A6M Zero's central drop tank attachments

The Japanese Navy 7.7mm mg was a licensed copy of the British Vickers Model E mg which used the same .303 British cartridge.

https://en.wikipedia.org/wiki/Type_97_aircraft_machine_gun

The Japanese Navy 13.2mm mg was a copy of the American M2 Browning which as noted before had a dual left or right feed system.

https://en.wikipedia.org/wiki/Type_3_aircraft_machine_gun

More Type 97 info:

https://www.rockislandauction.com/detail/68/1647/japanese-97-machine-gun-77-mm-japanese

It seems that the Type 97 was made in left or right only feeds, not like the Browning dual feeds.

stikpusher:

From what you say and from the Watanabe drawing, some gun models seem to be switchable in the field, and others come in two versions out of the factory.

buitre

stikpusher:

The drawing from Mikesh's book chapter (artist = Rikyu Watanabe) shows every single item in the wing cannon to be a mirror image of the equivalent item from the other cannon.  This cannon seems to be something that came straight out of the factory in two versions: a right-handed gun and a left-handed gun.

 

 

I don't know the specifics of the Japanese designs. But on the M2 .50 cal., a design that I used to be very familiar with, the gun could be switched from right hand to left hand feed by the operator. That was a feature of the design, out of the factory. That's how it can look that way in photos, artwork, etc. I was shown how to do that, but it's been over 35 years since I've played with a Ma Deuce, so I'd need a manual to do it again if I had the opportunity. 

stikpusher:

The rear end of the right wing gun, which got chopped off in my scan (book wider than scanner!) also shows perfectly mirror-imaged parts.  EVERYTHING is a perfect mirror image.  (Well, maybe the spiral groove inside the barrel was not mirror-imaged. )

stikpusher:

The drawing from Mikesh's book chapter (artist = Rikyu Watanabe) shows every single item in the wing cannon to be a mirror image of the equivalent item from the other cannon.  This cannon seems to be something that came straight out of the factory in two versions: a right-handed gun and a left-handed gun.

Right.  I have seen the light!

Many aircraft machine guns have the ability to be switched between right or left hand feed by the armorers. This come in handy for aircraft installation. Just look at the twin mount .30's and .50's used on many US aircraft for obvious examples.... or wing gun installations, with the guns installed at the some position on both wings, and the ammo trays on opposite outboard sides. Otherwise to acheive the same balance that would require the gun to be installed inverted, and introduce a new host of jam inducing headaches.

jeaton01:

I just noticed in the Tamiya instructions that the two hood machine guns are mirror-images of each other, at least as far as can be seen in the drawings.  (And I just realized that, in my Bandai Zero, I made --from scratch-- the two cocking levers be symmetrical, both on the inside of each 7.7 mm gun.  So at some point I was subconsciously? aware that in at least some aspects the hood machine guns were mirror images of each other.  DUH!  Why was that concept such a mental block for me?)

jeaton01:

Until this thread, I had never given any thought to shell pathways inside airplanes.  I had always assumed that any given type of machine gun model always got fed through one particular side, and ejected through the other one.  I never considered the complexities that such rigidity might create for airplane designers.

The attached graphic, taken from Robert C. Mikesh's "Zero Fighter" chapter in "The Great Book of World War II Airplanes", shows very clearly that the Zero's wing-mounted cannon existed in mirror-image versions.  By extension, I think it's very likely that the "hood" machine guns were also mirror-image versions of each other.

So that is clear to me now, Duh!

What I still can't picture is the paths followed by the shells in the drawings from the Tamiya instructions.  For instance, for the port gun I see a channel that goes up from just left of the midline of the plane, then goes horizontally toward the port side, then goes down, then goes up along the port edge of the fuselage, and finally reaches the port gun.  OK, so that could be either (a) the "feed" channel or (b) the "exit" channel.  But where is the other channel, the exit channel in case "a", or the feed channel in case "b"?  Or is that channel a double-channel, with internal inbound and outbound halves??

Looking at the chute I think the guns were both fed from the outside and the spent cartridges both came out in the middle.  I guess in that event the guns could either feed both ways or they were different for left and right.

