US 20mm Problems and the P-38

[Stoney]

If the early American attempts at designing a 20mm cannon proved so difficult, how was the weapon successfully used on the P-38?  Different weapon?  I've always thought that was strange.

[Murdr]

Reliabilty in a wing mounting config was one factor.

[Hooligan]

They lubricated the ammo belts in the US 20mm.  It was an inelegant solution but it worked.  A detailed description is provided in:  The Machine, by George Chinn, published by US Bureau of Ordnance.

Hooligan

[Furball]

They never found out there were any problems because the 38 was always the one being fired at.

[Cthulhu]

Reliabilty in a wing mounting config was one factor.

Murdr, can you elaborate? Was it a vibration issue? Feed issue?

[Enker]

Im guessing it was a issue that it would tear off the wings because of the kick back.

[AquaShrimp]

US 20mm cannon was actually regulated by the artillery department, due to antiquated regulations.  Years ago, someone posted a big article on the differences between the US and British version of the Hispano 20mm cannon. 

[Cthulhu]

Im guessing it was a issue that it would tear off the wings because of the kick back.

You have any idea how strong/stiff the wings are when loaded in that axis? Run a strong enough pair of intercostals between wing ribs... extremely stiff & plenty strong. Recoil loads could potentially twist the wing, spraying rds all over the place (especially if it excites a fundamental frequency), but that's just a matter of gun placement (aligning the recoil forces with the shear center of the wing).

[Hooligan]

Sorry for the somewhat ambiguous reply but as I recall it was a headspacing/feed issue.  UK and USA both made different modifications to the original design to fix it but the UK modifications were superior and USA hispanos suffered somewhat because they were manufactured to looser tolerances.  Anything 20mm and above in the US inventory was classified as a cannon and manufacturing tolerances for cannon were not as strict as those for machineguns.

If you get a copy of the book I referenced it will describe the problems and solutions in great detail.

Hooligan

[Murdr]

Murdr, can you elaborate? Was it a vibration issue? Feed issue?

This is just from memory by the way.  IIRC there was a feed/jambing issue from it being mounted on its side to fit inside a wing.  That was not an issue with the nose mounted configuration.

[Cthulhu]

This is just from memory by the way.  IIRC there was a feed/jambing issue from it being mounted on its side to fit inside a wing.  That was not an issue with the nose mounted configuration.

Thanks Murdr. That makes sense, especially given what Hooligan said about looser tolerances.

[Spikes]

From my thread in the super secret SAPP forum...
Basically saying that under high G's the 20mms were jammed for odd reasons. The side mounted 20mms to fit in the wing caused problems as well, as the feed was different then intended. Having the 20mm in the nose allowed for more space, meaning the 20mm would be upright and it would feed in better...

[Anaxogoras]

What about temperature?  I read that early version of the HS cannon had jamming problems in cold weather, and since the P-38 saw a lot of action in the pacific was that a reason to arm it with the M2?

[Karnak]

This is just from memory by the way.  IIRC there was a feed/jambing issue from it being mounted on its side to fit inside a wing.  That was not an issue with the nose mounted configuration.

Mounting it on its side only applied to Spitfire MK Ib and Mk IIb aircraft.  From the Mk Vb on they were mounted upright and that cause of jamming was eliminated.

[Murdr]

What about temperature?  I read that early version of the HS cannon had jamming problems in cold weather, and since the P-38 saw a lot of action in the pacific was that a reason to arm it with the M2?

It's cold at 22,000 feet where ever you are.  The P-38 had a heat system for the nose compartment.  It was always slated to carry a cannon, but the Oldsmobile M9 37mm was dumped in favor of the 20mm M2 for availiblity, and because the 37mm had problems of its own.