Firing MG's and cannons

[Thorlifter]

I'm just wondering what the experience was when flying and firing the MG's or cannons. My guess is the MG's just gave the plane a vibration while I can imagine the cannon's must have felt like hitting a speed bump at high speed. Any experiences?

 

[derek45]

In Hub Zemke's book he said it would slow the big heavy P-47 down.

Another book I read, ( maybe Johnson's ??) He stated the first time he fired 8 .50 cals, he thought something was shaking the aircraft apart.

 

[GregP]

I've fired a lot of guns including bigger cannons, but never from inside an aircraft. So, I can imagine, but don't know.

 

[yulzari]

IIRC firing a full drum of 40mm from x2 S Guns in a Hurricane IID knocked about 45 knots off the speed and a 4 degree drop in the nose.

 

[GregP]

45 knots! Wow! Better not shoot if you're anywhere near a stall, huh?

 

[fastmongrel]

IIRC firing a full drum of 40mm from x2 S Guns in a Hurricane IID knocked about 45 knots off the speed and a 4 degree drop in the nose.

Surely that cant be right you are firing at most 90 pounds of projectiles from a 7,500 pound aircraft doing about 160 to 180 knots. If that was the case a P39 firing its 30 rounds of 37mm would never have hit anything with the 2nd shot.

 

[Lefa]

Kyösti Karhila has said, "When the first time I shot all BF109G6/R6 machine guns, it felt like the plene should be stopped, and continued to fly again when I lift fingers off the trigger.

 

[Garyt]

Five .50 cal Mguns have a kinetic energy of about 100 kiloajoules.

A P-51 fully loaded traveling at about 300mph is at about 37,483 kj when being calulated for kinetic energy.

 

[KiwiBiggles]

From a kinetic point of view, firing the guns can be considered as an engine providing power, acting to slow the aeroplane.

Back-of-an-envelope calculations for muzzle horsepower of various weapons (based on ½mV² x RoF):

Browning M1919 58 hp
Browning AN/M2 318 hp
Hispano-Suiza HS.404 787 hp
MG151/20 440 hp

Multiply those by the number of guns carried, and you should get a rough idea of the effect on the plane. So going by these numbers, firing four Hispanos in a Tempest (3148 hp) would have much the same effect as switching off the engine altogether.

 

[tyrodtom]

Every one of those weapons utilize the recoil to power their mechanism to fire the next shot. It's not like you're firing a bolt action rifle.

I've been in a Huey when both M-134 miniguns were firing. When the co-pilot would slew them from side to side, both miniguns would fire at the low rate, 33 rps, when one gun would go a little past straight ahead it would quit firing and the gun on the other side would continue firing but step up to it's high rate of fire of 66 rps.

According to what you guys are implying that should have turned the aircraft sideways, but the pilot had no problem correcting with pedals. As a passenger I never even felt a correction. And i'll point out the M-134 was totally externally mechanically powered, none of it's recoil was soaked up through working the action.

With the all going on at the time, vibrations, wind buffet, and doing your job, you're sort of on sensory overload, I won't say you don't notice when the guns fire, but it's not the big event some of you are making it out to be. And these guns were only about 3 feet away from where I sat.

Even the one minigun on the much, much, lighter OH-6 that I usually flew in wasn't that dramatic when it fired.

 

[fastmongrel]

Hispano-Suiza HS.404 787 hp
.

787hp Surely the gun would turn into a puddle of steel if you pumped 787hp through it

 

[KiwiBiggles]

No, the energy is going into the projectile, not the gun. And as mechanical recoil to the airframe.

 

[Shortround6]

I think we have been through this one before guys.

The recoil is proportional to the momentum, not the energy kinetic energy. Bullet weight times bullet velocity equals gun* weight times gun velocity. Now you can figure out the recoil kinetic energy once you know the gun* weight and recoil velocity.

The next "problem" is the asterisk * ie, gun weight, I can reduce the recoil of a bolt action rifle by hollowing out the stock and putting a couple of pounds of lead in it. The gun is not bouncing around loose in the aircraft getting a running start before it crashes into something. Depending on how solid the mounting is the entire airplane may become part of the recoil equation. If a .50 cal MG had the recoil some people are claiming the P-47 would have been unflyable and a quick reality check can be made by looking at a Jeep with .50 cal or even one mounted on a ground tripod, yes they move but not at the rate some people think.

 

[GregP]

Reference post #8.

