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					Flight control systems:


        The systems, which operate the control surfaces, tabs and flaps, include flight
control system hardware linkages and mechanisms. These items connect the control
surfaces to the cockpit’s control.


        Flight control systems are basically divided into two types (a) Conventional controls /
Mechanical controls (b) Powered controls.


Mechanical FCS


        These are most basic design used in early aircrafts. This system includes cables,
push-pull rods, pulleys and sometimes chains to transmit the force of the cockpit controls to
the control surfaces.


Cables


   1.      Aircraft control cables are fabricated from carbon steel or stainless steel wire.
   2.      The diameter of the wires determines the total diameter of the cable.
   3.      7 or 19 wires wound as a strand in helical or spiral shape. Then 7 strands are
           wound as a cable.
   4.      Cable designations are based on no of strands and no of wires. If a cable
           consists of 7 strands and 7 wires it is designated 7x7. If it consists of 7 strands
           and 19 wires it is designated 7x19.
   5.      A conventional cable assembly consists of flexible cable, terminals (end fittings)
           for attaching to other units and turnbuckles. Turnbuckle is a device used to adjust
           tension.


Push-pull rods


        1. Control rods in FCS to give push-pull motion.
        2. This type of linkage eliminates the problem of varying tension and permits the
           transfer of either compression / tension through the tube.
       3. It consists of a hollow aluminum alloy or steel tube with an adjustable end fittings
          and a check nut at either ends.
       4. The rod end / clevis permits attachment of tube to flight control surfaces.
       5. The check nut when tightened prevents rod end from loosening.
       6. The control rod should be perfectly straight.
       7. Usually push-pull tubes are generally made in short lengths to prevent vibration
          and bending under compression load.


Powered flight controls


       Powered flight controls are employed in high-performance aircraft and are generally
of two main types (a) power-assisted and (b) power-operated. The forces required to
overcome the aerodynamic loads acting on the flight control surfaces govern the choice of
either system for a particular type of aircraft. In basic form, however, both systems are
similar in that a hydraulically operated servo-control unit, consisting of a control valve and
an actuating jack, is connected between the pilot’s controls and relevant control surfaces.
The major difference, apart from constructional features, is in the method of connecting
actuating jacks to control surfaces and this may be seen from figure.


              In a power-assisted system, the pilot’s control is connected to the control
surface, e.g. control column to elevators, via a control lever. When the pilot moves the
control column to initiate a climb say, the control lever pivots about point ‘X’, and
accordingly commences moving the elevators up. At the same time the control valve
pistons are displaced and this allows oil from the hydraulic system to flow to the left hand
side of the actuating jack piston, the rod of which is secured to the aircraft’s structure. The
reaction of the pressure exerted on the piston causes the whole servo unit, and control
lever, to move to the left, and because of the greater control effort produced the pilot is
assisted in making further upward movement of the elevators.


              In a power-operated system the pilot’s control is connected to the control
lever only, while the servo-unit is directly connected to the flight control surface. Thus, in
the example considered the effort required by the pilot to move the control column is simply
that needed to move the control lever and control valve piston. the effort required to move
the control surface is supplied slowly by servo-unit hydraulic power.
‘Fly-by-wire’ system


       Another system which may be considered under the heading of powered flight
controls, is the one referred to as a ‘fly-by-wire’ (FBW) control system. Although not new in
concept, complete re-development of the system was seen to be necessary in recent
years, as a means of controlling some highly sophisticated types of aircraft coming into
service. A FBW system is one in which wires carrying electrical signals from the pilot’s
controls, replace mechanical linkages entirely. In operation movements of control column
and rudder pedals, and the force exerted by the pilot, are measured by electrical
transducers, and the signals produced are then amplified and relayed to operate hydraulic
actuator units, which are directly connected to the flight control surfaces.


              Example of a FBW principle used in boeing 767 for wing spoiler panels is
given in the figure.


       For lateral control the deployment of panels is initiated by movement of pilot’s control
wheel to left or right as appropriate. This movement operates the position transducers, in
the form of a rotary variable differential transformer (RVDT) via mechanical gear drive from
the control wheels. The RVDTs produce command voltage signals proportional to control
wheel position and these signals are fed into a spoiler control module for processing and
channel selection.


       The spoiler control module output signals are then supplied to a solenoid valve
forming an integral part of a hydraulic power control actuator. The valve directs hydraulic
fluid under pressure to one or other side of the actuating piston, which then rises or lowers
the spoiler connected to the piston rod. The actuator is then mounted so that it pivots to
allow for the required angular movement of the spoiler panels. As the actuator piston rod
moves it also actuates a position transducer of the linear variable differential transformer
(LVDT) and this produces a voltage feedback signal proportional to spoiler panel position.

				
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