Aircraft guns and bullet behavior

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islandee
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Aircraft guns and bullet behavior

Postby islandee » 04 Mar 2014 11:16

I've been looking at a bridge here in northwest Thailand which still displays minor damage from WWII in the form of three shell holes.

I've completed a first draft for a webpage on the subject here: San Khayom Bridge. But the page is weak regarding aircraft armament and bullet behavior --- ie, what might have caused the damage. I have necessarily made a number of assumptions which I'm hoping someone might comment on. For example:

  1. Could BMG50 rounds have produced Hole Nos. 1 and 2? (if not, what?)
  2. Do shells behave as I've guesstimated when they hit structural steel (enlarged holes, circular and oblong)? (what might alternatives have been?)
  3. What might have caused the two-inch plus diameter Hole No. 3?
  4. What aircraft, or otherwise, might have used these guns?

I thank you.

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LWD
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Re: Aircraft guns and bullet behavior

Postby LWD » 04 Mar 2014 14:22

If you don't get an answer here soon you might try:
http://forums.delphiforums.com/n/main.a ... 2125131432

The field of terminal ballistics can get rather arcane at times.

A general rule if the bullet impacts at an oblique angle is that if it penetrates it's vector will be closer to perpendicular post impact. Yaw will also likely be induced so that any secondary impacts are verry unlikely to produce circular holes. For AP rounds that I've seen the hole left in the plate is usually quite close to the diameter of the penetrator. That would suggest either 20-25mm for the 23mm hole. I'd guess 20mm as the most likely (remember I'm not an expert). HE rounds of course can make bigger holes. Are you sure the damage is WWII and aircraft related? Are their any records of the combat in this area?

islandee
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Re: Aircraft guns and bullet behavior

Postby islandee » 04 Mar 2014 16:04

LWD: You ask a valid question, of course. Here, now, in this rural area particularly, one can never be sure of much historical.

However, there is much tangential evidence that the damage is from WWII.

Jack Newkirk, Flying Tiger Ace, crashed near here while attacking another bridge (N18°35.100 E99°01.370) about 6 km north of San Khayom in March 1942. A Thai Airforce Base was only 2.5 km north (N18°34.06 E99°02.48). Locals remember aircraft buzzing the area and shooting; but when is less than clear. No other bridges in the area display such holes.

Attacking isolated bridges along the railway seems to have been favored as a USAAF tactic. The bridge at Mae Chang (N18°16.480 E99°47.435) was bombed, 86 km ESE, and was sufficiently damaged that one section was replaced after the war. The bridge at Kueng Luang (aka Den Chai) is another example, 110 km ESE. There is one more farther south that I haven't written up yet. Whatever, the San Khayom falls in the category of relatively isolated. This sort of targeting intensified as the Allies came to enjoy great equipment superiority and could make commands to search out "targets of opportunity", and not to return with bombs because it was too dangerous landing with them. To be fair, all the above examples were effectively damaged by bombing, not strafing. Truss bridges were almost impossible to destroy with strafing --- bombing of abutments and piers was required, generally, to bring down bridges. Trusses received damage usually only incidentally when pilots were pursuing rolling stock, which did qualify equally under the term "targets of opportunity". As such, bridges should not have been expected to have received much "incoming".

And rail facilities in general were targeted: Chiang Mai railway station, about 28 km north, terminus of the Northern Line, was destroyed in a large Allied bombing effort in December 1943. More than 300 were killed, mostly Thais. After the war, the station house had to be replaced.

I'm in correspondence with a Thai who is well informed and acknowledges that this is probably the only railway structure standing today in northwest Thailand that displays damage from WWII; which is to say, quite a few railway bridges and stations were replaced after the war.

Nothing here is certain about what happened nearly 70 years ago; but I think the assumption that the damage to the bridge at San Khayom was WWII-related is reasonable. And perhaps the failure to seriously damage the bridge with strafing made it a monument to a "lesson learned" (but then that's rather a dramatic statement).

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LWD
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Re: Aircraft guns and bullet behavior

Postby LWD » 04 Mar 2014 20:06

It could also be that a train was being strafed at the time and the bridge wasn't the primary target. Sounds like you've got a pretty strong position there. The only two USAAF fighters that I know of that carried guns bigger than .50 cal were the P-38 with a 20mm and the P-29 with a 37mm or occasionally a 20mm. British fighters were more likely to carry a 20mm as were some of the Japanese fighters. Do you have a list of squadrons serving in the area?

islandee
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Re: Aircraft guns and bullet behavior

Postby islandee » 05 Mar 2014 01:43

For that, I don't have an answer right at hand; but I should be able to assemble a list: let me do the homework and I'll be back to you.

