P-61 cannons "aimable"?

[Stephan Wilkinson]

Somewhere I read--can't remember where--that a Black Widow's belly-pod cannons could be deflected by the pilot a quarter of a degree left, right, up or down (which minuscule amount would, of course, have a substantial effect on where the fire went a hundred or more yards out from the airplane). The pilot could thus choose converging, diverging, elevated or declinated fire. Does anybody know if this is true, or was it maybe an experimental feature briefly tried?

And if it -was- true, would P-61 pilots actually complicate their already-busy lives by choosing to use this feature rather than simply firing straight ahead?

 

[vikingBerserker]

I have not heard of this. Now on the ground I would imagine this could be done when synchronizing them.

 

[Capt. Vick]

Hmmm...weird

 

[Shortround6]

a quarter of a degree is 15 minutes of arc, for all practical purposes 15 inches at 100yrds of 60 inches (5 feet) at 400yds.
In other words there be NO practical effect when firing at a twin engine bomber at any effective range (under 600-800yds).

Now maybe the 15 minute figure is in error and the guns were much more flexible (150 minutes or maybe even 15 degrees?) but it sounds like something that should never have made it into a service aircraft even if drawn on paper. The gun mounts should have had that much travel (but not movable in flight) if not more to allow for proper zeroing. Not all guns/barrels will hit to the same place even if the receivers are mounted identically. In fact the vast majority will not.

 

[Stephan Wilkinson]

Good points. I think I'll figure this one is a myth.

 

[GrauGeist]

The cannon in the P-61 were mounted in static mounts, only ground-adjustable by the armorer much like MGs in fighters or the MG pods aboard the B-25 (and A-20, etc)...once airborne, the only "adjustment" made, was by the aircraft's control surfaces.

 

[Stephan Wilkinson]

But wait, there's more. I have found my original reference, and it is the very authoritative, 606-page "America's Hundred Thousand: U. S. Production Fighters of World War II," by Francis H. Dean. To quote from page 402: "The cannon were controlled by the pilot and could be set to fire forward through a horizontal angle range of plus or minus 1/4 degree from dead ahead and a vertical range of plus 1/4 or minus 1 degree from the horizontal, making possible converging, diverging, elevated or depressed lines of fire."

Maybe Dean should have written "The FIRING OF the cannon was controlled by the pilot, and THE CANNON COULD BE GROUND-ADJUSTED to fire forward...[etc.]"?

 

[Slam]

"...and could be set to fire..."

I take that as meaning they could be adjusted within a given range but not necessarily by the pilot in flagrante delicto, as it were.

 

[mikewint]

From Warren Thompson's Combat aircraft:
At the time of introduction the Northrop P-61 was the largest fighter in US inventory. It was also the first twin-engined dedicated night fighter designed by Northrop and proved to be an excellent weapons platform. The heavy armament of fixed cannons in the belly and machine guns in the top turret made getting kills easy, when they could be found, that is. By the time the P-61 entered service, the skies were pretty much owned by the Allies and pickings were pretty slim for the night fighter units.
Perhaps these "cones of fire" are what is refered to?
As an aside the 20mm Hispano M2 cannons never functioned properly and were subject to constant jams. To function at all the 20mm shells had to be coated with grease/wax to aid in shell removal

 

[Shortround6]

Actually the grease/wax helped the cartridge fit tighter in the chamber which allowed for a better firing pin strike. The US guns had a longer chamber than the British guns and the cartridge, depending on exact diminsions, could move forward under the firing pin impact lighting the effective blow to the primer and causing misfires.
The grease/wax attracted dirt which would prevent chambering as would extreme cold which solidified the grease/wax so the grease/wax brought it's own problems.

 

[redcoat]

As an aside the 20mm Hispano M2 cannons never functioned properly and were subject to constant jams. To function at all the 20mm shells had to be coated with grease/wax to aid in shell removal

Why did the US feel the need to further alter the design after the British had already spent a lot of time getting them to work reliably.

 

[mikewint]

Actually Torch it was the reverse. The British were very anxious for the US to begin production BUT the first US versions were the AN-M1 which had none of the British modification and had so many problems they were declared useless (after over 56,000 had been produced). The next generation the AN-M2 had a 1mm shorter chamber and other mods. Jams continued and under combat conditions reached 1 in 500 rounds. The SB2C Helldiver was a prime example.
Part of the US gun problems was due to Army regs which classified any gun over 15mm (.60 cal) as artillery and as such were made to artillery tollerances.
None of the M1s and only a fraction of the M2s were ever mounted in aircraft. Production ceased in 1944.
Post war the M2s were modified by the Navy (lightened and shortened) becoming the Navy's T31 20mm cannon