USAAC Project C

[KJ_Lesnick]

I just got the book "P-51 Mustang: Development of the Long-Range Escort Fighter" by Paul A. Ludwig and I noted a mention on page 23 of the following

Project C -- The Twin-Engined Escort

Having the V-1710 in production allowed aircraft designers to
offer a wide range of fighter prototypes which flowed to
Wright Field like a flood. Following the XP-37, P-38, P-39 and
P-40 came the useless XP-46 and, later the XP-51, all powered
by the V-1710. But the most anticipated prototype using the
V-1710 in 1936, was a twin-engined escort and multi-mission
fighter known as the Bell XFM-1 (Fighter, Multi-place)
Airacuda. It was expected to satisfy three needs: The Army's,
because it mounted two 37 mm cannon; escort advocates,
because it was an escort; and bomber barons, because it
carried bombs. Kelsey had anticipated it. It was a failure
because it was slow and underpowered. Nine YFM-1s and
three YFM-1As were ordered but the first of these did not fly
until eight months after the XP-38 flew for the first time.

I'm curious about a couple of things

• Why did the Army want 37mm cannons
  
• Did the Bomber Barons want an internally carried payload, or did they not care
  

I'm curious why the US Army wanted 37mm cannon

[tomo pauk]

The main job of the USAAC fighter force in the 1930s-early 40s was to protect the USA from the perceived bomber force of any likely enemy. The 37mm cannon was seen as the gun suitable to bring down any bomber. Hence the Airacuda, Airacobra and early P-38s with it.
But not only the 37mm was discussed, the Madsen 23mm also featured prominently, as well as the high velocity .60 in cannnon, plus the Hispano.

[KJ_Lesnick]

Tomo Pauk

The main job of the USAAC fighter force in the 1930s-early 40s was to protect the USA from the perceived bomber force of any likely enemy.

Well, some felt that way: Others felt they were supposed to support ground operations.  There were several schools of thoughts.

The 37mm cannon was seen as the gun suitable to bring down any bomber.

Heavy hitting, but slow firing

the Madsen 23mm also featured prominently, as well as the high velocity .60 in cannnon, plus the Hispano.

The Hispano fired only a little slower the 12.7mm's but delivered 3 times the amount of damage; the 23mm fired considerably slower than the 20mm, I'm not sure what the ballistics of the 0.60 was.

I'm surprised that the bomber-guys seemed to think that fighters had to have rear-firing armament to protect against fighters attacking from the rear: You'd figure they'd put a tail-gun on the bomber to take the edge off the bomber; then position the fighters in locations to allow them to nail attacks from the front, and from the rear and use maneuver and speed to do the rest.

[tomo pauk]

Once the shooting started, many pre-war doctrines (and threats) failed to materialize. The USAAC/USAAF started using fighters as they saw fit. The support of ground forces was, per pre-war doctrine, a job of attack aviation.
The RoF of 37mm was just fine for bomber busting, even if the bomber is an 'usual' 2-engined one as fieleded by most of beligerents in ww2. The Madsen 23 mm fired a 175 g shell at 350-400 rpm, again a considerable firepower against any late 1930s and early ww2 bomber.
The .60 have had similar ballistics as the German MG 151/15, or the Soviet 14.5 mm - a light projectile fired at very high speed. The cartidge was from the experimental anti tank rifle, so high muzzle velocity is not a surprise. After the war, the casing was necked out and shortened (surprise, surprise), in order to accomodate 20 mm shell, that, electrically primed, is used in the M61 gun, the well-known Vulcan among others.

The rear gun on the Airacuda is just another thing that makes us go 'what were they thinking??'.

[KJ_Lesnick]

tomo pauk

Once the shooting started, many pre-war doctrines (and threats) failed to materialize.

Just surprises me that people that those actually knew loads of stuff about aerial combat (including how to use maneuvering) were ignored or dismissed (Chennault).  Plus you'd think people with good senses of spatial orientation (pilots) would know that if you were in a bomber penetrating their territory they'd first be heading at you head on going up then as they pass you they'd swing onto your tail and start firing there (provided you didn't have a tail-gun).  It seems common sense to perch some planes ahead and have a small group behind (the group in the front would basically nail them as they come up and head at the bomber), and the group in the back would bag the plane should it manage to make it into the tail position. 

Of course some of the doctrines were conceived when planes were slower, but radar didn't exist -- so in practice the enemy aircraft were often at altitude when the bombers cruised in.