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Re: Aircraft guns and bullet behavior

Postby islandee » 05 Mar 2014 16:00

I'll try to answer by nationality; I've got tentative lists for The Royal Thai Air Force (RTAF) and the IJAAF. These are based largely on Edward Young's Aerial Nationalism. Units and aircraft types listed may or may not have been present in Northwest Thailand during the whole of WWII. A similar list for the Allies will take longer because of the vast number of potential units and their mobility.

The Thais (FB (Foong Bin)= squadron):
  1. FB 11: Ki-30
  2. FB 12: Ki-30
  3. FB 16: Hawk 75N & Ki-27
  4. FB 21: Corsair
  5. FB 22: Corsair
  6. FB 32: Corsair
  7. FB 33: Corsair
  8. FB 41: Hawk II
  9. FB 41: Hawk III
  10. FB 42: Hawk III
  11. FB 43: Hawk III
  12. FB 52: Corsair
  13. FB 61: Martin
  14. FB 62: Ki-21
  15. Support Wing: Ki-55

The Japanese:
  1. 12th Sentai: Ki-21
  2. 31st Sentai: Ki-30
  3. 77th Sentai: Ki-27
  4. 51st Independent Chutai: Ki-46
  5. 70th Independent Chutai: Ki-15

Aircraft types, with guns (this info is all from Wikipedia):
  1. Vought O2U Corsair, biplane, described variously as scout, observation, and light bomber.
    Guns: 3 x .30 (7.62mm) cal BMG (one facing forward; two on a mount at the rear cockpit)
  2. Curtiss Hawk II, Model P-6, biplane, fighter.
    Guns: 2 x .30 (7.62mm) cal MG
  3. Curtiss Hawk III, Model 68B, biplane, multipurpose.
    Guns: 1 x .30 (7.62mm) cal BMG and 1 x .50 (12.7mm)
  4. Martin 139W bomber.
    Guns: 3 x .30 (7.62mm) cal BMG (located nose, dorsal, & belly positions)
  5. Ki-15: Mitsubishi, reconnaissance ("Babs").
    Guns: 1 x .303 (7.7mm) cal MG
  6. Ki-21: Mitsubishi, heavy bomber ("Sally"/"Gwen").
    Guns: 4 x .303 (7.7mm) cal MG (located nose, beam, & tail positions + 1 x .50 cal (12.7mm) MG in dorsal turret)
  7. Ki-27: Mitsubishi, fighter ("Nate").
    Guns: 2 x .303 (7.7mm) cal MG, or 1 x .303 (7.7mm) cal + 1 x .50 (12.7mm) cal MG
  8. Ki-30: Mitsubishi, light bomber ("Ann").
    Guns: 2 x .303 (7.7mm) cal MG (located one (fixed) in wing + 1 rear cockpit)
  9. Ki-46: Mitsubishi, reconnaissance ("Dinah").
    Guns: 1 x .303 (7.7mm) cal MG
  10. Ki-55: Tachikawa, advanced trainer ("Ida").
    Guns: 1 x .303 (7.7mm) cal MG (forward mounted)

In reviewing all this, I don't see anything that would explain the 2"+ diameter hole; but Wikipedia's specs for each type of aircraft cannot cover all variations. Perhaps such could be relevant.

I'll check IJA AA guns next.

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Re: Aircraft guns and bullet behavior

Postby LWD » 05 Mar 2014 16:53

For American units look for ones using the P-38 or the P-39. If HE ammo was used either might have produced that big of hole with their cannon armament. Spitfires started carrying 20mm fairly early on as well I believe.
This page and/or some of the pages off of it may prove useful:
http://www.quarry.nildram.co.uk/ammotables.htm
In particular:
http://www.quarry.nildram.co.uk/ammotable2.htm
and
http://www.quarry.nildram.co.uk/ammotable5.htm
If you can find the candidate squadrons especially for the US or Britain it's possible that you might even find the attack mentioned in the after action reports. Although it may be listed as an attack against a train or obscure in some other way.
I definitly think about posting this on other forums as well. The one I mentioned above is gun related but the following might be usful as well.
http://www.ww2f.com/
http://www.j-aircraft.org/smf/index.php ... ww;board=1
http://propnturret.com/tully/
Some of them have people who have specialised more in Pacfic matters.

islandee
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Re: Aircraft guns and bullet behavior

Postby islandee » 06 Mar 2014 05:41

Thanks for the leads. Most of them are new to me.

islandee
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Re: Aircraft guns and bullet behavior

Postby islandee » 06 Mar 2014 15:35

With regard to my comment above that I would check AA shell types: I withdraw that comment --- I have been reminded that the RTA unit in charge of rail security in that area used water-cooled heavy machine guns.

