Mitsubishi Ki-46 'Dinah'

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The Mitsubishi Ki-46 'Dinah' was the Japanese Army's main reconnaissance aircraft of the Second World War, and was one of the most aerodynamically perfect aircraft of its era.

The Ki-46 was designed to replace the Mitsubishi Ki-15 'Babs', a reconnaissance aircraft that entered service in May 1937. Although the Ki-15 was a successful design it was already clear that it was not fast enough to elude the new generation of monoplane fighters, and so in 1937 officers from the Technical Branch of the Koku Hombu (Japanese Air Army Air Force HQ) began work on a specification for a replacement.

On 12 December the specifications were given to Mitsubishi. The new aircraft was to be able to cruise at 250mph between 13,125ft and 19,685ft for up to six hours. It was to have a top speed of 373mph at 13,125ft. Defensive armament was limited to a single rear-firing machine gun as the aircraft was designed to avoid combat. Mitsubishi were given a choice of three engines and could use one or two engines at their choice.

Mitsubishi had already done some work on a design for a twin-engined reconnaissance aircraft during 1937. This had evolved from their work on a design for a twin engined fighter (the Ki-39) and had been given the designation Ki-40. Neither of these aircraft progressed beyond the initial design stage, but when the new specification was received the project was revived and became the basis for the Ki-46.

Tomio Kubo, the designer of the Ki-46, had a very difficult task. The top speed of the new aircraft was to be faster than that of the new Hawker Hurricane I or Curtiss P-36A, and Japan lacked the sort of high power liquid cooled engine then associated with high speed aircraft. Tomio selected the Mitsubishi Ha-26 radial engine, which had the smaller diameter of the available engines, and paired it with a new cowling designed by the Institute of Aeronautical Research at the University of Tokyo. This both reduced drag and improved the pilot's sideways view from the cockpit. The University also carried out some work on the fully retractable landing gear.

To improve the performance of the aircraft it was given thin wings and as small a fuselage as possible. The fuel tank was placed close to the centre of gravity, with the pilot in front of it and the radio-operator/ gunner behind.

The first prototype was completed by November 1939 and made its maiden flight later in the month. Its maximum speed, of 335.5mph, was lower than specified, and lower than that of the most recent fighter aircraft, but it was still faster than the Japanese Army's newest fighter, the Ki-43-I, or the Navy's A6M2, and so it was ordered into production, while at the same time Mitsubishi were ordered to work on installed a more powerful engine in the aircraft.

The Ki-46-I entered service as the Army Type 100 Command Reconnaissance Plane Model 1. It was used for pilot training and service evaluation, with mixed results. It was a much more advanced aircraft than the Ki-15, and was harder to maintain in the field. Its climb rate was not as fast as expected, and the controls were sluggish and sometimes unresponsive. However as the Ki-46 was expected to use its speed and altitude to avoid combat none of these problems were particularly important.

The Ki-46-II, with more powerful engines, appeared in March 1941, and immediately reached its expected speed. It was this version of the aircraft that entered service in July 1941, and it remained in use to the end of the Second World War. Early combat experience came in China, where the aircraft's high speed and lack of opposition meant that it was able to operate in total freedom over China.

The same was true in the early part of the Second World War. The obsolescent fighter aircraft available to the Allies in the Far East lacked the speed and rate of climb to reach the Ki-46. Radar might have made the difference, but that wasn't available early in 1942. After pre-war flights over Malaya on 20 and 22 October 1941 the Ki-46 saw service on just about every front in the Pacific, operating in small packets. A small number were taken over by the Navy, and operated as far south as Northern Australian from bases on Timor, while Army aircraft reached the Bay of Bengal.

Despite this early success the JAAF realised that the Allies would eventually catch up, and so in May 1942 Mitsubishi were ordered to produce a faster version of the aircraft. The Ki-46-III was given a more powerful engine and a revised canopy, and entered service during 1943. By this time the first P-38Fs and Spitfire Vs had reached the Far East, and the -II was beginning to become vulnerable. The -III restored the situation, and was still able to operate in relative safety well into 1945 - the last regular duty performed by the Ki-46-III were a series of reconnaissance flights over the 20th Air Force bases on the Marianas Islands. Even with the high performance radar-assisted fighters of 1945 the Allies found it difficult to catch the Ki-46 (although the aircraft was very vulnerable once it had been caught). 

Ki-46-I

The Ki-46-I was the first production version of the aircraft, and was powered by two 900hp Mitsubishi Ha-26-I radial engines. It was not fast enough for front line service, but 34 were produced for training and evaluation before production switched to the Ki-46-II.

Ki-46-II

The Ki-46-II was the main operational version of the aircraft. It was powered by two Mitsubishi Ha-102 radial engines, a development of the Ha-26 but with a two-speed two-stage supercharger. The first -II made its maiden flight in March 1941, and achieved a top speed of 375mph at 19,000ft, above the original specification for the Ki-46. The Ki-46-II entered service in July 1941, and although it was eventually replaced on the production lines by the -III it remained in service to the end of the war. A total of 1,093 Ki-46-IIs were built before production ended.

The design was so successful that Germany attempted to buy a licence to produce the aircraft, but without success.

