Why so few planes that fired thorugh the propeller hub?

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Hello Stona
my points are
a) usually pilots underestimate the range
b) to hit from those distances one had to have guess the range fairly accurately, otherwise they could not correctly compensate the shell drop by aiming the certain amount above the target necessary for a hit. In Puhakka's case, because the fairly low muzzle velocity of the Oerlikon the amount was IIRC some 3m over. The sight helped on this as in range estimation, in both cases pilots knew the span of their targets. In hub mounted cannon that was what was neede but with wing mounted cannon one needed also take into consideration the amount of lateral off needed which was IMHO more difficult to estimate. So there was a considerable amount of luck needed. When I now think it, maybe Puhakka was helped by the fact that only one of his cannon worked, the jaw this produced might have helped him by reducing the lateral off.

Juha
 
Hi Juha.
Point one, absolutely correct. RAF pilots in the BoB were revealed to be opening fire at 1500 yards when they had estimated the range at 300 yards. Things did get better but the tendency to underestimate remained.
Point two. I don't believe any pilot correctly estimated the various criteria needed to make a successful air to air shot at extreme ranges. They took a best guess and hoped. Those with more experience were more likely to guess correctly, but it was still a slim chance. The occasions when this actually worked were few and far between. The arrangement of the weapons had little to do with it, luck had a lot.

My point is equally simple. The average pilot, firing from ranges for which his weapons have been harmonised, is more likely to score hits with his numerous wing mounted guns than an average pilot with a centreline weapon. In other words Sorley, way back in the mid 1930s, was correct.

Cheers

Steve
 
Hello Stona
I more or less agree with the first part of your message and completely agree with the end. In theory during the fairly long chase Duke would have had opportunity to utilise the range estimation feature of his sight because he knew the span of 109. I don't know if he did that. Anyway his purpose was to try to slow down the enemy by hitting him or missing him so closely that he would began to wave. Both Duke and Puhakka very good fighter pilots, the DB was Puhakka's 5th confirmed kill (and his 4th in reality) and he ended the war with, using the British system, 41 confirmed, 5 probablies and 10 damaged.

Yes, it was generally thought that an average pilot could only hit from fairly close range with no deflection. The sight really helped an average pilot only if his wings were level otherwise he needed to compensate the error produced by the bank.

Juha
 
First of all, the numbers I quoted for machine guns were not estimates. They are from what I recall from gun tests in various publications over the years. Perhaps my memory is faulty but I don't believe so.
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It would be interesting to see these "tests". There can be a world of difference between the mechanical accuracy of a gun bolted down to a large immovable bench and a gun fired from a service bipod/tripod sitting on the ground. In the first case you are testing the accuracy of the gun and ammunition, in the second case you are testing the ability of the bipod/tripod to support the gun. It may give you the "practical" accuracy of the gun/gun mount in service conditions but tells you very little about the gun in question.


Some rifles do better than average but there was a minimum acceptable standard that a gun had to pass for acceptance and it was 3-4 minutes of angle in some countries. Again this is with "test" ammo, whatever that was (selected service lots?) . Ammo was often tested in special test guns, Universal receivers with large, heavy barrels.


Think about what you just claimed for your water cooled Browning: perhaps a 1 inch 10 shot group at 27 yards? Would that be around 4 MOA? I am curious as to how these guns were tuned. Perhaps all shots touching is a bigger group than that?
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Not one inch but but more like 1 1/4- 1 1/2.

Information is from an article published in the "Infantry Mailing List" of 1937-38 and reprinted in Johnson and Haven's book Automatic arms. Section is 12 pages long and includes diagrams of shot patterns for examples of loose packing gland, proper headspace, headspace 3 notches loose, head space 6 notches loose, Play in elevating mechanism, Play in traverse mechanism, looseness in the cradle, Cycle rate ( each gun needs to fire smoothly, faster or slower is not always better), supporting the trail and riding the gun (unnecessary pressure on the grip). Please note that of the 8 things listed only 3 have to do with the gun itself. The rest have to do with the mount and firer.

