Why did the US ww2 fighters have 50 cals instead of 20mms like their european cousins

[Trav02]

As far as I know it was the drag of the protruding cannon that made the difference. In 1940-41 when they started fitting cannon to RAF fighters, the 0.5cal wasnt a viable option, not reliable, low rate of fire and USA not in the war. The Spitfire could carry 4 cannon and 4 x 0.303 " there were mock ups with 6 x 20mm cannon

Wasn't the 4 20mm and 4 303 an overload for some of the malta spits of the USS Wasp? I thought they weren't carrying any ammunition and 2 of the cannons were removed as soon as they landed. This was from a flight journal article so take with a liberal amount of salt....

 

[varsity07840]

four cannon OR two cannon and four .303s.

 

[Trav02]

four cannon OR two cannon and four .303s.

Thanks. Was wondering, yeah , seemed a tad ludicrous. I think there's a report somewhere on here that it (Spit V)was too unwieldy with 4 cannons. Will see if I can dig it up.

 

[Jugman]

UK had to move all this around, Commonwealth small arms calibers and Lend Lease received from USA

38 S&W
9mm
455 Webley
45 Auto
303
30-06
8mm Mauser
50 Browning
50 Vickers
55 Boys
15mm BESA

True but they were moving relatively small quantities. Keep in mind the shortest east to west distance across the United States is ~10% greater than the distance between Paris and Moscow. The US is huge. The economics and logistics of the US is far more complicated than it is for the UK or any other nation for that matter. It's far easier to ship something 20 miles or 200 miles from a factory to the front line than it is to ship something 1000 miles to a port then have it prepared for a long ocean voyage. Effectively the US had many weeks of extra delays built into any of their decisions.

 

[ThomasP]

There was room for 4x 20mm + 4x .303 cal and this combination could have been fitted - you can see in the following diagram of the 'C' wing that there was room for the guns and ammo boxes. How bad the decrease in performance would be, and whether weight and/or dynamic stresses on the wing structure would have allowed it, I can not say.

 

[PAT303]

There was room for 4x 20mm + 4x .303 cal and this combination could have been fitted - you can see in the following diagram of the 'C' wing that there was room for the guns and ammo boxes. How bad the decrease in performance would be, and whether weight and/or dynamic stresses on the wing structure would have allowed it, I can not say.

View attachment 759997

The Spitfire needed more fuel not more guns.

 

[pbehn]

Mk.XII with 6 Hispano mock up, to test possible handling deficiencies
View attachment 759929

You can have too much of a good thing.

 

[Shortround6]

You can have too much of a good thing.

There were a number of schemes/proposals for planes with six 20mm Hispano's. (Meteor was supposed to have six) but they came to nothing.
The Axis never came up with planes that Four Hispano's couldn't handle. There weren't enough Hispano's. Nobody wanted to take the Performance penalty. More ammo (firing time) was important than more barrels.
Take your pick of one or more
The MK V Hispano offered the fire power of 5 guns for less installed weight than 4 of the old ones.

 

[J.A.W.]

In service the 4x 20mm of the Hurricane Mk IIC lowered the Vmax by about 1.5-2 mph per 20mm ( total of about 6-8 mph). I have read that the Spitfire Mk VB/C & Mk IX had their Vmax reduced by about 2-3 mph per 20mm installed (total of about 4-6 mph for the 'B' wing and 8-12 mph for the 'C' and universal wing).

Yes, & test-pilot R. Beamont instigated the adaptation of the Spitfire cannon fairing to the Typhoon, did the RAF's cannon Mustangs get 'em?

Edit:
Only 3mph lost (& combined penalty of drag/weight increase) for the Hurribird toting the big guns, according to the ADS.
But its not like the ol' dog was 'some kind of bleedin' speed-merchant or nuffink.'

 

[ThomasP]

These are excerpts from the A&AEE tests re the drag caused by the IIC cannon installation. The flights were performed with a IIC under 2 conditions at the same weight - with the 4x 20mm installed and with the 20mm uninstalled and the leading edge of the wing faired over. Note that the IIC aircraft was flown in clean condition (ie no pylons or bomb racks) at approximately same weight as a Hurricane Mk IIB used in previous speed tests (~7300 lbs) also in clean condition.

My original upthread comment on the loss of 1.5-2 mph per cannon is based on a report by Hurricane Mk IIC and IIB Sqns regarding performance of aircraft in service, which report said they were observing a typical decrease in Vmax of 6-8 mph vs the Hurricane Mk IIB. The same report indicated the IIB was typically achieving 330-334 mph in service.

