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The Comanche radar cross-section (RCS) is less than that of a Hellfire
Radar cross-section (RCS) is reduced by tilting all the airframe surfaces away from the vertical - including the tail, which has an asymmetric cant angle - and by incorporating a retractable landing gear and internal weapons. The Comanche carries missiles on weapon-carrying doors in the side of the fuselage, and the turreted gun rotates through 180' to stow in a fairing beneath the nose. Flush air inlets, a convoluted inlet particle separator and the exhaust system conceal the engines from radar. "One of the biggest challenges" in the program, according to Blake, has been the development of shrouds for the rotor head, which conceal the complex machinery of the head from radar without hampering access for maintenance. The first Comanche prototype flew with a number of rotorhead designs, comprising root cuffs and pyramidical fairings around the mast, but a definitive design was selected late last year and has not been flight-tested.
'Stealthy' blades
The main rotor and FanTail blades of the production aircraft will also be shaped and treated to reduce RCS. At one point in the long development program, there was a possibility that Comanches would use untreated blades in peacetime and be fitted with low-RCS blades for combat operations, but it is now the intention to fit all aircraft with 'stealthy' blades. The Hexcel company provides 'special-process honeycomb' materials for the Comanche program, possibly for use in the rotor blades.
The design of a stealthy helicopter is described as "at least a thousand times more time-consuming" than a fixed-wing aircraft, "because the rotating blades, both the main rotor and the tail rotor, must be modeled in thousands of positions,"according to one Sikorsky statement. Sikorsky has developed in-house codes to perform RCS predictions for the Comanche, and installed an SGI Origin 2000 computer server in February 2000 to support this work. The computer has 128 processors and two terabytes of storage. Sikorsky now claims that "our level of efficiency and productivity in stealth calculations is unequalled in the airframe industry."
A full-scale RCS model built by MicroCraft has been tested at Boeing's outdoor RCS range at Boardman, Oregon. (Located in the northern part of the state, 270km east of Portland, the Boardman range has been a well-kept secret for some years. It uses a rail-mounted mobile shelter over the main model pylon, similar to the well-regarded McDonnell Douglas range at Grey Butte, California.) Both partial and full-scale tests have been carried out.