Two seater gunner well question

[hoodoo]

Designers went to a lot of trouble to dispose of empty belts and cartridges regarding the forward fixed guns but at least with the German two seaters at least ground pictures suggest that the belt and spent cartridges from the Parabellum machine gun just plopped into the gunners "well". While I can see an empty 100 round belt being thrown over the side or stuffed into a bag by the gunner when convenient, I can't quite picture 100 loose rounds bouncing around in the fuselage. Does anyone have the skinny on just how the empty cartridges were handled with the parabellum? Most of the mounts seem to have he gun set up where the falling cartridges (looks like they dropped out the bottom like allied guns rather than spit forward like the Spandaus) fall into the well. Was there a cut out in the bottom of the fuselages of these birds under the gunner?

[Patrick]

The lMG Parabellum Modell 14 ejected the hemp ammo belt from the left side through a small rectangular opening. The free end of the belt was doubled over and stitched to capture a D- or O-shaped steel ring. This ring was hooked to a short piece of light chain that had spring clips at either end. One of the chain clips hooked to the ring, the other looped the chain around part of the gun mount (there are pictures showing it looped to one of the arms of the pintle, the yoke that held the gun to the mount's base). So when the last round fired and the belt came free, it would trail out aft into the air stream but it wouldn't be lost. The observer could pull it in and stow it before reloading the gun.

The Parabellum ejected spent cartridges through a slot in the bottom of the breech. I have been unable to find a picture showing a collection bag attached to this gun. It's likely they weren't used because the bag could interfere with movement of the gun mount and aiming. The cartridges were simply blown aft by the air stream although a few probably did fall into the observer's cockpit. Many of the later-style mounts used with the Parabellum positioned the breech opening well-outside of the observer's circular opening which makes it difficult for a stray cartridge case to fall in.

[hoodoo]

Great answer, Patrick. Thank you! That answers the belt question. I did notice some installations as you mention and figured they helped to get the guns out there more putting spent cartridges out but I presume the primary reason would have been flexibility. There are issues though as those guns had a lot of travel due to the lesser slipstream of WWI aircraft and greater observer flexibility and without a bag a lot of cases are going to get into the well. It just may not have been that big a problem as those aircraft had less to jam up in that section of the fuselage. I don't see slipstream being a big help in that context though as the gunner would be either in the way of the slipstream or if twisted around the slipstream would put the rounds back onto him and then down into the well. Of course, ground photo may not tell the whole story as what you see in a ground shot may not be what is going on in the air. I wonder if there are any publications of instructions to German observers re use of the guns, the training I am sure was extensive.

[hoodoo]

After looking at some cross sections of some models of some of the two seaters it looks like the observers well, because he had so much different equipment, looks more like an enclosed room, if anything came loose you would not want it escaping so it looks pretty enclosed and a reasonable trap for any stray loose cartridges that could be policed after a flight.

[Patrick]

Originally Posted by hoodoo

. . . I don't see slipstream being a big help in that context though as the gunner would be either in the way of the slipstream or if twisted around the slipstream would put the rounds back onto him and then down into the well.

I think you're way underestimating the force of the air stream produced by wind and prop wash in a plane moving 90-100+ mph at thousands of feet of altitude. And the fact that the cartridges are ejected downward from the gun; they have almost no sideways movement component when they exit the breech opening, while the plane is moving forward at speed. In other words, the instant that the cartridge exits the gun, the plane is flying away from it. I think it unlikely that a lot of cartridges found their way into the observer's cockpit; I can't picture any scenario where the observer would be pelted by spent cartridges that are for some reason moving in a forward stream -- the physics just don't work that way.

[Riffster]

Originally Posted by Patrick

And the fact that the cartridges are ejected downward from the gun; they have almost no sideways movement component when they exit the breech opening, while the plane is moving forward at speed. In other words, the instant that the cartridge exits the gun, the plane is flying away from it.

Point of clarification: while the gun might not impart any sideways force to the spent casing on ejection, the casing does emerge with the same forward velocity as the plane and everything else attached to it. Unless the plane is accelerating, the plane does not "fly away from" the casing as soon as it is ejected. In a bizarre hypothetical world where this was happening in a vacuum, the casing would drop straight down with respect to the plane.

What does happen is that the ejected casing is no longer attached to the airplane, and so there is no longer any force (the airplane's thrust) to counterbalance the "negative acceleration" provided by air resistance. To a stationary observer on the ground, the casing slows down, the airplane does not. To the observer in the plane, the casing "accelerates" in the direction of the tail of the plane in addition to the acceleration of gravity.

I can't picture any scenario where the observer would be pelted by spent cartridges that are for some reason moving in a forward stream

If the observer is firing ahead of the plane - say at an enemy approaching from the classic 12 o'clock high position, then the observer is directly behind the gun with respect to the slipstream. It is possible for the spent casings to hit the observer.

