Ki-21 Sally remote-control tail "stinger"?

[Johnny .45]

Hey all. I've been looking into the Ki-21 since I'm hopefully receiving a die-cast version of it in the mail soon (although it's been three weeks since they said they were shipping it, so I wonder...). One thing I can't figure out for the life of me, even with the excellent cutaway drawing I found elsewhere on this site, is how the tail-armament worked. It's a "stinger" type 7.7mm machine gun, mounted in the extreme tail of the plane. There is no gunner. I've seen FIXED installations like this before (typically Luftwaffe), but this is specifically called "remote-control" in most of the references I've seen of it. Remotely controlled by whom? There is no crew member with a direct sight line to the rear, unless perhaps the dorsal gunner, and the view would be terrible. What do they use to control the gun? Somehow an electronic-type remote control system such as the B-29 had seems unlikely for a mass-produced early war Japanese type, even without a complex firing computer to compensate for range and deflection. Mechanical linkages are possible, but also seem implausible. And none of this explains who controls the weapon, regardless of how it's controlled. Without a remote gunsight and controls directly linked to the weapon, there would be no way to tell where it's aiming except the tracers.
Could this be a mistake, and it's actually a fixed defensive gun meant mostly to scare fighters away from the dead-astern position and into the field-of-fire of one of the other weapons? Or at best, maybe it's one that can be adjusted to fire higher or lower, but not really aimed per se? No matter how I think about it, it is not making any sense. Just wondered if anyone here had any thoughts or theories on this. Or the facts would be welcome, if you have them!

[RSwank]

Interesting question. I looked at the diagram shown here:

http://forum.12oclockhigh.net/attach...3&d=1295643763

I have a couple of ideas. There is an item labeled 105 "control linkage". I don't think this is the control linkage for the tail as there are other items 96 and 97 that seem to be the flight control linkages, so I think that 105 is the linkage for control of the tail gun. It is hard to track the item "forward" to see where it ends and the door seems to hide the path. So.... who might control it? I think it is "controlled" or linked to the dorsal gun. That gunner would have to have an "interrupter" on his gun to prevent him shooting off the rudder as he swings his gun from side to side as he tracks a plane approaching from the rear. I suspect that the "remote" tail gun will "follow" his dorsal gun in that narrow arc during which time "his" gun can't be fired as it would hit the rudder, but the tail gun could be fired at the rear attacking plane.

In the simplest case, the tail gun may only track the up and down movement of the dorsal gun (not side to side) and may only fire when the dorsal gun is being interrupted, i.e., when the dorsal gun is pointed back at the rudder and is trying to fire at an attacker directly behind the bomber. In this situation, to tail gun "takes over" for the dorsal gun during the time the dorsal gun can't be fired.

There seem to be two photos of two different versions of the gun on this link. Note in both cases there would not be much (if any) movement side to side but there could be some up and down movement.

http://www.4wdaction.com.au/forum/vi...502&start=3315

[Johnny .45]

Well, I suspect the "Control linkage" is just for elevator or rudder control, as it's location and the term "control linkage" are exactly the same in 90% of the cutaways in the book I have. Usually if if a cable for something unique, like an arrestor hook, it'll specify that it's for the arrestor hook, you know? That and I've found that one can't take these illustrations too literally. Usually they're pretty close when it comes to locations of things (not always though), but as for detail, they can be lacking in accuracy, especially with the more obscure aircraft. Hard to draw the details when there are no examples existent or accessible to the illustrator I guess! You have things like how many of the guns look strongly like Browning .50cals! Type 99/MG FF/MG51 ought to be tubular, the Hispano's receiver isn't much wider than the breech, yet most of them are your typical "box with a barrel" in many drawings. To be fair, many drawings are extremely accurate. In any case, I don't think they show the gun controls on this cutaway (this is the one I mentioned that I'd found on this site).
After I'd posted my question, I was reading about the "stinger" installation on the Do 217, and it was described as "remotely fired from the cockpit". It made me wonder if perhaps "remote control" really means "remotely fired". That's certainly possible, as I've seen installations like that on other aircraft like the Do 217. In all these cases, the gun is fixed and simply acts as a (slight) deterrent to attacks from the rear. That seems more plausible than an actual remote-control installation to me, especially as the stinger was something that was included on all production Ki-21's, which were introduced before the war even started. If the US and Britain and Germany had difficulties getting remotely-aimed guns to work properly DURING the war, it's hard to imagine the Japanese successfully putting a system into use before the war. If nothing else, the costs and weight would have been prohibitive in the Japanese way of thinking, especially for the potential gains.
I do have to say though, the fabric shrouding around it DOES suggest a movable installation, and in the (quite excellent) photo you linked to, it does LOOK like some sort of moving gun mount. If it is, your description could be correct; I'd guess a simple system set up with electrical switches (such as they used in interrupters), so it covers the area directly behind the tail. It may be that the stinger is set up so whenever the dorsal gun interrupter kicks in, it OPENS the firing circuit to the tail gun. When the circuit is opened, the gun automatically moves to the same azimuth and traverse as the dorsal gun is aimed and fires (while moving wherever the dorsal gun is aimed). As soon as the dorsal gun interrupters shut off, the tail gun is disabled. Or it may constantly follow the dorsal gun's azimuth and traverse (within it's limits of travel), so it wouldn't have to return to zero and back again if the gunner was forced to swing from one side of the tail to the other repeatedly. This way it's "ready" when the dorsal gun moves into the tail zone, and the interrupters simply enable it to fire. This sounds complex (and is!), but it's NOTHING compared to later-war fire control. Boggles my mind, that stuff.
Just seems like if it was that simple to do cheaply and effectively, that others would have used similar systems as well. And you'd think that it would warrant more mention when people write about the Ki-21 nowadays! It'd be far more advanced than any other aerial gunnery system put into production before the war, even if it's pretty basic compared to later Allied computer-controlled remote systems.
So try as I may, I can't answer my own question! I'd still love to know for sure, and to find out how it worked, if it did in fact exist.

