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RAH-66 Comanche

RAH-66 COMANCHE DESIGN

Comanche was Army aviation's first "clean sheet of paper design" in two decades. The RAH-66 Comanche helicopter's primary role would be to seek out enemy forces and designate targets for the AH-64 Apache Attack helicopter at night, in adverse weather, and in battlefield obscurants, using advanced infrared sensors. The helicopter weighed about 8,600 pounds empty and carried another 3,000 pounds with pilot, navigator and combat load. The Comanche was packed with state-of-the-art technology that would help it survive over the battlefield to locate the enemy and pass on information to friendly forces. At around $14 million each, the Comanche isn't cheap.

The Comanche was powered by two Light Helicopter Turbine Engine Co. (LHTEC) T800-801 engines. These advanced engines and a streamlined airframe would enable the Comanche to fly significantly faster than the larger AH-64 Apache. The Comanche's twin turboshaft engines each produced about 1,500 horsepower, enabling the helicopter to reach 190 mph at full throttle.

The aircraft was designed to emit a low-radar signature (stealth features). The Comanche would perform the attack mission itself for the Army's light divisions. The RAH-66 would be used as a scout and attack helicopter to include an air-to-ground and air-to-air combat capability. The Comanche was slated to replace the AH-1 Series Cobra light attack helicopter, the OH-6A Cayuse, and the OH-58A/OH-58C Kiowa light observation helicopters.

The Comanche mission equipment package consisted of a turret-mounted cannon, night-vision pilotage system, helmet-mounted display, electro-optical target acquisition and designation system, aided target recognition, and integrated communication / navigation / identification avionics system. Targeting includes a second generation forward-looking infrared (FLIR) sensor, a low-light-level television, a laser range finder and designator, and the Apache Longbow millimeter wave radar system. Digital sensors, computers and software would enable the aircraft to track and recognize advesarys long before they are aware of the Comanche's presence, a key advantage in both the reconnaissance and attack roles.

Aided target detection and classification software would automatically scan the battlefield, identifying and prioritizing targets. The target acquisition and communications system would allow burst transmissions of data to other aircraft and command and control systems. Digital communications links would enable the crew unparalleled situational awareness, making the Comanche an integral component of the digital battlefield.

The armament subsystems consisted of the XM301 20mm cannon, and up to 14 Hellfire anti-tank missiles, 28 Air-to-Air Stinger (ATAS) anti-aircraft missiles, or 56 2.75 inch Hydra 70 air-to-ground rockets carried internally and externally. Up to four Hellfire and two Air-to-Air Stinger (ATAS) missiles can be stowed in fully-retractable weapons bays and the gun can be rotated to a stowed position when not in use. This design feature reduces both drag and radar signature.

Mission management, status, and control information was provided over the MIL-STD-1553B databus between the mission equipment packages and the Turreted Gun System. The Comanche would have enhanced maintainability through it's modular electronics architecture and built-in diagnostics.

Comanche's Mission Equipment Package (MEP) segment included the mission computers, navigation subsystem, communications subsystem, targeting subsystem, aircraft survivability subsystem, night pilotage subsystem, controls and displays subsystem, and display generation subsystem. The Comanche MEP was based on a concept of electronic Line Replaceable Modules (LRMs) located in functional avionics racks. These LRMs permit a two level maintenance approach, i.e., remove and replace LRMs at the flight line and repair them at a depot. Comanche's avionics LRMs are based on the Standard Electronic Module - Format E (SEM-E) form factor with various electronic components mounted on a two-sided board. Comanche's current data processing capability was provided by dual Pentium CPUs on data processing modules provided by Northrop Grumman. Signal processing was accomplished by Northrop Grumman developed array processor modules.

Comanche's data link capability was provided by Link 16, SINCGARS, and HAVEQUICK; via Link 16/TADIL-J messages and via SINCGARS and HAVEQUICK with Joint Variable Message Format (JVMF) messages. These capabilities were being developed by TRW as part of Comanche's Integrated Communications, Navigation, Identification Avionics (ICNIA) subsystem. ICNIA consists of SEM-E modules with varying pitch (RF components being larger than digital components) that provide a software programmable waveform capability. Waveforms include HF, VHF-AM, VHF-FM, UHF, SINCGARS/SIP/ESIP, HAVEQUICK I/II, SATCOM 5 Khz and 25 Khz DAMA, and EPLRS. The IFF capability was similar to a Mark XII.

