CINCPAC - CINCPOA Translations - Nakajima J1N1 "Gekko" (Irving)

This is the first installment in a new series featuring primary source material we recently located in the National Diet Library. 

Since our current contest is about "Japanese Night Fighters", in this posting, we present a 14-page document regarding the installation of upward and downward cannons on the Navy Type 2 Reconnaissance Aircraft or Nakajima J1N1-C.

First, here's what Wiki has to say:

"In early 1943, Commander Yasuna Kozono of the 251st Kōkūtai in Rabaul came up with the idea of installing 20 mm cannons, firing upwards at a 30-degree angle in the fuselage. Against orders of central command, which was skeptical of his idea, he tested his idea on a J1N1-C as a night fighter. The field-modified J1N1-C KAI shot down two B-17s of 43rd Bomb Group attacking air bases around Rabaul on 21 May 1943."

And the report:

CONFIDENTIAL CINCPAC - CINCPOA TRANSLATIONS

Item No. 9311, page 1. 

Source: Unknown. 

Subject: Fighter Unit, YOKOSUKA Air Croup: Reference Notes on Upward and Downward Inclined Fixed 20 mm MG's for Type 2 Land Recce Plane; dated August 1943; 14 pages, mimeographed. 

Editor's Note: 

The installation described in this important document, issued by the tactical research center of the Japanese Air Force, became standard on Japanese night fighters in the latter part of 1943. The document has previously been summarized and commented upon in "WEEKLY INTELLIGENCE" Vol. 1, No. 2, 21 July 1944. The arrangement of inclined cannon described in it represents a modification of "Irving's" armament, replacing the remote controlled after-turret guns found to be unsatisfactory for either day or night fighting. In "WEEKLY INTELLIGENCE" Vol. 1, No. 8, 1 September 1944, two photographs confirm and amplify the documentary evidence. Of interest is the fact that the 20 mm fixed nose cannon is retained. 

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Military - Very Secret (GUNGOKUHI)                                          Fighter Unit, 

Copy #7 of 50                                                                 YOKOSUKA Air Group 

                                                                                              August 1943 

REFERENCE NOTES ON UPWARD AND DOWNWARD INCLINED FIXED 20 mm MG's FOR TYPE 2 LAND RECCE PLANE 

Contents 

I. Inclined Fixed Machine Gun Installation 

II. Electric Firing Circuit 

III. Pneumatic Charging System 

IV. Rules for Bore Sighting 

V. Firing Tactics and Battle Lessons 

I. Inclined Fixed Machine Gun Installation 

1. As proposed by Commander KOZONO, all of the initial armament of the Type 2 Land Recce Plane was removed from the fuselage and it was equipped with 2 upward inclined and 2 downward inclined 20 mm Mark 2 fixed MG's (100 round magazines) as described below: 

(2) The machine gun installation is virtually fixed; and slight adjustments can be made in the rear mounting sufficient to correct errors in the installation. 

(3) Method of harmonization: 

The machine gun and the sight are completely harmonized along parallel lines by the ABUNE (TN: phonetic) water level (TN: sight). The methods of correcting the sight, in accordance with the particular aerial combat conditions, and the harmonizing and correcting of sights so as to orient the line of sight to the center of the shell's impact in accordance with the results of actual aerial firing in the anticipated aerial combat are still in process of investigation. 

V. Firing Tactics and Battle Lessons 

(1) The relation between the speeds of the two aircraft and the point of impact of the MG shell: 

(a) The rate of fire of the 20 mm Mark 2 machine gun is 480 rounds per minute (8 rounds per second) and so if the speed of the plane is 250 knots, the shells will hit as follows: 

(b) Stationary land target - 1 shell every 16 meters. 

(c) Against a target on the same course at the same speed - all shells will hit at the same point. 

(d) If your speed is 250 knots and that of the enemy plane is 230 knots on the tame course, your hits will be 1.25 meters apart on the target. 

(e) If the planes are both flying at 250 knots in opposite directions, your hits will be at intervals of 36 meters. 

(f) The above figures are based on one machine gun, and since this plane has 2 machine guns firing in the same direction, each shot puts 2 rounds within the above spaces. 

(g) In accordance with the above, the most advantageous attack tactics for this plane are obviously to pursue the target on the same course and fire while keeping it continuously in the sights. 

(3) Battle Lessons 

In April, 1943, two planes of the 251st Air Group were equipped for the first time with lower guns at an angle of 40 degrees and upper guns at an angle of 30 degrees. In May, 1943, they went out into a combat area. We select battle lessons which can be examined below. 

(a) While on night patrol on 21 May 1943, at about 0237 an enemy large model plane was discovered at an altitude of 2500 meters, our altitude being 2,000 meters. We closed to a firing range of 300 meters. One burst by the upper guns started a fire, and one burst by the lower guns shot it down into the sea. The upper guns fired 30 rounds, and the lower guns 10 rounds. 

(b) The same night at about 0328, seeing four parachute flares dropped from above the clouds, we discovered an enemy plane. The upper guns, in two bursts, shot down an enemy B-17. The upper guns fired 140 rounds. 

(c) After that, the same air unit using a total of nine planes shot down five enemy planes (two of them unconfirmed) out of a total of 45 large model planes which conducted four night raids between 1 June and 15 June. 

(d) That it is profitable (to use) this plane as a night fighter is confirmed by the above battle results. By gradually increasing the number of planes, notable results will soon be achieved. 

(e) The following opinions are presented in accordance with subsequent actual results: 

1. It is desirable to install both the upper and lower guns at an angle of 30 degrees. 

2. It is necessary to increase immediately the number of rounds of ammunition carried. 

3. Since the Type 2 land recce plane is too slow to repulse daylight attacks, we are planning to install this tape of armament on the Type Zero land recce plane or possibly on the Type 2 carrier recce plane. Since the windshield is cut of curved glass and it is difficult to see the front sight of the upper gun, make this part of flat glass. 

4. It is necessary to install inclined fixed machine guns in single seater fighters. 

(End) 

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According to "Moonlight Interceptor" by Robert C. Mikesh & Osamu Tagaya, the 251Ku aircraft was flown by "Superior Flight Petty Officer (SFPO)" Kudo Shigetoshi in the pilot seat and "Lt. (jg)" Sugawara Akira.
The two B-17s  were flown by Major Paul I. Williams (his bomber was named "Honi Kuu Okole" Hawaiian for "Kiss my ass", s.n. 41-9244 ) and Capt Joseph W. Geddes (his unnamed bomber had s.n. 41-9011) of the 64th Bomb Squadron.

Wiki continues:

"The Navy took immediate notice and placed orders with Nakajima for the newly designated J1N1-S nightfighter design. This model was christened the Model 11 Gekko (月光, "Moonlight"). It had a crew of two, eliminating the navigator position. Like the KAI, it had twin 20 mm Type 99 Model 1 cannon firing upward in a 30° upward angle, but added a second pair firing downward at a forward 30° angle, allowing attacks from above or below. This arrangement was effective against B-17 Flying Fortress bombers and B-24 Liberators, which usually had Sperry ball turrets for ventral defense. The Gekko's existence was not quickly understood by the Allies, who assumed the Japanese did not have the technology for night fighter designs. Early versions had nose searchlights in place of radar. Later models, the J1N1-Sa Model 11a, omitted the two downward-firing guns and added another 20 mm cannon to face upward as with the other two. Other variants without nose antennae or searchlight added a 20 mm cannon to the nose."