20/20 Hindsight - different armament?

[Jank]

Tony, I have read your work and it is very illuminating.

To cut to the chase, do you think that recoil forces would have prohibited the P-47 from carrying six 20mm cannons (three in each wing as opposed to the two in each wing that the Hurricane carried)?

 

[delcyros]

Old question.
wOULD IT BE POSSIBLE? YES IT WOULD.
Would it be possible without structural reinforcements? Probably not.
Would the recoil effects have an impact for prolonged firing aiming in hi alt?
Depends. The mountings will play a role. But I suspect that the dispersion field of the 20mm would be larger in such an event. Nose / engine mounted guns do behave somehow better than mid wing mounted ones.

regards,

 

[Jank]

To put a sharper focus on the question, you contend that the structural reinforcements (if they were necessary) would inhibit performance due to excess weight.

I have already shown that the weight of 8 .50's with maximum reserves of ammo in a P-47 would weigh about the same as six 20mms with 175 rounds per gun. (My earlier post follows in Italics)
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From: http://www.geocities.com/CapeCanaver...n/fgun-pe.html

"Eight M2 .50's weigh in at 510.4 lbs.

Six Hispano Mk. V 20mm's weigh in at 554.4lbs. (I chose the Mk. V because it would have been later in the war after the P-47 assumed the air to ground role where six 20's would come in handy. If anyone prefers that I use the Mk. II 20mm instead, just add another 150lbs total weight for all six guns.)

The P-47 was designed to carry 425 rounds per gun of .50 cal ammo.

The projectile weight of the 20mm is just under three times that of the .50. If we assume the entire catridge weight of the 20mm is three times the entire cartidge weight of the .50, a 175 round 20mm load would weigh the same as 525 .50 rounds.

That ammunition weight x 6 guns would be equivalent to 3,150 .50 rounds.

With a full ammo load, a P-47 is already carrying the weight of 3,400 .50 rounds.

The point here is that the extra 45 pounds of gun that six Mk V 20mm's would have over eight .50's would be pretty well offset by the reduced weight of the total ammunition load because the 20mm cannon ammunition weight at 175 rounds for six guns would be less than the total ammunition weight of 425 .50 cal rounds for eight guns."

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The only issue then is the recoil of the six 20mm's and whether structural reinforcments (if even necessary) would be too heavy and inhibit the performance of the P-47.

I say no. You say yes.

Shaving the 20mm ammmunition load from 175 to 150 rounds per gun would decrease the weight by an amount equivalent to 450 .50 cal rounds. (six 20mm guns times 25 rounds each = 150 20mm rounds. Each 20mm round weighs as much as three .50 cal rounds so that's equivalent to 450 .50 cal rounds.)

Bottom line is that you could easily add over 100 pounds of steel to each wing with no adverse effects on performance.

 

[delcyros]

I say no. You say yes.

Shaving the 20mm ammmunition load from 175 to 150 rounds per gun would decrease the weight by an amount equivalent to 450 .50 cal rounds. (six 20mm guns times 25 rounds each = 150 20mm rounds. Each 20mm round weighs as much as three .50 cal rounds so that's equivalent to 450 .50 cal rounds.)

Bottom line is that you could easily add over 100 pounds of steel to each wing with no adverse effects on performance.

I am open to change my mind for good arguments, Jank. You very well factored gun weights and projectile weights but what about the space? It certainly is possible to storage 175, maybe even as much as 200 rounds pg into the wing. So, I admit that this is possible in the layout You suggest.
Revising the entire thread isn´t necessary.
However, let´s assume there are no significant recoil issues performance issues, would this armement be benefitial?
For ground attack purposes without doubt, for interceptions as well but what about dogfights in hi alt? The dispersion pattern for 20 mm´s is larger than for .50´s, the effective firing distance drops, with reaching Mach 1.5 at ~450m for the MK V and ~ 600 m for the MKII (compare 900 m for the .50 cal), implying a less favourable projectile trajectory beyond these distances. Still, I believe 450m are well beyond usual firing distance prior to the introduction of computing gunsights late in ww2. However, increasing the velocity by 50% means doubling the chance of hitting in deflection shootings. The battery output of the P-47 drops from ~100 rounds per second to ~70 rps. Take notice that convergance firing requires a good output. All in all the chances to hit are reduced in effective firing range and probability to hit for a rapid improve in destructive force. I personally question that this is a good development, at least with the thread scenario for the P-47 in mind. There were no bombers to take down but Luftwaffe fighter A/C.

