Guns

[Maxrobot1]

I was told by an ex-P47 pilot that as the guns were fired electrically they all fired at the same time and that the combined recoil made the plane shudder.
He also said that firing all 8 .50s slowed the plane down by 60 MPH. I believe it was he also who said that Hurricanes with the 4 20MMs had to be careful as the loss of speed from recoil could stall the plane.
He was one who joined the RAF in Canada before transferring to the 9th Air Force. (His name was James Forrest.)
Have any stories about recoil come up? I recall reading in Bob Johnson's book that he was in combat the first time he fired all 8 guns in his P-47. The shuddering made him break off for fear that his plane was coming apart.

 

[bobbysocks]

i find it hard to believe that it slowed the plane down that far. mustangs had only 2 less 50s. i can tell you that they made the plane shake. you can tell the difference in gun camera footage when the gun is firing and when not ( the cams ran for several seconds after the guns stopped ). in a lot of dog fights you were turning tight and balancing on the threshold of a stall to get the shot. if shooting dropped airspeed that much...and you shot in that situation...you would have stalled and most likely gone into a spin. i have never read or heard of that being the case ( grant you i am not the most well read person here). i know 50s had some kick but you also never pulled the trigger very long...you only had like 1800 rounds for 6 guns. cant remember the rate of fire but it had to be in the area of 600 rpm....so you would have less than a minute of shooting. i may be proved wrong but that seems far fetched.

 

[lesofprimus]

Gross exaggerations on all accounts....

 

[mikewint]

i have fired a .50cal machine gun and seen quad 50's in action (4 - .50's mounted together, 2 to a side in a truck bed).
the bullets have tremendous energy but most of that goes into advancing the belt, ejecting the used case, cambering a new round and cocking the pin. in addition the is usually some type of muzzle brake installed. when all is said and done the recoil is like a hunting rifle (.30-06). on the quad the shaking is quite pronounced since the guns to not fire in sequence as each has a slightly different rate. since momentum is conserved the bullets combined momentum (M x V) is equal to the recoil momentum also (M x V). .50 bullets are in the range of about 600grain (45g) (about 1/10 pound) while the aircraft in in the range of several tons traveling at 300 - 400 mph
20mm cannon are not much heavier a bit over 100 gram since they are drilled out to accept an explosive charge.
30mm cannon shells are about 3 times that mass but carry 80 - 90g of explosive. the germans fitted the MK108 30mm cannon to some aircraft and thae average b-17 or b-24 was toast after 4 - 5 hits, less if an incendiary round hit a fuel tank

 

[Maxrobot1]

The.50 Cal. AN M2 machine guns were made with lighter internal parts and had much lighter barrels than the M2 HB ( Heavy barrel) version seen on rifle ranges. This would make the felt recoil greater on the aircraft weapons. Plus the guns fireing round-for-round simultaneously x 8 must have slowed the plane noticably. Or it jus may have seemed that way when the pilot felt his harness pull back on his shoulders!
How did the Me163 in glider mode fare when firing its twin 30mms?
I know that on U.S. bombers the other crewmen could feel the vibrations when another gunner cut loose. Also the 75mm guns in the B-25s caused some damage after a few rounds but that aircaft had a bit of weight behind it.
How about the P-39? I have never read any anectdotes about pilots experiences actually firing their 37mm.

 

[Shortround6]

Lets think about this.

eight .50 cal guns firing at 800-850 rpm are supposed to slow a 7 ton Thunderbolt by 60mph in how many seconds?

Does this mean that pair of .50cal guns mounted on a jeep could accelerate it from a standing start to 50-60mph in the same number of seconds

 

[krieghund]

Nicely put!!

However what is the recoil of a M2 .50cal MG firing a three second burst. That info should put the nails into the discussion.

I do know when I worked A-10 Warthogs the GAU-8 30mm did produce a hefty recoil but then that is a 'Manly Gun!!'

 

[billswagger]

http://findarticles.com/p/articles/mi_qa3897/is_200310/ai_n9324510/pg_7/

"We fired in a 10-degree dive at a small island target in the Atlantic Ocean from an altitude of about 400 feet. I watched Carl fire, and I followed him a few minutes later. The unbelievable racket of eight guns firing scared the hell out of me, and I was glad that I had only 20 rounds per gun because I didn't release the trigger until long after I pulled out of the dive! I was also amazed that the P-47 airspeed was slowed as much as 30mph by the gun-recoil forces when they were fired-so much for almost heeding unsolicited advice."


Its entirely possible the dive pull out could also slow the plane but 10 degrees is not all that steep.

Bill

 

[mikewint]

the GAU-8/A gatling gun (not chain) 30mm cannon mounted in a modern warthog is the size of a VW, fires at 3,900 RPM and has a recoil of 10,000 lbs. because of the off center firing (about 9 o'clock) it could and did push the aircraft to the side hence it was mounted off center (plus the nose wheel needed room). exhaust gases from the cannon were so extensive that initially they caused flame-outs in the engines. yet when in use the aircraft speed is only slowed by a few mph
how then are .50 cal machine guns. even eight, going to produce such dramatic effects in aircraft speed?

 

[billswagger]

how then are .50 cal machine guns. even eight, going to produce such dramatic effects in aircraft speed?

Thats a good question and probably has more to do with the flight speed of the plane and the angle of dive.

