The Comanche was intended to replace the US Army's relatively unsophisticated OH-58D Kiowa Warrior and AH-1 Cobra with a stealthy multi-sensor platform able to carry out scouting and attack missions, shoot down enemy helicopters and pass data directly to the Longbow Apache attack helicopter. Slow funding of the programme encouraged more roles and capabilities to be added, increasing the weight and cost. An early plan envisaged procurement of as many as 5023 Comanches, later reduced to 1400, then 1213 and finally 650. As the numbers fell, the per-unit cost rose from $12.1 million to $58.9 million. In the end, 16 years and the expenditure of $8 billion only achieved little more than two flying prototypes and a partially completed test programme.

Jim Winchester "The World's Worst Aircraft", 2005

Boeing and Sikorsky began collaboration on what later became the RAH-66 in June 1985 and received a contract for the demonstration/validation programme in 1991. In early 2002, Boeing Sikorsky announced selection of Bridgeport, Connecticut, as final assembly location for the production RAH-66; also in 2002, the Joint Program Office moved from Huntsville, Alabama, to Bridgeport.

Programme cancelled 24 February 2004

Light Helicopter Experimental (LHX) design concepts requested by US Army 1981; numerous changes of programme; original plan for 5,000 to replace UH-1, AH-1, OH-58 and OH-6; reduced in 1987 to 2,096 scout/attack only, replacing 3,000 existing helicopters; further cut to 1,292 in 1990 (with another 389 possible) and then to 1,096 in 1999, although subsequently raised to 1,213 (including eight for US Army operational evaluation) by mid-2000. Under latter procurement plan, initial production examples to so-called Block I standard for armed reconnaissance; definitive Block II standard to follow with heavy attack capability; Block III to introduce added mission capabilities, including sensor fusion and AAMs. Most recent restructuring of programme has resulted in further cut in planned procurement to 650 over 12 year period, according to defence acquisition review that was completed on 7 October 2002; Army still intends to field Comanche in Blocks incorporating incremental improvements with low-rate initial production to begin in 2007. Three LRIP batches, totalling 73 aircraft, are to be built to Block I standard, followed by 108 Block II helicopters, whereupon Block III will be introduced.

LHX request for proposals issued 21 June 1988; 23 month demonstration/validation contracts to Boeing Sikorsky and Bell/McDonnell Douglas. Boeing Sikorsky selected 5 April 1991; to build four YRAH-66 demonstration/validation prototypes in 78 month programme, plus static test article (STA) and propulsion system testbed (PSTB).

LHTEC T800 engine specified October 1988. LHX designation changed to LH early 1990, then US Army designation RAH-66 Comanche in April 1991. Several versions of programme timetable were considered prior to that currently implemented.

Prototype critical design review, completed in December 1993, authorised production of three YRAH-66 prototypes (first item for which manufactured in September 1993). At same time, however, further R&D economies under study; December 1994 decision reduced dem/val phase to two prototypes (lacking Longbow/Hellfire capability).

Prototype construction began 29 November 1993 with forward fuselage at Sikorsky, Stratford; Boeing built aft fuselage in Philadelphia. STA airframe delivered to Stratford 1994, at which time PSTB under construction there. PSTB trials commenced in 1995 at West Palm Beach, with 100% torque from both engines achieved during first 10 hours of running. PSTB subsequently suffered failure of left input bevel gear, which disintegrated and punched hole in main gearbox housing during 110% power test; resonance blamed for failure.

Front and rear sections of prototype joined at Stratford 25 January 1995; completed helicopter (94-0327) rolled out 25 May 1995. Following transfer to Sikorsky's Development Flight Test Center in West Palm Beach, Florida, during June 1995, first flight accomplished on 4 January 1996. Prototype retired from flight test duty on 30 January 2002, by which time it had accumulated 387.1 flight hours in 318 sorties.

Aft fuselage section of second prototype (95-0001) delivered by Boeing to Stratford in early December 1996 for mating with forward fuselage; completed helicopter exhibited at Army Aviation Association's annual meeting in April 1998 and then to West Palm Beach. Made international debut when displayed statically at Famborough Air Show in September 1998; flew for first time on 30 March 1999. Completed initial test schedule in April 1999, recording 4.9 hours in five sorties before temporary lay-up, due to funding constraints; also used for vertical rate of climb demonstration later in year and will test integrated mission equipment package (ÌÅÐ), including digital avionics, communications, navigation and target acquisition systems. By mid-December 2000, had logged almost 53 flight hours in 50 sorties; these figures had risen to 93 and 103.5 respectively in May 2001 when it was removed from flight status to be prepared for flight testing and validation of ÌÅÐ. This phase of development began on 23 May 2002, when second prototype made first flight with ÌÅÐ and new engines installed. Near-term objectives to be achieved by the second prototype include flight with the night vision pilotage system by October 2002, as well as completion of total weapon system critical design review in May 2003, including the Lockheed Martin Electro-Optical Sensor System (EOSS), which due for delivery in first half of 2003.