A similar issue might occur when the observer is firing directly behind the plane - while the casing won't hit the observer, the observer may provide enough deflection of the slipstream to prevent it from carrying the casing away.

I seem to recall pictures of WWII waist gunners surrounded by spent casings and disintegrating links, and at least one post-mission picture of the ground crew pushing them out a waist door with a broom. Too much material to contain in a bag that wouldn't get in the way? I'm sure the WWII gunners learned to shuffle rather than step so as not to trip on a loose casing. Maybe the WWI observers did too?

Then, too, there's the question of just how short each side was of the raw materials (or time/energy/facilities) to make new brass casings. Does anyone know? If this was a concern, maybe having the spent brass drop into the well was intentional. Just a thought.

-Larry

[hoodoo]

The more I think of it, the question I raised is somewhat moot as most of the mounts as mentioned by Patrick pretty much ensured that the majority of rounds fell into the slipstream or were immediately acted upon by such. I have seen some mounts (early?) that appear to be much more likely to result in cartridge case buildup but these are probably the exception and not the rule. Even in cases were the gunner might be blocking some of the air stream, the cartridge cases would be hitting the fuselage and then getting picked up. Considering that the standard belt was 100 rounds you are not looking at a lot of spent cartridge case production (reloading slows down empty cartridge case production as when you reload the fight is often over) although the issue was critical with the fixed forward guns which definitely got rid of the cartridges by various means into the slipstream although I don't yet think I have found a WWI painting showing this (probably wrong). I'm personally pretty satisfied that bags for the parabellum were not necessary so there is nothing new to find here. As for WWII, I will stay away from that awful subject. :-) It is interesting to watch ammo pour from the bottom of a vickers or browning and spit out the front of a maxim and pile up, a different environment from the open cockpit two seater with the observer way out there and generally firing short bursts with a lot of gun swinging. I just learned that the lmg 08/15 ground gun, the round spit out forward but hit the bi-pod and fell down while with the 08 the rounds piled up in front of the gun, although when the short tube was attached that helped the rounds get down to the ground faster and in a shorter distance.

[Patrick]

Originally Posted by Riffster

Point of clarification: while the gun might not impart any sideways force to the spent casing on ejection, the casing does emerge with the same forward velocity as the plane and everything else attached to it. Unless the plane is accelerating, the plane does not "fly away from" the casing as soon as it is ejected. In a bizarre hypothetical world where this was happening in a vacuum, the casing would drop straight down with respect to the plane.

What does happen is that the ejected casing is no longer attached to the airplane, and so there is no longer any force (the airplane's thrust) to counterbalance the "negative acceleration" provided by air resistance. To a stationary observer on the ground, the casing slows down, the airplane does not. To the observer in the plane, the casing "accelerates" in the direction of the tail of the plane in addition to the acceleration of gravity.

Hi Larry, as you point out the cartridge has the velocity of the plane at the moment it becomes a free body. However the cartridge's acceleration is zero relative to the direction of the plane's flight at the instant it is free of the gun and instantaneously goes negative since there is no force acting on it that will maintain the exit velocity (conservation of energy in a closed system). With negative acceleration, the velocity decreases. If the plane were flying in a vacuum -- no force vectors due to air movement -- the only force acting on the free body cartridge is gravity. The cartridge would drop in a parabolic arc relative to the still-moving plane as it's forward velocity decayed and gravity pulled it straight down. And since the plane continues to fly, it is moving away from the cartridge at every point of it's descent. Put the plane back into its real environment and, in addition to gravity, there are strong moving air currents from wind and the propeller that instantly act on the cartridge. Instead of a gradual negative acceleration as in the vacuum scenario, there is instantaneous large negative acceleration from real forces acting on the cartridge so that the forward velocity almost immediately drops to zero and then just as quickly climbs in whatever direction these forces point. The forward force that the cartridge has to resist the force of moving air currents is F = ma. But the mass of the cartridge is very small and the acceleration component is zero to negative. Fair to say that the plane is flying away from the cartridge the instant it leaves the gun.

If the observer is firing ahead of the plane - say at an enemy approaching from the classic 12 o'clock high position, then the observer is directly behind the gun with respect to the slipstream. It is possible for the spent casings to hit the observer.

A similar issue might occur when the observer is firing directly behind the plane - while the casing won't hit the observer, the observer may provide enough deflection of the slipstream to prevent it from carrying the casing away.

Yes, if a gunner was firing forward over the wing they might get hit by spent casings. But I can't believe that the observer would ever provide enough deflection to negate air currents. If they could, then they could also get into the same position and light a cigarette. Just not possible, air currents are going to wrap right around him.

I seem to recall pictures of WWII waist gunners surrounded by spent casings and disintegrating links, and at least one post-mission picture of the ground crew pushing them out a waist door with a broom. Too much material to contain in a bag that wouldn't get in the way? I'm sure the WWII gunners learned to shuffle rather than step so as not to trip on a loose casing. Maybe the WWI observers did too?