Jeaton01:

Yes, I am aware of the Tamiya 1:32 Zero, I have heard that it is an outstanding kit.  When I was looking for an A6M5 to build, I almost went for this kit.  But then I decided that I wanted a kit at the same scale (1:24) as a FW-190F3 from Heller that I had just completed.  So I bought an ancient 1:24 scale A6M5 from Bandai.  Ugh, it has given me a lot of headaches, and I am still working on it!  I think it will work out OK in the end.

In regard to the feed chutes, in spite of the graphics I still don't quite understand in which direction the shells move prior to and after reaching each machine gun.  Were the two machine guns mirror images of each other?  If they weren't (and I assume that they weren't!), how could the spent casings travel to the left and down from the port gun and to the right and down from the starboard gun?

By the way, looking at the Tamiya instructions (I don't have the kit but I do have the instructions), I just noticed that "scene #52" shows beautifully the fat pin of the drop tank going into the big hole of the "keyhole" area, which confirms what we had concluded here.  In this model, the drop tank can actually drop: It can be pulled out and stuck back in again!

At this point, I have no questions about the drop tank connections, I consider it a resolved issue.  But I'd like to pin down what is the story with the centered vs not-centered rectangular hole.

Tcoat:

Here are all the graphics references that I have found that are relevant to "the" rectangle:

the following planforms viewed from below:  (1) an "A6M5" (definitely not an A6M5c) in the Sakaida/Osprey book (see above); (2) an A6M5a by T. Skwiot (I can't remember where I found it); (3) an A6M5c by T. Skwiot (https://naval-encyclopedia.com/naval-aviation/ww2/japan/mitsubishi-a6m-zero.php); (4-9) 6 views from below that I downloaded (with some trepidation!) from a Romanian website (http://www.mediafire.com/file/lu9da8pbvub2cw9/Mitsubishi-A6M-Zero-Scale-Model-Aircraft-Plans-Drawings.zip) that show two A6M2s, two A6M3s and two "A6M5" (definitely not A6M5c); and also (10) several drawings in the instructions for Tamiya's 1:32 scale model of the A6M5 (definitely not an A6M5c).  

All but one of the above show the rectangle centered on the midline.  The only exception is the Tamiya instructions, which show the rectangle offset to starboard.

Interesting points:

#1. The off-center rectangle hardly ever shows up in line drawings (one out of ten drawings).

#2. The off-center rectangle shows up in a Zero that has 2 hood machine guns, which argues against my hypothesis that the off-center rectangle occurs when there is only one hood machine gun.

#3. The fact that another one of those graphics shows an A6M5c with a centered rectangle also argues against my hypothesis that the off-center rectangle occurs when there is only one hood machine gun.

I have seen other line drawings for Zeros, but sometimes they don't show THE rectangle at all, and other times they present a hybrid top+bottom view (like the one provided by HooYah Deep Sea in the present thread) that can be very useful for many purposes, but that by their very nature can't address asymmetry questions.

Can you give me references to more line drawings that show the elusive asymmetric rectangle?  I'd love to find a logic that tells us when the rectangle will be centered and when it will not.

Here's a page from Tamiya's 1/32 Zero kit #60309, which shows the feed chutes from the two 7.7mm guns (it is an A6M5A, with the 7.7's in the fuselage and belt fed 20mm cannon in the wings. It does appear that both 7.7's feed to a common chute which is just in line with the rectanhular hole in the wing you all speak about.  This is an outstanding kit.  There's no help on the drop tank stuff, the kit has the fairing on the tank.

Tcoat:

What leads you to think that the off-center smaller port is for the 2-gun versions?

From measurements that I have taken on the graphic from the Sakaida book, the rectangular hole would be about 144x84 mm in real life. We can also think of the hole as two144x42 mm side-by-side holes if we assume that the final hole that we see is the combination of two chutes, one coming from each gun.

I have looked up casing sizes in the web (https://en.wikipedia.org/wiki/7.7×58mm_Arisaka for the 7.7 mm casing, and https://en.wikipedia.org/wiki/13.2×99mm_Hotchkiss_Long for the 13.2 casing).  The dimensions (rounded off to the nearest mm) are 58 mm length and 12 mm maximum width for the 7.7 mm casing, and 99 mm length and 20 mm maximum width for the 13.2 mm casing.  I graphed out their sizes and the size of the hole (whole size and split-in-half size).  See below the relative sizes of the casings and of the two hole sizes.  (I am sure that there are other sizes of 7.7 mm and 13.2 mm casings, but I would not expect them to be too different from the ones I found.)