Yes. Yes. Service rifle bullets were somewhat different. a bit more later.

Your 14 MOA correction is interesting but a bit simplistic. The correction isn't a constant MOA. The MOA increases as range increases.
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I beleive I covered that with " doubling the range usually increases the wind drift about 3 1/2 times for the same value wind"

In any case, the REAL question is how different a rifle caliber round flies as compared to a 20 mm cannon round, and I do not believe there is a great difference.
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Problem here is that there is a lot more variation between the 12.7-13mm bullets and the various 20mm shells than there were between the various rifle rounds. The champs among the high production guns are the US .50 and the Russian 12.7mm, high velocity and well shaped bullets, good enough that they beat any WWII 20mm gun for truly long range fire( over 1000yds). German 13mm and Italian/Japanese/British 12.7mm guns are not only lower velocity they use lighter bullets of poorer sectional density and poorer shape. The 20mm cannon are even worse as far as difference goes. The 20mm Hispano will match the US .50 out to around 600yds pretty well (within a few inches) It's higher sectional density offset by it's much worse shape. The 20mm Hispano is as good as it gets. The rest of the 20mm shells are either as good (roughly) as the Hispano and fired at a much lower velocity or they are lighter with poorer sectional density and no better shape and still fired at lower velocity (except the Russian 20mm)

Time of flight for German rounds to 300 and 600 meters.

7.92mm AP 10 g............0.453..........1.159
13mm HEI 34 g.............0.49............1.22
15mm HEI 57.5g...........0.357...........0.816
20mm HEI 92g..............0.551..........1.428 (from a MG/FFM)
20mm HET 117g...........0.477...........1.101 (from a MG 151)
30mm HEI 330g............0.696...........1.66 (MK 108 )

From a different source: time of flight to 600 yds for US .50 cal. 0.72 sec and for the 20mm Hispano 0.84 sec. Granted it is a 10% shorter distance.

I believe the discussion was about whether there was an advantage for centerline armament for an expert marksman which presumes he knows how to judge distance and proper lead.
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At 300 yds/meters most guns will hit the same ( with a few exceptions) at 600yds/meters the differences are starting to show up a lot more and at longer ranges it gets worse quick. From a German test the 20mm Hispano was still doing 500m/s at 600 meters, the 20mm mine shell from the MG/ffM is doing 281m/s, the 20mm HET from the 20mm MG 151 is doing 422m/s and the 30mm mine shell is doing 254m/s. The Hispano shell is traveling at 600meters as fast as teh 3MK 108 shell was at the muzzle.

The Germans used mixed belts of ammo. They mixed the 92g and 117g shell in the same belt. Now in either the MG/FFM or the MG 151 the muzzle velocities changed for the two shells which means the difference in timing/trajectory isn't quite what the above figures suggest but our intrepid pilot in either a German or Soviet plane trying to fire at 600 meters plus had better pick which gun he is going to try to hit with because different guns in his plane will NOT hit to the same time and place. It the case of the German pilot he may have to decide which type of ammo he is trying to hit with from his 20mm MG 151 as the mine shell and the heavier rounds will not arrive at the same point in space/time.

Before I get jumped on, this is referring to long range fire, like in excess or well in excess of 400 meters. At closer ranges the differences fade to insignificant the mis-matched guns co-ordinate very well. This is not which country was better ( I have left out rates of fire, weights of guns, weights of ammo, weight of HE, etc) this is only referring to problems in getting hits at long range.

In theory the fuselage mounted weapon had a lot going for it, in practice it had a lot of problems with implementation in WW II. Designing a weapon system (gun/engine/airplane) that only a very small percentage of pilots can make use of is not the smart way to go.
 
Shortround6 said:
Designing a weapon system (gun/engine/airplane) that only a very small percentage of pilots can make use of is not the smart way to go.
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Yes, and having that small percentage use the same system that is designed to optimise the performance of the majority will not suddenly render them less effective.
Cheers
Steve
 
A good marksman - an expert - will be able to take his weapon of choice and use it to his advantage every time regardless of what can or can't be done. If Hartmann had a spitball gun that was only effective at 1200 yards then that would be the parameters that he would work within to succeed. If I remember that was one reason Hartmann was so successful- he was able to get in very close without being noticed. In that instance I'm sure many of us would be successful. He was a master of it - and I would say regardless mostly of what the weapon was.
 

