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Wednesday, March 1, 2006

Military Spin

Why It's So Hard To Hit The Target

Lt. Col. Steve Colby

I'm often asked if it's hard for our gunners and flight engineers to hit targets during helicopter aerial gunnery. My answer is unequivocally, "Yes."

Crew-served weapons introduce exterior ballistic effects that seriously complicate Kentucky windage* and aim-point solutions. These effects are unusual and not intuitive for weapons fired perpendicular to the helicopter flight path. They include trajectory shift, drag shift, tip-off, Magnus effect and, for Gatling-style multi-barreled guns, tangential throw. All these effects are varied by helicopter speed, muzzle velocity, barrel twist and firing rate.

The exterior ballistic with the greatest affect on bullet path is trajectory shift. This is the instantaneous horizontal velocity of the bullet perpendicular to the bullet's flight path induced by the forward airspeed of the helicopter. This is the effect for which the greatest "lead" must be used. At 80 kt., the instantaneous crossing velocity for the round is approximately 134 fps. sideways in the helicopter's direction of flight. At 2,300 ft. (700 m.), a 7.62mm bullet time-of-flight is 0.8-1.0 sec., so there is significant time for this horizontal component to manifest itself in lateral cross-track error.

Countering this trajectory shift is the horizontal aerodynamic drag working against the bullet's tendency to fly its diagonal path to the target. This drag-shift effect takes away some of the horizontal component of the trajectory shift. But this drag also imparts a tendency for the bullet to try to align with the relative wind.

When a bullet leaves a barrel perpendicular to the relative wind, the tip of the bullet tends to turn toward the relative wind and the tail lags due to higher drag. This momentary tip-off causes a slight precessing wobble during the initial few feet of bullet travel. This wobble increases the circular error probable (CEP), making the weapon's accuracy less predictable and potentially less effective. Overall, the bullet's inertia and rotation overcomes this momentary vector change.

Another significant impact to bullet path is Magnus effect. This is introduced by the rotational motion of the bullet while it travels perpendicular to the relative wind. Viewed from behind (see diagram), a clockwise-spinning bullet fired from the left side of the helicopter is influenced by the airflow attaching to the lower side of the bullet and the disturbance being created above the bullet. This generates a downward lift vector the same way that a baseball with top spin will drop as a "sinker". A bullet fired from the helicopter's right side will do the opposite, as it is influenced similarly to a ball thrown with backspin that causes it to climb. This effect produces a significant component of snake-like bullet tracer path.

The last effect we'll discuss is tangential throw. Electrically-driven Gatling-style guns like the GAU-2 7.62mm minigun spin counterclockwise (viewed from the gunner) to produce firing rates of either 2,000 or 4,000 rounds per minute. This six-barreled gun spins at either 333 or 666 rpm to produce these firing rates. The diameter of the barrel cluster is approximately 3 in. At 4,000 rounds per minute, the barrels are rotating in a counterclockwise circular path at 104 in. per second. This equates to an instantaneous sideward velocity of 8.7 fps. in the direction of rotation when the bullet leaves the muzzle. For left-mounted weapons, this equates to a down and aft vector because the bullet leaves the barrel at approximately the 8 o'clock position in the cluster. For right-mounted guns, a downward and forward vector results.

The combined effect of all these influences makes for a rather bizarre, twisting, snake-like tracer trail for helicopter crew-served gunnery. These trail-shape characteristics vary considerably from left to right guns and, in fact, drive quite different initial aim points to counter the combined effects discussed. Add to these ballistics any input caused by changes in helicopter pitch, roll and yaw and you make the firing solution very difficult for the gunner.

The consequences of these strange flight paths make it essential for frequent and challenging gunnery training to effectively employ weapons from a flying helicopter. That training must include live gunnery and patterns that allow dwell periods with stable flight conditions.

* Kentucky windage, or hold-off, is an estimate of the modified aim point required to compensate for wind or for target movement.

The views expressed are the author's and not an official U.S. Air Force position. USAF Air Warfare Center public affairs approved this article for release.

Lt. Col. Steve Colby is commander of the USAF 34th Weapons Sqdn. based at Nellis AFB, Nev., which trains combat search-and-rescue helicopter instructor-pilots.

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