Ki-46-II KAI

The appearance of the Ki-46-III meant that a number of -IIs were available for use as trainers. They were converted into three-seat radio and navigation trainers, and were given a stepped-up rear cockpit mounted behind the pilot's seat. They were produced by converting existing aircraft, and entered service as the Army Type 100 Operation Trainer.

Ki-46-IIIa

Work on the Ki-46-III began in May 1942, when the Japanese Army Air Force ordered Mitsubishi to begin work on installed a new engine in the Ki-46. The engine chosen was the Ha-112-II, a version of the Ha-102 with a direct fuel injection system, and that could produce 1,500hp. The target was a speed of 404mph, combined with a longer range. To achieve this Mitsubishi installed a second fuel tank in front of the pilot, five tanks in each wing and a ventral drop tank. The machine gun was removed to save weight.

The airframe was modified in two ways - a new engine cowling was installed because of the larger engine, while a new canopy was designed, curving gently from the nose to the top of the fuselage and eliminating the 'step' present on the -I and -II. A total of 654 -IIIs were built, entering service as the Army Type 100 Command Reconnaissance Plane Model 3. The -III was produced at Nagoya until December 1944, but a combination of earthquake and bomb damage then meant that production had to move to Toyama, but only around 100 aircraft were built at the new location.

Ki-46-III Kai

The high speed and good altitude performance of the Ki-46 meant that it was almost inevitable that a fighter version would be produced, especially once it became clear that Japan would soon come under direct attack by high flying American bombers. Work on converting the Ki-46 into a fighter began with a study in June 1943, but didn't become a priority until May 1944. The new fighter was armed with two 20mm Ho-5 cannon mounted in the nose and a single 37mm Ho-203 cannon mounted in place of the forward fuel tank and firing up and forward at 30 degrees. The new aircraft resembled the Ki-46-II more than the -III, reverting to the earlier nose but retaining the more powerful engines.

The first Army Type 100 Air Defence Fighters were completed in October 1944 and entered service in Japan in November. They were a disappointment in combat, lacking the rate of climb needed to reach the B-29s, while the unprotected fuel tanks made it very vulnerable to the firepower of the American bombers. Once the B-29s were switched to low-level night attacks its lack of radar meant that the Ki-46-III Kai was of no further use.

Ki-46-IIIb

The Ki-46-IIIb was a ground attack version of the aircraft developed in March 1945. It was armed with two 20mm cannon, and entered service in small numbers as the Army Type 100 Assault Plane.

Ki-46-IIIc

The Ki-46-IIIc was a design for a fighter with two obliquely mounted 20mm cannon. None were built.

Ki-46-IV

The final version of the Ki-46 was designed after the failure of the Tachikawa Ki-70, its intended replacement. It was to be powered by the Ha-112-II Ru engine, a version of the Ha-112-II that was given an exhaust-driven turbo-supercharger, mounted behind the engine in the nacelle. The Ki-46-IV had more fuel capacity than the -III, but was otherwise identical.

Four prototypes were built in 1943-44, and flight tests begin in February 1944. The turbo-supercharger meant that the engine maintained its performance at much higher altitudes, giving it a very impressive turn of speed above 26,250ft. In February 1945 two of the prototypes flew from Peking to Yokota in 3 hours 15 minutes, covering 1,430 miles at an average of 435mph!

Despite the success of the design the -IV never entered production. The high performance engines were urgently needed for high-altitude fighter aircraft, and none were available for reconnaissance aircraft. Two versions were planned - the IVa reconnaissance aircraft and the cannon-armed IVb.

Stats

 

Ki-46-I

Ki-46-II

Ki-46-III

Ki-46-III Kai

Ki-46-IVa

Engine

Ha-26-I

Ha-102

Ha-112-II

Ha-112-II

Ha-112-II Ru

Power

900hp

1,050hp

1,250hp

1,250hp

1,250hp

Crew

2

2

2

2

2

Wing span

48ft 2 3/4in

Length

36ft 1 1/16in

36ft 1 1/16in

36ft 1 1/16in

37ft 8 3/15in

36ft 1 1/16in

Height

12ft 8 3/4in

12ft 8 3/4in

12ft 8 3/4in

12ft 8 3/4in

12ft 8 3/4in

Empty Weight

7,449lb

7,194lb

8,446lb

8,446lb

8,840lb

Loaded Weight

10,631lb

11,133lb

12,619lb

13,730lb

13,007lb

Maximum take-off weight

-

12,787lb

14,330lb

-

14,330lb

Max Speed

335.5mph at
13,340ft

375mph at
19,030ft

391mph at
19,685ft

391mph at
19,685ft

391mph at
32,810ft

Cruising Speed

-

249mph

-

-

280mph

Service Ceiling

35,170ft

36,170ft

34,450ft

34,450ft

36,090ft

Climb to

16,405ft in
7min 45sec

26,250ft in
17min 58sec

26,250ft in
20min 15sec

26,250ft in
19min

32,810ft in
16min 30sec

Range

1,305 miles

1,537 miles

2,485 miles

1,243 miles*

2,485 miles

Armament

One 7.7mm rear-firing machine gun

None

One 37mm and
two 20mm cannon

None

* and one hours combat

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How to cite this article: Rickard, J (19 February 2010), Mitsubishi Ki-46 'Dinah', http://www.historyofwar.org/articles/weapons_mitsubishi_ki-46.html

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