POWERED FLIGHT CONTROL SYSTEMS

INTRODUCTION
As the airspeed of later model aircraft increased, as compared to the earlier model ones, actuation of controls in flight became more difficult. It soon became apparent that the pilot needed assistance to overcome the airflow resistance to control movement. Spring tabs which were operated by the conventional control system were moved so that the airflow over them actually moved the primary control surface. This was sufficient for the aircraft operating in the lowest of the high speed ranges (250-300 mph).
For high speeds a power assist (hydraulic) control system was designed. Subsequently, with the increase of more speed and size of the aircraft fully powered flying controls came into practice.
Because of the high loads imposed, modern transport aircraft are invariably provided with power-operated or power-assisted controls. Because of the importance of the flying control system, hydraulic power to each control surface is provided by at least two independent hydraulic systems (sometimes using separate actuators) plus an emergency system operated by electrical power or by ram air turbines. In addition, some systems allow for reversion to manual operation of the control surfaces, or tabs, in the event of all hydraulic systems failing.

SYSTEM COMPONENTS & ARRANGEMENTS
Components/units: A powered flight control system has following major components/units:
a) Cockpit controls
b) Servo control unit
c) Mechanical system
d) Hydraulic supply system.
Cockpit controls: Cockpit controls for flight control system are wheels/steers, Columns, handles etc which are the means to give input signal into the system for actuating flight surfaces.
Servo Control Unit: It is a hydraulic unit consisting of a selector valve and the actuator. The selector valve is here known as the servo control valve or simply servo valve which is a spool type slide valve. It is directly or indirectly operated by the cockpit controls through the mechanical control system.
Mechanical system: Mechanical system, consisting of different mechanical linkages (e.g. push pull rods, teleflexes, cable system, pulleys, torque tubes, bell cranks etc) connect cockpit control to the servo valve.

Hydraulic supply: Hydraulic supply is ensured from the aircraft hydraulic system in modern aircraft. However, a complete power pack may have its own (self contained) hydraulic power system.

Operating Principle: Operating principle of hydraulic operation of flight surface through the use of servo control unit is illustrated by simplified schematic in Figure 5.1. The servo valve receives input signal from the cockpit controls through mechanical control system. Thus servo valve gets a selection being shifted from its neutral position. The pressurized hydraulic fluid is directed to act on one side of the piston of the actuating cylinder other side of the piston being connected to the return line of the hydraulic system. The flight surface is thus actuated by the force obtained by the mechanical force produced by hydraulic pressure in the operating a cylinder/piston assembly

In practice, the system is designed with a “feedback” or “follow up” mechanism to restore the initial neutral position of the servo valve, closing the supply and return ports by the spool/pistons of the servo valve, making a “hydraulic lock” to the actuator in the newly selected position of the flight surface.

POWER ASSISTED FLIGHT CONTROL SYSTEMS
General: Power assisted flight control, also called Power boosted flight control is a system where power is used as assistance to the pilot.

Description & Operation: In the system, the cockpit control is directly connected to flight control surface with the hydraulic control unit operating in parallel to the mechanical connection in order to lower the steering forces. The pilot feels the airloads. A typical system is shown in Figure 5.2
When the cockpit controls are operated, the servo valve is displaced and the surface is deflected hydraulically. This movement now is mechanically fed back to the input mechanism where it tries to null the servo valve. As long as the controls in the cockpit are operated, the servo valve remains open and the surface is travelling until the controls are stopped. Then the servo valve is nulled. (moved in the opposite direction) by the small further travel of the surface via the mechanical feed-back action on the input mechanism. In this system, during the movement the servo valve is just kept open until the input movement stop. This type of system to close (null) the servo valve are called “feed back” system.

POWERED FLIGHT CONTROL SYSTEMS
General: Powered flight control is normally meant for the “fully powered” flight control system which is used in the modern aircraft in most cases. In this system, flight surfaces are solely actuated by hydraulic force and no air load is felt by the pilot. For a relative “feel” of the airloads, some “artificial feel” is designed in the mechanical control system that connects cockpit control to the servo control system.
Description & Operation: The system has a servo control unit with a servo valve and actuator with hydraulic connections. The servo valve is operated by the cockpit control. Once cockpit control is deflected, piston within the valve is moved to give way for the hydraulic pressure to deflect another piston that will deflect the surface.

