Guns and Rotors

By Will Dabbs, MD

The floor of the aircraft rocks violently as the big Army helicopter screams up the dark, frozen Alaskan riverbed, rolling viciously to follow the contours of the landscape at fifty feet above the snow-covered ground and one hundred forty miles per hour. Attempting to compensate for the motion, you cling tightly to the spade grips of your D-model M-60 and keep your knees bent as you scan the green image flashing by through your ANVIS-6 (Aviator’s Night Vision Imaging System) night vision goggles. Abruptly the aircraft rolls into a steep bank and rounds a bend in the river, bursting into a wide flood plain. As you’d anticipated, a formation of two dozen NATO-standard silhouette targets stands clustered some three hundred meters off your route of flight. The stark shadows they cast in the electronically-intensified moonlight of your goggles make them easy targets.

You punch off the safety on your -60 and lean back against the grips, allowing the weapon and mount to take your weight. Taking a rough bead slightly below and behind the cluster of targets, you tighten around the gun’s twin triggers and chug out a ten-round burst. The pair of tracers that light out of the burst arc brilliantly out of the muzzle and impact a bit short, leaving glowing green trails in the phosphor of your goggles. You note to yourself with a grin that the muzzle flash off of your -60 looks really cool through your NVGs. A minor adjustment and the next burst chews into the center of the formation, tossing snow and dirt liberally among the plastic soldiers.

Your next target is a Conex-a big metal shipping container. It rests five hundred meters or so out in the riverbed and is as large as a small house. You fire another sounding burst and then walk your tracers toward the dark metal box. Once you are on target you hold the triggers an extra second or two; there’s no being too careful when dealing with hostile shipping containers.

In the minutes that follow you ventilate assorted junked cars, trucks, vans, and construction equipment at a variety of ranges out to about a kilometer. You are having so much fun that you almost don’t notice the one hundred twenty-knot slipstream tearing across your face. The wind itself wouldn’t be so bad were it not for the ambient negative twenty five degree temperatures typical of the Alaskan wintertime. These two elements combine to yield a wind chill you conservatively estimate at one hundred thousand degrees below zero. No matter, that which does not kill us makes us stronger and you’re having too much fun to notice the pain.

Sooner than you would like, the plain starts to tighten back around the river and the range nears its close. There is nothing quite like firing belt-fed automatic weapons out of a moving aircraft. For the recreational machinegunner it is very nearly nirvana. To do so at night under night vision goggles is cool beyond my capacity to describe.

The M-60D, the Army’s standard defensive helicopter armament, is at its heart a slightly modified version of the M-60 General Purpose Machine Gun. The barrel and receiver group of the D-model are essentially those of a ground -60 but the pistol grip/trigger assembly is replaced by a set of spade grips and a mechanical linkage connecting the gun’s sear to the trigger rings on the grip. The sight is a fold-up spider web type device comprised of a pair of concentric aiming circles. In practical application the weapon can be finicky. Some pieces, dependent upon age, wear, and, I suppose, the phase of the moon, are notoriously unreliable.

A canvas ejection control bag mounts on the right side of the gun to catch brass and links, preventing potential damage to delicate aircraft systems by these discarded items. The weapon employs a rather cumbersome crossbolt safety that is difficult to manipulate with a link bag installed and foregoes the familiar rubber-covered forearm of the ground version. The ejection control bag itself is effective but was obviously designed as an afterthought. The operator must reach his hand through a zippered opening in the rear of the bag to access the weapon’s charging handle and it can be quite frustrating reducing multiple stoppages on a crotchety gun with the link bag installed.

The physics of aerial gunnery differ significantly from that which governs the more orthodox employment of ground-mounted automatic weapons. While the M-60D comes equipped with the aforementioned small fold-up ring sight, I have found that most effective aerial engagements result from simply tracking one’s tracers and walking long bursts into a target. As helicopter door guns are by their nature suppressive weapons, this technique projects maximum fire into the target area, hopefully keeping an enemy’s head down long enough to allow the aircraft to egress the area. The most striking aspect of helicopter door gunnery, however, is the behavior of bursts fired on opposite sides of a moving aircraft.

WARNING: Physics-intensive discussion follows. Skip this part if you don’t care about the details.

A bullet launched from an M-60 leaves the muzzle at roughly 2800 feet per second imparted with a clockwise spin of one turn every 12 inches. This results in a rotational velocity of roughly 228,000 rpm. When fired from a stationary platform the projectiles arc toward their targets based solely upon Newtonian physics. The bullets accelerate toward the center of the earth at 32.17 feet per second squared, their range being therefore governed by their initial velocity and the distance above the ground at which they were fired. When the firing platform is moving through the air, however, each individual projectile becomes its own little flying machine, interacting with the lateral airflow to generate specific characteristics based upon the direction of spin and wind speed.

