20/20 Hindsight - different armament?

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May 16, 2006
This is mone of those hindsight questions that don't take into account the realities of why fighters ended up with the armaments they did.

Do you think the Americans would have been better off with 20mm's on their planes? Could the P-51 have handled four 20mm's? Could the P-47 have handled six 20mm's? Would the P-38 have been better with three 20mm's in the nose?
Answers in order;


Not without changing the wing, 2x20mm and 2x0.5" easier

No, but could carry 4x20mm

Probably not as the 20mm they were armed with wasn't that good and only had a 120round drum. It was often removed.
P-51A's did in fact have 4 x 20mm cannons.

One look at the wings of the P-51 and Hurricane (both of which had models sporting 4 x 20mm cannons) leads me to believe that the P-47 would have done just fine with 6 x 20mm cannons. It would have been pretty devastating too. As it was though, eight .50's did just fine.

The USAAF toyed with the idea of less cannons vs more .50's when outfitting the ground attack versions of the A-26 and settled on a nose package of 8 x .50's.

The .50 is a fine weapon. Very flat trajectory and decent AP capability.

3 x 20mm cannons in the nose of the P-38 is an interesting idea.
I am confident that such an improvement wasn´t really necessary for the USAAF in ww2 over europe. If the GAF would have deployed far more bombers than they historically did against the western allies, this would make more sense. With the historcial thread scenario in mind there is no justification for this step. Fighter and fighterbomber would be the prime enemy and these can be dealed with by 0.50.
Beside of this, a 20 mm armament would reduce the performance of the Mustang, P-38 and P-47 considerably by pure weight of gun ammo, not to speak of the structural reinforcements necessary in order to deal with the heavier recoil forces of mid wing / outer wing placed 20 mm guns.
This would make more sense to me for the P-38 anyway.
The American, British and German forces generally had well armed planes and probably wouldn't have bothered making any changes. The Russians had excellent weapons and took a deliberate decision to limit (in most cases) the number carried.

Now if you were to ask the Italians and JAAF the same question, I am sure they would have made some changes. The Machi 202 was crying out for a couple of 20mm, it ruined what was a perfectly good fighter.

As for the JAAF you have to almost feel sorry for those pilots going up against B25/B26/B17/B24's with only a couple of HMG's and pretty poor HMG's at that.
On the Mc-202 -- Two .50 Bredas and two 7.7mm's is pretty pathetic.

The Mc-205, too late to see really widespread service replaced the 7.7's with 20mm's. That and the improved engine made it a truly top notch fighter.

The final evolution of Italian aeronautics, the G.55, had three 20mm's, two .50's and an even more powerful powerplant. Very few saw combat though.

I'm not sure that structural reinforcement would have been necessary on the P-47 for six 20's. She might have just limited her bomb load to 500 pounders under each wing instead of 1,000 pounders. There would, of course, have been some trade off in performance due to the additional weight of the guns and ammo.

As has been indicated already. Eight .50's were more than ample for dealing with axis fighters. I suspect that the air to ground role wopuld have been enhanced with a complement of six 20's though.
For 6 20mm guns? Absolutely sure. Take the off center recoil forces into account, that´s pretty much a doubling of recoilforces in case of the P47. No good news for a mid wing configuration. The airframe as a whole would hardly be able to susatin more than 0.1 recoil/weight ratio (above that margin the gunplatform isn´t stable enough for prolonged shootings). And there would arise a need in wing modification to carry the guns, the ammo and more important: To distribute the recoil forces. 234% the recoilforces of a "normal" P47 would make structural reinforcements necessary.

P-47D: recoil force each wing: 167.04 Kp per salvo
gas effects: 8.352 Kp
Total energy: 350.7 Kp

P47-Hispano MK II mod (3 guns each wing, mid wing configuration):
recoil force each wing: 343.2 Kp per salvo
gas effects: 68.64 Kp
total energy: 823.68 Kp

The heavier wings would probably reduce the performance somehow (higher wingload), a significant reduced rate of roll is obvious.

"As has been indicated already. Eight .50's were more than ample for dealing with axis fighters. I suspect that the air to ground role wopuld have been enhanced with a complement of six 20's though."

Agreed upon that.
All that math and those numbers make me dizzy.

All I know is that if a Hawker Hurricane Mk IIC weighing in at 8,000lbs combat loaded can do just fine with 4 twenty milimeter guns, a 14,500lb combat loaded P-47 would handle 6 twenty milimeter guns no problem.

The P-47's wings and other structure is a hell of a lot more robust that a Hawker Hurricane.

