Although built in fewer numbers than the PZL P.11, the PZL P.24 was for a period during the 1930's the fastest and most heavily armed single-seat fighter in the world. Having acquired early notoriety at the Paris Salon with their innovative wing design, the P.24 represented the ultimate development of the family of fighter planes designed by Zygmunt Pulawski and saw service in the air forces of four countries: Bulgaria, Greece, Romania and Turkey.
When PZL (Panstwowe Zaklady Lotnicze, or National Aircraft Works) opened for production near Warsaw's Mokotow airfield in 1928, twenty-seven year old Zygmunt Pulawski was hired as the main designer. In 1925 he had graduated with honors from the Department of Mechanics at Warsaw Polytechnic and was awarded an apprenticeship at the Breguet airplane factory in France. Having graduated in 1927 from Air Officer Cadet Reserve School, where he earned his pilot's license, he was hired by the Central Aircraft Machine Shops in Warsaw, which later became PZL.
Pulawski's talent finally had a platform at PZL and his first project was to design an all– metal fighter plane. At the time, most planes were either wooden or mixed structure, with welded fuselages and wooden wings. Duralumin was used on very few planes.
Pulawski's concept fighter
Pulawski's original fighter design, the P.1, brought him and PZL recognition for its innovative gull-shaped wing (thin and canted or 'gulled' upwards at the fuselage, gradually becoming wider and leveling out at the wingtips with underwing struts) which ensured excellent forward visibility from the cockpit, great strength and light weight. They were later called "Pulawski wings" or "Polish wings."
Another design innovation was the "scissors" undercarriage. It allowed the shock absorbers to be hidden in the fuselage, thus reducing drag. The shocks were compressed by levers and rods in a system that is used to this day in airplanes around the world, including the Polish PZL-106 "Kruk" used in agriculture.
Pulawski's imitators around the world
There were many imitators of the "Pulawski wing" around the world with the most similar being the French-made fighters Loire 43 (1932), Loire 45 (1933) and Loire 46 (1934), the Czechoslovakian Aero A-102 prototype (1933) and the German Henschel Hs 121 (1934), all based on the Pulawski design. The "Pulawski wing" was also used on the following airplanes: the Yugoslavian Ikarus IK-1 (1935) and Ikarus IK-2 (1936), the French Mureaux 170 (1933) and Mureaux 180 (1934), Gourdou Lesseure GL-482 (1933) and the Arsenal-Delanne 10 (1940). In 1930 the American observation plane Douglas O-32 made use of the "Pulawski wing" with cable supports instead of the usual struts. The German Dornier Do-C1 used the "Pulawski wing" mounted on small posts away from the fuselage. In 1934 two planes, the British Armstrong Whitworth AW 32 and the Hungarian Weiss WM-18, were designed with the "Pulawski wing," but never made it into production. The only twin engine planes to be outfitted with the "Pulawski Wing" were twelve American Douglas O-37/B7's (1931).
Pulawski's gull wing design was also adapted for the following biplanes: Soviet fighter planes including the Polikarpov I-15 (1933, 450 built), I-153 Czajka (1938, 3,400 built) and the Nikitin-Szewczenko IS-2 (1941), the Italian fighter plane Romeo 41 (1933, 180 built) and seaplanes Romeo 43 (1935, 100 built) and Romeo 44 (1938, 25 built), the French seaplanes Romano R-90 (1935) and R-92 (1936), the Czechoslovakian Avia B-422 (1938), the British Westland F7/30 (1931), the American-built Curtiss F9c-3 Sparrowhawk (1932), the Canadian Gregor FDB-1 (1935) and the American-built Lamson L-101 Air Tractor ag-plane.
Pulawski's gull wing concept was also used on multi-engine seaplanes, although its purpose was different from that in the fighter; the gull wing design allowed the engines to be mounted further away from the waterline as seen on the British Short S-18 (1933), the German Dornier Do-26 (1938, 6 built), the American Martin Mariner (1939, 1,325 built) as well as the Soviet Berijew Be-6 (1949) and Be-12 Czajka (1963, 132 built).