 

[Jank]

delcyros,

In post #19 above, you said, "A six 20 mm upgrade for the P-47 would therefore have more worse tradeoffs than benefitials, except for the use as specialized ground attack plane."

I responded in the following reply at post #21 saying, "I thought it was clear that that was what we were talking about.

Since the P-47 did just fine in the air to air role with eight .50's, (Almost all of the other fighters in the U.S. arsenal did fine with six .50's.) an outfit of six 20's would have been an improvement for the air to ground role which the P-47 assumed heavy responsibility for later in the war."

Yes, all along, I have been talking solely about increasing the air to ground destructiveness.

 

[Tony Williams]

I have no specific information relating to the installation of cannon in the P-47; I don't know if it was ever considered. However, on the face of it I doubt that there would have been any problems in fitting six Hispanos. Don't forget that a six-cannon installation was plannned for the Spitfire Mk IV (and a mock-up of the installation constructed) and the Martin Baker MB3 was designed around six wing-mounted Hispanos. These were intended to be air-superiority fighters, not ground attack planes. As they were very small and light aircraft by comparison with the P-47 (which also had a reputation for great strength and ruggedness) then I can't imagine that the big fighter would have had any problems with such an armament, and it would have significantly magnified its destructiveness in both aerial combat and ground attack.

I doubt very much that accuracy would have suffered either: the Hispano was actually a more accurate gun than the Browning (which was one of the least accurate aircraft guns).

The absolute range of the .50 Browning was greater than the Hispano, simply because the bullets had a superior aerodynamic shape compared with the rather blunt cannon shells - the muzzle velocities were similar. However, this was not an issue at normal combat ranges and, of course, the higher the altitude, the less significant air resistance becomes.

The RAF seemed satisfied with four cannon, because they found them enough to do the job. The first jet, the Meteor, was planned to have six Hispanos (possibly because it was expected that the high speeds of jet combat would have minimised firing opportunities) but the installation of the extra two caused problems so they were dropped.

Tony Williams: Military gun and ammunition website and discussion forum

 

[Jank]

Tony, I obviously agree with your opinion on the issue of six 20mm's in a Thunderbolt.

On another note. I'm curious as to your statement that "I doubt very much that accuracy would have suffered either: the Hispano was actually a more accurate gun than the Browning (which was one of the least accurate aircraft guns)."

I was under the impression that the M2 was quite accurate. The .50BMG cartridge is considered an inherently accurate round. I have seen guys consistently hit 8" gongs at 600 yards with rifles chambered for the round.

Why was the M2 not considered accurate? Was it poor mounts that failed to hold the gun securely in place under fire?

Can you point me to some souces for information on the lack of accuracy of the M2?

 

[Tony Williams]

The problem was not the ammunition but the gun. The Browning was a short-recoil design in which the barrel moved to and fro - it was not fixed in relation to the rest of the gun, as it was in the Hispano. Short-recoil guns can still be accurate if they are assembled carefully with tight tolerances, but tight tolerances reduce reliability, so the guns were made deliberately 'sloppy'.

A friend of mine made some studies of aircraft gun dispersion and came up with figures of 4 mils for the Browning and 3 mils for the Hispano (for 75% of the shots - double for 100%). Mountings - and especially wing mountings - added more dispersion due to flexibility, and what the RAF called 'aim wander' (the tendency for the plane to slide off-target while the pilot was firing) had even greater effects. I discovered some official stats about the RAF's fighters with eight .303 Brownings which showed that in ground tests each gun had a dispersion of 10 mils as installed (which is to say a spread of one metre at 100 metres range), with 5 mils for the 75% figure.