That is an awfully small telescope for high precision work.
How many shots in your groups?
I am typically shooting 5 shot groups because I believe it is enough to be fairly representative without being overly wasteful of ammunition. Statistically you can work out what a 10 shot group size would be from this also.

A lot of the lack of accuracy in military rifles is due to the ammunition that was issued. As an example with a particular M1 Garand (glass bedded): With fairly good handloads 1.00 inch to 1.25 inch. With M72 Military Match ammunition: 1.75 inch. With Lake City M2 Ball ammunition 2.5 inch. With French .30-06 FMJ ammunition, 3-5 inch groups were more common. The accuracy varied a bit but this was typical with this gun shooting off sandbags and using a 3-9X telescope.

In theory, a sporter stock should improve accuracy but personally I have not found that to be the case. My experience is that unless there is a problem with a pressure point somewhere, a fully military stock will often shoot as well as what you are describing at least with Mauser 98s, M1903s, Swedish Mausers, Mosin Nagants and Lee Enfields in both No.1 and No.4 and with Pattern 14 and M1917s. I don't have enough experience with Japanese or French bolt actions to come to any conclusion. This is not to say they all shoot this well, but at least on a couple examples that did not, taking the rifle apart revealed something that was wrong.

Hope this helps.
Pardon the digression.
- Ivan.
 
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Shortround6 said:
Not one inch but but more like 1 1/4- 1 1/2.
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That works out to 4.5 to 5.4 MOA which is still amazingly accurate.


I actually have the book "Automatic Weapons" by Melvin Johnson. Is this the book? Do you happen to know where in the book. Now I just need to find where the book is hiding.


Sounds like the centerline gun arrangement on the P-38 is quite good!
If the US and Soviet .50 cals were quite good and 20 mm and 12.7 mm guns from other countries were worse, then how different would a German MG 151/20 and the MG 131 be?


Many years back, I was interested in getting good accuracy from the Sierra 168 grain MatchKing HPBT bullet out to 1000 yards when launched from a .308 Winchester. This bullet has a reputation for excellent accuracy as long as it stays above the speed of sound. The ballistics programs I was using were predicting that when launched at 2600 fps, the velocity at 1000 yards was still around 1300 fps (400 M/s).

Turns out that the bullet starts to misbehave when it enters the trans sonic range and not just when it drops below the speed of sound. The point here is that 1300 fps is still fairly decent velocity though actual field conditions would likely bring the velocity down a touch.


Not disagreeing with you here. The general issue weapon has to suit the majority of the operators. Thus the AK-47 is a fairly good weapon for general issue. The point I believe we were discussing is whether or not an expert would be able to better utilise a different weapon. I believe an expert marksman would be more effective with something like a M14 rifle even if the average soldier would not. Similarly I believe an expert aerial marksman would see an advantage in centerline armament on a fighter even if the average pilot would not.

- Ivan.
 
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Ivan1GFP said:
folks are playing all kinds of tricks to juice up the MV to keep a HPBT Match bullet above the trans sonic range out to 1000 yards.
.
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Keeping off topic (apologies). This is exactly what musket target shooting requires. Except at 100 metres instead of 1,000 yards. As soon as a round musket ball drops into the transonic range it destabilises and accuracy flies out of the window.

The usual choice is a large load of powder to chuck the thing down range hard enough for a reasonably flat trajectory as far as the target so that it is still supersonic at 100 metres (or 50 metres in my case).

In principle the alternative is to load light to ensure it leaves the muzzle below transonic speeds and thus remains stable until it simply runs out of puff. However you have trajectory of a badminton shuttlecock and the time of flight leaves the ball vulnerable to wind even at 100 metres. With care you can actually see the thing in flight.