A few concepts inverted Pulawski's idea with a low-wing design strutted upwards with the wings narrowing at the fuselage: the German Henschel Hs 125 (1935), the Soviet CKB-4 (TSz) and the Polish Szpak-2, -3 and -4 (1945-48, 13 built).
Gull wings were often used in the 1930's on stunt gliders to raise the frail wingtips thus avoiding damage during landings. Gull wings also increased the stability of gliders during rolls. It was first used on the German glider Fafnir in 1930. Some examples of Polish gull wing gliders are: CW-5 bis, SG-3 bis/36, SG-7, Orlik, Mewa, PWS-101, PWS-102 Rekin, PWS-103, WWS-2 Delfin, B38 and the Jastrzab.
To what extent did Pulawski's concept influence the development of airplanes around the world? Although his fighter design was state-of-the-art, its heyday was brief. Advances during the 1930s were so rapid that by 1936 cantilevered mid-wing designs with retractable landing gear were the design of the future. Pulawski's arrangement was used in only 730 high wing fighter planes, but close to 4,000 biplane fighters and almost 1,500 flying boats were built with gull wings. It can be safely stated that Pulawski influenced the construction of at least 6,000 airplanes and 2,000 gliders.
Pulawski's fighter planes with in-line engines
As a result of an initiative in 1928 from the head of the Department of Aeronautics in the Ministry of Army Affairs, Col. Ludomil Rayski, the newly opened PZL factory cut its teeth on a production license for the French Wibault 7 fighter plane. (Designer and engineer Michel Wibault would, 30 years later, outline a concept for vectoring thrust by means of nozzles, which evolved into the Pegasus engine powering the first successful VTOL plane, the Harrier). The Wibault 7's construction was innovative with a 0.4-0.5 mm duralumin skin strengthened with corrugations placed at regular intervals. In addition to this, Wibault, came up with the idea of using over-sized flat ribs in the wings with "shelves" for the skin to be riveted to the ribs instead of riveting the skin to the folded edge of each wing rib (which made for difficult riveting). This made it possible to rivet from the outside with easy access to the rivets. Pulawski used the idea in his own design with some improvements. He increased the number of corrugations in the plates and eliminated the raised ribs on the wing upper surfaces and control surfaces without compromising ease of riveting.
Pulawski incorporated the following ideas into his first fighter design: gull wings, Wibault's wing and control surface ribbing, a "scissors" undercarriage and a 600hp in-line engine that allowed for an aerodynamically refined fuselage design and better visibility than a radial equivalent. The PZL design office was so busy in the first half of 1928 getting the Wibault 7 ready for production that the documentation for the PZL P.1 was drawn up in the second half of the year. Together with the start of production of the Wibault 7, at the beginning of 1929 the workshop began construction on the first prototype of the PZL P.1. The tempo was fast, with the first machine being ready for test flying in August 1929. The first test flight was made by Boleslaw Orlinski, a well-known aerobatics pilot (225 loops during one flight) and experienced in long distance flying (22,600 km [14,043mi.] Warsaw – Tokyo – Warsaw in 1926). That first flight was dramatic and would decide the further fate of Pulawski's fighter plane. In order to reduce the weight of the plane the decision was made to use a new material, magnesium alloy, on all the wing and aileron leading edges. The alloy is lighter than duralumin but more brittle. When Orlinski had reached a sufficient altitude to test the plane's dive speed a section of the leading edge on the starboard wing crumpled, forming a flat surface much wider than the wing's thickness, which acted as a brake. As a result the wing lost a lot of lift and the damage-induced drag caused the aircraft to tilt and turn to starboard. The plane became very difficult to control and Orlinski would have been justified in using his parachute, but he was aware that such a catastrophe with the prototype would end all chances of further development on the plane. Fighting for control, Orlinski carefully nursed the plane back to Mokotow airfield and thus saved the prototype. The leading edges were replaced with duralumin and on September 25 Orlinski made a successful test flight. He climbed to an altitude of 2,620 m (8,595 ft.) in 3 minutes and 40 seconds for a climb rate of 11.9 m/sec. (2,340 ft/min) and attained a maximum speed of 295 km/h (183 mph) which at the time were excellent results for a fighter plane. A competing plane made at the time, the PWS-10, had a climb rate of 6 m/sec. (1,176 ft./min.) and a top speed of 241 km/h (149 mph). The first trials, however, indicated a need for some improvements. A second prototype was built in 1930 with modified vertical stabilizers and an elevator without aerodynamic compensation. Orlinski test flew the PZL P.1/II at the end of March, 1930, and in June Col. Jerzy Kossowski took the PZL P.1/II to the International Fighter Competition held in Bucharest, Romania. Although his bid was successful, Romania later abandoned its intention to purchase a fighter as a result of the country's weak economy. The competition in Bucharest was Pulawski's first international show, and his first success, but didn't insure the production of his fighter. The Polish authorities had decided that their air force would not utilize in-line engines, and that such engines would not be built in Poland.