The Republic XP-72 had the same wings and tail as the P-47D. It was ordered to be produced having four 37mm guns! The XP-72 had a loaded weight that was 70lbs less than the P-47D.
delcyros said, "The airframe as a whole would hardly be able to susatin more than 0.1 recoil/weight ratio (above that margin the gunplatform isn´t stable enough for prolonged shootings)."

What does this theory tell you about the Hawker Hurricane's ability to handle 4 x 20mm's? How about the P-51A?
Nope. The Hurricane was a stable gun-platform and the recoil off four Hispano II 20-mm cannon caused little, if any trouble to the plane. The Hurricane IIC was the most widely deployed Hurricane and this would not be the case if the plane had problems with the cannon armament.
For me about the P-38 L.
3 cannon in the nose and 2 placed in the wingroots. total 5 cannons. hows that?
I wonder if the two in the wing roots would be too close to the pilot for comfort's sake. The gun barrels would literally be within arm's reach!

All I did is a lot of research regarding the recoil-weight ratios of different fighter airplanes. From what I can summarize, the ratio over 0.1 was never exceeded (including Me-262 and Fw190-those variants to exceed that ratio had a worrisome performance, the MK 103 equipped Fw190 was dropped because it reached horrible recoil forces), except for specialized ground attack planes (e.g.Hurricane, Hs-129, Il-2), which may exceed this. However, all these designs had structural reinforcements to carry the additional loads and recoil forces. The Huricane is no example for a fighter. So if You want to stay with a fighter is primary role, You should care about a decent recoil-weight ratio. I do suspect that the 0.5´s improved the steadiness of the gunplatform for US fighters in comparison with german ones, since US fighters barely exceeded a ratio of 0.05. The gunlayout in midwing position is also worrisome, they have more impact on the steadiness (in direct relation to the off centre rule).
The XP-72 is a good example that You simply cannot put as many weapons on a n airplane as You want. This may also be a reason to drop the project.
The XP-72 was ordered by the USAAF in two armament variants. One was to have six .50's. The other was to have four 37mm cannons.

The project was dropped because the writing was on the wall with respect to what the USAAF wanted and that was extreme range. The P-47N fit the bill nicely with it's 2,300 mile maximum range and souped up performance.
syscom3 said:
Not Good. That would take away some of the fuel cells and decrease range considerably.
Ah well It looks like you have a point here when I see on a X-raydrawing on the P-38. the ammo is no problem if you place them behind the pilot but for the gun It looks like It have to make bout the main thank and the reserve port thank 5-10 % smaller wide. that mean losing 12,5-25 Imp gal (56,8-113,6 litres) fuel. I wonder if there was possible to place some or even more of the lost fuel betwen the supercharger and the engine coolant. like make some modifications in a way when they did on the P-51 B Mustang. Putting a Extra fuelthank in the rear. would that solve the problem whit 5 cannons on the P-38?
If You ask me, the XP-72 armed with four 37 mm would be good for some bad surprises.

We may discuss this more in detail, if You desire.
-impact of altitude-
low latitude=high airdensity=higher drag
-stabilising the airframe due to more airflow, the plane gets "stiffer".
One reason why the Ju-87 could use two 3,7cm BK, however, as such, the plane wouldn´t really execute prolonged sustained firing.
high altitude=low airdensity= low drag, rapidly increasing due to earlier Mach effects
-from hi alt dogfights -or interception- reports it is obvious that the recoil played a huge and often unnoticed role. There are several reports of hi alt engagements with Spits armed with two 20 mm guns. The most often recorded failure was loss of controll due to excessive recoilforces.

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. The Hi alt performance would be terrible when shooting. The recoil force is also much more pointed: originally: 350.7 Kp distributed to eight structural fixpoints in the wing (each 43.84 Kp), post refit: 823.68 Kp distributed to only six structural fixpoints (each 137.28 Kp). This causes a TRIPPLING of powerimpact for structural issues.

I do not question that it might be possible to fit 6 20mm guns into the wing of a Thud but I am confident that this could only be achieved with structural reinforcements of the midwing section, thus further increasing the weight and lowering the performance.
Wouldn't a mix of armament like a Spitfire provide a reasonable gun load, four 20mm is a lot of firepower against a fighter, though I guess you want a lot of quick killing power in a boom and zoom aircraft. 2x 20mm and 2x or 4x .50cal with extra ammo would be a good mix of armament for many situations. Many other fighters got on fine with 2 cannon and 2 mgs, the 109E and Zero for example, they also had tons of mg ammo.

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