Guns

[mikewint]

planes speed does not matter. all velocity is relative. before the trigger is pulled, bullet and plane are stationary relative to each other. after firing the bullet moves forward and the gun backward (recoil) to conserve momentum. the speed reduction in terms of percent increases as speed decreases 3mph out of 400mph is less than 3mph out of 100mph

 

[billswagger]

that makes sense, and i agree its a function of the speed, but its not a constant 3mph. The only thing constant is the energy coming out of the muzzle. It would react differently to the momentum of the plane, the amount of available thrust, and the angle of dive, if the weight is also constant. I don't think its as simple as your depicting it.

And so i understand better, your calculating 3mph per second of trigger time right?

So a 5 second burst would slow the plane 15mph.

Aside from those factors, you also have the muzzle flash displacing the air going over the wing and it would effect drag, which is also dependent on the speed. So its not just the energy but also changes in the air density outside of the barrels.


Bill

 

[bobbysocks]

that makes sense, and i agree its a function of the speed, but its not a constant 3mph. The only thing constant is the energy coming out of the muzzle. It would react differently to the momentum of the plane, the amount of available thrust, and the angle of dive, if the weight is also constant. I don't think its as simple as your depicting it.

And so i understand better, your calculating 3mph per second of trigger time right?

So a 5 second burst would slow the plane 15mph.

Aside from those factors, you also have the muzzle flash displacing the air going over the wing and it would effect drag, which is also dependent on the speed. So its not just the energy but also changes in the air density outside of the barrels.


Bill

3 mph per second...wouldnt the effects begin to decrease and cease at a certain point?

 

[mikewint]

Billswagger, 8 machine guns each fire one bullet. 8 bullets at 45g each are .36kg ejected. at 800rpm that's 13 - 14 firings per second, ejecting 5kg per second at a rate of 850 m/s. the p-47 is 9900 lbs or 4500 kg. the bullets momentum 4250 kg m/s must equal the planes momentum. divide by 4500 to get .9m/s backwards or 2mph not per second
the engine continues to produce thrust while the guns brake. like keeping your foot on the accelerator and hitting the brake at the same time. the brake produces a constant retarding force which reduces your speed.
the ejected gases are hot which is low density. if anything this would reduce friction as the wing is moving through less dense air.
in essence you are correct it isn't simple but a pretty close approximation of the real world. the aircraft will slow but not by the amounts claimed earlier nor will the braking effect accumulate unless you change the amount of metal ejected

 

[billswagger]

Billswagger, 8 machine guns each fire one bullet. 8 bullets at 45g each are .36kg ejected. at 800rpm that's 13 - 14 firings per second, ejecting 5kg per second at a rate of 850 m/s. the p-47 is 9900 lbs or 4500 kg. the bullets momentum 4250 kg m/s must equal the planes momentum. divide by 4500 to get .9m/s backwards or 2mph not per second

You say 5kg per second at a rate of 850 m/s, and then later mention it equates to 2mph not per second.
I'm finding that part a bit confusing.

Also, how is the speed of the bullet leaving the barrel an indicator of the energy? Shouldn't you be considering the energy it takes to move a 45g bullet 850 m/s? Afterall, you're only calculating the energy transfer to the bullets velocity, not the total energy being expelled from the barrel. I think its what's referred to as muzzle energy, expressed in joules, but what about the rest?

And leads me to think more about the air density outside of the barrel, where although its hot gas, its not less dense or the bullets wouldn't leave the barrel, they would be sucked in.



Bill

 

[mikewint]

Bill. .9m/s is about 3ft per sec in one hour 3600sec that's 10627ft or 2mi in one hour from the p-47's speed of 412mph
the bullet has KE or kinetic energy .5 M Vsquared or 16,256 Joules per bullet. KE is not conserved while momentum M x V is conserved always. thus the bullet moves forward and the gun kicks backward. pistols have more kick than rifles (same shell fired) due to the Mass increase Velocity must decrease.
hot air is less dense than cold which is why hot air rises and cold sinks. bullets are pushed out the barrel by the expansion of the hot gases within the barrel

 

[Shortround6]

And leads me to think more about the air density outside of the barrel, where although its hot gas, its not less dense or the bullets wouldn't leave the barrel, they would be sucked in.

