NA-91

[twitchy]

https://en.wikipedia.org/wiki/North_American_P-51_Mustang_variants#P-51/Mustang_Mk_IA_(NA-91)
The 150 NA-91s were designated P-51 by the newly formed USAAF and were initially named Apache, although this was soon dropped and the RAF name, Mustang, adopted instead. The USAAF did not like the mixed armament of the British Mustang Is and instead adopted an armament of four long-barrelled 20 mm (.79 in) Hispano Mk II cannon, deleting the .50 cal "nostril"-mounted weapons. The British designated this model as Mustang Mk IA, and would fit a number with similar equipment.[10]

Had no idea there was a P-51 variant with 4 20mm, but would be an absolute beast I'm sure.

[Dadtallica]

Four 20’s would make the AH P51 almost unstoppable lol.

Sign me up! 

[Mongoose]

It was a very early version of the P-51, used by Britain.  The four 20mm gave it lots of firepower, but performance above 15K wasn't very good.  Good for ground attack, not a dog fighter.

You can get more detail here:

http://www.joebaugher.com/usaf_fighters/p51_2.html

[oboe]

Interesting point, the P-51's Allison and later Merlin engines were vulnerable to ground fire due to being water-cooled.  Any damage to the cooling system resulting in coolant leaks could result in the loss of the aircraft. 

The Thunderbolt, however, with its air-cooled radial engine, could take the punishment down low and still get back home.  There are stories of Thunderbolts with multiple cylinder heads shot away and still making it back home.   

I hadn't thought about this before, but why didn't they try 4x20mm Hispanos in the P-47?  I guess it would've been basically the USAAF's counterpart to the Navy's F4U-1C.   You'd think it would've been discussed at least, especially with the ground attack role for the P-47 becoming so important in the last two years of the War...

[Greebo]

In 1940 the British tried using Hispano cannons in Spitfires but found they were hopelessly unreliable. The gun had originally been designed to be stiffened by the engine block of the French HS 12Y engine whose hub it fired through, but when mounted in a wing under load they flexed and jammed. The solution was not just to stiffen the structure of the wing around the guns but also to improve the gun's materials and tighten its tolerances to make it the Hispano Mk II. By mid 1941 the gun was working reliably in both the Spitfire and Hurricane, the only problem being insufficient gun heat in the Spitfire which could cause the gun to jam at altitude.

The USAAF also employed the Hispano in various aircraft (P-38, P-400 etc.) where it was nose mounted in a fairly stiff structure and so worked reasonably well. When the British requested the the US manufacture their more reliable Mk II version for them they were refused, the American ordnance dept not wanting to disrupt production to introduce the changes. So when US aircraft manufacturers started trying the gun in the wings of fighters they ran into the same problems as the RAF had in 1940.

The other reason the US made less use of the gun than the British was because the RAF were more focused on bomber interception and later ground attack roles for their fighters which suited a gun which fired bigger rounds. The USAAF however were more interested in fighter vs fighter combat and preferred a greater rate of fire over heavier rounds for this role. Like the RAF the USN needed to bring down incoming bombers quickly so persevered with the Hispano. However as it was never as reliable as the Browning relatively few F4Us and F6Fs were equipped with them during the war.

[oboe]

Thanks for that info, Greebo.

I just learned the 78th FG, while it was equipped with the P-47, tried a field modification of adding 20 mm gun pods under each wing of the Thunderbolt.  I think this was just a one-off and doesn't appear to have progressed further than field testing.

https://ww2aircraft.net/forum/threads/p-47-more-firepower.19962/

[twitchy]

A heavily perked P51 with 20mm would be a great addition to the game, anybody know how many rounds these things had?

[Greebo]

The Mustang Mk Ia carried 125 rounds per gun, enough to ruin anyone's day.

[drgondog]

NA-91 same as Mustang IA. AAF retained 58 of the original RAF order for 150.

It was the first Mustang to undergo Operational Suitability Testing at Eglin and deemed superior to P-47, P-38, P-40 and P-39 under 15000 feet.

Most of AAF ships went to Depots for K-24 camera mods and assigned to TAC, primarily for recon in MTO. One pulled for testing by USN, two pulled for XP-78/XP-51B Merlin conversion.

The Hispano II issues were US version related pointing to chamber and ammo issues. The Brits replaced with their version and beefed up the mounts. The slanted Hispano installation was not changed much for the two AN2 Browning 50cal for A-36/P-51A/B/C.

As late as November 1943, 20mm Oldsmobile (Hispano II) were considered for P-51F, and Australian P-51D - but AAF pretty adamant about using only 50 caliber as much as possible, leaving P-61 and P-38 to soldier on until GUNVAL mods to F-86 during Korea.