I am definitely no expert on this topic, but the casings seem to have pretty comfortable margins relative to the holes, even when we compare the larger casing (13.2 mm) with the half-size hole (shown in the lower right).

buitre

Tcoat:

I agree that, for our purposes, it does not matter much if those hood guns feed from the left or from the right.  What seems clear is that they eject their shell casings downward to the rectangular hole immediately ahead of the forward main spar.

Oh, and I have a theory for why some sources say that the rectangular hole should be in the midline of the fuselage (see the attached graphic, taken from Sakaida's Osprey book), whereas others (like the Tamiya instructions) say that it should be in an asymmetric position, shifted to starboard.  Models A6M2, A6M3, A6M5 and A6M5a ("Ko") had two 7.7 mm machine guns on the hood.  In the A6M5b ("Otsu"), the starboard 7.7 mm machine gun was replaced by a 13.2 mm machine gun. For the A6M5c ("Hei"), a 13.2 mm machine gun was added to each wing, outboard of the 20 mm cannon. However, this made the traditionally ultra-nimble Zero fly like a truck.  Therefore, the left side hood 7.7 mm gun was deleted, and this became the definitive A6M5c.  So the A6M5c had only one gun on the hood, on the starboard side, while all the previous models had two guns side by side on the hood.

In the Zeros that had 2 guns on the hood, every component of the starboard gun would logically be somewhere to the right of the corresponding component of the port gun.  This would include the ejection chutes.  I think that in the 2-hood-gun Zeros, the ejection chutes converged near or at the end (as you also suspect), and expelled the shell casings through a rectangular exit hole centered on the midline of the fuselage.  But the A6M5c, with no hood gun on the port side, did not need a chute on that side.  Given the obsession with minimization of the Zero's weight, I think it's likely that the left gun's chute was removed, and that the left half of the rectangular hole was sealed.  That would leave the A6M5c with a single, asymmetric rectangular exit hole to the right of the midline, and probably half the size of the exit holes of the 2-hood-gun Zeros.  What do you think?

 

I think that the off center smaller port is for the two gun versions. in scale the hole just doesn't look big enough for the larger casings. All pictures I could find of it were two gun aircraft. The larger center line ones do look large enough though. There is also the distinct possibility that the lower rate of fire and only one gun allowed to to collect the brass instead of ejecting it. When the very late models came out resource conservation was vital since they had lost all their mines in China so collecting brass would be important. 

In the end I think we have managed to answer the question of if that hole had anything to do with drop tanks.

I love research!

Cowl gun armament changed on the A6M5 series. On the 5A it was still the two 7.7mm machine guns. On the 5B, one of the 7.7's was replaced by a 13.2mm heavy machine gun. On the 5C, the remaining 7.7mm machine gun was deleted, leaving only the single 13.2mm HMG as a cowl gun. Obviously any ejection ports for spent brass and links will likely need to be enlarged for the 13.2mm gun compared to the 7.7mm guns.

Tcoat:

I agree that, for our purposes, it does not matter much if those hood guns feed from the left or from the right.  What seems clear is that they eject their shell casings downward to the rectangular hole immediately ahead of the forward main spar.

Oh, and I have a theory for why some sources say that the rectangular hole should be in the midline of the fuselage (see the attached graphic, taken from Sakaida's Osprey book), whereas others (like the Tamiya instructions) say that it should be in an asymmetric position, shifted to starboard.  Models A6M2, A6M3, A6M5 and A6M5a ("Ko") had two 7.7 mm machine guns on the hood.  In the A6M5b ("Otsu"), the starboard 7.7 mm machine gun was replaced by a 13.2 mm machine gun. For the A6M5c ("Hei"), a 13.2 mm machine gun was added to each wing, outboard of the 20 mm cannon. However, this made the traditionally ultra-nimble Zero fly like a truck.  Therefore, the left side hood 7.7 mm gun was deleted, and this became the definitive A6M5c.  So the A6M5c had only one gun on the hood, on the starboard side, while all the previous models had two guns side by side on the hood.

