51 while Jerzy Dąbrowski was to integrate the old fuselage into this new design. For speed, Misztal chose to incorporate the wing design from his PZL.38 Wilk - a stalled twin-engined fighter concept he had been working on. [8] The centre section of that 3-part wing was narrowed - giving a final span of 9.90 m (32.5 ft) and an area of 17.18 m2 (185 sq ft). The outer panels were also given more dihedral.
For simplicity, Misztal advocated a Seversky P-35 style main undercarriage. But, ultimately, that was Jerzy Dąbrowski's call ... and he chose a main gear designed by France's Société Messier instead. The legs of these units attached to the dihedral-less wing centre section while retracting outward to be housed in the outer wing panels. That made for much cleaner lines than the arrangement advocated for by Misztal. However, this outward-retracting main gear took up more wing 'real estate', adding complexity to fixed armament installations.
Prototypowanie - You've Gotta Start Somewhere
The first prototype PZL P.51/I 'recycled' a P.24G fuselage and was powered by a 920 hp IAR K14 radial enclosed within the same straight-sided cowling designed for the PZL P.24/III prototype. Since that engine type couldńt accommodate a synchronisation gear, all fixed armament was to be mounted in the wings. In this first prototype P.51, this consisted of twin 20 mm Oerlikon FF autocannons inboard of the wheels plus a pair of 7.92 mm PWU wz. 36 machine guns mounted outboard of those retracted wheels.
As developed into the production P.51H, the armament was revised to the same 4 x 7.9 mm Darne as Spanish P.11s . It was planned that this would, eventually, be changed to either 6 x 7.9 mm Darnes or a 4-gunned type with inboard 13.2 mm FN Browning heavy machine guns. In theory, a bomb pylon could be attached beneath the wing centre section although this was never done on operational P.51Hs. Officially, the handful of P.51Hs to arrive in Spain had been snuck through through the blockage into the port of Barcelona. But that was a ruse. In actuality, Gnome et Rhône had used its influence with the French government to have the P.51H airframes delivered via Bordeaux. [9]
Scion of the P.11c - The PZL P.51A for Poland
Risking a jinx, the fuselage of a wrecked P.11c was the starting point for building the second prototype PZL P.51/II. The 640 hp Mercury VI S.2 from the P.11c was retained along with its fixed-pitch, wooden Szomanski propeller. Armament was 2 x synchronised 7.92 mm PWU wz. 36 machine guns with another pair of wz. 36 mounted in the new wings (inboard of the retracted main wheels). Both powerplant and armament would be changed for production examples.
A third prototype PZL P.51/III was prepared from another P.11c airframe. This would become the definitive engine and armaments prototype for the Lotnictwo Wojskowe's production model P.51A. In contrast to the P.51/II, the third prototype featured a Bristol-built Mercury VIII radial (albeit still driving a Szomanski prop) and was armed with twin synchronised Belgian 12.7 mm FN-Browning M2 HB guns. [10] After acceptance the P.51/III was allocated to Warsaw's Instytutu Technicznego Lotnictwa for further trials.
Production P.51As would be essentially similar to the third prototype but were powered by a 725 hp PZL-built Merkury VIII engine (with an emergency rating of 840 hp). As planned, the synchronised fuselage guns became PWU-FB built 12.7 mm ckm wz. 38 FNs licensed from FN Hestal in Belgium. [10] Along with those Radom-built ckm wz. 38 FN medium guns were the twin, wing-mounted PWU wz. 36 rifle-calibre machine guns which fired outside of the propeller arc.
Stado Krogulców - Proliferating P.51 Variants
Priority was given to developing the first two prototyped P.51 variants - producing the production model P.51H Gavilán for Spaińs FARE and the Merkury-engined P.51A for Poland. However, a number of proposed P.51 variants were also in the offing. These included P.51 offerings for existing P.24 export customers as well as for the Lotnictwo Wojskowe itself.
For Poland's defencę the most important of these variants was the PZL P.51C which was to be powered by a 1,050 hp Gnome-Rhône 14N-07 built by PZL WS-1 as the Gnom. The closely-related PZL P.51D was developed as a safety measure - effectively being a PZL P.51C which retained the P.51H's Romanian 920 hp IAR K14 engine. As it happened, neither of these Polish alternative models would be realised. Records will show 'P.51Dś in Polish service in Sept 1939 but, in reality, these were P.51H airframes completed for the Lotnictwo Wojskowe after the March 1939 collapse of the Spanish Republic.
