Talk:Gun harmonisation

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Central Guns Convergence Point

TL;DR: Planes often have a vertical convergence point as well as a horizontal convergence point. For example, the P-38's vertical convergence point was about 375 yards. The wording of this article incorrectly implies that all convergence issues go away by moving the guns from the wings to the nose. Centering the guns only eliminates horizontal convergence issues. Alvint69 (talk) 05:19, 11 March 2023 (UTC)

Long Version: The article as it's written strongly implies that if guns are centered on the plane, convergence considerations are rendered moot. This is not the case if the plane employs more than one type of gun, and if the fired projectiles have different ballistic properties. For example, the P-38 employed both .50 cal machine gun and 20mm cannon ammunition. These bullets have different ballistic arcs, so you still needed to configure the guns with a vertical convergence point, and engaging a target away from the convergence point still renders the shot less effective. The effect is less severe than with horizontal convergence (wing-mounted guns) but still present. See picture.

vertical convergence

Alvint69 (talk) 03:03, 10 March 2023 (UTC)

Wikipedia exists to summarize published sources. If you can find WP:Reliable sources talking about vertical convergence, then you have some leverage here. If not, your suggestion would be a violation of WP:No original research. Binksternet (talk) 03:47, 10 March 2023 (UTC)

Yep, I'll see if I can find something. In the mean time, I've read the currently cited documentation. It only mentions the convergence issue of wing-mounted guns. I suspect that those reading it concluded that removing wing-mounted guns eliminated convergence issues, when in fact there isn't enough information to draw that conclusion. Alvint69 (talk) 08:38, 10 March 2023 (UTC)

I'm not sure if interviews counts as a proper citation, but according to former USAAC Captain Len Gold, this was indeed an issue. However, according to him, they generally "solved this" by simply not firing both weapons at the same time. I'm not sure how that could possibly solve the underlying issue though, and I suspect that they actually did set the convergence point whether they realized that was what they were doing or not (explained below).

Even with a single centrally-mounted weapon, the gun and sight would still have to be calibrated to a certain range. In other words (I'm simplifying a bit), there's always a "virtual convergence point" between where you're aiming (line of sight is a straight line) and where the bullet actually goes (the bullet travels in an arc). This point is at a fixed range depending on the gun elevation, bullet speed, etc. This is exactly the same concept why snipers must know the range to their target.

As far as I know, the P-38 gun sight did not have a separate "cannon mode" and ".50 cal mode" at the time, where you could have an independent range for each. So if the gun site was calibrated at (for example) 300 yards, and it worked for both .50 cal and cannons, he was unwittingly setting the convergence point. That is, all three lines (LOS and the the two different arcs) meet at the same point (300 yards).

Disclaimer: The information I received about this interview was second-hand. I'll keep looking for a cleaner source. Alvint69 (talk) 10:37, 10 March 2023 (UTC)

P-38 pilots very frequently used just .50 cal alone or 20 mm alone, because the speed and arc of the two projectiles were different. Firing both at once would only work predictably at very close range. I have a rather rare book on my shelf about the P-38 written by Warren Bodie. His exhaustive work never mentions the pilots or armorers choosing a distance where .50 cal intersects with 20 mm. My impression was that the two streams diverged and never intersected, the larger 20 mm rounds slowing down and losing elevation before the .50 cal rounds. The cannon shells started out traveling 840–880 m/s while the machine gun rounds were slightly faster at 890 m/s. The bigger round had more aerodynamic drag. It might have been possible to converge the two arcs, but I have never read of it being part of wartime practice. Binksternet (talk) 13:44, 10 March 2023 (UTC)

Hmm, if the two bullet streams diverged and never intersected, do you agree that the gun sight could not possibly be completely accurate for both .50s and cannons? We also know that the plane's guns were indeed pointed upwards slightly to compensate for gravity; that's very well documented, and the gun sight would not work at all if that weren't the case. So it would seem to me very strange if they chose not to elevate the canons a little more to make the sight as accurate as possible for both weapons.