Tony Williams: Military gun and ammunition website and discussion forum

 

[Jank]

I hope you stick around on this forum Tony.

I think we're all going to learn a thing or two from you.

What do you think of the proposed .60 cal gun that the USAAF was testing as a replacement for the .50 BMG? They appeared to be more impressed with it than the 20mm guns in use. The following information is taken from the Report of Joint Fighter Conference / Oct. 1944 pp. 165-166.

A 140 lb gun firing a .60 cal round with 3,600 fps (about 1,100 m/s) velocity and a ROF in the range of 500 to 700 rpm. Armor penetration was reportedly 1-1/4" at 600 yards and 20 degree obliquity.

 

[Tony Williams]

I'm happy to stick around for as long as there are interesting things to talk about

There's quite a history behind the .60 cal. The cartridge was originally developed for a tripod-mounted semi-automatic anti-tank rifle, but by the time it was ready ATRs had gone out of fashion. So the cartridge was then spotted as a potential aircraft gun round to replace the .50 BMG. Interestingly, the gun chosen to fire it was a modified Mauser MG 151, captured examples of which had been studied. This gun was designated T17 and about 300 were built, but it was not adopted. I suspect that this was because it was substantially heavier than the .50 Browning and also slower-firing, but it lacked the destructive power of a cannon (note that the Luftwaffe preferred the 20 mm version of the MG 151 over the 15 mm for exactly that reason).

However, the USAAF had a thing about high muzzle velocity, arguing that it would increase hit probability by reducing the effect of aiming errors. They even produced a necked-down .50/60 round with an extremely high velocity. After WW2 when the M39 revolver cannon (also based on a wartime Mauser design) was being developed, a .60 version of the gun featured. But eventually, it was decided to follow the Luftwaffe's example and neck the case up to 20mm calibre to take explosive shells. The result was the 20x102 cartridge which was adopted for the M39 and is still in service in the M61 'gatling' gun used in present-day US fighters.

You can find comparative photos of the ammo on my site (Ammo Photo Gallery, also Military Cartridge Relationships), together with basic cartridge data (Ammo Data Tables).

Tony Williams: Military gun and ammunition website and discussion forum

 

[delcyros]

Don't forget that a six-cannon installation was plannned for the Spitfire Mk IV (and a mock-up of the installation constructed) and the Martin Baker MB3 was designed around six wing-mounted Hispanos. These were intended to be air-superiority fighters, not ground attack planes. website and discussion forum

..and actually now you named the exceptions from the rule, I already mentioned. But as You know, none of them were used in combat so evaluation of them is impossible. No combat prooven fighter A/C exceeded the recoil/weight ratio that much. I have grave doubts about the usefuleness of this armement layout in dogfights.

 

[Tony Williams]

No combat prooven fighter A/C exceeded the recoil/weight ratio that much. I have grave doubts about the usefuleness of this armement layout in dogfights.

It's a question of the balance of advantages. A P-47 with four Hispano Mk V would be carrying less weight than with eight .50s, but would still have the edge in destructive power. A P-47 with six Hispanos would be a little less quick and agile, but enjoy a 50% advantage in destructive power meaning that the guns would only need to be 'on target' for two-thirds as long for each kill.

As far as 'recoil ratios' are concerned the Hurricane IIC had four Hispanos for a normal loaded weight of 7,300 lbs, while the P-47 weighed 13,500 lb (normal loaded). Clearly, a six-cannon P-47 would have had to cope with less recoil per unit weight than the Hurricane. I have never read any suggestion that the Hurricane couldn't cope with four Hispanos. The reason that it was eventually relegated to ground attack was that the basic design simply couldn't be made fast enough to compete with the latest German fighters (its performance was already pretty marginal even during the BoB) - but that applied to the MG-armed Hurri IIB just as much as it did to the IIC.

