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Common modules take control

ADVANCING THE STATE of the art in system design and integration has enabled the F-22 team to meet demanding weight and cost budgets "flowed down" to the subsystem level by the integrated product-team structure.

Many of the traditional boundaries between systems have be blurred in a bid to gain the benefits of integration. The vehicle-management system (VMS), for example, combines the flight and propulsion controls, while the integrated vehicle subsystem control (IVSC) operates the aircraft utilities via databus. The VMS, IVSC and stores-management system (SMS), which controls the weapons, take the physical form of modules plugged into the F-22's avionics racks. In all, some 18 common processor-modules are used across the VMS, IVSC and SMS, says Robert Reardon, air-vehicle product director at Lockheed Martin Tactical Aircraft Systems. Texas Instruments provides common 1750A-processor modules for these systems.



Reardon describes the VMS, developed by Lear Astronics, as "new, advanced and different." The F-22 is the first fighter with triplex digital flight-control computers and no electrical or mechanical back-up mode. There are two 276bar (4,000lb/in2) hydraulic systems, but only one actuator on each control surface, to save weight and cost. This is made possible by the use of numerous control-reconfiguration modes which provide safe flying qualities in the event of an actuator or hydraulic failure. Additionally, a compensator on each actuator provides stiffness in the event of hydraulic failure, Reardon says.

The VMS controls 14 surfaces: the horizontal tail, rudders, ailerons, flaperons, leading-edge flaps and inlet bleed and bypass doors. There is no angle-of-attack limitation, but load factor and roll rate are limited as a function of flight condition, fuel state and stores loading to prevent structural overloading. Not overstressing the airframe is important for maintenance, says chief test pilot Paul Metz. Whereas the F-16 has only symmetrical structural limiting, and can be overstressed in roll, he says, the F-22 will have full g-protection throughout the flight envelope. "The pilot physically cannot hurt the aircraft," he says.

Air data is provided via databus to the flight-control computers by a low-observable sensor system, supplied by Rosemount, which includes two angle-of-attack probes and four sideslip plates on the nose. Above about 30í angle-of-attack, and if the air-data system fails, the two Litton LN-100 laser-gyro inertial-reference systems in the nose are used to estimate angle-of-attack and sideslip.

Other VMS features include: a high pitch-rate mode which the pilot can engage by pulling or pushing through a "soft stop" on the sidestick; a speedbrake function which integrates drag-producing deflections of the rudders, flaperons and ailerons with normal operation of the surface; and nosewheel-steering software with low-gain, high-gain and bypass (free-castering) modes.

Software control of aircraft subsystems by the Lear Astronics-supplied IVSC modules eliminates the dedicated cockpit controls and displays previously required. Systems controlled by the IVSC include: electric, hydraulic, fuel, environmental control, life support, auxiliary power, landing gear including brakes, integrated caution/advisory/warning, diagnostics and structural-integrity and health monitors.

The engines drive two 65kW electrical generators - the F-22 is primarily a DC aircraft - and four 270litre/min hydraulic pumps, and are backed up by the 335kWAlliedSignal G250 auxiliary power-unit (APU), which drives a 27kW generator and 100litre/min pump. They draw fuel from eight tanks in the forward- and mid-fuselage, wing and tail booms, with growth space in saddle and fin tanks. The dorsal aerial-refuelling receptacle is concealed behind doors when not required, and the aircraft can be refuelled on the ground without ground power.



A "totally integrated" environmental-control system, supplied by AlliedSignal, provides aircraft thermal-conditioning throughout the flight envelope. There are three basic elements: an open-loop air-cycle system to air-cool the flight-critical avionics and supply the life-support system; a closed-loop vapour-cycle system to liquid-cool the mission-critical avionics, including antennas; and a thermal-management system to cool the fuel, which is used as a heat sink for the cooling loops, and to provide fire protection.

Bleed air from the engines or APU is cooled with outside air in the primary heat-exchanger. As avionics cooling is required as soon as the F-22 is powered up, injectors are used to suck air into the ram-air ducts when the aircraft is on the ground. Cooled bleed air is refrigerated and distributed to the avionics, including the VMSand IVSC, and cockpit. Liquid cooling of the hotter-running mission avionics to around 15íC, using polyalphaolephin coolant, increases reliability. Fuel is cooled by outside air in an air-cooled fuel cooler, with the fuel system regulating the temperature at which fuel is delivered to the engines to maximise efficiency.

The Menasco-supplied landing gear is electrically controlled and hydraulically actuated, with digital brake-by-wire and steer-by-wire. The "deceleration-command" braking system provides full-time anti-skid and protection against locked or spinning wheels.

F-22 systems are designed to the same damage-tolerant requirement as the airframe, with no single failure allowed to incapacitate the aircraft. Damage-tolerance schemes including back-up channels, independent power sources and redundant controllers ensure that it will take several dual failures to knock out an F-22, its designers say.