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Re: Aircraft guns and bullet behavior

Postby ChristopherPerrien » 06 Mar 2014 17:11

My guess for the first two would be a "pot shot" or any accidental discharge by some soldier with one of these, http://en.wikipedia.org/wiki/Type_97_automatic_cannon

or perhaps one of these, but I think a type97.

http://en.wikipedia.org/wiki/Type_96_25_mm_AT/AA_Gun

The angle looks all wrong for an air-attack for the first bullets. Looks like a flat trajectory holes, and I have seen enough holes in metal plates from a .50 BMG at various angles.

The second has a chance of being from an air-attack, or maybe the same soldier shot at the bridge from a different angle.

islandee
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Re: Aircraft guns and bullet behavior

Postby islandee » 22 Mar 2014 06:54

Thank you for your questions and comments.

With regard to my comment excluding AA shell types, I find that may not be valid after all and have included data about them on my webpage 2.

I've folded comments from this and other forums, plus additional information I've generated (photos and measurements; and a listing of Allied units in the area on page 2) into a major rework of the San Khayom Bridge webpage for possibly teasing out further comments. With regard to bridge engineering questions, I'm in contact with a fellow familiar with riveting methods who has promised comment. I'll also try to query additional forums, including the Int'l Ammunition Assoc.

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Re: Aircraft guns and bullet behavior

Postby islandee » 27 Aug 2014 05:20

After some interruptions, I'm back to the shell holes in the San Khayom bridge. I apologize for the length of this; but I don't know how to shorten it further.

With regard to the hole at Point A, I'm trying to visualize the actions of the aircraft that produced the hole. I'd like to include a sketch on my webpage of the possible / probable path of the aircraft during its attack.

The consensus is that the hole is apparently from a 20mm projectile; and in the Southeast Asia Theater both the P-38 and the Bristol Beaufighter carried HS.404 cannon which used that ammunition. I'll use the P-38 as a subject here simply because I can find more information about it (the Beaufighter might be the better candidate because, beyond its harmonization range, it would have offered a dispersion pattern with its four cannon: such a pattern might more easily have included Point F, the other 20mm impact point on the bridge. Does anyone know of an on-line presentation, including sketches, perhaps a manual, for harmonizing the guns on a Beaufighter?).

So, scenario: a P-38 pilot happened upon a "target of opportunity", a train traveling south out of Lamphun and approaching San Khayom bridge. The pilot reduced altitude so as to fire on the train. Coming into range, he fired for a certain period; then he broke contact and climbed away from the train and the bridge.

Angle of attack: The exit angle of Hole A measures about 12° to the horizontal. In the sketch below, I assume the bullet path was almost unaffected in passing through the comparatively thin bridge plate (projectile diameter was twice the thickness of the target plate). On that basis, I assume the pilot dropped to an altitude matching the 12° allowing an angle of attack of 12°.

A P-38's single 20mm cannon was centrally mounted in the fuselage and its operation was basically point (the plane)-and-shoot. The range of the cannon was limited primarily by the pilot's visual acuity and his properly adjusting for bullet drop (excluding weather, aircraft performance, pilot skills, whatever).

I assume an attack speed of 350 mph: a P-38's cruising speed was 275 mph and max speed was 414 mph. A velocity diagram would look like this for a 12° dive at 350 mph:

Image

The important information here is that, flying at a down-angle of 12° and at a speed of 350mph, the aircraft would have been dropping 107 feet per second.

The idealized flight path would have looked like this:

Image

Of course, the pilot would have disengaged at some point before his aircraft flew into the target.

Questions.

Do these assumptions and the scenario seem plausible? If not, I'd appreciate being corrected.

If the scenario is not too far off:

    At what range would the pilot likely have started firing? (I read that one B-25 gunner had his guns harmonized for 1000 yards: he cautioned that this was useful only for stationary targets)
    And at what altitude would the pilot have had to disengage (how much altitude would be burned up after the pilot tried to gain altitude; and how much spare altitude should he have allowed)?
    Tying these down would also establish how long the pilot might have fired.
Alternate scenario: With the above presented, there is a piece of data which could contradict this scenario, and I don't have the background to interpret it: the "floor" of Hole A is at an angle of about 7° to the horizontal. If 7° (not the 12° used above) were the angle at which the projectile entered the plate, and the exit angle was, as actually measured, 12°, then passing through the plate would have bent the projectile's course by 12° - 7° = 5° (see cross-section).

