Author Topic: Progress at Poland's PZL  (Read 496 times)

Offline apophenia

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Progress at Poland's PZL
« on: July 23, 2021, 11:40:52 AM »
In the mid-1930s, the Polish Air Force (Siły Powietrzne) contracted state aircraft firm PZL to produce a replacement for its in-service PZL P.11 gull-winged fighters. The task was assigned to engineer Wsiewołod Jakimiuk whose team sketched out a low-winged monoplane with enclosed-cockpit. This design emerged as the PZL.50 Jastrząb (Hawk). However, by the Summer of 1937, it was decided that progress on the PZL.50 was too slow. 'Jaki' was ordered focus his team's attention on a quicker-to-produce, low-winged derivative of the production-model P.11c fighter. This concept would originally be known as the Dolnopłat Myśliwski Jakimiuka (DMJ).

With speed the priority, Jakimiuk chose to adopt the wing structure designed for the twin-engined PZL.38 Wilk. [1] Stripped of its nacelles, this wing was simplified and modified to fit the base P.11 fuselage structure. To further speed development, a fixed undercarriage was adopted. The main wheels were to be clad in streamlined spats akin to those on the P.23 Karaś attack aircraft. Much of the engineering work was performed using a slightly damaged P.11c airframe to create what was effectively a 'flying mock-up' of the new fighter. Although the fuselage had to be slightly lengthened, other than an inserted extension frame it remained unchanged. That allowed what became the P.11d ( 'd' for dolnopłat or low-wing) to be completed very quickly.  By October 1937, the prototype P.11d was undergoing service trials with Siły Powietrzne personnel at Warsaw-Okecie airfield.

Top Prototype conversion PZL P.11d on service trials, Warsaw-Okęcie, Autumn 1937. Note windscreen retained from the P.11c and full main undercarriage leg spats. Just visible are the leading-edge slats - unique to this prototype.

While service trials were underway, a pre-production batch was approved under the new designation PZL.31a. [3] These aircraft were all to be based upon selected P.11c airframes returned from service use. Fortunately, only minor detail differences had to be incorporated into the design based on the Warsaw-Okęcie trials. By the late Summer of 1938, the first PZL.31a fighters were entering service with the 123 Eskadra Mysliwska (123 Fighter Squadron) at Poniatów near Warsaw. The PZL.31 was dubbed Krogulec (Sparrowhawk) in service.

Bottom One of the first PZL PZL.31a Krogulecs delivered to the 123 Eskadra Mysliwska, displacing older, gull-winged P.7a fighters. In late December 1938, 123.EM Commanding Officer Capt. Mieczyslaw Olszewski ground-looped 'White 3' at Poniatów. The airframe was repaired but did not return to the 123 Eskadra Mysliwska.

An obvious change for these pre-production fighters was their larger, broaden windscreen (taken from the export P.24). [4] Note too that the spats have been removed (to avoid the buildup of mud and other debris which could jam the wheels).

(To be continued ...)

__________________________________

[1] Work on this wing design had begun back in 1934 for the multi-purpose aircraft which predated the Wilk.

[2] Low-wing Monoplane = Dolnopłat as opposed to Górnopłat (or high-wing monoplane) also known as the płatem polskim or Polish wing.

[3] There is no explanation for this 'retrograde' designation. The PZL-31 designation had previously been assigned to a since-abandoned bomber concept.

[4] Although the P.24s hinged canopy and raised rear decking was not adopted for the PZL.31a conversions.
« Last Edit: August 08, 2021, 10:38:18 AM by apophenia »
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Offline Old Wombat

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Re: Progress at Poland's PZL
« Reply #1 on: July 23, 2021, 03:26:47 PM »
An obvious change for these pre-production fighters was their larger, broaden windscreen (taken from the export P.24). [4] Note too that the spats have been removed (to avoid the buildup of mud and other debris which could jam the wheels).

(To be continued ...)

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Offline Robomog

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Re: Progress at Poland's PZL
« Reply #2 on: July 23, 2021, 06:04:43 PM »
Absolutly !   ;)

  Mog
>^-.-^<
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Re: Progress at Poland's PZL
« Reply #3 on: July 24, 2021, 02:53:22 AM »
 :smiley:
All hail the God of Frustration!!!

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Offline apophenia

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Re: Progress at Poland's PZL
« Reply #4 on: July 24, 2021, 07:23:08 AM »
Brian would be appalled! :o

He would  :-[

Additional votive offerings have already been laid at the alter of St. Brian of Bash ... 'cuz the PZL spat situation is about to become even more dire   :o
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Offline apophenia

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Re: Progress at Poland's PZL
« Reply #5 on: July 26, 2021, 12:49:41 AM »
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 ...)
________________________________________________

[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.
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Offline apophenia

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Re: Progress at Poland's PZL
« Reply #6 on: July 26, 2021, 10:50:50 AM »
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 ...)

________________________________________________

[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.
« Last Edit: July 26, 2021, 10:54:03 AM by apophenia »
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Offline ChernayaAkula

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Re: Progress at Poland's PZL
« Reply #7 on: July 29, 2021, 08:37:44 AM »
Wonderful designs all round.  :-*
Cheers,
Moritz

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Offline apophenia

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Re: Progress at Poland's PZL
« Reply #8 on: July 30, 2021, 07:07:37 AM »
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.
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Offline apophenia

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Re: Progress at Poland's PZL
« Reply #9 on: August 08, 2021, 10:35:48 AM »
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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