In the Zeros that had 2 guns on the hood, every component of the starboard gun would logically be somewhere to the right of the corresponding component of the port gun.  This would include the ejection chutes.  I think that in the 2-hood-gun Zeros, the ejection chutes converged near or at the end (as you also suspect), and expelled the shell casings through a rectangular exit hole centered on the midline of the fuselage.  But the A6M5c, with no hood gun on the port side, did not need a chute on that side.  Given the obsession with minimization of the Zero's weight, I think it's likely that the left gun's chute was removed, and that the left half of the rectangular hole was sealed.  That would leave the A6M5c with a single, asymmetric rectangular exit hole to the right of the midline, and probably half the size of the exit holes of the 2-hood-gun Zeros.  What do you think?

For disintigrating link ammunition belts, there also needs to be either a catch bag/bin for the links, or an ejection port. Some machine guns, such as the M2 .50 caliber, will eject the spent shell casing downwards thru the botton of the receiver while the metal links are extracted out the opposite side of the feed tray as that the ammo belt comes in. I dont know any specifics of the Japanese weapons used on the Zero in this regard. If the guns use a non disintigating belt made from fabric or metal, there needs to be a catch bag/bin for those items.

buitre

 

 

Tcoat

OK that opening between the wheel wells is most certainly the spent casing ejection chute exit.

In this drawing you can see the chute (in green under front of windscreen) leading from the gun breech's down the side of the aircraft outboard and aft of the oil tank and lining up exactly with the opening.

This would be why it would be closed or even faired over on museum aircraft as they have no casings to eject. 

 

I am uncertain as to the path followed by the spent casings of the two 7.7 mm fuselage machine guns.  Each of the two guns will have an entry route and an exit route.  My guess (and it's only a guess!) is that the entry route is on the left side of each gun, and the exit route is on the right side of each gun.  If that is correct (which, I repeat, I don't know if it is) then what you are calling the ejection chute would actually be the entry route, and the spent casings would exit somewhere near the midline of the fuselage, and would go vertically downward from there.  I assume that the starboard machine gun would not be symmetric, and that its entry route would be an upward path near the midline of the fuselage, and the exit would be down the right side of the fuselage.

The exit holes for the two guns would be somewhere in the bottom of the fuselage, and theoretically they could be anywhere relative to the fuselage midline, because the channel has an inward bend near the bottom.

In the graphic, I have also marked what I have called before the "upper cheek" hole.  I now think that this hole has nothing to do with the ejection of casings.  All exit holes for the ejection of casings that I have ever seen are rectangular.  Why would the Zero use an elliptical exit hole??  Nevertheless, this tube seems to be connected to the machine gun --or maybe not!  Do you have any idea what it's for?

 

 

 

 

 

 

 

Most likely a vent for the oil tank as it is right on top of it. The guns ejection chute may be joined somewhere so they use a common exit. Really hard to find much on it so I think my theory is sound but am not an expert so could be way out to lunch. No doubt there is somebody someplace that know exactly how it works. The only thing that bugs me is I don't know where the guns feed from so I may have things backwards and the outer is feed and the extractor is inboard. All I am positive about is that the casings have to go someplace and that port is in the right place, looks like many other aircraft and even the cannon ports (closed on the model) on the wings. 

Tcoat

OK that opening between the wheel wells is most certainly the spent casing ejection chute exit.

In this drawing you can see the chute (in green under front of windscreen) leading from the gun breech's down the side of the aircraft outboard and aft of the oil tank and lining up exactly with the opening.

This would be why it would be closed or even faired over on museum aircraft as they have no casings to eject. 

 

I am uncertain as to the path followed by the spent casings of the two 7.7 mm fuselage machine guns.  Each of the two guns will have an entry route and an exit route.  My guess (and it's only a guess!) is that the entry route is on the left side of each gun, and the exit route is on the right side of each gun.  If that is correct (which, I repeat, I don't know if it is) then what you are calling the ejection chute would actually be the entry route, and the spent casings would exit somewhere near the midline of the fuselage, and would go vertically downward from there.  I assume that the starboard machine gun would not be symmetric, and that its entry route would be an upward path near the midline of the fuselage, and the exit would be down the right side of the fuselage.