When fired out of the right door of a moving helicopter the left-to-right airflow across the projectile interacts with the clockwise spinning projectile to produce a low pressure area on the top of the bullet. The practical result of this phenomenon, technically a derivative of Bernoulli’s effect, is to create lift and cause the bullet to fly upward, following a trajectory completely opposite that which one would expect in stationary gunnery. As such, an aerial gunner firing out the right side of a moving aircraft must actually aim below a target and allow his projectiles to fly up into the target. (see Fig I)

When firing out the left side of an aircraft the opposite is true. The lateral airflow creates a low-pressure area underneath the spinning bullet and exacerbates the plunging effect of a burst caused by gravity. The result is a decrease in effective range of the weapon. (see Fig II) The climb or descent of the projectiles is a function of the square of the airflow velocity so the effect is much more pronounced at high aircraft speeds.

CONGRATULATIONS! You made it through the physics part. Now back to the fun stuff.

The 100-plus-knot slipstream outside a helicopter in flight does an excellent job of cooling an air-cooled machinegun. I have seen air-60s digest truly phenomenal quantities of ammunition in long, sustained bursts without suffering the debilitating effects of excessive barrel heat to which ground guns are susceptible.

Mounts for the guns are unique to the airframe and employ cams and blocks that limit the field of fire of the weapon so as to preclude shooting up one’s own fuselage or rotor system. The UH-60 and UH-1 aircraft have provision for two guns each, one per side, while the twin-rotor CH-47 can sport a third gun in a tail stinger mount.
Most special operations aviation units employ either the General Electric M-134 minigun or Browning M-3 aircraft fifty-caliber machinegun as on-board suppressive armament. The M-134 is an electrically-powered, six-barrelled gatling gun which fires 7.62 mm rounds at rates of up to 6000 rounds per minute. The weapon is powered by the aircraft’s integral 24-volt DC electrical system and, unlike the -60, is rendered inoperative if separated from the aircraft and its power source.

The Browning M-3 aircraft gun is the same weapon our grandfathers employed in flexible mountings on B-17s and B-24s. The gun is a lightweight version of the M-2 heavy barrel machinegun, weighing in at 68 pounds and firing at an accelerated cyclic rate of 1200 rounds per minute. The recoil impulse of this fast-firing fifty restricts its use to relatively large aerial platforms such as the MH-47E Chinook and MH-53E Pave Low aircraft.

Most western European rotary-wing aircraft employ the FN MAG GPMG for on-board suppressive fire. Soviet-bloc aircraft such as the Mi-8 and Mi-17 have mountings in each passenger window for either the PK general purpose machinegun or the Kalashnikov series of assault rifles. Like most of the military materiel designed in the former Soviet Union, these mounts are simple, robust, and effective. Each mount clamps around the forend of the weapon and allows a limited traverse while preventing the weapon from falling out of the aircraft, a significant concern with enthusiastic young soldiers under stressful conditions.

Tracers fired under night vision goggles are absolutely spectacular. The ANVIS system mounts to a modified SPH-series aviator helmet visor and sports a separate battery pack that secures with velcro on the back of the helmet. While the entire setup is a bit heavy and cumbersome, generating some truly world-class neck strain in the uninitiated, it is remarkably sensitive and allows the contemporary Army aviator to fly any mission he might fly in daylight in practically pitch darkness. The goggles themselves are analogous to a pair of miniature television cameras with their associated screens suspended just in front of the aviator’s eyes. The faint image of the terrain ahead of the aircraft generated by the reflection of ambient moonlight or starlight is electronically amplified and presented in various shades of green monochrome on the eyepiece end of the goggles. For functionally lightless evenings Army aircraft are equipped with an infrared searchlight which can be independently gimbled by the pilot in flight via a coolie hat control switch on the collective pitch control. These searchlights are nearly all-illuminating under night vision systems but invisible to the naked eye. This might be something to think about the next time you plan on necking with your girlfriend out on a deserted road near an Army base.

Effective aerial gunnery is ammunition-intensive and an acquired skill and is therefore a rather challenging task on which to train in a military environment. The range space required to train on aerial gunnery tasks safely is obviously prodigious and it is a daunting task to configure targets is such a way as to be a challenging and realistic simulation of reality. The fire hazard of firing tracer ammunition into flammable woodland and grassland areas provides yet another impediment to aerial gunnery training. While stationed at Fort Sill, Oklahoma I found that the Oklahoma grasslands were much more susceptible to incineration via helicopter-fired tracer ammunition than to even white phosphorous artillery rounds.

Regardless of the training challenges, true aerial gunnery is just too cool for school. A computer game manufacturer who could accurately replicate the sights, sounds, and feelings of firing a machinegun out of an aircraft in flight would be a wealthy man. It is a shame that helicopter costs, range space, and draconian legislation stand between most civilian machinegunners and this unique shooting experience.

We American machinegunners are an innovative lot, however, and have developed countless ways to enjoy our sport while remaining within the confines of U.S. firearms law. Who knows, perhaps some enterprising recreational machinegunner with access to a Cessna may someday put a new slant on the Knob Creek shoot.