Tony Williams: Military gun and ammunition website and discussion forum

 

[Jank]

Deja vu.

I advanced the same argument at post #8 about the recoil forces per pound between the Hurricane (4 x 20mm) and the Thunderbolt (6 x 20mm) favoring the Thunderbolt. Delcyros, your counterpoints were as follows:

"Conclusion: This specialized ground attack plane exceeds the 0.1 margin in recoil/weight ratio, which would effect the fighter role of this plane severely." Post #24

"The VVS analyzed MK IIc and found the plane to be well suited in the ground attack and very worse for dogfights (the report quotes that prolonged deflective shooting is very difficult with the heavy recoilforces). Eventually the VVS DOWNGRADED their few MK IIc Hurricane to 0.50 cal guns which again fitted the critical recoil/weight margin of 0.1!" Post #27

I have not read anything, anywhere indicating that the 4x20mm armed Hurricane was a dog in the air to air role compared to it's lighter armed bretheren.

Now keep in mind that the Hurricane experienced an increase in weight by going to 4x20mm's over it's other configured armaments. This, though, would not be so with the P-47. And, adding 100 pounds of steel to each wing of the Hurricane for reinforcement would represent a greater burden than a 100 pound increase to each wing of the Thunderbolt (not that the Thunderbolt would even need such reinforcement as its wings are far, far stronger to begin with).

Just look at the difference in size between the Thunderbolt and the Hurricane!

I hope that made you smile.

 

[Tony Williams]

An afterthought: some Spitfires fought with a four-Hispano armament, which became standard with the Mk 21. As the weight of a Spit was around two-thirds that of a P-47, that obviously compares with six cannon for the Jug. The Mk 21 may have been a shade too late to see action, but I don't think that anyone would it regard as an unsatisfactory fighter...and the postwar Spiteful weighed little more.

Tony Williams: Military gun and ammunition website and discussion forum

 

[delcyros]

Speculation. By the time of Spiteful Spit XXI the thread scenario already changed. Dogfighting at hi alt with 4 mid wing mounted 20mm is an awesome idea in a Spit. The twin 20mm + .3 cal Hi alt Spit suffered from worrisome recoileffects according to RAF reports.
Four 20mm MK V would have around 2 times the recoil energy of two 20mm and four 0.33 cal. And because the recoil is spread out across half as many pulses per second, it's going to be much more severe. Therefore, it's going to be a lot harder to hold your aim with wingmounted 20mm cannons due to off centre recoil caused sideways slew.

 

[Tony Williams]

"Speculation" that the Spitfire 21 and Spiteful would have proved to be successful in combat if the war had lasted slightly longer? Well, I'm not a betting man but would regard a bet on that as being a sound investment, since they were no more than evolutionary developments of the most successful fighter design of the war.

You seem to be combining two issues: the total recoil generated by the armament, and the vibration caused by the firing pulses. These were related, but by no means the same. As I have pointed out previously, the use of the Edgewater mounting reduced the Hispano's peak recoil blow (the source of the vibration) to less than that of a .50 Browning with an ordinary mounting. Instead of being a quick, sharp hammer-blow the recoil-force graph became levelled off, more of a smooth oscillating curve than a sharp on/off.

As far as the total recoil is concerned, you are quite right that a gun jam would cause assymmetric effects which would make it difficult to hold the plane in the aim - but that's one of the penalties of powerful wing-mounted armament. As you can see from this article I favour armament concentrated in or close to the centreline - but that just wasn't possible with RAF or USAAF fighters (with the single exception of the P-39/63). They seemed to perform pretty well all the same.

Tony Williams: Military gun and ammunition website and discussion forum

 

[Jank]

If one of the six guns were to jam on one side, that would represent less of a recoil differential between the two sides than a single jam on one side of a 4 x 20mm armed plane.

If two 20's on one side jammed and one 20 on the other side all jammed, that would also represent less of a differential between the sides.