The exit holes for the two guns would be somewhere in the bottom of the fuselage, and theoretically they could be anywhere relative to the fuselage midline, because the channel has an inward bend near the bottom.

In the graphic, I have also marked what I have called before the "upper cheek" hole.  I now think that this hole has nothing to do with the ejection of casings.  All exit holes for the ejection of casings that I have ever seen are rectangular.  Why would the Zero use an elliptical exit hole??  Nevertheless, this tube seems to be connected to the machine gun --or maybe not!  Do you have any idea what it's for?

 

 

 

buitre

Tcoat:  Yes, we are in total agreement in regard to this.

 

Yep!

And I see your confusion with the square hole now as that one is oviously faired over. Gonna' have a bunch of brass stuck up there if they try to fire the machine guns!

Tcoat:  Yes, we are in total agreement in regard to this.

buitre

Now that I've learned how to upload images, here is my interpretation of the protrusions that go into the "keyhole" area's two holes.  Notice that the large protrusion has a flange; that is NOT what I meant by "keyhole".  The "keyhole" is the reddish area shown in the museum photo The flange would go into a small groove at the front end of the front hole.

 

Got it. The end result is that those are the only two attachement points for that style drop tank. Differnet styles may vary a bit but not much.

Now that I've learned how to upload images, here is my interpretation of the protrusions that go into the "keyhole" area's two holes.  Notice that the large protrusion has a flange; that is NOT what I meant by "keyhole".  The "keyhole" is the reddish area shown in the museum photo The flange would go into a small groove at the front end of the front hole.

buitre

Tcoat:

Wow, I am impressed at how many responses I have had within 12 hours of my initial posting!  Thank you, guys!

Your picture of the tanks showing the large protrusion + small protrusion really do seem to fit quite well (literally!) with the "keyhole" attachment point.  I don't seem to have picture-posting privileges here, or I'd show you a photo that I took 11 years ago of an A6M5 Model 52 Zero at the National Air and Space Museum, which shows the keyhole-shaped opening, which is rather shallow (maybe 1 cm or less deep in real life?), and inside of it two smaller holes that go deeper and seem to correspond to the large and small protrusions in the tanks.  For me, this has solved the question of the "keyhole" attachment point.

It still leaves open the question of the somewhat smaller rectangular opening immediately forward of the forward main spar.  Was it an open hole, or was it normally covered?

By the way, I am building the ancient Bandai 1:24 scale A6M5 kit.  I am modifying it to be an A6M5a instead of the intended A6M5c.  It's a challenge, because quite a few parts don't fit well with each other.  Also, some areas have required me to make substantial modifications.  The cockpit frame was internal, so I sanded out the entire frame, and then returned it to transparent.  So far, I am overall happy with the (slow) progress.

 

(Am practicing how to upload images.  Let's see if I can make my Zero from the National Air and Space Museum show up.)  The reddish area is what I have been calling the "keyhole" area.  Notice the two deeper circular holes within it, one (at the front) wider than the other.

stikpusher

Here's a A6M5 at San Diego Air & Space Museum. It has some stub points for the drop tank

 

 I believe those are bomb clamps not for drop tanks. Not one picture of a drop tank shows attachment points for those but all bomb rack do.

OK that opening between the wheel wells is most certainly the spent casing ejection chute exit.

In this drawing you can see the chute (in green under front of windscreen) leading from the gun breech's down the side of the aircraft outboard and aft of the oil tank and lining up exactly with the opening.

This would be why it would be closed or even faired over on museum aircraft as they have no casings to eject. 

HooYah Deep Sea

I'm tending to believe that the 'square' or 'rectangular' opening was open, as I believe that it is the same aperature used for the centerline bomb rack.

 I tried to contact three different aviation museums on the subject, but either they didn't have an A6M or they didn't answer the phone. Sorry.

 

The opening in question is about 3 scale feet forward of the attachement point shown in your drawing here. The drawing shows it aft of the panel ine and about lined up with the rear of the windscreen.

The opening is well forward of the wing panel line and in front of the windscreen. If placed in the opening the rack and bomb would hit the scoop and block the landing gear doors. 

I think the attachment point for the bomb rack is under the hinged panel just aft of the drop tank opening. You can see it below.