You can do it by energy or by momentum. I'll choose energy. A Joule is 1 kg-m^2 / s^2 and kinetic energy = ½ m v^2.

Let's say you are talking about the Browning 50-cal M2. The projectile masses 43.3 g (.0433 kg) and has an average muzzle velocity of 880 m/s. The low rate of fire is 750 rpm and the high rate of fire is 850 rpm. The average is 800 rounds per minute. That's 13.3333 rounds per second. One bullet has .5 * .0433 * 880^2 J. or 13,856 J, or 13.456 kJ. Six have 83.16 kJ.

Let's say a P-51 Mustang enters combat at 9,500 pounds and 300 mph. That is 4,309 kg and 134.1 m/s. So it has .5 * 4309 * 134.1^2 J, or 38,745,111 J or 38,745.111 kJ.

So the P-51 has approximately 466 times the kinetic energy of 6 Browning M2 MG's being fired for one second.

It started with 38,745.111 kJ and one second later has 38,662 kJ. Figuring backwards, the velocity comes out to 133.958 m/s, which equates to 299.68 mph.

So a 1 second burst costs the P-51 about 0.32 mph. I'd call that negligible compared with any of several other factors including gravity.

Those calculations assume the entire recoil energy is imparted to the airframe and it isn't. Some is absorbed by the recoil springs. I don't see how firing six 50's would affect much in the a P-51 no matter what was happening.

As Shortround says, you can do it with momentum, too. It just isn't a big factor.

Might be when you get to bigger cannons. The problem with the cannons is the low rate of fire, so you'd have to use at least the time to fire one round instead of exactly one second.

 

[Garyt]

So the P-51 has approximately 466 times the kinetic energy of 6 Browning M2 MG's being fired for one second.

It started with 38,745.111 kJ and one second later has 38,662 kJ. Figuring backwards, the velocity comes out to 133.958 m/s, which equates to 299.68 mph

That was my thought as well, that the firing of guns and or cannons have a negligible effect, but it probably does not seem that way for the one flying the plane!

The only thing is, I know we are missing some things in our calculation. I think the energy needed to stop the plane is two fold - it's momentum (a plane moving at 300 mph is easier to keep at 300 mph than it is to try to get a plane moving at 100 mph to 300 mph), + energy from the prop. This is countered by drag of course in the energy of the guns (I'm thinking K.E. but I am not sure)

Unfortunately I lack the knowledge of detailed flight physics to know the exact formula.

But I still think the effects would be somewhat negligible.

 

[fastmongrel]

Firing a Lahti 20mm anti tank cannon.


View: https://www.youtube.com/watch?v=OvsfT1_W6TE

fierce recoil but 4 of them firing at once is not going to bother a 5 ton plane doing 300mph.

 

[Shortround6]

You can do it by energy or by momentum. I'll choose energy.

you can't chose. You either pick the right way or the wrong way.

As an example pick two 9lb rifles. One is firing a 55 grain bullet at 3300fps (.223 or 5.56mm) the other is firing a 500 grain bullet at around 1100fps.(45/70 with heavy bullet or shot gun slug?) Both will have about the same muzzle energy (1330ftlb to 1343ftlb) yet the recoil energy will be way different.
The rifle firing the light bullet will be moving backwards at 2.88fps and the rifle firing the 500 grain bullet will be moving back at 8.73 fps. Now since the recoil energy is proportional to the square of the rifle's speed your shoulder is going to feel the the 45/70 a whole lot more than the .223.

Now since the guns are bolted to the plane you have to take the wight of the plane into account but then you also have a lot of rounds fired in a short period of time.

I am sure the pilots felt something including a whole lot of vibration (and often the nose dipping) but actually losing several dozen MPH?

 

[tomo pauk]

If the 8 HMGs were capable to slow the P-47 down, why shouldn't the 6 of the same HMGs be capable of slowing down much lighter F4F or P-40? Or, four Hispanos substantially slowing down the Hurricane? And we cannot read anywhere that was the case.
So I'd like to tag with 'Hollywood' sticker those stories.

 

[GregP]

Deleted .. not worth the effort.

 

[Garyt]

you can't chose. You either pick the right way or the wrong way.

Actually, I think the equation is more complex than just momentum or energy as I said above.

The Plane has Momentum going for it. It also has acceleration, or at least energy maintaining a speed forward, the prop and engine.

The guns I think would be best represented by energy, as they are accelerating the bullets out of the barrel.

How to combine the effects of energy and momentum (and drag), I am not sure.