Those hot gases are actually loosing pressure the further the bulletl travels down the barrel, from a peak of 42,000-50,000PSI to about 6,000-8,000psi at the muzzle. Pressure drops pretty quick after the bullet leaves the barrel.

 

[bobbysocks]

but the sudden heat causes extremely rapid expansion of the gases.....the powder is slow burning...not like black powder.. and still buring as the bullet is travelling down the length of the barrel...thus more expanding gases to propel the projectile. a lot of the heat from that expansion along with that from the friction of the bullet as it is travelling is absorbed into the chamber and barrel. you can burn the barrel out of a machinegun pretty quickly. when that happens you will see the bullets ( tracers) go way off course or cork screw down range.

 

[mikewint]

The reason for large grain slow burning powder is to keep pressure fairly constant in the barrel. once the bullet clears the barrel is open and pressure falls to zero in milliseconds.
the M2HB that we used had its rate of fire cut in half to reduce overheating plus it's barrel was exceptionally heavy (HB). at night you can see a red glow if the gun is continually fired. barrel changes were armory work if available but field barrel changes did occur. head space gauges were issued with each gun but usually lost. we counted the clicks to remove the old and then clicked in the new the same number. close enough
the quad 50 at night firing tracers, barrels glowing red hot was a sight to see. kinda like star wars for real.
during the day we fired at the ground about 50yds out so you could see where the rounds were going, they'd bounce and at 100yds or so be coming in at about chest height.
ask about foo gas

 

[bobbysocks]

never heard of foo gas until i saw "the green berets" with john wayne....HOLY CRAP! that ought to have been a sight!

 

[mikewint]

pitch dark night, then 55gal napalm + a pound or so of C4 no moe sappers

 

[bobbysocks]

gives a whole new meaning to "you light up my life." you ought to be able to feel the heat for a hundred yards!! sheesh!

 

[Degs]

I understand the US stopped carrying Gatling guns on the wings of F4 Phantoms in Vietnam because if one jammed the other pushed the plane into a flat spin which could not be recovered. That was a lot of recoil and way out on the wing for lots of leverage.

 

[marshall]

I understand the US stopped carrying Gatling guns on the wings of F4 Phantoms in Vietnam because if one jammed the other pushed the plane into a flat spin which could not be recovered. That was a lot of recoil and way out on the wing for lots of leverage.

That's probably another myth.

For example, F-15 has a non centrally mounted M61 and I don't think it spins every time it fires.

And this is what wikipedia has to say about stopping carrying the M61:

"Two gun pod versions, the SUU-16/A (also designated M12 by the US Army) and improved SUU-23/A (US Army M25), were developed in the 1960s, often used on gunless versions of the F-4. The SUU-16/A uses the electric M61A1 with a ram-air turbine to power the motor. This proved to cause serious aerodynamic drag at higher speeds, while speeds under 400 miles per hour (640 km/h) did not provide enough air flow for maximum rate of fire. The subsequent SUU-23/A uses the GAU-4/A self-powered Vulcan, with an electric inertia starter to bring it up to speed. Both pods ejected empty casings and unfired rounds rather than retaining them. Both pods contained 1,200 rounds of ammunition, with a loaded weight of 1,615 pounds (733 kg) and 1,720 pounds (780 kg) respectively. During service in the Vietnam War the pods proved to be relatively inaccurate: the pylon mounting was not rigid enough to prevent deflection when firing, and repeated use would misalign the pod on its pylon, making matters worse."

"The F-4's biggest weakness, as it was initially designed, was its lack of an internal cannon. For a brief period, doctrine held that turning combat would be impossible at supersonic speeds and little effort was made to teach pilots air combat maneuvering. In reality, engagements quickly became subsonic. Furthermore, the relatively new heat-seeking and radar-guided missiles at the time were frequently reported as unreliable and pilots had to use multiple shots just to hit one target. To compound the problem, rules of engagement in Vietnam precluded long-range missile attacks in most instances, as visual identification was normally required. Many pilots found themselves on the tail of an enemy aircraft but too close to fire short-range Falcons or Sidewinders. Although in 1967 USAF F-4Cs began carrying SUU-16 or SUU-23 external gunpods containing a 20 mm (.79 in) M61 Vulcan Gatling cannon, USAF cockpits were not equipped with lead-computing gunsights, virtually assuring a miss in a maneuvering fight. Some Marine Corps aircraft carried two pods for strafing. In addition to the loss of performance due to drag, combat showed the externally mounted cannon to be inaccurate unless frequently boresighted, yet far more cost-effective than missiles. The lack of cannon was finally addressed by adding an internally mounted 20 mm (.79 in) M61 Vulcan on the F-4E."

