Zero-Sen Inspiration
The war in China threatened to become interminable because of the great distances and the resistance offered by the Chinese Air Force (CAF).
Every year Japanese air superiority practically annihilated CAF during the summer months, but the Chinese rebuilt it during the winter by importing planes and pilots.
The reorganization was carried out in some Western bases far from the combat zones and the long-range attacks carried out by the Mitsubishi Ki.21 and G3M Japanese bombers were systematically repulsed by Chinese fighters manned by mercenary pilots.
To keep in touch, the Ki.27 and A5M2 fighters were equipped with belly fuel tanks that increased its operational range. They used to escort the bombers at a distance, but its extended range was insufficient to reach the bases furthest from the CAF.
In February 1939, bomber losses reached 10 per cent and the offensive was limited to night attacks against Chungking.
The special conditions of the war in China influenced the drafting of the Imperial Japanese Navy specification 12-Shi (May 19, 1937), calling for a long-range escort fighter with 500 km/h top speed, capable of protecting the bombers during the journey to the target and back to their bases.
In January 1938, veteran pilots of 12th Kokutai led by the Lieutenant-Commader Minoru Genda, demanded that the new fighter had light weight and maneuverability equivalent to that of the Mitsubishi A5M, but the IJN had good reason to recommend the use of 20-mm guns.
During the 30s, the Western powers that constrained the Japanese territorial expansion were developing four engine heavy bombers. The Tupolev TB.3, capable of reaching Tokyo taking off from Soviet territory, flew for the first time in 1930. In 1932 the French Farman 222 also performed its first flight; although ugly as all its kind, it was very effective to control navigation in the area to the south of Formosa, operating from Saigon. The Boeing B-17 prototype was on the air in 1935 and the British Sunderland and Stirling prototypes, capable of transatlantic flights, followed in 1938 and 1939. Even the Dutch worked on a four-engine version of the Fokker T-IX, named T-VI /115.
The B-17 had a 2,000 miles range, turbocharged engines and such a powerful armament that allowed this airplane autonomous defence. When the Imperial Japanese Army (IJA) was aware of these excellent performances, they realized that the American bomber was above the interception capabilities of the standard fighter Ki.27.
Given the potential threat that the B-17s based in Philippines could represent, the Imperial Japanese Navy (IJN) considered the American bomber, would attack Japanese naval bases on the periphery of the Empire.
In 1916, the French developed the SPAD S.XII fighter for the destruction of balloons. It was armed with a single-shot SAMC (APX) 37-mm cannon mounted between the V8 cylinder blocks of the 200 hp Hispano-Suiza 8c engine.
The moteur-canon was not very successful, due to its slow reloading capacity and excessive recoil shock, and the SPAD S.XII was abandoned, but the French retained the idea until the arrival of the right time. This came in 1932 with the 690 hp liquid-cooled engine Hispano-Suiza H.S.12 Xbrs of 12 cylinders in 60º 'Vee' configuration.
During the 20s, the Swiss firm SEMAG committed itself to the redesign of the 20-mm cannon Becker used during the WWI by some German bombers. During the first 30s, the research was continued by the Swiss firm Oerlikon who started to commercialize three basic models using cartridges of progressive length and power. They were the Oerlikon FFF type (72-mm case length), the Oerlikon FFL type (100-mm case length) and the Oerlikon FFS type (110-mm case length).
The Hispano-Suiza H.S.12 Xbrs was the right engine to combine with one 20-mm Oerlikon FFS cannon (manufactured under license as Hispano-Suiza H.S.7) mounted between the two-cylinder banks. The crankcase was strengthened, and the reduction gear modified to bring the hollow propeller shaft in line with the gun barrel.
The ammunition was the same than that of the Swiss gun, contained in a 60 rounds drum. The H.S.7 was heavier, due to the fixation system to the engine, and had a rate of fire of just 350 rpm (compared to the 470 rpm of the Oerlikon) to protect the engine from any destructive vibration. This engine was then known as Hispano-Suiza 12 Xcrs (‘c’ for canon = cannon) and had reduced its power to 680 hp, consequence of the modifications made in the gearbox. The new moteur-canon was installed in the Dewoitine D.510 fighter.
In 1935 a single D.510J, designated AXD1, was supplied to the IJN for evaluation.
The H.S.7 was a formidable weapon when integrated in a Hispano-Suiza engine, however, it was less resilient when installed in the wings of the single engine fighters, causing different problems of vibration, stoppage, freezing and dispersion of firing. And it was practically useless for the Japanese because it was not possible to install it in radial engines.
