History of the AH-1G Cobra

  The development of the AH-1G Cobra was closely linked to that of the UH-1. Some confusion about this has resulted from the fact that during initial development, the AH-1 was considered a “model” of the UH-1 line and given alphabetical model designations from the UH-1 series. Later, the AH-1 series used model letters, “E” and “F” in particular, that had already been used in the UH-1 series. This history will cover only the AH-1G and mention of the AH-1J, not the later models.

  As U.S. involvement in Viet Nam began to increase, and developments in the “airmobile” concept were proven, the Army decided there was a need for a dedicated helicopter gunship. Bell helicopter seemed to be the natural choice since the highly successful UH-1 series included the UH-1B that was being used as a gunship, and the UH-1C was being developed. But the Army, and the Pentagon, saw it differently. The contract was awarded to Lockheed, and became the AH-56 Cheyenne. The Cheyenne was a very advanced concept for the time. It had a “rigid” rotor, and a pusher propeller on the tail, which made it more of a “composite” aircraft than a helicopter. It had a nose turret, a belly turret, a “helmet sight”, and could carry the TOW missile as well as 2.75 in rockets.  It was fast, and agile, when it finally flew.

  Bell Helicopter looked at the Cheyenne project and saw the one flaw in it that the Army seemed to miss. With all its complex systems, the Cheyenne would take years to develop, and a better gunship was needed in Viet Nam sooner that that. Bell thought they could provide the solution.

  Bell already had three components they could use for a dedicated gunship, the basic engine, rotor and mechanical systems of the UH-1C, the slender body of their entry into the Cheyenne competition, the D-255, with tandem seating for a crew of two, and the turret system used on the Sioux Scout version of the H-13. The turret was made by Emerson, and they already had an improved version ready, that used the M-134 “minigun” in place of the two M-60 type machine guns. As an “in house” project, using their own money, Bell Helicopter built the model 209 prototype, N209J. This was the prototype of the AH-1G Cobra.

  The model 209 had a slender body with tandem seating, and the Emerson TAT-102 turret mounted under the nose with a single M-134. The tail and tail rotor were taken from the UH-1C, with the vertical tail lengthened slightly for better control. The rest of the systems, engine (the L11 in the prototype), transmission, electrical, and hydraulics, were all taken straight from the UH-1C with little change. But there were two major differences, the 209 had retractable land skids, and there was no stabilizer bar on the main rotor. Bell had developed an electro-mechanical stabilizer known as SCAS, or Stability (&) Control Augmentation System, that replaced the stabilizer bar in the 209. Once it was flying, Bell flew the 209 to Ft Wolters, the Army primary helicopter training center, and showed the Army what they had.

  The Army did hold what might be called a “competition” for an “interim” gunship, but the 209 was so far ahead of the other proposals there was really no contest. In 1967 the Army ordered the first 110 of over 1100 AH-1G aircraft from Bell. These incorporated a number of changes the Army wanted to the design. First, the retracting skids were replaced with normal fixed skid landing gear. The retractable ones were just too heavy and didn’t add much at all. Second, the Army specified the new T-53-L13, 1400 SHP, engine for all AH-1G models. The L13 was a major step up from the L11, and much better suited for the high hot environment of Viet Nam. Since the transmission was limited to 1100 SHP, it meant that the L13 could produce that 1100 SHP in much higher hotter conditions than the L11 could. Third the Army made a few small changes to the airframe, like widening the area directly behind the turret to give more room for ammunition storage, beefing up the "stub wings" to make them stronger, and removing the small ventral tail fin of the original 209, which wasn’t really needed. Finally, the Army insisted on having controls in the front seat so that the “gunner” could fly the aircraft, making the crew two pilots, instead of a pilot and a gunner.

  The front seat controls proved to be interesting. Since the sighting station for the turret was mounted in the floor where the “cyclic” (main control stick) would normally have been, the front seat cyclic was placed on the right side in front of the arm rest. It quickly got the name “sidearm” controls. The problem came with the fact that the sidearm cyclic was very short, limiting its movement. This meant that moving the sidearm cyclic one inch, moved the rear cyclic four inches, making the sidearm very sensitive. It also gave the rear seat a four to one leverage advantage. This would be addressed in later production versions, and the mechanical advantage removed.

  The Cobra proved to be a natural for Viet Nam, and became an immediate success. It could carry a much heavier weapons load than the UH-1C, fly out to an area faster, and stay on station longer. It’s very narrow profile also made it much harder to hit when flying toward a target on a firing run. To get these advantages, you gave up the two extra pair of eyes of the door gunners, and their suppressive fire, that the UH-1B/C always had. The argument over which was better goes on to this day.