Considering that Comanche was an armed reconnaissance and light attack helicopter, pilot workload was significant. The Comanche had two identical crew stations situated in tandem. Each crew station had independent displays and mission computer assets. The crew interface was primarily based on the Comanche displays. The Comanche would have two identical 6" x 8" color displays side by side in each crew station. These displays were referred to as Multifunction Displays (MFDs). Each crew station also had two color Multipurpose Displays (MPDs) situated below and out board of each MFD. Although reconfiguration capabilities permited a given display page to appear essentially on any one display, normally the right MFD was the Tactical Situation Display with its digital map background. The left MFD served as the targeting sensor display. The right MPD serves as the status page for communications and armament systems. The left MPD served as the menu display that drives the display pages on all displays. Also, when the MEP fails a "fly home" display would be provided to the MPDs by the Flight Control System via a 1553B interface.

The helmet had FLIR images and overlaid symbology that can be used as a headup display in nape-of-the-earth (NOE) flight. The pilots also wear a Helmet Mounted Display (HMD) for pilotage, especially at night as the night pilotage FLIR imagery was displayed on the HMD visor and integrated with flight symbology. developing the RAH-66 Comanche reconnaissance helicopter. This aircraft would utilize a partially overlapped biocular HMD, known as the Helmet Integrated Display Sight System (HIDSS). It consists of a pilot retained unit (PRU) and an aircraft retained unit (ARU). The PRU was the basic helmet with visor assembly. The ARU was a front piece consisting of two image sources and optical relays attached to a mounting bracket. The HIDSS development and validation phade design, which uses two miniature, 1-inch, CRTs as image sources, provides a 30 (V) by 52 (H) field-of-view (FOV) with a 17 overlap region. However, miniature displays based on flat panel (FP) technologies [e.g., liquid crystal (LC) and electroluminescence (EL)] would very likely replace the CRTs in subsequent program phades.

MANPRINT

The U.S. Army's MANPRINT (MANpower and PeRsonnel INTegration) program was designed to ensure that the soldier and unit needs are considered throughout the entire system acquisition process and life cycle. Nowhere had the new soldier-oriented partnership between Government and industry been more visible than on the Army's Light Helicopter Experimental (LHX) program. Better known to us today as the Comanche, the LHX in 1986 was the Army's true experimental program, testing where it was possible to introduce cutting-edge technology into its inventory without running headlong into the problems of unsatisfactory performance and runaway personnel costs. Even opponents of Comanche cannot ignore the great advances achieved in this program beyond the standard of normal acquisition practices.

Perhaps the first indication that MANPRINT was not only viable but could revolutionize the military's procurement process was the successful development of the comanche's T-800 engine. The MANPRINT approach fostered hundreds of design improvements affecting both maintenance and reliability. In one striking example, the tool kit for the organization mechanic was reduced from 134 tools to only 6. The trunk-sized caster tool kit used on other helicopters was reduced to a canvass pouch half the size of a rolled-up newspaper. Furthermore, this reduction cost Government and industry nothing and would save taxpayer dollars.

For the Comanche itself, MANPRINT resulted in more than 500 design improvements in system performance and logistics. The cockpit was designed outward, from the pilot seat, using simulations and modeling, lessons learned from previous aircraft programs, and user inputs. In addition, when fielded, the Comanche would allow the aircrew to select what information was needed during missions. The result was an anticipated system with a much improved pilot-crew workload. A typical performance benefit was illustrated in the reduced number of steps it takes for the pilot to acquire a target. The OH-58D Kiowa Warrior required 34; the Comanche, 5.

Incorporation of MANPRINT considerations during Comanche development also introduced entirely new concepts to the acquisition process. The source selection competition included MANPRINT in all evaluation areas. It became impossible for a company to win the contract without a plan to integrate MANPRINT in the design, development, and manufacture of Comanche. In addition, seasoned maintenance personnel and other soldiers with field experience in operational units were assigned to the contractor's plant as representatives of the users in the operating commands. These soldiers were invaluable in fitting the machine to the operator. For example, they completed a rotor design change in 30 days that would otherwise have taken 12 months to achieve contractor-Government approval.

MANPRINT was also responsible for technological advances. To provide for easy maintenance to aircraft components, Comanche was built around a box like, load bearing keel. In most helicopters, the load was carried by the external skin. In Comanche, the load bearing keel made it possible to locate easy-access panels almost anywhere on the aircraft. Consequently, maintenance personnel can easily reach all of the internal components. In this case, a maintenance requirement drove the technological design, which in turn resulted in an aerodynamic improvement.