The FFS was acquired by l’Armèe de l’Air to serve as a base for its own Hispano Suiza H.S.7 and H.S.9 designs which were both modified for aircraft engine mounting.
The FFF was chosen by the German, Polish and Romanian Air Forces to equip the Bf 109 E-2/E-3, Bf 110C, PZL P-24 C/F/G and IAR 80 fighters.
The FFL was acquired by the IJN in 1939 and manufactured under license by Dai-Nihon Heiki Company, as Type 99-2.
The designers in charge of the 12-Shi project knew that it would be necessary to build an extraordinarily light aircraft capable of carrying a large amount of fuel without losing performance, but the installation of cannons in the wings forced them to use very strong structures to absorb the recoil.
On November 16, 1935, l’Armée de l’Air issued the C.1 specification calling for a French single-seat fighter with 450 km/h top speed, armed with two 20-mm cannons.
This Chasseur Bicanon specification was divided into two categories: a light version equipped with 450-500 hp engines and a heavy version with 800-1,000 hp engines.
To compete in the first category, the Caudron C.710-01 prototype was built, which made its first flight on July 10, 1936, for the second category the Marcel Bloch firm built the MB.150. The prototype was scheduled to fly on July 17, 1936, but the test was delayed due to a broken landing gear.
During the summer of 1936, the IJN sent Commander Misuo Wada to Europe with the mission of investigating how the French had solved the problem of installing guns on the wings of single-engine fighters.
The Caudron was available for export, but the wooden structure of the wings was incompatible with the 12-Shi specification.
The Bloch's airframe was all-metal, but the wing design proved to be so poor that the manufacturer decided to redesign it, and the modified aircraft did not fly until December 1937.
After the failure of his mission, Wada requested information from the Swiss Antoine Gazda, sales director of the firm Oerlikon, and managed to find out that the American Vought V-143 prototype was equipped with a Northrop wing of great structural resistance that according to the builder could admit the installation of cannons.
In November 1936, the Japanese firm Mitsui based in New York began negotiations with Eugene E. Wilson, president of United Aircraft Transport Corporation and Chance Vought Corporation, for the acquisition of the V-143.
On January 4, 1937, the prototype was presented to a Japanese delegation at the East Hartford factory.
On April 26, 1937, a Japanese technical commission led by Misao Wada met with Eugene E. Wilson and Vought's director of engineering, C.J. McCarty, to inquire about the possibility of installing the Swiss Oerlikon FFL cannon on the wing of the V-143, the response was positive. Months later, Wada participated in the drafting of the 12-Shi specification.
On June 18, 1937, a negotiating commission consisting of IJN officers, Mitsui lawyers, and Mitsubishi engineers visited East Hartford to formalize the acquisition of the prototype.
On September 8, the commission also acquired the production rights of the V-143 from the Douglas firm, which owns the project.
In December all documents and technical drawings, including the procedure for installing the guns, were sent to Mitsui's offices.
When the prototype reached Japan in late 1937, it was designated AXV1and was flown in mock combats against Nakajima Ki.21 and Mitsubishi A5M fighters, but mass production was not considered.
When the Heinkel He 112 V1 prototype made its first flight in December 1935, the IJN became interested in the new aircraft because of its low wing load, robust landing gear and excellent visibility during landing, due to the Vee inverted design of its Jumo 210 engine, which constituted an excellent combination of factors for operating from aircraft carriers.
But the Heinkel fighter was not for sale because the manufacturer hoped to build it in large series for the Jagdwaffe.
When the competition was won by the Messerschmitt Bf 109, the RLM authorized the construction of a Heinkel series dedicated to export.
In the late spring of 1937, the prototype He 112 V9 was revealed to a Japanese military delegation.
By this time, the Mitsubishi A5M could already operate from aircraft carriers and was achieving air superiority in China, but the IJN's interest was directed towards the coaxial mechanism that allowed the He 112s of the pre-series A-0 to fire a 20-mm cannon through the propeller hub.
To study this new technology, the Japanese delegation decided to acquire the He 112 V5 prototype (Werk. Nr. 1951, D-IIZO) fitted with open cockpit, two bladed VDM/HWK propeller, one Jumo 210 Da engine and one engine-mounted Rheinmetall-Borsig MG C/30L 20-mm cannon with 100 drum-feed 20 x 138B rounds and 300-350 rpm rate of fire. The aircraft was shipped to Japan in December 1937.
