Modelling > 1920s/1930s GB or Between the Wars GB

Progress at Poland's PZL

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apophenia:
Although just entering squadron service, Poland's new PZL PZL.31a Krogulec fighter was something of a dissappointment. It was manouevrable enough but didn't really have sufficient speed advantage to warrant replacing the gull-winged PZL P.11c. As an interim fix, Wsiewołod Jakimiuk and his team quickly produced a refined undercarriage design. The result was that uncompleted PZL.31a fighters on the production were finished as PZL.31b Krogulecs. The top speed improvement was marginal but the addition of twin wz.33 wing guns at least matched the firepower of the heaviest-armed P.11c fighters. [1]

Bottom The prototype PZL.31b Krogulec which was a rebuilt P.31a fighter. [2] This aircraft has full wireless equipment installed and the fuselage Vickers gun is visible though its open hatch.

A higher than expected accident rate with the PZL.31b convinced inż. Jakimiuk et al that a conversion trainer was required. Another conversion concept was worked up based upon the gull-winged PZL P.7a fighter. This was referred to as the szkolenie dwumiejscowe (SDW, or training 2-seater) although, internally, the unofficial designation PZL.17s was applied. [3] The Siły Powietrzne considered the 2-seater concept but drew different conclusions. The official view was that discipline, not a lack of training, was at issue. In any case, the Air Force had its own plans for the now-surplus P.7a airframes.

Top Unrealized PZL.17s (szkolenie dwumiejscowe or SDW) concept. The SDW combined airframe elements from the PZL.31a and older PZL P.7a fighters. The P.7a fuselage was modified to accept the low-set wing and had its cockpit moved aft for the instructor. A new, student cockpit forward displaced the fuselage fuel tank (dictating wing tanks).

No fuselage guns were to be installed. A single, starboard wing gun was planned for armaments training.

(To be continued ...)
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[1] The fuselage guns remained Vickers wz. 09/32, pending sufficient availability of the domestic machine gun type.

[2] This airframe had originally been a P.31a assigned to the 123 Eskadra Myśliwska at Poniatów. That P.31a had been returned to PZL after its original undercarriage was damaged while ground-looping.

[3] This was another 'recycled' designation. The original, 1932 PZL.17 had been a projected PZL.16 derivative. In the 'new' designation, the 's' suffix was for szkolenie.

apophenia:
Lekki Samolot Myśliwski - Zalewski Lightweight Fighter

Despite their rejection of the PZL.17s trainer concept, the Siły Powietrzne received a second PZL P.7a airframe rebuild proposal. This was for a light fighter concept - the Lekki Samolot Myśliwski (LSM) - put forward by veteran PZL engineer Władysław Zalewski. Proposed was a more straightforward adaptation than Jakimiuk's P.31 series. The P.7 fuselage was not lengthened and the wing (developed with inż Franciszek Misztal) was based directly upon the structure of the P.7/P.11 gull wing. This wing was raised from the bottom of the fuselage to provide stowage space for the main wheels when retracted. Power would be provided by a small-diameter engine of French origin - the Gnome-Rhône 14M-05 Mars twin-row radial. [1]

The Sily Powietrzne rejected Zalewski's LSM proposal primarily because the service intended to convert all suitable P.7a airframes to a new ground-attack configuration. However, cursory wind tunnel testing at the University of Technology in Warsaw had also revealed problems - the Zalewski fighter was unlikely to be longitudinally stable and the fuselage rear decking was poorly integrated. [2] Reviews of a full-scale mock-up generated positive comments on the undercarriage arrangement [3] and especially on Zalewski's revised canopy design. The latter was derived from the P.24 canopy but rearranged to allow the canopy hood to slide (rather than be side-hinged as on the P.24 and PZL.31). A request was made that Zalewski's canopy and windscreen design be incorporated onto the PZL.31 series as soon as practical.

PZL.41 - Next Stage in Jakimiuk Fighter Development

Meanwhile, the Jakimiuk fighter continued evolving. The PZL.41 replaced the PZL.31b on the WP-1 production line at Okęcie. The change warranting a change in designation number was the introduction of a retractable undercarriage. The new main gear was arranged to retract aft into fairings on the underside of the wings. The actual undercarriage legs did not penetrate the wing structure when retracted. The underwing 'kajaki' (canoes) covered those legs but part of the main wheels was left exposed to reduce damage in the event of a wheels-up landing. A section of these 'kajaki' swung down when the gear was extended, acting as air brakes to slow landing speed.

The PZL.41 was quickly approved for production. The 4-gunned PZL.41b was the first to enter service. These aircraft were armed with 7.9 mm wz.33 machine guns in both fuselage and wing positions. In the event, due to high costs, the 20 mm Oerlikon FFS cannons required for the PZL.41a variant would never be procured. The prototype PZL.41-I was briefly fitted with Oerlikon guns 'borrowed' from the P.24/III demonstrator. But that was as close as the PZL.41a ever came to genuine existence.

(To be continued ...)

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[1] The 14M Mars was to be license-built by the new engine division, PZL WS-2, at Rzeszów. This engine was also to power the  PZL.48 Lampart twin-engined fighter-bomber (which had eclipsed the PZL.38 Wilk).

[2] Unbeknownst to PZL, France was not yet prepared to reveal the 'secrets' of its streamlined Mercier cowlings. An even bigger problem was that the G-R 14M engine had no provision for synchronized armament (Zalewski had assumed that PZL WS-2 would be able to introduce such a feature on license-built Mars).

[3] This retractable undercarriage distinguished Zalewski's LSM from a rival Mars-powered fighter design - the fixed-gear PZL.45 Sokół.

ChernayaAkula:
Wonderful designs all round.  :-*

apophenia:
Thanks Moritz! I've got a few more PZL concepts but they are nudging into September '39. So, I'll save those for after the GB.

apophenia:
And the story continues ... But too much WW2 content, so you can find the PZL.51 and PZL.61 fighters here:

http://beyondthesprues.com/Forum/index.php?topic=351.msg187430#msg187430

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