Romania's IAR had sought a license for domestic production of the P.51F. This type also provided the model for a more general export type - the P.51E (Eksport) - which would, ultimately, be offered with a range of engine types and armament options. More specific proposals were the P.51K (Kayseri) to be built in Turkey (aka the 'P.51T' for Turcja or Türkiye) and the P.51G (Grecja) to replace Ellinikí Vasilikí Aeroporía P.24Gs. None of these variants were ever realised. [11]
The PZL P.51 'Sparrowhawkś Into Service
The first PZL P.51H Gavilán were delivered with 4 x 7.9 mm FN Browning guns. Indeed, once the collapse of the Catalonian front began in earnest, confusion reigned. It remains unknown whether any 6-gunned Gavilanes ever arrived in Catalonia during those dying days of the Spanish Republic. The low-winged PZLs gave the 'Nationalistś and their fascist allies a surprise but it was an obvious matter of too littlę too late. The Condor Legion and Aviazione Legionaria already soaked up the skies.
Top: A captured P.51H Gavilán after the end of the Spanish Civil War in the colours of Franco's Ejército del Aire (established in Oct 1939). This Gavilán, found relatively intact in Catalonią has been completely stripped of its original paintwork. This trophy P.51H was to be displayed in a flypast over the Paseo de la Castellana during Franco's grand 19 May 1939 victory parade.
Prior to repainting, a test flight was made by the intended display pilot, civil war ace alférez Arístides García. This flight was made without a replacement lower cowling having been located. After departing the Aeròdrome del Prat, the upper cowling was seen to part company with the airframe. The stricken P.51H came down in the Balearic Sea. The body of Arístides García López Rengel was never recovered. [12]
Compared with the P.11h the higher-powered P.51Hs were a much better match for early-model Bf 109s ... but the Messerschmitts still held an edge. Although less immediately urgent, this imbalance in performance was greatly concerning for Polish planners. If, as anticipated, war between Poland and Germany broke out in the Summer of 1940, the bulk of the Lotnictwo Wojskowe interceptor force would still be made up of Merkury-powered PZL P.51A Krogulce fighters (backed up by those P.11c 'Jedenaste' gull-wings which had not yet been converted for the ground attack role).
The Luftwaffe's latest Messerschmitt Bf 109E model was cannon-armed (which sounded more impressive than it was). Due to a low muzzle velocity, the Oerlikon MG-FF cannon had an effective range was only 100-400 meters (comparable to 185 to 365 m for the Darnes arming the P.51Hs). By contrast, the higher muzzle velocity 12.7 mm ckm wz. 38 FN guns arming the Polish P.51A Krogulec had an effective air range of 550 to 900 m. When synchronised, the 12.7 mm machine guńs rate of fire was little more than the Oerlikon. But a Polish pilot could always open fire first and from further away. [13]
In Warsaw, P.51 development continued at a frenzied pace. So too did schemes to develop even more potent Polish fighter aircraft. But as the Summer of 1939 approached, the rest of Europe slipped towards the fate already suffered by Republican Spain. A deadly game board had been set and the Lotnictwo Wojskowe would be forced to fight with the aeronautical chessmen already at its disposal.
Bottom: The prototype PZL P.51/III was originally assigned to the Instytutu Technicznego Lotnictwa in Warsaw. In late Aug 1939, this aircraft was sent to an operational unit - 114 Eskadry Myśliwskiej at Poniatow. The swallow (jaskółczy) emblem of 114 EM has been applied to the fuselage side (despite orders from Lotnictwo Wojskowe HQ that all such unit identifiers were to be eliminated from combat aircraft).
During the retreat south, this aircraft was lost in aerial combat near Kozienice. The pilot, kpt inż Stanisław Dudziński - an ITL staffer - parachuted to safety. Note that the prototype PZL P.51/II retained the P.11's open cockpit, never receiving the canopy fitted to the production model P.51A Krogulec.
(Fin)
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[8] In the end, the PZL-Wytwórnia Silników (engine factory) would kill off its Foka IV-8 powerplant. That signalled the end of the PZL.38 Wilk (the similarly-powered LWS-4 - aka PZL.39 - light fighter having already been cancelled).
[9] At Bordeaux, the PZL airframes were offloaded for assembly and fitted with their French-made Ratier 3-bladed constant-speed propellers.
[10] The Polish air force would have preferred the 13.2 mm FN Browning but could not afford to wait another year for that Hotchkiss-cartridged gun.
[11] Although no Romanian P.51F were ever built, the ARR did take two 'refugee' PZL P.51D/P.51H fighters into service in Oct 1939. Fortunately for the ARR, IAR had persisted with an alternative P.24E-based low-wing monoplane design of its own - the IAR 80 series which entered service in Feb 1941.
[12] The 'stripped' PZL P.51H in Nationalist' colours profile is based upon a photo of a beautifully-made IAR 80 model build in 1/72nd by Polish britmodeller.com member, Yawimaya.
[13] The Bf 109E's secondary armament were a pair of synchronised 7.92 mm MG 17 guns with an effective range of 200 to 400 meters.