I don't think so. There would be too great a risk of loose cartridges jamming a control wire or pulley. I think if there were a lot of cartridges ending up in the observer's cockpit there would have been attempts to contain or eliminate the problem. I haven't seen any pictures showing this kind of system for the Parabellum gun -- there were attempts to scavenge heat from the exhaust to keep the breech mechanism warm, but I haven't seen even one picture of a cartridge collection system. My guess is that the number of cartridges dropping into the cockpit was few or none.

Then, too, there's the question of just how short each side was of the raw materials (or time/energy/facilities) to make new brass casings. Does anyone know? If this was a concern, maybe having the spent brass drop into the well was intentional. Just a thought.

Actually, even when it would have been practical to recover the spent brass cartridges, the Germans chose to eject them. For example, the Albatros D-series scouts used curved tubes to carry the cartridges from the two guns down until they fell out of holes beneath the plane. I always found this choice strange for a country that was blockaded and where all critical materials were in short supply. You've got to believe that a typical Jasta could have recovered kilos of recyclable brass every week.

[Ransom E. Olds]

Woodman's Early Aircraft Armament does show a brass-catching bag attached to a Parabellum, but the gun is mounted in the prow of a pusher and pointed forward. Woodman said they didn't want the cases to hit the propeller. Ransom

[Riffster]

Originally Posted by Patrick

If the plane were flying in a vacuum -- no force vectors due to air movement -- the only force acting on the free body cartridge is gravity. The cartridge would drop in a parabolic arc relative to the still-moving plane as it's forward velocity decayed and gravity pulled it straight down. And since the plane continues to fly, it is moving away from the cartridge at every point of it's descent.

I just want to make sure we get the physics right. In a vacuum, there is nothing to make the forward velocity of the cartridge decay. It would continue to move forward with the same velocity, always remaining directly underneath the plane (assuming the plane also maintains steady velocity). While the cartridge would follow the "ideal" parabolic arc as seen by an exterior, stationary observer viewing the event from the side, to the observer in the plane, the cartridge would appear to drop straight down.

One of the difficulties in bombing is to determine the "trail" of a bomb. This is the difference between impact point of the "ideal" curve (where the bomb is always directly beneath the plane) and the actual impact point caused by the decay of forward motion of the bomb (due to air resistance). This is a complex equation that takes into consideration the time of fall, density of air, aerodynamics of the bomb, and the initial speed of the airplane with bomb attached. (This is before you consider wind vectors, and only addresses the difference along a line parallel to the plane's direction of travel.)

A tumbling, spent case will have greater air resistance than a bomb and being relatively lighter as well, will decelerate much quicker. But I question whether the deceleration is as instantaneous as you claim. Of course, I haven't even done any back of the envelope calculations, much less tried dropping something similar from a plane. You may be right. Maybe we could settle this by tossing a .30 cal. cartridge out a car window at highway speeds and see if it immediately decelerates or keeps up with the car for a bit? In any case I think saying the plane "flies away from" the cartridge is ambiguous and potentially misleading.

I can't believe that the observer would ever provide enough deflection to negate air currents. If they could, then they could also get into the same position and light a cigarette. Just not possible, air currents are going to wrap right around him.

Maybe the observer can't provide enough deflection; I'm just speculating. I'm not saying that his body will create a "dead zone," but he will create some reduction of the full force of the prop wash/slipstream in some space "behind" his body (i.e., in the direction of the plane's tail). Yes, air currents will wrap around him, but they will be turbulent and chaotic for some distance and so may not provide enough force in the right direction to slow the spent brass enough that it doesn't fall in the observer's well. (This is a far cry from saying it would be calm enough to light a cigarette, even with a "windproof" lighter.)

As for the WWII gunners, this isn't the photo I was thinking of, but you can see there's no collection bag, and the description confirms that the spent casings collected on the floor of the plane, at least for the waist gunners.Skylighters, The Web Site of the 225th AAA Searchlight Battalion: Photo of the Week -- 10 March 2003 That said, I have seen other images of WWII waist guns on other planes that DO have collection bags attached to the ejection ports.

As hoodoo said, many of the gunner's cockpits seem to be separated from the rest of the interior of the plane, so if brass did fall in it wouldn't necessarily get caught in control lines or pulleys. Of course, looking at some of these openings in the WWI planes, they weren't much bigger around than the gunners themselves, so it wouldn't take much deflection at all for spent brass to miss the opening and fall outside of the well.

What it comes down to is this: I don't know. I'm just trying to come up with the different things to take into consideration. Short of someone with a working machine gun going up in a WWI-design plane and firing off some blank rounds, I suppose we either need a post-mission picture of brass on the floor of a gunner's cockpit or maintenance diaries specifying that some ground crewman removed the spent casings. Or, alternatively, someone remarking on the rarity of needing to fish them out after a mission. Of course, it's also possible that it wasn't worth documenting these things if they happened.

-Larry