Engineering and Manufacturing Development (EMD) officially began 1 June 2000, following RAH-66 meeting (on 4 April 2000) seven key Defense Acquisition Board Milestone 2 criteria, including a 107m/min vertical climb rate, a specified detection range for the FLIR sensors, a radar cross-section specification, ballistic vulnerability and tolerance specifications and tower-testing of the selected FCR. Weight reduction effort under way in late 2000, to reduce from current level of about 4,310kg to target weight of 4,218kg. Under original plan, EMD expected to take six years and include production of five RAH-66 specifically for EMD testing, followed by further eight for initial operational test and evaluation (IOT&E) by the US Army. However, EMD contract and plan restructured in mid-2002, at which time IOC forecast to occur in September 2009. The new plan includes a start of low-rate initial production (LRIP) in 2007, with full-rate production set at 60 per year from 2011-12 onwards. First EMD RAH-66 expected to fly in March 2005. In meantime, second YRAH-66 will assume increasing burden of test duty. Production of components for the first EMD RAH-66 began at Boeing's Philadelphia factory in early 2003, with work on assembling the first empennage beginning on 21 April 2003; on completion, this shipped to Bridgeport and mated with Sikorsky-produced elements.

First -801 growth version of T800 turboshaft began bench runs in March 1994; -801 preliminary design review completed May 1993; critical design review March 1995; prototypes originally fitted with less powerful T800-LHT-800 engines, but first flight with definitive -801 engine made on 1 June 2001 by first prototype. Same engine subsequently installed on second prototype in time for resumption of flight test duty in May 2002.

CUSTOMERS: US Army, two prototypes, plus, five systems development and demonstration (SDD) helicopters and four initial operational test and evaluation (IOT&E) aircraft for test duties, operational trials and training tasks. Planned procurement of 650 production aircraft. This total to include some dedicated for use by special operations forces, which may entail addition of in-flight refuelling capability.

COSTS: US$34,000 million programme, including US$1,960 million dem/val and US$900 million FSD but reduced to US$2,240 million dem/val/FSD between 1993 and 1997 by cancellation of three of six planned prototypes; US$8.9 million flyaway unit cost (1988 values), increased to US$13 million by early 1993. By 1993 (in 1994 dollars, estimated), procurement unit cost US$21 million, programme unit cost US$27 million. Appropriations for R&D in FY97-99 comprised US$338.6 million, US$282.0 million and US$367.8 million, with US$467 million in FY00 and US$614 million for FY01. EMD phase then projected to cost US$3.2 billion, with initial contract worth US$147.5 million awarded to Boeing Sikorsky on 1 June 2000. Under most recent programme plan (mid-2002), development costs expected to rise to about US$6.5 billion, some of which could be recouped from accelerated production; unit cost then forecast to exceed US$32 million.

DESIGN FEATURES: First combat helicopter designed from outset to have "stealth" features and target acquisition radar. Embodies low-observable (LO) attributes and stated to have radar cross-section (RCS) lower than that of Hellfire missile; frontal RCS reportedly 360 times smaller than AH-64, 250 times smaller than OH-58D and 32 times smaller than OH-58D with mast-mounted sight. Also has quarter of AH-64D’s IR emissions and is six times quieter, head-on. Maximum avionics commonality required with USAF Lockheed Martin F/A-22 Raptor programme.

RAH-66 is lighter, but only slightly smaller, than AH-64 Apache; specified empty weight of 3,402kg increased to 3,522kg by early 1992, as result of Army add-ons, including allowance for Longbow radar; mission equipment package has maximum commonality with F/A-22 Raptor technology. Design has faceted appearance for radar reflection; downward-angled engine exhausts; T tail with endplates; eight-blade fan-in-fin shrouded tail rotor; and five-blade all-composites bearingless main rntor system, with latter increased in diameter by 0.3m and gaining noise-reducing anhedral tips on forthcoming EMD aircraft. New rotor incorporating anhedral tips flown for first time on first prototype on 20 July 2001. RAH-66 also features internal weapon stowage.

Split torque transmission, obviating need for planetary gearing. Upper part of T tail folds down for air transportation. Detachable stub-wings for additional weapon carriage and/or auxiliary fuel tanks (EFAMS: external fuel and armament management system). Radar, infra-red, acoustic and visual signature requirements set to defeat threats postulated by US Army. Eight deploy able inside Lockheed C-5 Galaxy or four in Boeing C-17 Globemaster III with only removal of main rotor; ready for flight 20 minutes after transport lands. Combat turnround time 13 minutes.

FLYING CONTROLS: Dual triplex fly-by-wire, with sidestick cyclic pitch controllers and normal collective levers. Main rotor blades removable without disconnecting control system.