Unwilling to abandon his idea of a fighter with an in-line engine, Pulawski designed an experimental model designated the PZL P.8 powered with a 600 hp engine. It was test flown by Orlinski in the summer of 1931 and reached a maximum speed of 330 kph (205 mph). A second prototype, the PZL P.8/II with a 675 hp engine made its first flight in March, 1932, and reach a top speed of 350 kph (217 mph). In July, 1932, Orlinski was to fly the PZL P.8/II to the International Air Meeting in Zürich, but crashed near Konskie, Poland. A few days later he made a second attempt in the PZL P.8/I but crashed near Innsbruck, Austria. In December, 1932, the rebuilt PZL P.8/II was exhibited at the Paris Salon. When in 1935, Yugoslavia showed interest in buying the plane, improvements and a new model, the PZL P.28 were proposed by the factory but the order never came to fruition.
Pulawski's fighter planes with radial engines
At the end of 1929 the Department of Aeronautics in the Ministry of Army Affairs had chosen the Bristol Jupiter radial engine and all subsequent models for its fighter planes. It was well known that radial engines were simpler to construct, lighter, and air-cooled (which further reduced the weight by another 40 kg, or 88 lbs, of water). They were easier and cheaper to use, more resistant to damage from enemy fire (on account of the fact that they did not feature a water cooling system susceptible to small arms fire), and required less fuel. In addition, the radial construction moved the center of gravity much closer to the center of the airplane, which had a positive effect on maneuverability. The main drawback, though, was their large diameter. This had a negative impact on forward visibility and, as a result of the widened fuselage cross section, increased drag considerably. The license agreement with Bristol was valuable in that it gave the Poles rights to build all Bristol engines designed within the next 10 years, but also contained a clause that none of the engines could be built for export. This complicated the sale abroad of Polish designed fighters utilizing these engines.
In spite of his conviction that in-line engines gave the pilot better visibility, Pulawski was forced to accept the situation as it was. In the fall of 1929 the Polish army had placed an order for Pulawski's plane, but with a radial engine. Pulawski immediately set about designing the new plane, which was called the PZL P.6. The new engine forced him to redesign the fuselage in an "O" shape. The new shape made it possible to use a semi-monocoque design. Changes were made to the wings, control surfaces and undercarriage as well. Basically, the PZL P.6 was a completely new design, which utilized concepts and experience gained during the construction of the PZL P.1. By using an engine that was 175 kg (385 lbs.) lighter and an airframe that was 60 kg (132 lbs.) lighter the PZL P.6 weighed 235 kg (518 lbs.) less than the PZL P.1. During the planning stages there was a proposal to construct two variants of the plane: one with a 450 hp Jupiter VI engine for use at low altitudes and a second with a Jupiter VII which would give the plane a cruising altitude of 4,000 m (13,123 ft.). The low altitude version would keep the P.6 designation, while the high altitude version would get the P.7 designation.