[RSwank]

Here is an interesting link. Search for "dorsal" on the page.

http://www.researcheratlarge.com/Air...Summary13.html

This is a report from 1943 "Notes on Japanese Aircraft". In particular it describes a cashed "Sally" and some of its features. Here is the section on the guns carried.

"The armament consisted of;

1 x 7.7 mm free gun in the nose,

2 x 7.7 mm free lateral guns just below and forward of the leading edge of the tail plane.

1 x 7.7 mm ventral gun under the leading edge of the tail plane

1 x 7.7 mm free gun in the tail, operated by remote control from the dorsal turret.

1 x 12.7 mm free gun in a dorsal turret."

All sights are ring and bead sights except the tail gun which has the reflector type. The cone of fire from the tail gun is about 20 degrees. A report on one crash states the possibility of the aircraft being equipped with a power gun turret."

Not quite sure how to interpret the reflector "sight" "on" the tail gun except it say it just was not used used in the air. With the shroud on the tail gun any sight on the tail gun would be worthless. It was probably used to sight the tail gun and align it with the dorsal gun when the plane was on the ground and the shroud was off the tail gun. Since it was "linked" to the dorsal gun it would need to be aligned and "sighted in" and I suspect that was the purpose of the reflector sight on the tail gun.

[Johnny .45]