I suppose they could have used divergent paths and had the gun sight's LOS sandwiched between the two paths, but then the sight wouldn't quite be accurate for either weapon. And worse yet, the sight would be inaccurate in opposite directions depending on the weapon used. That would not be great adding to the cognitive load of the pilot at a time when they absolutely did not need additional cognitive load. This would also seem to contradict other parts of this Wikipedia article which states that they used convergence zones early in the war (this would fall under that category), but then quickly settled on convergence points.

I'll also point out that the average pilot in WWII was some 22-year-old kid off the farm who likely did not have any more than the minimum requirements in physics. The way that the P-38 manual is worded seems to reflect this. Knowing exactly how this stuff worked may have been best left to the "eggheads". Alvint69 (talk) 17:05, 10 March 2023 (UTC)

EDIT: Interesting! The most common P-38 variants during the war were the P38J at 2970 examples and P38L with 3810 produced. These models had (yet another) updated steering yoke and there was no longer a separate cannon button--the same trigger fired both weapons. The front button that used to be used for cannons was now used for bombs/rockets. They couldn't fire just one weapon or another if they wanted to. — Preceding unsigned comment added by Alvint69 (talk • contribs) 19:38, 10 March 2023 (UTC)

I've found a scan of the original A/N N2/N2A/N3/N-3A/N3B gunsight manual. P-38s up until the J5 used the N-3A, while later versions used an L-3 gunsight (which operated similarly) due to its differing windscreen configuration. This was a bare-bones (even for the time) reflector gunsight, with a fixed (not adjustable) reticle and no gyro input for computing lead.

The way gunsights worked at the time is the pilot would achieve optimal range (the range where the gunsight is most accurate) by either (a) adjusting the reticle size itself or (b) maneuvering, so that the target's wingtips just touched the reticle's perimeter. Since the reticle on these variants is fixed, the optimal range is also fixed and cannot be changed depending on the weapon. The fact that this gunsight was intended only to be accurate at that fixed range on the P-38 seems to be confirmed by the N-3A variant's explicit omission of a ladder from the gunsight view (see page 13).

Someone who remembers their physics equations better than I do could probably compute exactly what this optimal distance was, based on the reticle size (70mm), its apparent distance from the pilot's eyes, and the average wingspan of likely targets.

This is fairly conclusive proof that gunsight was designed to be most accurate only at a single fixed distance. This by definition means that the bullets must also be the most accurate at that same fixed distance, since that is how gunsight accuracy is measured. The rest is just the physics and math I mentioned earlier; the only way this can be achieved is if the bullets converged at that point. Alvint69 (talk) 01:05, 11 March 2023 (UTC)

It looks like it was about 375 yards. Alvint69 (talk) 04:23, 11 March 2023 (UTC)

Great find! Binksternet (talk) 15:55, 11 March 2023 (UTC)

Alvint69, I introduced your requested concept into the article here. Feel free to improve it, keeping in mind that we shouldn't take this topic too far off on a tangent. Binksternet (talk) 01:31, 14 March 2023 (UTC)

Yards?

When discussing gun harmonisation ranges for RAF machines pre-war or early WW2, wouldn't the distances in feet which are divisible by three actually be ranges in yards rather than feet, for purposes of simple grasp? (For example, 1,200 feet is obviously meant to be 400 yds for a pilot). While I understand that yards are not commonly used now, back in the 1940s, the yard in multiples of tens or hundreds was an easy-to-grasp and commonly-employed standard for distances, which could be estimated visually.

Might it perhaps be worthwhile giving ranges which are listed in feet as yards in addition (to assist readers who do understand and work in them, as well as for general completion or context)? 2A00:23C7:3119:AD01:7C56:8FAE:B945:170D (talk) 00:33, 7 September 2023 (UTC)

You are correct that yards were certainly used back then. In fact the article says "the projectiles met at one or more areas several hundred yards or metres in front of the fighter's nose". But the math to change feet to yards is trivially easy to do in one's head. I don't think we need to make a change. Contemporary American sources use feet rather than yards. Binksternet (talk) 01:03, 7 September 2023 (UTC)