The USAAC/USAAF started using fighters as they saw fit. The support of ground forces was, per pre-war doctrine, a job of attack aviation.

Well, some felt they were to assist attack-aviation, others felt they could be fighter-bombers, others felt they could be used to attack bombers.

I know those interested in destroying bombers would want the 37mm's, was there any ground-guys who wanted them for shredding tanks and stuff?

The rear gun on the Airacuda is just another thing that makes us go 'what were they thinking??'.

It had to do with the belief that escort fighters should have rear-guns with sophisticated fire-control systems to essentially deal with the enemy coming at the bombers from the rear: They didn't seem to realize that if the plane was fast and agile enough it could just quickly maneuver at and blow up the fighters, then maneuver back into the formation and keep on going.

[wuzak]

I know those interested in destroying bombers would want the 37mm's, was there any ground-guys who wanted them for shredding tanks and stuff?

The muzzle velocity of the 37mm M4 was low so any penetration of armour would depend on the weight of the shell.

The RAF used Vickers 40mm Type S autocannon for tank busting in the Hurricane in North Africa, and that had a similar muzzle velocity and higher round weight.

The low muzzle velocity affected the trajectory of the round.

The USA did fit one 37 mm gun to its aircraft - the M4. However, this fired low-powered 37x145R ammunition and was not very effective against tanks, being intended for air combat.

http://www.quarryhs.co.uk/tankbusters.htm

The rear gun on the Airacuda is just another thing that makes us go 'what were they thinking??'.

It had to do with the belief that escort fighters should have rear-guns with sophisticated fire-control systems to essentially deal with the enemy coming at the bombers from the rear: They didn't seem to realize that if the plane was fast and agile enough it could just quickly maneuver at and blow up the fighters, then maneuver back into the formation and keep on going.

The Airacuda was a bomber destroyer. Rear firing guns were not meant for bomber destroying, but self defence.

[KJ_Lesnick]

Wuzak

The muzzle velocity of the 37mm M4 was low so any penetration of armour would depend on the weight of the shell.

Okay

The low muzzle velocity affected the trajectory of the round.

Yeah it covers little distance but continues to drop...

The USA did fit one 37 mm gun to its aircraft - the M4. However, this fired low-powered 37x145R ammunition and was not very effective against tanks, being intended for air combat.

That I didn't know

The Airacuda was a bomber destroyer. Rear firing guns were not meant for bomber destroying, but self defence.

I thought it was to protect bombers...

[tomo pauk]

USAF doctrine of second part of ww2 was not the same as the doctrine of USAAC before the USA entered the war. The doctrine, for the bombers, was that they will swarm enemy territory without much of trouble, having speed, altitude and armament to help with the task. The intended job of then-considered fighters was to prevent the perceived enemy doing the same to the USA - hence the attraction of the cannon for the planners. An escort fighter was either LW or IJA/IJN invention from 1940, USAAC/USAF needed couple of hard lessons to figure it out.

The ballistics of the 37mm M4 was better than of MG FF, MK 108 or Type 99-1 20 mm - fine if the target is a bomber, problematic if the currently-maneuvering enemy fighter is the target. As wuzak said, the M4 was a lousy AT cannon, the British Type S fired at same MV a projectile that was twice as heavy. BTW - why would anyone try to do gound pounding in a turbocharged aircraft? That is a response for those (whom?) that were seing an USAC fighter being a fighter bomber, that was not outfitted with bomb racks in the 1st place.

[KJ_Lesnick]

tomo pauk

USAF doctrine of second part of ww2 was not the same as the doctrine of USAAC before the USA entered the war. The doctrine, for the bombers, was that they will swarm enemy territory without much of trouble, having speed, altitude and armament to help with the task.

Of course, however with things like radar and high performance fighters it doesn't work as well as advertised

The intended job of then-considered fighters was to prevent the perceived enemy doing the same to the USA - hence the attraction of the cannon for the planners.

Which was acceptable for the intended purpose

The ballistics of the 37mm M4 was better than of MG FF, MK 108 or Type 99-1 20 mm - fine if the target is a bomber, problematic if the currently-maneuvering enemy fighter is the target.

Correct

BTW - why would anyone try to do gound pounding in a turbocharged aircraft?

Not sure, but the P-47 did it okay

[tomo pauk]

Of course, however with things like radar and high performance fighters it doesn't work as well as advertised

Bingo.

BTW - why would anyone try to do gound pounding in a turbocharged aircraft?

Not sure, but the P-47 did it okay

So did the P-38, but not in, say, 1940.