The Bell (model 409) YAH-63 (1976) was Bell Helicopter's entry in the Advance Attack Helicopter (AAH) competition against the Hughes YAH-64.
The Bell (model 309) KingCobra was a scaled-up version of the AH-1G Cobra attack helicopter. It had a lengthened and strengthened fuselage and changes in the nose to accept a stabilized multi-sensor gun sight. It was evaluated against the AH-56A Cheyenne and the Sikorsky S-67 Blackhawk in 1972, but none were accepted by the Army. The Bell model 309 KingCobra had a laser day and night sight, infrared fire control system, and night vision TV. Armament was similar to that of the Cheyenne and the Blackhawk. The KingCobra was powered by a single Lycoming T55-L-7C 2850 shp engine. One of only two prototypes crashed. The other took part in comparative trials in 1972 with the Lockheed AH-56A Cheyenne and the Sikorsky (S-67) Blackhawk, but the US Army decided none matched its requirement. The Bell (model 309) KingCobra evolved into the Bell (model 409) YAH-63.
In November 1972 the Army solicited design proposals for a new Advanced Attack Helicopter (AAH) intended for the all-weather anti-armor role. The Army's specifications required that the new aircraft be powered by twin General Electric T700 turboshaft engines and armed with up to sixteen Hellfire or TOW anti-tank missiles in addition to a single 30mm cannon. Preliminary design proposals were submitted by Boeing-Vertol, Bell, Hughes, Lockheed, and Sikorsky, and in June 1973 Bell and Hughes were selected as finalists and were each awarded contracts for the construction of two prototype aircraft.
From the Model 309, Bell derived a new prototype attack helicopter, the Model 409 (US Army designation YAH-63), which it entered for the AAH (Advanced Attack Helicopter) competition won by the aircraft proposed by Hughes. The YAH-63 had a different profile from the Kingcobra, including a conspicuous ventral fin and a tail plane at the top of the tail fin.
The Bell YAH-63 featured a tandem cockpit with flat-plate canopy for reduced glint detection in combat. The pilot sat in the forward cockpit for better visibility while flying nap-of-the-earth. The two-blade tail rotor and drive train were supported and protected by the tail boom and an I tail. The I tail played an important role in stabilty and control, and it also prevented inadvertent ground strikes. The YAH-63 had a high flotation tricycle wheeled undercarriage with oleo struts equipped with 'strut cutter' crash energy absorber to meet the design impact velocity of 12.8m/sec.
Power was provided by two widely separated 1500shp General Electric YT700-GE-700 turboshafts driving wide-chord, two-bladed semi-rigid main and tail rotors. Main rotor blade chord was 1.08m and an FX-69-H-083 aerofoil was used. The wide-chord had been selected mainly because it met performance requirements, permitted the spar separation required for 23mm survivability and was less complex by a factor of two. The 'flat-pack' transmission had large slow turning herringbone gears for increased survivability, reduced noise and a 30-minute fly-dry capability. The main rotor mast quickly retracted into the transmission for air transport.
The weapon systems consisted of up to sixteen Rockwell AGM-114A Hellfire air-to-ground missiles or seventy-six 70mm FFAR rockets could be carried on the four wing stores. The turret-mounted triple-barrel 30mm XM-188 rotary cannon was mounted ahead of the stabilised sight to minimize damaging muzzle blast effects. The XM188 was a Research and Development project for a three-barrel 30mm Gatling gun for use on the Bell YAH-63, which participated in the Advance Attack Helicopter competition with the Hughes YAH-64. It had a firing rate of 600-750 spm (other sources reporting a fire rate 600 to 1800rpm).
The cruising speed at 1200m was to be 270km/h, the vertical rate of climb with eight Hellfire missiles and 320 rounds of 30mm ammunition was to be 135m/min and endurance was to be 1hr 50min.
The first prototype YAH-63 (serial 73-22246) made its maiden flight on 1 October 1975, but crashed during a test flight the following June. The aircraft was repaired and, along with the second prototype (73-22247), entered the official Army 'fly-off' against the YAH-64. On 10 December 1976 the Hughes machine was selected as the winning AAH design, and both YAH-63 prototypes were subsequently returned to Bell for disposal.
In 1981, a full-scale crash test of the YAH-63 prototype helicopter was conducted at the Impact Dynamics Research Facility (IDRF) located at NASA Langley Research Center in Hampton, Virginia. The crash test was performed to evaluate the energy-absorbing and load-limiting features of the airframe, landing gear, and seats. Under its ongoing crash research testing program, the Army conducted drop test T-41 using a YAH-63 prototype helicopter as a test article. The YAH-63 was residual hardware from the AAH competition of the mid 70's and incorporated many crashworthy features, including a high energy landing gear, crushable fuselage structure, stroking crew seats, high strength retention of large masses, and a crash-resistant fuel system. The test was conducted in July 1981 at the NASA Langley Impact Dynamics Research Facility. Many onboard experiments were also evaluated in the drop test with participation from NASA, the Navy, and the Army. The KRASH results were compared to test and showed generally good agreement for landing gear energy absorption, fuselage crushing, nose structure failure, and copilot/gunner seat stroking and bottoming. The acceleration levels in the fuselage agreed well in the mid fuselage impact, but predicted levels were lower than test levels in the forward fuselage, probably due to the hard armament structure on the test article not represented in the KRASH model.
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