Selling the AH‑1G to the US Army relied partly on its 80 percent commonality with the UH‑1, but the gunship’s original features made it the first helicopter designed specifically for the attack role. For this it had to survive close engagements with ground defenses. Its 38in. fuselage width reduced the helicopter’s vulnerability head‑on, although the NVA used triangulated gun positions to fire on it from the sides. The narrow fuselage also increased the AH‑1G’s susceptibility to high side winds when static. The Model 540’s wide‑chord rotor design, based on “pitch cone coupling” and “transient torque” technology, allowed the rotor to effectively unload itself in high‑g maneuvers without overtaxing the system and inducing negative g forces that would destroy the transmission system. For the AH‑1G, this enabled the helicopter to make hard “wing‑over” maneuvers at angles approaching 135 degrees, leading to a dive at up to 220mph for rocket launching. The pilot pulled the nose up and slowed to 80mph, rolling almost inverted. He then let the nose drop, made a 90‑degree turn and rolled out toward the target. He had to pull out at a sufficient altitude to avoid structural damage, which could be caused if he exceeded the 220mph “red line” speed limit. A direct dive also gave AAA gunners a more predictable target.

Early Cobras had attitude indicators whose gyroscopes were disabled for up to ten minutes by a wing‑over dive, disorientating a pilot in poor visibility and persuading him to invert the aircraft, severely overstressing the rotor and transmission. Improved gyroscopes were supplied by 1968.

AH‑1G ARMAMENT

The TAT‑102A turret in early AH‑1Gs housed a single M134/GAU‑2B/A six‑barrel 7.62mm Minigun with 8,000 rounds, capable of firing either 2,000 or 4,000 rounds per minute, depending on trigger pressure, in bursts of around three seconds, punctuated by a few seconds’ cooling time. Every fifth round was a tracer, giving the effect of a “laser” when fired. At 4,000 rounds per minute, all the ammunition could be fired in 30 seconds. Shorter bursts could jam the feed mechanism. The Minigun was often used as a preliminary suppressor while the pilot aligned the aircraft for a rocket attack.

On AH‑1Gs delivered to Vietnam from October 1968, an Emerson Electric M28A1 turret with two M134 guns was fitted. Either gun could be replaced by a Hughes M129 40mm grenade launcher, known as the “chunker” due to its sound when firing, with 231 grenades fired at 450 rounds per minute in ten‑second bursts. The M129 was angled upwards to lob grenades at a pre‑determined angle, but accuracy was variable. The recommended dive angle for grenade firing was 30 degrees, with a short “gun run.” An interlock mechanism interrupted fire from the weapons, which could be fired simultaneously. Used for “area fire” rather than pinpoint accuracy, the M129 was never fired against targets at the same time as 2.75in. rockets so as to avoid the latter being struck by grenades.

Very occasionally the grenade feed mechanism jammed, causing an internal grenade explosion that destroyed the turret. Grenades traveled at slower speeds than 7.62mm rounds, which meant they exploded on a target just as the Cobra passed near it – when the helicopter was at its most vulnerable to ground fire.

Heavier firepower could be supplied by a General Electric M195 20mm rotary cannon within the M35 armament system, the weapon generally being considered more accurate for ground attack than rockets. This comprised the gun on a mounting under the left inboard wing pylon, with a streamlined fairing attached to the lower left fuselage side containing 1,000 rounds. It could be combined with an M18E‑1 Minigun pod on the left inboard position with 1,500 rounds. The M195 had a stand‑off range of up to 6,000ft, out‑ranging the NVA’s lethal ZSU‑23. Wiring for the M35 system was housed under raised panels on the forward fuselage side. A chute below the cannon ejected shell cases away from the tail rotor.

The 196th Assault Support Helicopter unit had 20mm‑equipped AH‑1Gs in January 1972, using them to devastating effect on enemy troop concentrations. During the summer of that same year, B Troop, 2/17th Air Cavalry often flew a two‑Cobra team that saw one helicopter equipped with rockets and the other with the M195 in missions against vehicles. Some M195s had also been used by Cobras in Lam Son 719 in January 1971 despite armor‑piercing ammunition being unavailable during that operation.

Although the various turret combinations were effective at fairly short range, the Cobra relied on rockets when tasked with performing the role of airborne artillery. A 40mm grenade had a bursting radius of 15ft, but a 2.75in. rocket could cover a 45ft radius, making rockets ideal for launching near troops in contact with the enemy. “Short round” incidents in which friendly troops were hit by misplaced rockets were very rare. Tactical fighters delivered heavier ordnance and bombs.

M157 seven‑tube rocket launchers were usually mounted on the outer pylons, with 19‑tube M159 or M200 “Hog pod” launchers on the inboard pylons. The M158, comprising seven separate tubes, could replace the M157. Four M200s delivered a barrage of 76 rockets, fired in pairs at six per second. In an emergency, the wing‑mounted ordnance could be jettisoned at speeds below 138mph, although the inboard stores had to be shed at a 40‑degree attitude to avoid them hitting the landing skids.

Each 2.75in. rocket flew at 2,200ft per minute, propelled by a Mk 40 rocket motor. In 1969, rockets with warheads containing 17lb of High Explosive Composition B4 and radio‑triggered proximity fuses were shipped to Cobra units to supplement the standard “Black Beauty” rockets with ten‑pound warheads. With a similar destructive force to a 105mm artillery shell and a bursting radius of 30ft, the B4‑equipped rockets were far more effective against “hard” targets including armored vehicles. Their heavier warheads meant that pilots had to compensate by “lofting” rockets higher to avoid them falling short if they were fired from a range of more than 3,000ft. If rockets had to be fired into wind, preferably head‑on to avoid deviation, the pilot could use a high angle of attack to enable them to reach their maximum range. Ten‑pound warhead‑equipped rockets typically ran out of propellent some 4,500ft from where they were launched.

Extract from AH-1 Cobra Gunship vs NVA Armor: Vietnam 1967–73

With an intended speed of 250mph, a four‑bladed rigid main rotor and a novel “rotorprop” pusher propeller/tail rotor, the prototype AH‑56A Cheyenne first
flew on September 2, 1967. It had a helmet‑mounted weapons sighting system, a rotating seat and periscope for the gunner’s cockpit, and weapons on six external pylons. Here, three of the ten examples built by Lockheed fly in formation for the camera during an air‑to‑air photo sortie north of Santa Barbara, California. (Lockheed)

A Cobra’s M28A1 turret, which replaced the TAT‑102A from mid‑1968, featured an M134 Minigun (left) and an M129 grenade launcher (right). The
open ammunition bay door was a useful general purpose shelf on which aircrew could leave their equipment prior to strapping into the cockpit. (US Army)

An M195 (cut‑down M61A1) 20mm rotary cannon unit hangs alongside a seven‑tube M158 rocket launcher on this AH‑1G. For a four‑gun configuration, an M18 7.62mm Minigun pod could be mounted on the right inboard pylon. The M195 was capable of firing 750 rounds per minute, with ammunition fed via two canisters attached to either side of the fuselage above the skids. Each of these held 950 rounds, as well as the crossover ammunition feed chutes. (US Army)