The PZL P.6 prototype was flown by Orlinski in August, 1930. It was easy to handle and achieved a top speed of 292 kph (181 mph) and a climb rate of 10.3 m/sec. (2,028 ft./min.). The P.6 was exhibited at the Paris Salon in December 1930, where it drew attention for its gull wing design and duralumin semi-monocoque construction. On December 22 Orlinski flew an amazing display routine in the P.6 at Le Bourget airfield. It was not long before the international aeronautical press was proclaiming the P.6 the best fighter plane in the world. In August, 1931, Orlinski took part in the National Air Races in Cleveland, Ohio and competed against the best aerobatic pilots in the world with Udet representing Germany, Bernardi – Italy, Atcherly – England, Williams – U.S.A. and Kubita – Czechoslovakia. Orlinski's daring aerobatics brought him personal success and enthused the crowd. The local press wrote that "Orlinski delighted the crowd and won respect for the Polish airplane industry." Without a doubt Orlinski's two appearances brought fame to PZL and Pulawski. The P.6 prototype was destroyed in a crash in October 1931, outside of Olsztyn (Czestochowa), when the propeller and engine fell off. Orlinski was able to parachute to safety. Only one prototype of the PZL P.6 was built and no further work was done on the plane.
The PZL P.7 was first flown by Orlinski in October 1931, at Mokotow airfield in Warsaw. It was fitted with a 520 hp engine providing 70 hp more than the P.6 and was capable of a maximum speed of 317 kph (197 mph) at 4,000 m (13,123 ft.). After intensive testing of two P.7 prototypes some modifications were made and the design was sent to production. The wings and control surfaces were fitted with a new type of corrugated skin. Out of 150 P.7a planes ordered by the Polish army the first were ready in the fall of 1932 and the last were delivered in November 1933. With the P.7 the Polish air force became the first in the world to be equipped with an all-metal fighter plane.
It was assumed at the beginning of the Thirties that the average speed for fighter planes would increase by 10 kph /year and the Department of Aeronautics was aware of the fact that as soon as a fighter was introduced its successor would have to be on the drawing board. When Bristol designed the new Mercury engine developing 600 hp in 1930, which Poland had the rights to build, the Department of Aeronautics ordered the P.6 to be adapted to this engine. During production of its prototype, designated P.11, in March 1931, Pulawski was killed in an accident while flying the PZL.12 amphibious plane, also one of his designs. Further work on the PZL P.7 and P.11 was directed by Wsiewolod Jan Jakimiuk, Pulawski's deputy. Jakimiuk emigrated to Canada after the war and designed the DHC-1 Chipmunk training plane in 1946 (1,292 built), the DHC-3 Beaver transport plane in 1947 (1,718 built). In 1948 he designed the DH.112 Venom sea fighter plane for England (381 built) and in France the SE 5000 Baroudeur fighter plane.
The first PZL P.11 prototype was similar to the P.6 and was flown for the first time by Orlinski in August 1931. The P.11 was modified to reflect advances made on the P.7 and newer prototypes were tested for the next two years (since the P.7a had just entered service, there was apparently no need to speed up development). When Romania showed interest in buying a fighter from PZL a fourth prototype, the PZL P.11/IV, was fitted with a 550 hp Gnôme Rhôme K9 and sent to the Paris Salon in December 1932. At the beginning of 1933 the Polish Ministry of Army Affairs ordered 50 P.11a fighters, while in April Romania ordered fifty P.11 fighters with the Gnôme-Rhône K9 engine. The decision was made to give the Romanian order for fifty PZL P.11b fighters priority over the order from the Polish government. The first forty-nine P.11b fighters and plans for the P.11f (to be built under license) were delivered in the first quarter of 1934. An improved version, the P.11c was built in a series of 150 fighters between the fall of 1934 and the summer of 1936. Romania built ninety-five P.11f fighters between 1937-38. The last stage of development for the P.11 was the P.11g Kobuz prototype with a 800 hp Mercury VIII engine, and production was scheduled to begin in the fall of 1939.
