Evidence and Theory Presented by Jim Long
In the October 1974 issue of the Koku-Fan Monthly came the meaty evidence to prove that the carburetor air scoop on the A6M1 was not sticking out into the airstream from the accessory section, but was in fact routed under the engine cowling in a more modern manner.
The October magazine installment included drawings on page 188 of the Zero's engine fire-fighting device, which worked by fogging the entire accessory section with carbon dioxide forced through small holes in some aluminum tubing that also injected CO2 directly into the carburetor air intake pipe. The tubing encircled the accessory section and ended at two fittings on the air scoop, one on each side. Naturally the air scoop had to be included in the drawing of the device in order to show the routing and connections of the tubing. And fortunately for us, the drawings that were in the files of the naval air technical arsenal were of the fire-fighting device as used on the A6M1, made recognizable by the presence of the same profile of the Zuisei's downdraft carburetor that the draftsman used in illustrating the A6M1's cowl-mounted machine guns in Part Three.
As you can see, the relative positions of the air scoop and the curve of the cowling aft of it make the picture perfectly clear that the intake pipe was under the top part of the engine cowling. This is proof positive. The scoop was under the cowling, and it is conceivable that the forward portion of the air intake was formed by baffling built into the upper cowling piece. Although this authentic WWII diagram doesn't provide the details, it is possible to get an idea of how it may have been done by looking that the installation on the F1M2 Type Zero Observation Plane, another MItsubishi product of the same period, and one that also mounted the Zuisei 13 engine.
Maru Mechanic No. 20 of 1980 was on the F1M Type Zero Observation Plane, the Pete, and had some technical drawings of the plane's air scoop to offer. These items are on page 38. The drawings show an air scoop installation that was probably very similar to that used on the A6M1. Notice that this arrangement permitted some control over the temperature of the air going into the carburetor by drawing it either from under the front lip of the cowl or from the area around the top engine cylinders. The two A6M1s undoubtedly had a similar facility; it is certain that the A6M2s did.
You may be wondering whether or not the A6M2s had an engine fire-fighting device like that of the A6M1. The answer is affirmative. But a diagram of it was not with the material published by the Koku-Fan Monthly. For that you have to go back the Model Art Publication No. 323. Pages 144-45 have Diagram 173 that shows the device as it was used on the A6M2. The CO2 tubing had to be rerouted so that its t-shaped fitting was at the top. This permitted the two tubing sections to circle the accessory section by curving down on each side to termini at the carburetor air scoop. It was just the reverse of the A6M1 installation.
While you are at pages 144-45 in the Model Art book, you might want to notice that diagram 172 points out that the two A6M1s had their flotation devices arranged in the same manner as the A6M2. Except that the fuselage flotation bag ended at fuselage station 12 in the A6M1, whereas in the A6M2 it ended at station 13. Notes on the drawing point this difference out to the reader.
8. Maru Mechanic Book No. 20, Maru Magazine Publishing Co., Tokyo, 1980. As you flip through the pages of this booklet and take a closer look at the Pete, you will see other similarities between it and the A6Ms in the arrangements and the equipment. There are three good pictures of the Zuisei 13 on page 36, at least that is true according to the labeling. They could also be pictures of the Japanese army's Ha-26, which was the same basic engine.
to Part V