The P.Z.L. P.45 Sokol (Falcon) was a lightweight fighter designed to replace or supplement the increasingly outdated P.Z.L. P.11 that had almost reached the prototype stage when the Germans invaded Poland in 1939.
The idea of producing a lightweight fighter came from Kazimierz Korsak, a member of P.Z.L.’s fighter team, who was inspired by the success of the lightweight and low powered RWD 9 and P.Z.L.26 touring aircraft in the Challenge International de Tourisme 1934, which was won by the Polish team.
During 1935 Korsak came up with a design for all metal low wing monoplane, to be powered by a 400-600hp inverted vee engine, and with an emphasis on dog fighting and manoeuvrability instead of speed or heavy firepower.
In 1936 Korsak was given permission to carry out a preliminary design study, and the new aircraft was given the designation P.45 and named as the Sokol (Falcon). Three versions were studied.
The first had a fixed undercarriage and used either a 420hp Ranger SGV-770 or 500hp Ranger B-6 engine and an estimated top speed of 260.9mpg at 3,280ft.
The second used a 450-600hp P.Z.L. Foka air cooled inline V engine, which was then under development and a fixed undercarriage.
The third used the same Foka engine but with a retractable undercarriage. The first version would also have been able to swap its Ranger engine for the Foka.
Both of the possible engines soon ran into problems, with the Foka engine suffering a series of problems that eventually led to the cancellation of the Foka A, while attempts to begin licence production of the Ranger engine in Poland weren’t going well. As a result the Polish Aviation Command decided to focus on the P.50 Jastrzab, a rather more conventional fighter powered by the Bristol Mercury radial engine.
Towards the end of 1936 the Poles received details of the new 700hp Gnome-Rhone 14M Mars double row radial engine. Negotiations for the licensed production of this aircraft in Poland were more promising, and fifty were ordered from Gnome-Rhone for use in the L.W.S.3 Mewa (Gull). Korsal began to revise the P.45 to use this small radial engine, which had a diameter of only 0.96m, making it perfect for the lightweight fighter.
Some progress was made during 1937. A scale model was built and tested in the wind tunnel of the Aerodynamic Institute in Warsaw. Two one-third scale model wings were produced to test two different aerofoil sections. Detailed theoretical analysis was carried out on the Ranger and Mars powered versions of the aircraft.
Work began to speed up in 1938. The P.50 was starting to run into problems, so the Aviation Command was interested in an alternative design. At the same time the Air Force wanted bigger reserves for their fighter squadrons, which were starting to run short of P.Z.L. P.11s. One solution was to produce the P.45 and use it to replace the P.11 in fighter training units, and as an emergency fighter.
In the autumn of 1938 the construction of two prototypes was ordered, the P.45/I with a fixed undercarriage and the P.45/II with a retractable undercarriage. A gull scale mock-up of the P.45/I was built by January 1939. Construction of the P.45/I prototype was approved. At the same time P.W.S. and D.W.L. were asked to produce designs to fit the same requirement, coming up with the P.W.S.42 and the RWD 25, both of which used the same Mars engine. All three aircraft were to be armed with four guns, and they were to be evaluated in the spring of 1940.
In June 1939 the first Mars engines were delivered. In August it was decided to put the P.45A into production at P.Z.L. and P.W.S., well before the original trials were due. By the start of September work on the prototype was quite far advanced and it was expected to be completed by November. However the German invasion stopped all work, and the P.45/I never flew.
The P.45 would have been a low wing cantilever monoplane of all metal construction. The wing would have been built in three sections, with the inner section forming part of the fuselage. Two guns would have been carried in the inner wing section and two in the fuselage, for a total of four 7.7mm KM Ws 36 guns. It would have had a fixed undercarriage with streamlined fairings and spats over the wheels, although a retractable undercarriage was under development. It was to be powered by a 660hp-730hp Gnome-Rhone 14M05 Mars 5 radial and the estimated top speed was 323.1mph at 11,482ft.
P.45/ I (performance date estimates)
Engine: Gnome-Rhone 14M05 Mars 5 radial
Power: 660hp at 11,975ft, 730hp at 11,975ft
Span: 39ft 10.25in
Length: 25ft 10.25in
Height: 8ft 4.75in
Empty Weight: 2,440lb
Loaded Weight: 4,277lb
Maximum Speed: 323.1mph at 11,482ft
Guns: Four 7.7mm machine guns