Firing MG's and cannons

[pinsog]

If a 50 Browning weighing 127 pounds, sitting on a tripod on the ground doesn't slide off across the battlefield, then how would 4, 6 or 8 slow down an 8,000 to 16,000 pound fighter plane doing 200 to 400 mph?

GRAUGEIST

I have fired a 12 gauge shot gun multiple times off the back of an ATV moving 20 to 30 miles an hour and moving happened? Was that the point you were trying to make?

 

[GrauGeist]

GRAUGEIST

I have fired a 12 gauge shot gun multiple times off the back of an ATV moving 20 to 30 miles an hour and moving happened? Was that the point you were trying to make?

I'm sure you have.

If I have to explain further, the point is lost.

 

[Shortround6]

The mass of gases leaving the barrels should be added to the mass of the projectiles leaving the barrels. In the equation it is the sum of all mass being projected from the aircraft that is relevant. I would contend (but I don't know much about firearms) that this mass of gas is a very small component of the total mass being ejected. Any gases or anything else that remain in the aircraft, or weapon, are not relevant.
Cheers
Steve

The mass of the propellant gas can vary widely (or even wildly) depending on the firearm. For some extremes a .45 automatic can use under 6 grains of propellant to drive a 230 grain bullet while a .220 Swift can use over 40 grains to drive a 45-46 grain bullet.

To get more on track the American .50 used about 240 grains of propellant for it's 670 to 715 grain projectiles. Since powder varies a bit from production batch to production batch exact amount varied a bit while MV was held constant. The American .50. Russian 12.7 and German 15mm used the highest ratio ( I am meaning the most propellant per unit of projectile weight) of propellant to projectile. Not necessarily in that order. The early Japanese and German 2Omm and the German 30mm MK 108 rounds used the lowest ratios of the common guns. High velocity needs a lot of propellant. There may be more than one constant for the escaping gas depending on the velocity class of the weapon but the guns we are concerned with 1200 m/s seems to be an accepted figure.

 

[muscogeemike]

Come on guys! When I posted that B-25 crews reported their planes paused when the 75mm canon was fired I thought everyone would take it a humorous example of the thread subject.

But if we are going to discount or deeply analyze all personal reports by vets then I think much of what we think we know could well by false.

---
For the first time I have seen "History" at close quarters, and I know that its actual process is very different from what is presented to Posterity. From the WW I diary of Gen Max Hoffman

 

[GregP]

For those of you interested in continuing with the analysis, the typical 50-cal round had about 14.5 grams of powder in it. If you add that, you might affect the 3rd or 4th decimal place. Go for it.

Mike brings up an important point. Recorded history is usually the viewpoint of the victor. That is, the viewpoint of the loser is almost always lost to posterity since the people writing the history texts must usually deal with the winning side when they publish their books. If they stray too far from what has been said by the people in power, then their text is almost never adopted by schools or bought widely. A true historian might not care, but most textbook writers aren't able to ignore possible sales.

I have a history text written only 4 years after the US Civil War and it is completely different from what is being taught in schools today. If you were to read a modern version and then the book written at the time, you'd swear they were talking about two different conflicts.

To me, personal accounts are the recollections of someone who was there and must be viewed as such. Never mind the overall battle, their recollections are of the events that happened to them. They are mostly true with the bad parts somewhat exaggerated and the good parts also somewhat exaggerated, but essentially true accounts of what they saw. Most of the exaggeration comes for the fog of time, not from a desire to make it sound better.

I've heard maybe 100 former WWII pilots talk about WWII over the time I've been at the Planes of Fame Museum, and their stories are vivid, with little quarter given or asked. Some mentioned the strain of fear of having missed some enemy who was stalking you in combat since it was usually the guy you didn't see who killed you. Some didn't mention any combat stress at all. A couple mentioned the unreal sensation of bailing out of a plane going down as one of the most memorable experiences of their lifetime, particularly about the time it came to pull the ripcord. Both of these gents worried about being shot at while hanging in their parachutes and neither actually was shot at. One said the German who got him flew past and waved.

I got the distinct impression they hated the enemy while fighting him, but if they won and he bailed out, he became a man again and they left him alone in his parachute. That is, the man and aircraft together were despised, but once separated, they were just a plane and a guy. Whether or not they continued hating him when he was out of the plane depended largely on whether or not he had just killed one of your best friends or not.

 

[Shortround6]

For those of you interested in continuing with the analysis, the typical 50-cal round had about 14.5 grams of powder in it. If you add that, you might affect the 3rd or 4th decimal place.

You have 14.5 grams of propellant exiting the muzzle at around 1200m/s and you have a 41-46 gram projectiles exiting at 870-900 m/s. The Propellant (in the case of the .50 cal) makes up about 31% of the recoil momentum or recoil impulse.

 

[GregP]

I went back and recalculated. I was right, it made a change in the 4th decimal place ... absolutely NO effect on the calculation for all practical purposes. Makes a big difference in recoil, but infinitesimal compared with the overall calculation.

Momentum = m v, and when the mass is 4,309 kg and the velocity is 1,400 m/s, changing the mass to 4,309.045 versus 4,309.060 doesn't do anything to speak of to the result.

 

[Garyt]

Momentum = m v, and when the mass is 4,309 kg and the velocity is 1,400 m/s, changing the mass to 4,309.045 versus 4,309.060 doesn't do anything to speak of to the result

I think we should be looking at kinetic energy here, not momentum.