There were good reasons why musket fire beyond 100-150 yards was not a battlefield norm and this is one of them. Once the ball drops to transonic speed it is beyond the control of even the best marksman and the holdover is measured in yards not inches.

Smooth bore muzzle loader hunters adjusted fixed sights with variable loads so that the point of aim and point of impact matched the sights at different ranges. But now we are veering too far off the topic.
 
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yulzari said:
But now we are veering too far off the topic.
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I dont think so the topic has become more about range than the original post. Good topics mature and spread out and so far this is a good one long may it continue in any direction the discussion takes it. Neville Dukes Golden shot sounds unbelievable but is backed up by some evidence. Stranger things have happened in life, I once saw someone take aim with an 100 year old plus Enfield Martini .303 lever action single shot at a Pigeon at approx Football pitch distance and blow it out of the sky, no one was more surprised than the shooter.
 
I actually have the book "Automatic Weapons" by Melvin Johnson. Is this the book? Do you happen to know where in the book. Now I just need to find where the book is hiding.
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"Automatic Arms" is copyright 1941. "Automatic Weapons" is copyright 1945, it may have a number of changes.



Sounds like the centerline gun arrangement on the P-38 is quite good!
If the US and Soviet .50 cals were quite good and 20 mm and 12.7 mm guns from other countries were worse, then how different would a German MG 151/20 and the MG 131 be?
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Again, thanks to Zeno's.

Time of flight is within 1/10 sec at 600yds, I don't believe the German guns matched quite that well.
In one of Tony Williams books there is a trajectory chart for the Me 410B-2/U1 armed with two MG 131s, Two MG 151/15s and a 5cm BK 5. the last two are rather out of the ordinary but the MG 131 has a trajectory that starts about 90cm below the line of sight, crosses the line of sight at about 120 meters, peaks at about 80cm above the line of sight at just over 300 meters and drops to the line of sight at 500 meters. The curve on the chart stops at that point but if you were to extend the curve it may be 80cm low before it gets to 600 meters and over 160 cm low before it gets to 700 meters. In a single engine fighter using it as cowl gun with the guns closer to the line of sight you either have a shorter distance to the the 2nd crossing of the line of sight or an even higher 300-350 meter trajectory point. 80cm=32in= 2 2/3 feet. On a bomber ypou will still be on target but it seems a lot is made of wing guns being off 3-4 feet laterally at combat distances.


Off topic but many years back I pulled pit for a man using 168 grain bullets from a semi auto at 800, 900, and 1000yds. He did pretty good at 800 and 900, probably better than I could with a service rifle. At 1000yds 16-17 out of 22 rounds fired showed some evidence of tipping as they went through the target. Everything from slightly oval holes to full profiles of the bullet, size of the "group" went from several feet at 900yds (extreme spread) to just about the entire 6' x 6' target at 1000 yds. I can't remember if there were one or more misses. I don't think any one who was in the pits that day would be tempted to use 168s at 1000yds.
 

.308 Winchester/7.62 NATO goes subsonic at around 880 yards, from all the data I have ever seen (give or take a few yards depending on bullet choice, and I am talking about high ballistic coefficient bullets) so beyond that point accuracy is going to suffer very badly. I believe US snipers consider 800 meters to be the max effective range for a .308 Winchester using a sniper rifle.
 
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Bullets behave differently. The old 172-173 grain boat tail military bullet may not have had the sub one minute of angle accuracy of the Sierra 168 grain but it tended to stay point on when when it went subsonic. Or at least with the same MV as the 168 grain Sierra it stand supersonic longer.
The other things affecting "max effective range" is range and wind drift.

The difference between 900 yds and 1000 yds with a .308 and 2600fps MF using 168-172 grain bullets ( generic ballistic calculator) is about 100 inches of drop ( 8ft) and in a 10mph cross wind another 20 inchs of drift. Without a good laser rangefinder or GPS on known co-ordinates even a 50yd error in range estimation can mean a vertical miss of several feet and horizontal miss of about 1 foot even the wind is guess 100% right.
 
Just getting nack to this. Replying to Marcel's post #14.