In January 1938, the RLM authorized the export of the He 112 B-0 series and the IJN acquired six aircraft equipped with enclosed cockpit and Jumo 210 C engines.
This operation allowed the Japanese to access the Dural/Electron technology for airframe construction and the structural solutions developed in Germany for the installation of Oerlikon FFF 20-mm cannons on elliptical wings.
The export license also allowed the IJN to study the secrets of the Daimler-Benz DB 600 engine by acquiring the He 112 V11 prototype in May 1938.
When the Sudeten crisis ended, the RLM allowed the export of the He 112 B-1 series and the IJN acquired twenty-one units, powered by Jumo 210 Ea engines, which were delivered throughout 1938.
In Japan the He 112 was marked A7He1 and tested in mock combats against the Mitsubishi A5M2. During these trials the German fighter proved to be faster but much less maneuverable than the Japanese fighter. The pilots of the 12th Kokutai refused to fly it in combat over China and the IJN decided to cancel the acquisition of a series of 100 machines planned for 1939.
To achieve the performance demanded by the 12-Shi specification, at the end of January 1938 the Mitsubishi design team led by American trained engineer Jiro Horikoshi, began working on the concept of a low wing monoplane fighter of all-metal construction with enclosed cockpit and retractable landing gear.
The main guideline of the project was to reduce the weight of the airframe as much as possible and the design team decided to take inspiration from the construction system of the Northrop Gamma 5A acquired in October 1935 by the IJN: flush-riveted stressed-skin structure with reinforcing rings and longitudinal stiffeners.
The two-spar high aspect ratio wing, with NACA B-9 (23012) airfoil, was built in one piece with the fuselage center section built as an integral part of the wing.
The front and rear fuselage sections were removable, the long semi-monocoque fuselage provided longitudinal stability and a very stable firing platform for 20-mm cannons.
To lighten the airframe, structure engineers Yoshio Yoshikawa and Yoshitoshi Sone used ESD aluminum alloy in the wing spar and the lightest gauge of SD aluminum possible in the skin.
The SD was equivalent to the American alloy Alclad 24 ST and the ESD was a zinc/aluminum alloy equivalent to the American alloy 75S (7075).
These construction materials were the product of technology transfers between Hamilton-Standard y Sumitomo Metal Industry.
The engineers Denichiro Inoue and Shotaro Tanaka decided to use the same chassis as the Vought V-143 in the power plant installation.
The cylindrical engine cowling used from October 1936 by the Nakajima Ki.27 fighter proved to generate less drag than the cowling used by the Mitsubishi A5M from 1935 and Japanese engineers decided to use a Wright cowling developed in 1934 for the Northrop Gamma 3 (XFT-2) fighter.
The constant-speed Sumitomo propeller was a result of a cooperation between Sumitomo and Hamilton Standard (CS 40B).
Instruments and engine accessories were license-built Sperry, Bendix and Kollmann.
In 1937, winter operations in China necessitated the installation of an enclosed cockpit on the Mitsubishi A5M2b fighter, but the design was unfortunate because it limited the pilot's rear visibility during combat.
The streamlined cockpit hood developed for the Nakajima Ki.27 was much more aerodynamically efficient but was too narrow for the pilot to turn his head to watch the rear hemisphere, often the center section of the canopy was removed.
To solve the problem, Horikoshi decided to use a 360-degree-view tear drop canopy, with drag-reducing properties, inspired by that of the Westland Whirlwind prototype that was presented to the British media in October 1938.
By early 1938, the Japanese had acquired four models of aircraft with retractable aircraft abroad landing gear:
-Northrop Gamma 5D (c/n 291) in September 1936.
-Seversky 2PA-B3 in March 1937.
-Vought V-143 in September 1937.
-Heinkel He 112 V5 in January 1938.
Engineers Sadahiko Kato and Takeyoshi Mori, in charge of the design of the 12-Shi fighter landing gear, discarded the system used by Seversky because it was not completely retractable and generated too much drag penalizing the range.
They also did not consider the German system suitable because it had been designed for an aircraft with W-wings configuration and the main members were too short and heavy.
The hydraulically operated, completely retractable inwards gear used by the Gamma bomber it was calculated for an aircraft larger and heavier than a fighter and it was estimated that its adaptation to the 12 Shi would have required a lot of time and design work.
It was finally decided to copy the system used in the V-143, improving it aerodynamically so that the wheels were completely hidden inside the wings.