That IS very interesting. One has to wonder how accurate it is though; sometimes you can't tell whether they had it right at the time, or the modern references are the accurate ones. This listing is for a "Sally II", i.e. one of the later types with the hemispherical dorsal turret, not the one I am most interested it, which is the early "greenhouse" type with the machine gun deployed from a hatch (I didn't specify this however, and it may make little or no difference to the tail armament used). But above this listing, they mention a "Sally I", which they describe as being armed with a "20mm cannon" in the tail stinger. None of the references, pictures or drawings I've seen agree with this; they universally claim a 7.7mm machine gun, which appears to be a Type 97 complete with barrel shroud. (The belt-fed type that the Zero fighter uses, not the Bren-gun style Type 97 infantry gun). It is, however, entirely possible that some Sallies were re-fitted with one of the several different Japanese 20mm cannons in the field, or even from the factory. There is a sad lack in surviving information about Japanese aircraft, particularly info that is easy to find, accurate, or both. The more I read about them, the worse it gets. So much is contradictory, you frequently find information that is flat-out wrong even when you CAN find it, and most of it is just quick references here and there. I've just about lost faith in Wikipedia for anything regarding the topic of Japanese weaponry.
But I digress. While searching, I've also found another reference to the tail gun, which agree with the dorsal-position idea. At: http://www.j-aircraft.com/faq/ki21.htm they refer to a "tail gun controlled from the dorsal position as well". Still lacking in technical details, which is what I really lust for. Perhaps it's gone forever; there's a lot of stuff that people seem to just not know for sure, since the US military (so wisely...) scrapped every single example of most Japanese types after the war.
Anyway, the "20deg field of fire" and "reflector sight" are a great clue, and I thank you for that. Unless they are totally off (possible considering the "20mm cannon" claim...this was wartime after all), it certainly indicates a true remote control tail gun.
I would say, however, that the "reflector sight" reference probably doesn't mean a sight on the gun ITSELF, rather that there was a separate, remote sight within the dorsal position, much like B-29 or Me 410 used. In this case, it was likely a simple reflector sight like early war fighter-types used, where a "pipper" is projected onto a pane of glass, rather than the complex analog-computer sights that later war fighter aircraft and the B-29 used, which automatically corrects for range, speed and deflection (not sure about the Me 410). With a hand-aimed machine gun, the distance rear and front sights gives you an acceptable "sight-radius", or distance between the two sights. The shorter the distance, the more any slight variation in aim is magnified. You could set up a remote sight using a long rod as a stand-in "gun", where the remote gun automatically moves to point exactly where the rod is aiming, but you'd have to make the rod a foot or two in length in order to get an acceptable sight radius. Or you can use a reflector sight, which just like a "red-dot" sight today is parallax-free, meaning that when you look at it, wherever the "pipper" is, that's where the gun is aiming (not countering for bullet drop or deflection). This means you can use a pane of glass and a couple grips to aim the remote gun. It's far more compact, especially in a position already containing a REAL gun. The other benefit is that it's far easier to set up a computing sight using an easily-moved "pipper" on a pane of glass than an actual sight. They tried it, it doesn't work! (Note: the B-17 used a semi-remote "rod" with sights mounted to aim the rear guns, but this was mechanically linked to match the actual guns directly below, which is far different from a true remote mounting...still, it can't have helped accuracy.)
In any case, I envision a position where the dorsal gunner is given both a handheld Type 89 machine gun AND a separate, compact reflector sight to aim the tail stinger gun for when the vertical stabilizer is obstructing his view. The questions now are:
"how was the sight linked to the rear gun?",
"was this on all Ki-21's or just the 'Sally II' version?",
"was the gun in question a 20mm or 7.7mm, and which type was it specifically?"
"what was the ammunition capacity, etc?"
"what problems arose in use, and why is this not a better-known feature, considering that it's far more novel than anything I've seen in just about any other aircraft at the time?"
This was a time when most aircraft were using ventral-tunnel armaments or periscopes to aim ventral "turrets", or forgoing them altogether. The early B-17's lacked tail guns at all. I've never seen a remote-control system that was even STARTED by the US until after the war began (I'll have to check though...I have a VERY detailed book on US armament systems somewhere around here). In any case, once again I can't help but wonder if Japan was really so technologically backwards as typical accounts suggest. If they were behind, it wasn't for lack of trying. The older I get, the more I wonder at how obscure Japanese WWII aviation really is. Sure, they lost, but so didn't the Luftwaffe...and look at THEM!

[RSwank]

There is a pretty complete description of the mechanism (or at least one version of it) here:

http://www.lonesentry.com/articles/t...se-bomber.html

The dorsal turret gunner actually had to drop "his" gun and use a pistol grip mechanism to control a ring and bead sight which was connected to the tail gun by linking rods that ran down the sides of the fuselage along with cables to control other functions of the tail gun.

"A 7.7-mm Vickers-type machine gun is mounted in the tail on a free-traversing bracket, the elevation and depression of which is controlled by another bracket attached to the forward end of the gun barrel. Both brackets are connected by link mechanisms to rods running down both sides of the fuselage. These, in turn, are coupled to a curved arm carrying a pistol grip which is operated by the gunner standing in the (dorsal?) turret. Coupled in the linkage is a ring-and-bead sight, which protrudes through two holes in the turret, and movement of the pistol grip controls the gun and the sight. The maximum movement appears to be 10 degrees in any direction, giving a cone of fire with a 20-degree angle. On the control arm are two "T" handles connected by Bowden cable to the cocking handle and to the stoppage clearing handle, respectively. The gun is fired by a trigger mechanism in the arm, connected by Bowden cable to the front sear on the gun."

An explanation of the Bowden cable is here.

http://en.wikipedia.org/wiki/Bowden_cable

All in all it would appear to be a totally mechanical mechanism with no electrical components at all. Apparently there was no "connection" to the turret gun at all. The turret gunner had to switch between the two "guns" to operate them both. probably not very easy to do when under attack.

[Johnny .45]

Wow, thank you. That is exactly what I was looking for. So, perhaps not all that "advanced", but certainly a pretty clever way to deal with the "blind spot" behind the tail of most bombers. It may have been difficult to operate, but it's better than a fixed rear-firing gun, or no gun at all. And switching between guns was probably still easier than the task of gunners in planes like the Do 17 or 217, where one gunner was expected to operate both lateral guns as well as the ventral gun by moving around between the guns (I still can't imagine how they manage that, unless given several seconds warning as to the direction of attack). Still, a lot of very good stuff in that article. I'll have to presume the setup was similar or the same in the earlier "Sally I" versions. Thank you again.