Two of the 20mm's jamming on one side only would present a problem that could be easily overcome by the use of a firing trigger than allows the four inboard guns to be fired either independently of or in conjunction with the two outer guns. You would still have firepower, albeit less, without significant recoil differential.

This would also be an easy fix (lots of guns had similar alternative/conjunction firing switches) to the issue raised by Delcyros about six 20mm's shaking the plane too much for air to air applications. I don't agree with Delcyros on that point but it would provide a solution to what he views as a problem. You could bring 4 20mm's to bear in air to air (for perfectly suitable lethality) and all six in air to ground applications.

The chance of a single 20mm jamming (which would have been much more common that two 20mms jamming on one side only) on the several 4 x 20mm armed planes was never a consideration that prohibited that armament arrangement. The loss of one 20mm, is a loss of 50% recoil forces on one side. Again, apparently this was not significant enough of a problem to keep Hurricanes, Tempests, Typhoons, F6F Hellcats and Corsairs (4x20mm armed versions of the latter two may have not seen combat but were in the pipeline) from being pressed into service.

 

[Tony Williams]

This would also be an easy fix (lots of guns had similar alternative/conjunction firing switches) to the issue raised by Delcyros about six 20mm's shaking the plane too much for air to air applications. I don't agree with Delcyros on that point but it would provide a solution to what he views as a problem. You could bring 4 20mm's to bear in air to air (for perfectly suitable lethality) and all six in air to ground applications.

Actually the requirements tended to be the other way round. Maximum firepower was needed in aerial combat, because the firing opportunity might only be a fraction of a second. In ground attack, there was time to line up the target and a longer burst was possible. I have read that pilots of the Hawker Hunter jet (possibly overgunned with four 30mm Adens) switched off two of the cannon for ground attack, in order to conserve ammo.

Tony Williams: Military gun and ammunition website and discussion forum

 

[Twitch]

All the P-47 Pilots I ever talked to agreed that the 8 fifties was ample for any role. With bombs and rockets added for ground pounding duties the Jug had enough power ti do the job. But pretty much in the end it didn't matter what any attacking plane had as armament since every German airfield bristled with a huge variety of defensive weapons and they brought down many of the aces who dared to strafe them.

If ever cannons were to be added to the P-47s 4 20mms would have been ample. There is enough space to have 250 RPG or more that way. A few smaller Italian fighters carried 250 RPG for 2 wing-mount 20s. The Thunderbolt certainly could have.

It was not unheard of for one wing's weapons to not fire for assorted reasons. Pilots simply adjusted rudder and went about their business. No big deal.

 

[delcyros]

"Speculation" that the Spitfire 21 and Spiteful would have proved to be successful in combat if the war had lasted slightly longer? Well, I'm not a betting man but would regard a bet on that as being a sound investment, since they were no more than evolutionary developments of the most successful fighter design of the war.
forum

Succesful is relative, by the time of their arrival they would have been totally outdated by first gen. jets, at least over europe.

Do You have an idea about hi alt decremental effects of recoil?
Lunatic actually studied this before. A long time ago he calculated that each round coming out the pair of Ho155's on the Ki-84-1c would slow the plane approximately 11 mph. Such huge recoil forces are a real issue for effective weaponry in any aircraft.
Note that a single Ho-155 has around 1.4 times the recoil of a Hispano 20mm MK II, a six 20 mm upgunned P-47 would have TWO TIMES AS MUCH recoil force, even if it is a slowly applied force as the gunmount Tony suggest, imply. (By the way, interesting aspect, Tony!)
What to expect in dogfights? I suspect that it would have been hard to hold Your aim for more than 2-3 shots.
I would still prefer the eight .5 cal for dogfighting, against ground targets 4 20 mm are decent, as Twitch suggests. Only in case a large air target (heavy bomber) has to be intercepted in a single pass such weaponry makes sense, but their was no such thread scenario in 1944/45 requiring this weaponry.
best regards,