In October 1936, the Heinkel He 51B-1 biplane fighters began using a ventral drop tank, with a capacity of 50 liters, during their combat operations in Spain.
The design team considered the German system to be more aerodynamically effective than the one used in China by the Japanese Ki.27 and A5M fighters and decided to equip the 12-Shi with a light alloy/plywood ventral tank with a capacity of 330 liter.
Construction of the 12-Shi prototype began during the summer of 1938, and it was completed in February 1939.
On April 1, 1939, the aircraft was flown under the IJN denomination Mitsubishi A6M1 Zero-Sen.
The new fighter weighed 25 percent less than the Grumman F4F and was tested in mock combat against Heinkel He 112 B-0, Seversky 2PA-B3 and Vought V-143 fighters.
Zero was already flying 2,250 km missions eighteen months before the attack on Pearl Harbor.
Early in 1941, P-40 fighters from the American Volunteer Group entered combat over China against Zeros of 12th Kokutai.
Warnings had repeatedly been sent to Washington since the appearance of the new Japanese fighter in China. General Chennault and French observers in occupied Indochina sent back sketches and data.
In early 1942, Eugene E. Wilson was invited to examine the wreckage of a Zero from the Akagi carrier, marked AI-154, shot down during the attack on Pearl Harbor, Wilson identified the engine mounting and the undercarriage as the technology used in the construction of the Vought V-143 and Vought Chief Engineer Fred Dickerman was of the same opinion.
In the spring of 1941, the Chinese managed to shoot down an A6M2 Model 21 over Chengdu, very accurate data sheet and three-view drawings were delivered to the U.S. Navy Department and the U.S. War Department, but experts did not consider the data to be credible, and the report was ignored. Nobody in the Western countries believed that a fighter with these characteristics could exist until the attacks to Pearl Harbour, Philippines and Java.
On November 26, 1941, two A6M2 fighters from 22nd Kokutai, marked V-172 and V-174, landed by mistake on the Liuchow Peninsula and were captured by local Chinese militia.
The V-174 was partially destroyed during landing, but the V-172 (c/n 3372) was captured intact and was repainted with the olive drab/pale grey camouflage scheme and markings of the C.A.F. and marked P-5016.
During the summer of 1942 it was revealed to the USAAF, tested by AVG pilots and turned over for a complete overhaul to the technicians of the 23rd U.S. Fighter Group. The Japanese fighter was then sent to the United States for further evaluation but was damaged during the voyage and did not arrive at Eglin Field until September 1943.
On June 4, 1942, a Zero from the aircraft carrier Ryujo strafing shipwrecks near Dutch Harbor, was hit by ground fire and crash-landed in a swamp off Akutan Island.
Six days after the accident, the Japanese fighter wreckage was located by a PBY flying boat of VP-41 Sqn, yanked out of the swamp and taken to North Island Naval Air Station, San Diego, there the Zero was repaired and flight-tested between September 20 and October 15, 1942.
During testing, American pilots discovered that the Zero had superior maneuverability only at the lower speed used in dogfighting, with short turning radius and excellent aileron control at very low speeds. It rolled to the left much easier than to the right, however the ailerons froze up at speeds above 200 knots, and the engine cut out under negative acceleration due to its float-type carburetor.
The captured aircraft was tested in mock dog fights with American fighters P-38, P-39, P-40, P-51, F4U and F4F and the best combat tactics against the Zero were developed for each type.
As a result of these investigations on October 19, 1942, the U.S. Navy Bureau of Aeronautics sent to War Department a memorandum giving Zero performance data.
On November 4, the U.S. Navy released the Technical Aviation Intelligence Brief No. 3 “Performance and Characteristics Trials. Japanese Fighter”.
In December 1942, the U.S. Army published the Intelligence Summary No. 85.
In February 1944 he was captured at Kwajalein a Japanese manual of the Zero’s flight characteristics, dated October 1943.
Several sources claimed that the Zero was copied from the Northrop FT-1, the Vultee Vanguard, the Gloster F.5/34 or the Chance-Vought V-143.
These data were published by the U.S. Popular Press in alarming terms that forced the government to create the Pearl Harbor Committee to find out if the Zero was a copy of the V-143 prototype sold to Japan by Douglas and Vought.
In 1946 journalist Drew Pearson published documents from the Japanese Export-Import Company dated 1938 that revealed Commander Wada's performance and his interest in mounting cannons on the wings of the V-143.
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