 

[mikewint]

marshall, definitely correct, not that we hung out with pilots but an SF rode with the FAC to help with ground communications and understanding exactly what was going on and what was needed by the teams on the ground. plus the VC and NVA had plenty of captured Prick-25 (PRC) radios and (nasty buggers) pretended to be us to lure choppers/aircraft/rescue teams into ambush. i even got to ride in SPOOKY on a night mission. the pilot controlled the three gatling guns and i can definitely state that with all three firing the tail of the c-47 was NOT spun around by the recoil, course they were only .30cal but we could hit a football field in 3sec and put a bullet every foot. anything on the ground was hard to recognize as having been human

 

[Degs]

Why no photos of F4 with 3 x SUU gun-pods then ? one centre-line and 2 instead of Sgt Fletchers? Ideal for digging up the garden and soft targets.

Also big difference between wing root mounted F-15 and 2 on the outboards of an F4

 

[mikewint]

Degs, can't answer about the pics but two things occur to me, one, vibration caused by the recoil could certainly cause problems with delicate electronic components and, two, exhaust gases from the pods getting into the air intakes causing a flame-out as happened initially on the A-10 with the 30mm
US gun pods go back to WWII when .50cal "blisters" were mounted on B-25s to increase fire power. The germans also employed waffentrager Mg81 pods and the massive bordkanone, for tank-busting, in anything from 37mm to 57mm on BF109s, FW190, and the stuka. (none of which stopped the aircraft or knocked it backwards.
the advent of air-to-air missles changed US policy on dogfights and multimillion aircraft strafing the ground was deemed to costly so the f-4 came to VN without guns. As often happens field modification are made and vulcan gun-pods were fitted on wing-pylons. The pilots complained about not being able to aim and after a couple of uses the mounting began to loosen and the pods began to wander.
eventually, factory internally mounted cannon with gun sights were added
i also saw quad .50s mounted on outboard pylons on Hueys and don't recall any of them being knocked backward or spun in a circle unless the tail rotor failed

 

[davparlr]

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?

10,000 lbs of thrust is a lot of thrust. The T-38 generates 7700 lbs of thrust in full afterburner and can propel the 12,000 lb aircraft from zero to 185 mph in 25 sec. I wouldn't be surprised if the gun is generating more thrust than the A-10 engines at flying at 300 kts. Of course you are not going to operate the guns very long.

 

[marshall]

Another quote from wikipedia but a sourced one:

"The recoil force of the GAU-8/A[14] is 10,000 pounds-force (45 kN),[3] which is slightly more than the output of one of the A-10's TF-34 two engines (9,065 lbf / 40.3 kN each).[15] While this recoil force is significant, in practice cannon fire only slows the aircraft a few miles per hour.[13]

13. ^ a b Jenkins, Dennis R (1998). Fairchild-Republic A/OA-10 Warthog. North Branch, Minnesota: Specialty Press. ISBN 1-58007-013-2."

 

[mikewint]

problem with getting old is you remember (hopefully) things you read but not always where. when i made the statement i knew i had read it somewhere but not where.
thanks Marshall, at 70 rounds per sec, firing a mix of API and HEI rounds, i'd think a 1-2 sec burst would take out just about any tank
10,000 lbs of rearward force acting on the A-10s 42,000lbs produce an acceleration of 2.3m/s or 5.2mph each second the gun is fired considering zero net forward thrust from the aircraft's engines
At 55% throttle the 9,065lbs engines would produce 10,000 lbs of forward thrust. i know drag is a factor and determined by speed but i cannot find any info on it. the A-10 is not a high speed craft going mainly after ground targets so i would think? speed would be kept low when attacking making drag minimal.
anyone know the drag coefficients? or attack tactics?