If the angle of attack were a very lean 7°, then at a range of 1000 yards, the gun would have been only 366 feet off the ground --- with bullet drop compensated for by either, an extra 24 more feet for a total of 390 feet, or a slight tilt upward in angle of attack. And at 500 yards 183 feet (compensated for 4 feet of bullet drop). On the other hand, the aircraft's altitude loss while changing from a down-angle of 7° to actually moving upward would have been less than recovering from a down-angle of 12°. But operating that close to the ground at 350 mph or more seems unlikely (which is why I didn't include it in my main presentation above); though I am willing to be corrected.

Are there any comments about this alternative?

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Re: Aircraft guns and bullet behavior

Postby LWD » 27 Aug 2014 13:49

There are some video's /images of P-38 strafing trains in this google:
https://www.google.com/#q=p-38+strafing+train
perhaps some information can be extracted from them. As far as altitude goes it wasn't unheard of for strafing planes to hit trees.

I'm not the best at photo interpretation but some here and on j-aircraft are quite good at it.

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Re: Aircraft guns and bullet behavior

Postby Felix C » 27 Aug 2014 20:24

I hope this may be relevant although it deals with WW1 .303 but Leon Bennett wrote an interesting piece on aerial ballistics in his http://www.amazon.com/Gunning-Red-Baron ... 158544507X

Follows some of the posts above.
Quite a bit is available on googlebooks here http://books.google.com/books/about/Gun ... hwoz6H9vIC

Now back to your regular postings.

islandee
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Re: Aircraft guns and bullet behavior

Postby islandee » 30 Aug 2014 17:14

Totally by chance, and only after I put this question to the forum, I finally came across a fairly detailed description of the strafing process on the Internet for the 13th Bomb Squadron website in its Combat Tactics: Strafing (Area). The aircraft used was the Douglas A-26/B-26 Invader, apparently for combat in Korea:

". . . from a low level approach to the target. . . . a desired 320 mph. . . . At approximately 3,000 yards from the target, the pilot pulls up to 300 ft. above the terrain, spots his target, . . . At approximately 1200 yards from the target, the pilot enters a gentle dive, lines up the target in the sight and commences his strafing run. Firing bursts should be short, starting at 1,000 yards, and breaking off at 400 yards. Gently fan the rudder . . . to effectively cover the target area. . . . Airspeed . . . is the best defense against enemy ground fire while on a low-level strafing run. . . ."

Key factors in the description are a starting point, one, at an altitude of 300 feet (100 yards) and, two, at a distance of 1200 yards from a target. If the pilot flies a straight line toward his target, his angle of attack will be (rounded):

arcsin(-100 yards / 1200 yards) = -5°

The minus sign is a convention indicating he's flying downward, not upward. At the 400 yard breakaway point, he would have used up two-thirds of his 300 foot altitude; so he would be at 100 feet.

The conclusions to be drawn are: one, an angle of attack as shallow as -5°, and two, a breakaway altitude of 100 feet, are both realistic, especially during actual combat conditions. Moving into terminal ballistics, an assumption I think is reasonable is that a comparatively weak, thin walled target, such as 10mm of structural steel will not much alter the course of a 20mm AP round; if true, then a projectile's exit angle from a target should be close to the angle of the source gun; in the case of an aircraft, that would be its angle of attack. As a consequence, the 12° exit angle measured at Hole A in the 10mm plate at the San Khayom Bridge indicates the bullet came from the air, not the ground. If the assumption is wrong and say instead that the 7° "floor" of the hole represents the impact angle, that 7° angle still does not reach the 5° angle calculated above, and so is still not in conflict with the conclusion that the projectile came from the air; ie, an aircraft.

Dramatic confirmation of the 13th Bomb Squadron's strafing instructions lies in the gun camera footage suggested by LWD. As he noted, "it wasn't unheard of for strafing planes to hit trees." From Aerial Dogfights and Strafing, this sequence where a pilot parts the trees to finally come about level with a locomotive he's firing at (frames enhanced by IrfanView):

Image

There is much more gun camera footage of low-altitude strafing, especially at 1945 New US Gun-Cam Footage from Germany!: while the video has been dramatized with the addition of pilots' heads and unrelated frames of aircraft in general flight, the gun camera footage seems genuine.

Another item in the 13th Bomb Squadron instructions suggests the same source for Hole A could have caused Hole F: during the firing run, "Gently fan the rudder . . . to effectively cover the target area." But that only makes differentiating the P-38 from the Beaufighter more difficult; and I don't think that a harmonization chart for a Beaufighter will help in choosing the more likely attacker. The 13th's instruction is confirmed in an excerpt from the Military Channel at American warplanes strafing enemy trains during World War Two at 04:08-04:19 and again at 04:40-04:59.

I guess my question now to the forum is: does anyone see any major problems in this tentative explanation?


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