The PZL P.24 fighter plane
Foreign interest in Pulawski's fighter planes led to the development of special versions for export. Obviously the Bristol engines couldn't be used, since the British didn't want their newest technologies to be sold abroad, and declined to allow Poland to sell licensed engines. France, on the other hand, was very willing to sell its newest engines, and so the Gnôme-Rhône was chosen as the export engine for PZL fighter planes. For Gnôme-Rhône it was a chance not only for export. They hoped that the PZL fighter would be chosen for the French army air force and their orders would grow. In the spring of 1932 the director of PZL, Witold Rumbowicz, while in Paris, received a formal proposal from Gnôme-Rhône to use their engines in Pulawski's fighters. Gnôme-Rhône gave Rumbowicz an engine for fitting in a prototype which Wsiewolod Jakimiuk, head of design since Pulawski's death, would adapt for the P.7 and P.11. The new plane was assigned the number P.24 and was fitted with the 14 cylinder 760 hp Gnôme-Rhône 14Kds. Construction of the first prototype began in July 1932.
Much of the P.24 prototype came from the P.7a already in production. Unchanged were the wings, rear fuselage, horizontal stabilizers and elevators, rudder and vertical stabilizer. The undercarriage was slightly raised to accommodate the changes made to the forward end of the fuselage and the increased weight and speed of the plane. To correctly locate the center of gravity with the heavier engine, a 45 cm section was added between the wing mounts and the cockpit. The cockpit was redesigned and the pilot's seat was raised 50 mm. The wings were not strengthened, as static trials showed that the wing could withstand an ultimate load factor much greater than the required 12.8. With a total weight of 1,420 kg the wing's limit load factor was 19 and at a factor of 12.8 would have been acceptable for a plane weighing 2,100 kg. A slight modification was made to the wing by thinning it at the fuselage to fit a machine gun mount and the P.7 flaperons were eliminated. Because a 14 cylinder engine doesn't have gaps between the cylinders it was impossible to mount machine guns on the fuselage, as on the P.7 which had barrels fitted between the cylinders. With the mounting of two Swiss-made 20mm Oerlikon canon the P.24 was one of the most advanced fighter designs of its day. Flaperons were adapted from the P.1 (which were shorter than those on the P.7) and could be used as flaps to reduce the take-off and landing speed. A larger stabilizer was developed to compensate for the more powerful engine and longer fuselage and the oil shock absorber on the tail skid was changed to a rubber one.
Boleslaw Orlinski first test-piloted the P.24/I prototype in May, 1933, at Mokotow airfield in Warsaw. An interesting note is that although the plane belonged to PZL it was painted in the khaki color of the Polish air force, and carried the red and white checkerboard military insignia. This resulted in the "myth" that the Polish air force actually put the PZL P.24 into service, which is untrue, and the national insignia were later removed. During the first flight Orlinski achieved a climb rate of 12 m/s (2,340 ft/min) but flew too long at full throttle with the supercharger running and as a result of the vibrations the wooden propeller cracked and broke off. When Orlinski landed the mechanics found that only one of the engine mounts was still holding. Both the engine and airframe needed overhauling. During subsequent trials fairings were mounted on the wheels, a pitot tube was mounted under the wing and ventilation openings were introduced to the front of the fuselage underneath the exhaust manifold. Flight testing on that prototype ended in February, 1934.
The PZL P.24/II was fitted with a Gnôme-Rhône 14 Kfs engine which output a nominal 900 hp and maximum 930 hp. At first a two blade wooden propeller designed by Szomanski was mounted, later to be replaced by a Gnôme-Rhône metal three blade propeller. It was test flown in June, 1934, by Orlinski and named the "P.24. Super" On June 28, 1934, Orlinski set a new world speed record for radial engine fighter planes overseen by the FAI. At first a speed of 404 km/h (251 mph) was announced but when adjustments for atmospheric conditions were made a speed of 414 km/h (257 mph) was officially given. Orlinski later reached 416 km/h (258.5 mph). During flight testing it turned out that the oil was overheating and a cooler was added to the fuselage under the starboard wing and in the summer of 1934 the armament was installed. Two Vickers 7.9 mm machine guns were mounted forward of the cockpit and two 20 mm Oerlikon cannon were mounted in fairings under the wings.
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