There were NO radials with cannons firing through the hub. The center of the master rod was NEVER in line with the prop hub at any point during rotation and you can't fire through the center of it since there are rods in the way.

The FEW radials with guns firing through the engine all had single-row radials with the guns firing between the cylinders, as on the Boeing P-26. It was one solution but not a good solution since fighters were very quickly going to require multi-row radials, effectively meaning no space between the cylinders at all. They had to find other places for the guns.
 

Properly stated: A 168 grain Sierra (or Hornady or Speer or similar) HPBT bullet at a muzzle velocity around 2600 fps goes TRANS-sonic at around 880 yards.

The launching cartridge doesn't matter. The muzzle velocity does.
The same cartridge in different guns has different effects: There is going to be a velocity difference when launched from a 18 inch barrel (H&K G-3) versus a Remington 700 Varmint Gun with a 26 inch barrel versus a Palma rifle with a 30 inch barrel.

One other thing worthy of note is that the velocities specified on US Match / Sniper ammunition is actually instrumental velocity at about 78 feet from the muzzle. Actual MV is typically about 50 fps faster. The velocities I have recorded are at 7 feet from the muzzle which I do not correct back to actual MV. The difference is so little (guessing 3-5 fps) as to be below the shot to shot variation and about 1/5 of the standard deviation in velocities (about 15 fps) on a good load.

There may be others, but these are the issued US 7.62 mm NATO Sniper cartridges I know about: M118 (173 grain FMJ-BT), M852 (168 grain Sierra HPBT or similar), M118LR (175 grain Sierra HPBT). The Federal .308 Gold Medal Match I do not believe is issued, but very similar to the M852. There is a difference: The M852 is not SAAMI spec because it is typically loaded to OAL of 2.810-2.815 inch which is over max of 2.800 inch.
Of these, I believe only the 168 HPBT is known to have serious yawing / stability issues when dropping through the trans sonic range.

Target shooters have all kinds of ways of working around this issue and the fact that the .308 / 7.62 cartridge has so little case capacity. Most of these methods exceed SAAMI specifications one way or another and thus can no longer be considered a .308 Winchester cartridge in my opinion.

I can go on quite a bit more about this topic, but I am pretty sure this already is pretty seriously off the original topic. I picked examples of .308 Win ballistics to illustrate what could be expected of Rifle Caliber machinegun projectiles such as those used in the US .30 cals and the Germal 7.92 x 57 mm.
If this topic (and I am sure a bunch of us like to talk about guns), we should move this to another thread and post a link in this thread.

Thanks.
- Ivan.
 
Getting way back to topic: apparently the RAF did consider the use of a central cannon but rejected the idea because it was calculated that a short burst from one cannon wouldn't put enough shells on target to do decisive damage; a paper was written in 1935 by Sqn. Ldr. Sorley stating:

(bold underline added)

National Archives: AIR5/1137 Air Fighting Committee, 15th meeting Further Review by O. R. of the Use For Air Fighting of a Gun of Larger Calibre than Machine Guns 26/07/35

From: Niall Corduroy Whirlwind: Westland's Enigmatic Fighter [Fonthill 2013] page 13
 
Tell us how you really feel DonL!

Ivan, I believe point blank means the bul;let will hit where it is aimed exactly. So a point blank range of 5800 yards doesn't say anything about rise and fall, it says the bullet will hit the belt at 500 yards if aimed exactly there.

To find the bullet rise between the gun muzzle and the impact point at 500 yards, you have to go to the ballistics characteristics of the round with the length of the barrel or, more correctly, the distance between the chamber and the gas port known. However, if you are interested in hits, the rise doesn't matter if you can hit the target ... unless you are shooting under something. That's rare in combat.

As for the differening ballistic characteristic of armament in fighters, if you shoot anywhere NEAR the point blank range, it doesn't matter. If you shoot closer or farther away by a significant amount, it does. That;s why the Zero allowed the pilot to select the MG, the cannon, or both ... to account for shooting at bombers from a distance. He could turn off the MG and just use the cannon.
 
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