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Aviation History
1998
1998 - 2339.PDF
HELICOPTERS Functionally, the NH90 FBW system is divided into a primary FCS (PFCS) and auto matic FCS (AFCS). The PFCS is dedicated more to hands-on flight, and the AFCS to hands-off operation. The PFCS is the focal point of the FBW system because it is responsi ble for pilot interfaces, basic and degraded con trol laws, actuator control and actuation - all of the functions relating to the handling qualities of the aircraft. "The PFCS can be considered as the electronic equivalent of a conventional mechanical FCS enhanced with a large author ity stability and control augmentation system", saysVidal. For maximum redundancy and survivability, a quadruplex in-line monitored system was cho sen, as opposed to cross-channel monitoring (which relies on voting to decide correct func tioning). There are two dual digital and two dual analogue channels, integrated into two identical flight control computers (FCCs), the analogue channels providing back-up - but widi limited flying quality - to the digital. The use of an analogue system provides yet another redundancy feature, says Vidal. "In a pure digital system, redundancy is achieved by using different software in each channel. When you add analogue, you can do it by using differ ent hardware and by taking components from different sources". Analogue is also more sur- vivable against radiation damage, he adds. Each dual channel consists of two lanes, each being responsible for computing the position of the actuators which move the rotor blades to provide collective and cyclic pitch control. The problem of providing actuator redun dancy has been overcome in a novel way, since a conventional swashplate was chosen, which only allows for single actuators operating in each control sense. Quadruple redundancy for actuator control has therefore been provided within the actuator itself, with extreme atten tion paid to prevention of internal jamming. Four torque motors in each actuator operate two rotary hydraulic control valves mounted on the same shaft (see diagram). The four (ana logue) actuator control channels feeding them, each composed of two lanes (one "command", the other "monitor") must agree. Undetected failure of any one channel or motor is min imised by a "force fighting" system in which the superior force of at least two other motors over rides a faulty channel or motOr. A detected fail ure results in that channel being switched off. The two hydraulic control valves feed into separate cylinders of a tandem-bodied actuator. Jamming of any motor can be overriden by a built-in clutch, while jamming of a control valve is overriden by an intermediate sleeve which actuates within the control valve. Any jamming incident is detected by specific devices on the actuator ram and on the motor shafts. Sidestick controls were rejected, says Vidal, not only because they gave "no appreciable improvement in handling quality", but because of the need, particularly acute in a helicopter, NH90 FBW FCS Layout Flight sensors i Inceptors \t Force feel system Digital computers upper modes Digital computers Control laws Positions ' transducers »» Analogue computers " Back-up Control law Flight Sensors Automatic flight control system (AFCS) Actuator control computers (ACCs) Main rotor actuators Electrical link C30 Tail-rotor actuators Primary Flight Control System (PFCS) 1 FCS architecture AHRS -i IRS1 - IRS2 - Inceptors Force feel Trim Flight control computer (1) CMD —i DIG channel 1 MON •ANA channel 1 Position sensors CMD • MON CMD • DIG channel 2 MON . .... . ..CMD' ANA channel 2 MON Flight control computer (2) I I I I Actuator control computer (1) , CMD Channel 1 MON Channel 2?M-D- I MON L , CMD _ Channel 3- MON CMD _ Channel 4 MON Actuators Motor 1 Actuator control computer (2) Motor 2 Motor 3 Motor 4 if ^ Actuator layout Control valve position Hydraulic power supply 1 ACC control signals Hydraulic power supply 2 Control module Actuator position Cylinder 1 Cylinder 2 for synchronisation of the pilot and copilot con trols to ensure that stick position accurately reflects the rotor position. "When the programme was launched, indus try was not able to provide a synchronisable sidestick controller at a reasonable volume, weight and cost," he adds. Conventional sticks were therefore used, with artificial force feel provided by one trim actuator per axis. For redundancy, information on the stick position is provided to the FBW system by two different types of sensor. The control laws selected for the NH90 pro vide ADS3 3-specified attitude response in pitch or roll, blending progressively to a rate response as speed increases. Besides the basic flying qual ities, the control laws provide for nap-of-the- earth flying, and for system degradation. The helicopter pilot is informed constantly of which law he is flying. • FLIGHT INTERNATIONAL 2 - 8 September 1998 99
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