 

[MIflyer]

Some of the Hurricanes had two cannon deleted because of the adverse effect on performance.

 

[Clayton Magnet]

The sweet spot for WW2 era aircraft seems to be 3-4 20mm cannons.
Perhaps the best all around compromise between weight of fire and all-up weight was probably the Yak-3P, or Fw 190 D-13, with 3 centrally mounted 20mm auto cannon.

 

[PAT303]

The sweet spot for WW2 era aircraft seems to be 3-4 20mm cannons.
Perhaps the best all around compromise between weight of fire and all-up weight was probably the Yak-3P, or Fw 190 D-13, with 3 centrally mounted 20mm auto cannon.

My only criticism would be the different points of impact between the wing and centrally mounted guns, one way to overcome it could be harmonizing them into a cone shaped POI maybe. My opinion, wing root mounted Hispano's like the 190's cannons zeroed to shoot parallel straight ahead would be my choice.

 

[MIflyer]

I think the P-38 would have been better off with one or two more .50 cal in the nose rather than the 20MM cannon. A 15MM cannon like the BF-109F had might have been a good compromise, and hence the value of up-scaling the .50 cal. I do not recall ever reading where a P-38 pilot decided to shoot just his 20MM cannon because it was more effective, although most of the airplanes were equipped with separate machine gun and cannon firing buttons. I recall reading of one unit that used one button for both the .50 cal and 20MM, used the extra button for push to talk radio, and used the former PTT "horn button" in the center of the wheel to jettison the drop tanks rather than the previous location where it was hard to find when under duress.

 

[Shortround6]

I do not recall ever reading where a P-38 pilot decided to shoot just his 20MM cannon because it was more effective, although most of the airplanes were equipped with separate machine gun and cannon firing buttons.

I have no idea why he would?
He had less firing time for the 20mm gun (about 15 seconds worth) and he had 23-38 seconds of firing time for the .50 cals depending on the load out.
One can see the opposite. Firing the .50 cal and saving the 20mm.
But the 20mm and .50 cal had about the best ballistic matchup (time of flight and trajectory) of any two guns used in WW II aircraft. Just about anything else was worse, sometimes way worse.

 

[MIflyer]

Just about anything else was worse, sometimes way worse.

Yes, the the p-39 was famous for having .50 cal, .30 cal and 37MM, all of which were ballistically different enough to make hitting a target that was not dead nuts ahead and at close range something of a challenge. One pilot who flew P-39's said it was VERY important to only fire the 37MM when you had positive G's, not negative. The gun needed gravity to keep the shells being fed properly; to do otherwise risked a jam. You did not dive and open fire. You dove, then pulled up a bit as you fired the 37MM.

 

[Shortround6]

One pilot who flew P-39's said it was VERY important to only fire the 37MM when you had positive G's, not negative.

On this plane they actually designed to nose so that the .50 cal could tilt up and down to get them to hit at the place the two 37mm guns would hit depending on range while flying.
No Idea how they synchronized the gunsight, gun housing and range input (Pilot's eye MK I?)
Sometimes simple was better than a lot of schemed people came up with.

 

[MIflyer]

No Idea how they synchronized the gunsight, gun housing and range input

Supposedly they used a "Christmas Tree" gunsight with different marks for each type of gun.

 

[Clayton Magnet]

I think the P-38 would have been better off with one or two more .50 cal in the nose rather than the 20MM cannon. A 15MM cannon like the BF-109F had might have been a good compromise, and hence the value of up-scaling the .50 cal. I do not recall ever reading where a P-38 pilot decided to shoot just his 20MM cannon because it was more effective, although most of the airplanes were equipped with separate machine gun and cannon firing buttons. I recall reading of one unit that used one button for both the .50 cal and 20MM, used the extra button for push to talk radio, and used the former PTT "horn button" in the center of the wheel to jettison the drop tanks rather than the previous location where it was hard to find when under duress.

Perhaps the P-38 would have been better off by deleting the four .50 cal's, and adding one or two more 20mm cannons instead. The installed weight would be similar, but increased lethality

 

[pbehn]

It can be a very interesting discussion involving a lot of science, energy, velocity, rate of fire etc, but could any aircraft withstand 1 seconds fire from 8 x 50Cal or 4 x 20mm cannon?