On 11/07/2020 08:21, nightjar wrote:
On 11/07/2020 08:15, Chris Hogg wrote:
On Sat, 11 Jul 2020 08:02:32 +0100, Harry Bloomfield, Esq.
wrote:
BBC News article claiming a 13 year olds intricate calculations
persuaded them to fit 8 browning guns rather than the four planned.
Obviously chances are improved of bringing down an enemy aircraft, the
more guns are being fired, but what is the intricate calculation needed
to prove that?
It seems the guns were aimed to coincide at a range of 250 yards. 250
yards seems awfully distant to me, what they show of the films of the
time, it seems much less than 250 yards, maybe as little as 50 yards,
unless the used telephoto lens?
The convergence 'point' was actually a 12' x 8' rectangle, known as the
Dowding spread, as he believed it gave even a mediocre pilot a fair
chance of hitting the enemy. With Spitfire guns so widely spaced on the
wings, there was also quite a wide area of less concentrated fire both
before and after the convergence point. Aces would normally have the
guns set to their own personal preferences, while some Spitfire
squadrons set different convergence distances for each wing, to give a
greater depth of concentrated fire.
Apparently, officers could and did give their ground crews orders to
adjust the aim of the guns to suit their preferences and experiences,
usually abandoning the official 'spread' for convergence at 100 yards.
Sergeant Pilots didn't have the same authority, so had to work with the
standard settings. Consequently, Sergeant Pilots didn't so so well at
getting high scores.
I rather wondered the same. And FMI what's the difference between a
Browning machine gun and a cannon, that some Spitfires were apparently
equipped with? Better for bringing down German bombers, the chap said,
which were more heavily armored than their fighters. Do cannons fire
heavier bullets, or have higher muzzle velocities, or what?
The Browning machine guns fitted to early Spitfires fired .303 rifle
rounds. The cannon fired 20mm (3/4") explosive rounds.
When the Germans started fitting armour to their bombers, the machine
guns proved to be inadequate and cannon were tried as an answer.
However, the early cannon were prone to jam, so, for a while the
standard fit became two cannon and four machine guns.
The concept of the cannon-armed fighter was proposed before WWII, and
the HS404 autocannon was fitted to the Beaufighter in 1940. The Battle
of Britain showed that the 303 round was not adequate for the task, and
so the move to equip the single-engined fighter force with cannon ramped up.
Unfortunately, there was an issue with the HS404 that meant that in
combat the guns could misfire. These failed guns were taken out of
service and passed to the Royal Small Arms Factory for investigation.
This job fell to my father, who was given a building full of these guns,
and he spent some time finding a cause. I recall him telling me what
he'd discovered, but it didn't appear to be the reason later put forward
by officialdom. The official view was that the rounds were being too
lightly struck by the (fairly light) bolt, the solution being to machine
1/16th or so from the face of the receiver, so that the bolt would be
travelling faster and make a misfire less likely.
I understand that something like 100,000,000 rounds of 20mm ammunition
had been made in advance, and this was put at risk by the unreliability
of the guns.
The Americans also produced the HS404, and they were still debating the
misfire issue at the end of the war, although they had gone some way to
adopting the British solution to the problem.
The later 'universal' or 'c' wing Spitfire could be fitted with either
eight machine guns, two cannon and four machine guns or four cannon, as
well as being able to carry 2 x 250lb bombs. The 'e' wing Spitfire had a
development of the 'c' wing and could carry either four cannon or two
0.5" machine guns and two cannon.
Here's 3m46 of British ground attacks in 1944. Locomotives made
attractive targets...
https://www.youtube.com/watch?v=19R3xVJ_iBY
--
Spike