In another instance MANPRINT and transport considerations suggested the need for an improved rotor blade removal capability. The contractor design team already had a rotor blade design which met Government specifications and was concerned about the added expense. Nevertheless, because of soldier concerns, MANPRINT prevailed. A new blade was designed at a cost of approximately $60,000. Life cycle cost calculations have indicated that the new blade would remain easier to manufacture and should save approximately $150 million in personnel, maintenance, and transport costs from the original design.

From the outset soldier safety had been a major design objective. Safety experts studied more than two decades of helicopters accident reports to determine how the designers could make Comanche a safer aircraft. As a result of their efforts, the Comanche's safety-related design features are projected--when compared to other helicopters such as the OH-58 Kiowa and AH-1F Cobra--to save 91 soldiers lives and avoid at least 116 disabling injuries. A 1995 report by the Analytic Sciences Corp. - Minninger, et. al.--documents the performance improvements and savings on Comanche attributable to MANPRINT. The report found Comanche cost avoidance in manpower , personnel, training, and safety to be a whopping $3.29 billion. This return resulted from a design investment of approximately 4 percent of the Comanche R & D budget. Calculated as a return on design investment, MANPRINT in the Comanche program yielded over an 8,000-percent return. Moreover, if the costs of the remaining MANPRINT disciplines--health hazards and soldiers survivability--are included in the calculation, the return on investment for the entire program remains well over 4000 percent.

Design Issues

Considering the history of the program and the MS II Exit Criteria test data, it was highly unlikely that the Service can deliver the expected system performance within the current budget and schedule. Lacking an operational assessment of an integrated system, it was difficult to predict with any degree of confidence whether the individual subsystems can be successfully integrated, whether the subsystems would function properly in an operational environment, or whether, in concert, they would provide the anticipated benefits in operational performance.

The results of the MS II exit criteria testing were encouraging, but areas of concern remain. One of the MS II exit criteria for the CR was not demonstrated, and the CR's performance against stationary targets was a particular concern. Other important issues include the potential impact of: (1) aircraft weight growth on flight performance, (2) vibration and lack of directional stability and the effect of these on flight performance and target acquisition, (3) aircraft reliability, and (4) the ability to integrate the many MEP components effectively.

Comanche Radar Performance

During the program restructure in 1998, the Service moved the development of the CR forward in time five years and added MS II exit criterion for radar development. The CR MS II exit criterion was for Comanche to achieve the specified probability of detection (Pd) and probability of correct classification (Pcc) of at least 80 percent of the moving target performance range specified for "typical threat vehicles." An expedient test article was assembled, consisting of the one-of-a-kind developmental electronically steered antenna (ESA) and an early model Longbow radar. This system was a development testbed, not a production prototype of the objective system, and it was expected that few components of the test article (hardware or software) would be common to the objective system. To reduce cost and disruption to system development activities, the test was performed with the radar mounted on a tower located near Baltimore-Washington International Airport. The use of these expedient facilities allowed only helicopters as moving targets and, for reasons of availability, a UH-1 helicopter was used as a surrogate for all air and ground moving targets. In this test, the CR failed to meet the Pd exit criterion, achieving 49 percent (vice 80 percent) of the required range because of excess losses in the ESA and lower target RCS than predicted. During subsequent testing at Yuma Proving Grounds using tank targets, the CR did demonstrate performance that exceeded the MS II exit criterion for Pd. Because only one target type was available for each test, the Pcc estimates that were produced were not meaningful and could not be compared to the Pcc exit criterion.

Although the contractor was on a path to identify ESA problems and improve moving target detection performance, stationary target detection performance was a significant, and yet untested, program issue. A validating data collection campaign had been planned for 2002.

Weight Growth Impact on Flight Performance

There was a concern that weight increases would prevent the aircraft from meeting its VROC requirement. Provided the production aircraft does not exceed the predicted empty weight of 9,300 pounds by more than (approximately) 115 pounds, it should achieve the MS III VROC requirement (500 feet per minute), if weight growth exceeds that which was now forecasted then it would not. DOT&E's assessment of the Comanche's weight projections found several questionable areas, including overly optimistic expected weight reductions and questionable estimates of future weight growth. Overall, DOT&E concluded that, although the contractor may achieve 80 percent of their projected weight savings, the helicopter's weight would grow more than anticipated, and thus the final weight may be approximately 9,500 pounds. If this occurs, DOT&E estimates that the aircraft's VROC under the stated conditions would be approximately 430 feet per minute, rather than the required 500 feet per minute.