The plane has momentum due to it not being at rest. The prop would be measured by it's kinetic energy, as would the recoil.

Just my 2 cents, but I think it's correct. How to add KE to momentum I am not sure. And of course drag would play a role.

 

[tyrodtom]

The original poster asked what it was like to shoot a machine gun, or cannon, in a aircraft.

He probably didn't want, or expect anyone to get into quantum physics to answer that question.

 

[KiwiBiggles]

...when the mass is 4,309 kg and the velocity is 1,400 m/s...

1400 m/s? Nearly Mach 5?

 

[GregP]

No, 300 mph is about 134 m/s and the Browning 50 has a muzzle velocity of about 880 m/s. So the total is about 934 m/s, I was just posting the speed as a talking point. When the velocity is many hundreds and the mass is several thousands, hen adding or subtracting .04 kg means nothing whatsoever to the answer.

ANd I believe we have a good post just above, The poster never meant to get into Physics ... so I'll decline to pursue it father in this thread in agreement.

 

[Maxrobot1]

I recently re-read "God is my Co-pilot" by Col. Robert Scott (1943) after thirty years. He had flown a P-43 with 4 .50's. This how he described his experience firing the six .50's of his new P40E, 41-1496.
" I'll never forget the first time I pressed the trigger of my guns and heard the co-ordinated roar of the six fifty-calibre machine guns."
"The sense of their power impressed me as the recoil slowed me many miles per hour in my dive; I could feel my head snap forward from the deceleration. Sometimes when the guns on only one side would fire, the unequal kicks from the recoil would almost turn the ship."

 

[mikewint]

This question was debated to death a few years back. I don't recall the exact thread. This is strictly a question of momentum or m(mass in kg) times v(velocity in m/s).
Momentum is always conserved. The momentum of the forward moving bullet is equal to the momentum of the rearward moving gun (recoil). Jump upward you have upward momentum the Earth MUST move downward. Since your mass (m) is small and the Earth's Mass (M) is large the Earth's downward velocity (v) must be miniscule by comparison
Recoil is indeed the mass of ALL ejecta (bullet + gas) X ejecta velocity. Thus a 0.0458kg bullet X 856 m/s = 39.2kg m/s momentum X 6 guns = 235kg m/s
Ejected gases velocity = A x Vo where A = 1.25 to 1.75 depending on propellent and Vo is muzzle velocity. Thus gas ejecta momentum = 0.0145 x 856 x 1.25 = 15.515kg m/s X 6 = 93.09kg m/s
or 0.0145 x 856 x 1.75 X 6= 130.326kg m/s
The aircraft at 4309kg and 134m/s has 577406kg m/s momentum
Thus taking the higher bullet constant summing forward momentum = 235 + 130 = 365kg m/s that must equal 4309kg X rearward velocity or 365 divided by 4309 = 0.0847m/s to the rear or a reduction of .2 mph for every 6rnd salvo. The aircraft version had a cyclic rate of 750 to 850 rounds per minute thus 13rnds per sec on average. With all guns firing overall speed reduction would be 2.6mph

 

[Koopernic]

The equations from rocketry for thrust are:
F = dm/dt x V is the thrust (or recoil) with the answer Force being in Newtons.

dm/dt is the mass flow rate (ie bullet weight in kg times cadence rate per second)

V = velocity in meters/second.

Of course the propellant mass and propellant velocity needs to be added in as well.

The exhaust gas velocity of the Saturn V's F1 were about 2890 meters a second giving an Isp specific impulse of about 280 whereas that of the Space shuttle on LH2+LOX was about 455 (in vacuum). I suppose the cartridge propellant of a typical gun would be more like 2000m/s.

 

[Shortround6]

There are constants (or approximations) for the velocity of escaping propellant gas in other threads on this subject and already figured into available online recoil calculators.

There are at least 2 threads on this subject already.

 

[Gixxerman]

The original poster asked what it was like to shoot a machine gun, or cannon, in a aircraft.

I imagine the noise vibration was something else, especially in planes those upgunned multi-cannon Fw190's the like.

 

[derek45]

The First Hellcat Ace [Kindle Edition]
Cdr Hamilton McWhorter, III, USN (Ret)
with Jay A. Stout

http://www.amazon.com/dp/B002IT6LB8/?tag=dcglabs-20

 

[swampyankee]

I race cars, and I let other people race my car. For 4 years now there is always a Go-Pro camera inside the car viewing the driver and a lot of the action out front and to the sides.

Before installing those Go-Pro cameras we relied on videos taken of the race.

Viewing those videos , from both views often tell a different story from the driver, even when the driver was me.

I'm come to the conclusion that the more excited, or excitable, a person is, the less reliable his recall is of events.

Not surprising -- psychologists have found that eyewitness testimony, in general, is not that reliable for that very reason.

 

[GrauGeist]

Just saw this video and it reminded me of the several threads here regarding the effect of weapon recoil on an airframe.

This video shows a good example of just how much 6 fifties will rock the airframe during a short burst. Also note just how far back the P-51D pulls against the lanyards during the burst.

It's also a cool video for showing the armorer's weapon prep and installation, too!


View: https://www.youtube.com/watch?v=niJ82YCiuYU