P-51 6 x .50 ammo increase to 400 rpg complications?

Found that in a WWII document. Use a different weight if you want to.

Pre- and early-war load out sheets for the US often listed the weight of 100 rounds of .50 cal as 25 lbs - they only used the ~average weight of the cartridges. As the war progressed they updated the weight & loading/balance sheets and used 30 lbs per 100 rounds - which included the weight of the links.

Long ago I crunched the late-great Frank Olynyk's numbers in several of his US aerial victory volumes. they all show summaries of destroyed-probable-damaged by a/c type though some are generic.
Anyway:
The optimum WW II fighter armament was four .50 calibers. The six-gun a/c with 50 percent more firepower (F4F-4, F6F, F4U, P-51Ds and most P-40s etc) only averaged 10 percent more lethality, accepting Destroyed claims. Probables and damaged were lumped into the total numbers.

BarnOwlLover said:

Problem was US made HS404s rarely worked as intended, though US makers never adopted the shortened chamber that solved most of the issues that the British initially had.

Of course, on secretprojects I've read that the USAAF wanted eventually to mount 4 20mm cannons on the P-51H and the P-82B/XP-82, but NAA said "nope". Of course, if there's any truth to that on either end, I can say that NAA did propose armament fits for the P-51 and A-36 that included 20mm and 37mm cannons or a mix of the two, per several posts made my drgondog when discussing Mustang armament fits. But each time NAA proposed anything other than .50s, the USAAF ignored them.

Thus if the USAAF wanted 20mm cannons on the P-51H or the P-82B (due to both aircraft's potential as interceptors with 5000+ fpm climb rates on normal max take off weight and WEP), and NAA told them no (story is that NAA spec'd out for 6 .50s on both planes already when the USAAF suggested it), North American had been burned before on doing such work expecting/hoping the USAAF would come around to the cannon armed P-51.

And yes, If you've read the chapter about the P-51H in Michael O'Leary's book "Building the P-51 Mustang", there were mock ups of the NA-117 (pre-production P-51H design, production models were the NA-126, with the P-51L being the NA-129), there's photos there (as well as probably in Boeing's archives, and maybe someone else's archives) of a P-51H mock up with 4 20mm cannons.

Click to expand...

IIRC the only issues re: 20mm armament for P-51s were a.) AAF always said no thanks, and b.) the original wing spar for XP-51F/G/J/H needed more opening space to accomodate the Oldsmobile Hispano. While I know USAAF considered 20mm or the P-51F, the spar was mentioned as an 'affected' assembly.

I have never seen correspondence to/from Wright Field discussing 20mm for P-51H

I've seen photos in Building the P-51 Mustang that shows the NA-117 mock up (pre production P-51H) with four cannons in each wing. But one, it was a mock up, and two, it was the NA-117, which evolved into the NA-126 (production P-51H), which though very similar, I doubt that the NA-117 and NA-126 were completely identical Thus, what were the notable differences between the NA-117 design and the NA-126?

BarnOwlLover said:

I've seen photos in Building the P-51 Mustang that shows the NA-117 mock up (pre production P-51H) with four cannons in each wing. But one, it was a mock up, and two, it was the NA-117, which evolved into the NA-126 (production P-51H), which though very similar, I doubt that the NA-117 and NA-126 were completely identical Thus, what were the notable differences between the NA-117 design and 125

Click to expand...

I believe that the two images with 20mm provided by O'Leary are of the XP-51F 20mm mock up.

That said, and still doing research, I think that the difference was the September 1943 mandate from Materiel Command for all Musangs delivered after January1944 have te fuselage fuel tank. For the P-51B/D that trnslated to 85gal - which is why NA-106 morphed to NA-109/111. But, the NA-105 already provided for 20+gal more fuel in wings.

I'm still trying to find and pull NA-117 dwgs to verifiy but this is my theory:

NA-117 was intended for production version of NA-105A and had more or less the same General Arrangement and dimension as XP-51F/G with no fuse tank. Again speculatively, NAA decided to solve the stability issues imposed by fuse tank creating aft cg - and began design on the 'stretch vesion of the XP-51F/G' and got permission to install only the 50 gal fuse tank in the P-51H.

Both the P-51H (NA-126) and J were the same length and both moved the wing back about 6" more from spinner tip to 25% chord line compared to all previous mustangs incl F and G

if you look at image on pg 207 you can see the spar reference line on NA-117 root chord well forward of the cockpit inst panel. when yo turn the page, the technical illustration shows spar fwd attach well aft of the firewall.

Comparing Three Views of NA-73 through NA-109 as well as 111, 122, 124, the reference distance to 1/4 chord is 133+ in and the same reference for the H/J is 139+ inches. If we had a 3 view of 117, I suspect that the dimensions are nearly the same as the earlier Mustangs.

I repeat, theory - not yet fact - on the difference between NA-117 and 126. The relevant anology is NA-106 folded into NA-109 with no airframe difference other than the production instl'n of the 85 gal fuse tank.

This is why I'm looking forward to Volume 1 of your Mustang book arriving Monday or Tuesday, and for Volume 2 when it's ready. I'm looking forward to such little details and such coming out about the "unsung heroes" of the Mustang saga, both man and machine.

As to what you said above, I did check, also using the wing leading edge, and it seems that the wing on the mock up was indeed several inches ahead of where it would be on production aircraft.

So it seems that the NA-117 was going to be a "shorty" P-51H (about the same length as the F/G or any other Mustang up to that point), or, if you will, a F/G with a different canopy, P-51H windscreen, and the raised cockpit position of the H.

Thinking about this now, this makes me wish I knew of photos for sure of the NA-117, NA-126 and XP-51F/G mock ups or technical illustrations. I know that Boeing might have them (though not for public consumption unless you're a writer and want to pay for them), and maybe AirCorps has them in the liberated NAA documents that they're working on digitizing.

Granted, I know that this will be covered in details in Volume 2, but (and I don't mind spoilers--and the book is in Ohio as of last check in the afternoon) how much gets covered on the XP-51F in the later stages of Volume 1 (I did read references to charts on page 326)? I know that the first prototype only flew a few weeks before the Normandy Landings, which is where the first volume roughly ends.

The book end date is June 6th. The prototype first flew on February 14th.

Look forward to checking that out and the rest of the book later--it's out for delivery this morning at my post office.

Kind of OT, but I do remember reading that the LW-based P-51s would've needed an altered front wing spar to accommodate 4 20mm cannons. Would such changes have needed to be made to the earlier wings, given that the Mustang IA had 4 20mm cannons, and it was looked at as armament for the A-36 as well as 37mm cannons (albeit in underwing mounts for the latter--I saw that in the book)? Or even for the F-82? I wonder what such an armament arrangement would've looked like in both cases (maybe like the CA-15's arrangement?):



Source: Internet find, possibly from a document posted here.

BarnOwlLover said:

Kind of OT, but I do remember reading that the LW-based P-51s would've needed an altered front wing spar to accommodate 4 20mm cannons. Would such changes have needed to be made to the earlier wings, given that the Mustang IA had 4 20mm cannons, and it was looked at as armament for the A-36 as well as 37mm cannons (albeit in underwing mounts for the latter--I saw that in the book)? Or even for the F-82? I wonder what such an armament arrangement would've looked like in both cases (maybe like the CA-15's arrangement?):

View attachment 762580

Source: Internet find, possibly from a document posted here.

Click to expand...

Not a real issue.