Vibration and Directional Stability. Flight-testing associated with envelope expansion had gone well. The prototype aircraft's demonstrated flight envelope significantly exceeds that of the OH-58D, which should result in improved operational capability. As an example, increases in forward, rearward, and lateral airspeeds would allow the Comanche to take off and hover at higher crosswind speeds than the OH-58D, with attendant improvements in maneuverability and controllability.

However, challenges remained. Flight-testing revealed a noticeable tail buffet as the aircraft's speed reached 80 to 100 knots. Although this does not immediately and directly affect flight safety, it was clearly undesirable from the user's perspective (vibration levels may interfere with weapon targeting, and buffet loads can contribute to tail structural fatigue). A reshaped pylon, first flight tested in 1999, reduced tail buffeting but compromised directional stability. Additional corrective changes have been identified and evaluated in wind tunnel testing, but have not been flight-tested. Furthermore, these corrective actions may have unintended consequences such as increased aircraft weight or increased RCS. Also, later aircraft would be equipped with larger rotors (an increase of one foot in diameter) and blades fitted with anhedral tips. At this point, it was difficult to predict what effects the CR, larger rotor, and blade tip changes would have on the tail buffeting/directional stability problems.

MEP Integration

Overall, the Comanche had a risky test and evaluation strategy for integrating the MEP components on the aircraft. Most testing involving the integration of the complete MEP on the aircraft would not occur until the end of the EMD phade. The resulting schedule compression allows little reserve in the timetable, thereby increasing the impact of unforeseen events/delays.

Reliability

Given the importance of reliability to the eventual assessment of the Comanche's operational suitability, this issue received considerable scrutiny at the MS II review. Although the program office/contractor had put into place a comprehensive system to identify and correct failure modes, corrective actions for identified failure modes generally would not be implemented except at several discrete points in time, because of the compressed developmental schedule. Because of this lag in applying fixes, it would be difficult to demonstrate improvements in aircraft's reliability by means of testing. Consequently, there would be little evidence that Comanche would meet its reliability requirements before the MS III decision.

Ballistic Vulnerability

The LFT&E MS II Exit Criteria Ballistic Vulnerability tests demonstrated structural damage tolerance potential for five critical components via ballistic impact, followed by structural fatigue testing and/or analysis. The MS II exit criterion called for damage tolerance potential, to be demonstrated via ballistic testing against the primary threat and engineering analysis for the main rotor flexbeam, fuel tank panel, composite panels, main rotor mast, and fantail drive shaft. The goal was to demonstrate component ability to maintain safe operation, following ballistic impact, for a minimum of 30 minutes. The plan was to conduct ballistic testing against the primary API threat under static loading, and to perform post-impact fatigue and residual strength tests. The initial ballistic damage characterization and subsequent damage growth was to be determined both visually and using ultrasonic, non-destructive inspection (NDI) techniques.

The main rotor flexbeam and the fantail drive shaft components were tested under simulated flight loads during ballistic impact, evaluated for post-impact fatigue and residual strength, and subjected to NDI of the damaged articles. The fuel tank panel test consisted of a ballistic impact into the fuel tank of the full-scale static test article filled (two-thirds level) with water to determine the hydrodynamic ram effects on the fuel tank structure and surrounding support structure. A visual inspection was made subsequent to the ballistic impact. The remaining two components (main rotor mast and composite panels) were supported in a realistic representation of the actual vehicle configuration, but were not loaded at the time of impact. Structural damage during the ballistic tests was monitored visually. These components did not include a post-test structural investigation. The composite panels included ultrasonic NDI. The post-impact investigation of these two components consisted of a review of the damaged specimen and a correlation of this damage against the contractor's predictive methodology to determine residual strength.

More recently, the contractors initiated an additional series of Risk Reduction ballistic and structural tests on evolving design configurations for several major components (i.e., main rotor blade and mast, tailcone and shroud, composite panels and Fantail) against larger caliber threats. The purpose for the redesign of these components was to reduce weight and cost.

The remainder of the LFT&E program included component qualification and subsystem level ballistic testing for 27 critical components, as well as full-up system level dynamic testing on a pre-production-configuration aircraft. However, because of the compressed nature of the EMD phade, it would be extremely difficult to correct any weaknesses discovered during the LFT&E, and there was a schedule risk to accomplish the stated goals of the ORD.



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