I have a problem trying to make the analogy with modern aircraft which are tens of thousands of pounds heavier and have jet engines, but it could be presumed that a plane flying in a level cruise at 200 indicated is probably going to slow up more than the plane pitched in a steep dive above 400mph.


Bill

 

[marshall]

how then are .50 cal machine guns. even eight, going to produce such dramatic effects in aircraft speed?

It's simple, these guns won't produce such a dramatic effects.

I don't know much about physics, and I'm sure some more knowledgeable people around here will be able to explain it better than me, but the energy of recoil of a one M2 will be around 18kJ (so eight M2s around 144kJ, and the energy of a flying P-47 will be around 43000kJ (speed 400km/h, mass 7000kg), so those are some quite big differences in the energy, so the effect of firing will be very small, also we can change the energy of these eight firing M2s into power and we have 144kW and a P-47 has something more like 1800kW, so again it's quite a big difference. Though I have to say I don't know what is more important the difference in the energy or in the power and how to exactly calculate how much will the plane slow down but I guess it won't be much.

 

[fastmongrel]

The Infantry in the early years of WWII used Anti Tank rifles with similar recoil to a .50 cal M2.

If 8 x .50s could knock 60 mph off a 6 ton fighter then a WWII A/T rifle should break every bone in the firers body and turn his insides into red jelly.

 

[bobbysocks]

that was a Boys antitank rifle...


View: https://www.youtube.com/watch?v=ZheQexrFX8E

 

[riacrato]

Anti-tank rifles don't fire at several hundred rpm. And yes, they can break a collar bone if not used carefully.

 

[Shortround6]

Anti-tank rifles don't fire at several hundred rpm. And yes, they can break a collar bone if not used carefully.

I saw the aftermath of an AT rifle "accident" that resulted in a broken collar bone. A German AT rifle had been re-barreled to take American .50 cal ammo. The AT rifle was a take-down (barrel will detach from receiver for transport) and the safety interlock that supposed to prevent the rifle from firing if the barrel was not properly locked in place failed. While I did not witness the shot I was on scene with a few minutes and saw were the barrel had landed (several feet in front of the gun), the fired case, and I assisted the man (what can you do for a broken collar bone?) while waiting for the ambulance to arrive.

But launching a .50cal barrel and bipod, even at low velocity is somewhat different than firing a bullet.

 

[cherry blossom]

If we assume a 5 second burst from 8 M2 guns firing 800 45g bullets per minute at 900 m/s, we seem to get a momentum of 21600 kgM/s. A P-47 weighed about 7 tons, so it would have the same momentum at 3.08 m/s or 6.9 mph. Thus firing a 5 second burst should slow a P-47 by roughly 6.9 mph if momentum were the main factor. However, the gases from the guns might also change the drag from the wings.

 

[mikewint]

billswagger marshall, the physics is quite simple M x V = M x V. mass of bullet times velocity of bullet = mass of plane times velocity. a .50 cal is about 45 gram times 8 = .36 kg times about 850 m/s is 306 kg m/s. since momentum is conserved the mass of the aircraft (P-47 about 9,900 lbs or 4500 kg) times recoil speed is equal or the aircraft is slowed by
.068 m/s or about .2 mph
the browning .50 put out about 500 rpm or about 8 - 9 per sec so the total effect is 8X to 9X say 2 mph to be very generous
so a p-47 pilot would slow from 412 mph to 410mph with all 8 machine guns firing. hardly dramatic

 

[Shortround6]

billswagger marshall, the physics is quite simple M x V = M x V. mass of bullet times velocity of bullet = mass of plane times velocity. a .50 cal is about 45 gram times 8 = .36 kg times about 850 m/s is 306 kg m/s. since momentum is conserved the mass of the aircraft (P-47 about 9,900 lbs or 4500 kg) times recoil speed is equal or the aircraft is slowed by
.068 m/s or about .2 mph
the browning .50 put out about 500 rpm or about 8 - 9 per sec so the total effect is 8X to 9X say 2 mph to be very generous
so a p-47 pilot would slow from 412 mph to 410mph with all 8 machine guns firing. hardly dramatic

The Browning aircraft guns fired at about 800-850rpm and you do have to count the weight of the propellant and the velocity of the escaping propellant gas but that is only going to add about 20-25% to the recoil of the guns. But you are correct in saying the that speed change would hardly be dramatic even if the noise and vibration were

 

[mikewint]

shortround, i was thinking of the M2HB browning which i am more familiar with not the AN/M2 used on aircraft so give it 800 rpm so 13 to 14 rounds per sec which ups my estimate to 3 mph
my thought on the propellant was that recoil forces are used to do work on the gun mechanism itself, i.e. eject, reload, advance, so they do not act to slow the aircraft.
the M2HB recoil is easily controlled and while you have to either set the brake or put the deuce and a half in gear the quad 50 does not cause the truck to shoot forward.

 

[billswagger]

Does your calculation factor in speed of the plane?
Wouldn't the velocity (weight X speed) dictate how much it actually slows?

I'd equate it to stopping a freight train, where it can take miles to stop a train moving only a few miles an hour because of the weight. An airplane weighing between 6 and 7 tons is going to involve the same principals of weight and speed, where basically you aren't going to have a constant velocity because speed changes. However the energy expelled from a gun(s) is constant.

I think its more a function of velocity, not just the energy of the gun(s) which i why i mention the differences of firing at 400mph in a dive or at a level cruise.



Bill