  Once in service in Viet Nam, some areas were identified where the Cobra could be improved. Bell had intended, almost from the start, to replace the single gun TAT-102 turret with a new version Emerson was developing that carried two weapons. This became the M-28 system, or TAT-141. This turret could mount either the M-134 minigun or the M-129 40mm grenade launcher in either side of the turret. So you could mount one of each or two minis or two 40mms. The normal configuration was the M-134 minigun on the right side and the M-129 on the left. Almost no one ever mounted two 40s, and it was soon noted that a minigun mounted in the left side was very prone to jamming. Both turret systems had electronic compensation for the effects of wind (slipstream) and muzzle velocity of the weapon being fired. It was actually a very accurate system. Also at some point, most Cobras had the nose mounted landing lights removed, and the nose painted over. This was mostly because the glare from those lights would totally blind the front seat pilot. This change was made in later production aircraft as well.

  The next improvement was a major one from a pilot’s standpoint. The Cobra became the first Army helicopter to be air conditioned. The canopy of the Cobra almost acted as a “green house” in the heat of Southeast Asia. Unless you could get up to higher altitude, where the air was cooler, it could become a real “sweat box”. The solution was simple, for someone who understood physics! They took a small amount of “bleed air” off of the final compressor stage of the engine and fed it into what was called the ECU, or Environmental Control Unit. This air was under very high pressure, and was about 750 degrees F. In the ECU, the bleed air was allowed to return to normal pressure very rapidly. This has the somewhat odd effect of super cooling the expanding air, and what you ended up with was air, at normal pressure, that was very cold! The ECU then mixed that air with outside air depending on how the controls were set to cool the cockpit. With the humid air in Viet Nam, the ECU unit would actually blow ice crystals at you when turned up full! Needless to say, pilots loved it. The ECU did have to be turned off during high power operations like hovering, since it did rob a little power from the engine.

  By 1971, two more improvements were being made to the Cobras serving in Nam. First was the replacement of the original green tinted canopy with a clear one. This wasn’t a major issue, but the green canopy could make it hard to see at night, and with the ECU, it wasn’t needed. The other change was to move the tail rotor from it’s original position on the left side of the tail over to the right side. When hovering, the Cobra had a problem if there was much wind from the right rear. The tail blocked the tail rotor, and caused turbulence that could cause a loss of yaw control. Moving the tail rotor to the right side eliminated this problem. Some plastic model kits, and a few pictures, show the Cobra with a "frisby" shaped disk attached to the rotor mast to cover the hole in the top of the "dog house" structure. I don't recall ever actually seeing one of these in real life, but I have no idea when they were removed. 

  The only other major change to Army Cobras in Viet Nam was in weapons systems. The minigun pods carried under the wings on some early Cobras all but disappeared, and in 1971 the 238^th AWC didn’t even have them. What we did have as the XM-35 system. The XM-35 (later M-35) system mounted a six barrel 20mm cannon under the left wing inboard. The ammunition was carried in boxes attached to the sides of the aircraft just above the skid tubes. The 20mm was a cut down version of the Air Forces “Vulcan” cannon, with the rate of fire reduced to 550 rounds per minute. This system was developed mainly to take on the NVA .51cal machine guns that shot down many of our aircraft. With the XM-35, you could stand off at a range where the .51 wasn’t so accurate and still hit the target easily. My favorite aircraft was an XM-35 system Cobra, serial # 68-17031, it really was an awesome bird to fly!

  In 1971 Bell introduced an entirely new version of the Cobra. This was the AH-1J produced for the Marine Corps. The AH-1J was quite different from the AH-1G. It had a "twin pack" engine, or two engines linked together, that produced 1800 SHP total. It also had a three barrel 20mm cannon mounted in the chin turret in place of the minigun and 40mm. This cannon would become standard on later model Cobras, but was totally new in 1971. One other difference was that the AH-1J did not have the magnesium alloy body of the AH-1G, because it was too prone to corrosion from salt water. This became the forerunner of today's Marine "SuperCobras".

  In 1973 IR suppressor kits were fitted to some Cobras that remained in Viet Nam. When the U.S. withdrew from Viet Nam, we took all the Cobras with us, none were left for the VNAF. I don’t have exact figures, but I have seen estimates that something less than 700 of the 1100 AH-1Gs built actually served in Nam. Since the contracts for the 1100 extend through 1971, that may be correct. Records show that 279 Cobras were lost to all causes, or about 40% of those that served there. That is actually a lower rate than any of the other UH-1 models to serve in Viet Nam. Almost all surviving Cobras were updated to TOW Cobras so very few original G model Cobras exist today.

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