STRUCTURE: Largely composites airframe and rotor system. Fuselage built around composites internal box-beam; non-load-bearing skin panels, more than half of which can be hinged or removed for access to interior (for example, weapons bay doors can double as maintenance work platforms). Eight-blade Fantail rear rotor operable with 12.7mm calibre bullet hits; or for 30 minutes with one blade missing. Main rotor blades and tail section by Boeing, forward fuselage and final assembly by Sikorsky.

LANDING GEAR: Tailwheel type, retractable, with single wheel on each unit; main units retract aft, with tailwheel retracting forward. Main units can 'kneel' for air transportability.

POWERPLANT: Two LHTEC T800-LHT-801 turboshafts, each rated at 1,165kW. Transmission rating 1,639kW. Internal fuel capacity 1,142 litres. Two external tanks totalling 3,407 litres for self-deployment; total fuel capacity 4,548 litres. Additional fuel to be contained in two 424 litre tanks in side weapon bays, which in preliminary development in mid-1999. Main rotor tip speed 221m/s; 355rpm; normal operation from 95% (quiet mode) to 107% (load factor enhancement).

ACCOMODATION: Pilot (in front) and WSO in identical stepped cockpits, pressurised for chemical/biological warfare protection. Crew seats resist 11.6m/s vertical crash landing.

SYSTEMS: Williams International subsystem power unit (SPU), mounted aft of starboard engine, drives hydraulic pump for actuation of landing gear and weapons bay doors; further two hydraulic pumps provide pressure for flight control systems.

AVIONICS:

• Comms: Package known as Comanche Integrated Communications, Navigation and Identification (ICNIA) systems comprises dual anti-jam VHF-FM and UHF-AM Have Quick tactical communications, VHF-AM, anti-jam HF-SSB; IFF.
• Radar: Miniaturised version of Longbow fire-control radar to be installed in one-third of fleet although all to have carriage provision.
• Flight: Litton PAGAN jam-resistant four-channel GPS and radar altimeter. Litton AHRS, comprising two fibre optic LN-210C and one LN-100C gyro platforms.
• Instrumentation: Kaiser Electronics Helmet Integrated Display Sighting System (HIDSS); NVG-compatible lighting; integrated cockpit, night vision pilotage systems, and digital map display. Two 152x203mm multifunction flat-screen LCDs in each cockpit (left is monochrome for FLIR/TV, right is colour for moving map, tactical situation and night operations), plus two 89x185mm multipurpose display (MPD) flat-screen monochrome LCDs for fuel, armament and communications information in each cockpit. Microvision awarded contract by US Army to provide in 1998 a prototype helmet-mounted display incorporating virtual retinal-display technology as potential alternative to conventional helmet-mounted displays. Three redundant databusses: one low-speed (MIL-STD-1553B), one high-speed and one very high-speed (fibre optic-based) for signal data distribution.
• Mission: Airborne target handover system. Laser range-finder/designator. Nose-mounted Lockheed Martin electro-optical target acquisition system with two second-generation FLIR sensors and a solid-state TV camera. Northrop Grumman TASS (Target Acquisition System Software).
• Self-defence: Goodrich Electro-Optical Systems AN/ AVR-2A(V) Advanced Laser Warning Receiver, chemical warning and AN/ALQ-211(V)3 radar warning receivers; RF and IR jammers.

EQUIPMENT: Side armour for cockpit fitted as standard; optional armour kit available for floor of cockpit.

ARMAMENT: General Dynamics stowable XM-301 three-barrel 20mm cannon in Giat undernose turret, with up to 500 rounds (320 rounds normal for primary mission) and either 750 or 1,500 rounds per minute firing rates. Aiming coverage of gun is +15 to -45° in elevation and ±120° in azimuth. Integrated retractable aircraft munitions system (IRAMS) features side-opening weapons bay door in each side of fuselage, on each of which can be mounted up to three Hellfire or six Stinger missiles or other weapons. Four more Hellfires or eight Stingers can be deployed from multiple carriers under tip of each optional stub-wing, or auxiliary fuel tank for self-deployment. Will be compatible with Starstreak and Mistral air-to-air missiles. Maximum of 56 Hydra 70mm FFARs or Sura D or Snora 81mm equivalents. All weapons can be fired, and targets designated, from push-buttons on collective and sidestick controllers.

Jane's All the World's Aircraft, 2004-2005

- The tail surfaces went through many changes to avoid problems with buffeting, eventually being reduced in size and having endplate fins.

- The 'flowerpot' on top of the second prototypes main rotor hub contained a version of the Longbow radar.

- The Comanche's main weapons were housed in internal bays to reduce radar signature and popped out when needed.

- The tail rotor was of the fenestron type originally developed by Aerospatiale in France.