A typical servo-control unit is illustrated in Figure 5.3. With hydraulic power available, operation of a pilot’s control moves the spool in the selector, thus directing fluid to one side of the actuator and opening a return path from the other side. Movement of the actuator operates the control surface, and at the same time moves the selector back towards the neutral position. When control surface movement corresponds to the deflection of the pilots’ control, the selector is in the neutral position, and fluid is locked in the actuator.

In this case, the mechanism that brings the servo control valve to its null position is ‘follow up” mechanism where the hydraulic cylinder follows the movement of the selector valve to have the null position.
When no hydraulic pressure is available, the interconnecting valve opens under spring pressure and the actuator is free to move. The control may then be operated by alternative servo-control units, or by manual linkage, depending on the particular installation.
A hydraulic sub-system for the operation of the flying controls, is often fed through a priority valve, which ensures that fluid under pressure is always available; the sub-system may also have a separate accumulator.

PFCU: An alternative method of operating the flying controls is by means of self-contained powered flying control units (PFCU’s). Control surfaces are divided into sections, and each section is operated by a separate PFCU, thus providing duplication to guard against failure of a unit. Each unit is controlled by mechanical linkage from the pilots’ controls, and some units also accept electrical inputs from the auto-pilot and autostabilizer. The mechanical input rod to each unit is telescopic and spring loaded, so that failure of one PFCU will not prevent operation of the associated control system. In the event of failure, or when a unit is inoperative, the actuating ram is mechanically locked in the neutral position, thus preventing movement of the associated section of the control surface. Actual operation is basically similar to that of the servo-control unit described previously, but each PFCU is a self-contained hydraulic system, and is not connected with the main hydraulic system, or with other PFCU’s. The main body of each PFCU acts as a reservoir, and houses all the components necessary for operation of the unit, including electrically driven pumps and hydraulic actuator.

Duplication/Triplication of Power Controls: Failure of a power control component (especially the jack or servo-valve) in a system will result in a complete failure of a flight operation if it is not at least duplicated. Modern aircraft makes mostly triplication of the powered control system to operate each primary flight control surface.

Artificial Feel system: This is another feature installed in powered flight controls.
A. Feel: ‘Feel’ is the resistance felt at hand to move control surfaces due to aerodynamic forces on them. Pilot experiences this ‘feel’ at the control wheel, control column and rudder bar in case of a non-powered flight control.
B. Artificial Feel: In case of a powered flight control system, Pilot’s hand force does not go to the deflecting surface, so he does not feel any operational load. In the absence of this feel, Pilot tends to over-deflect the control which may cause over loading and damage to the structure. So, in powered flight control, artificial feel is created by installing spring or other mechanism to the servo valves. By installing such spring/mechanism, resistance is felt at hand when actuating controls in the cockpit.
HIGH LIFT DEVICES
General: High-lift devices, as indicated in week 2, are used in order to reduce the takeoff or landing speed by changing the lift characteristics of an airfoil during the landing or takeoff phases. When these devices are no longer needed they are re-turned to a position within the wing to regain the normal characteristics of the airfoil.
In the larger aircraft, where powered flight control system is used, high lift devices are normally the slats and the flap system.

Flaps and slats: Wing flaps and slats are used to give the aircraft extra lift. They reduce the landing speed, thereby shortening the length of the landing rollout to facilitate landing in small or obstructed areas by permitting the gliding angle to be increased without greatly increasing the approach speed. I addition, the use of flaps during takeoff reduces the length of the takeoff run.
Most flaps are hinged to the lower trailing edge of the wings, inboard of the ailerons. Some common types of flaps are shown in Figure 5.4 and Figure and Figure 5.5.

Some aircraft has leading edge flap attached to the undersurface of the wing leading edge. When they are in the “up” (or retracted) position, they fair in with the wings and serve as part of the wing trailing edge. When in the “down” (or extended) position, the flaps pivot on the hinge points and drop to about a 45° or 50° angle with the wing chord line. This increases the wing camber and changes the airflow, providing greater lift. (see Figure 5.6)
Leading edge flaps adjacent to the wing-fuselage joint as well as wing-pylon junction are generally called Kruger-flaps or simply the Krugers.

Slats are the leading edge high lift devices that extend forward and drops down creating a slot as well as increase the wing camber.
The flap and slats may be controlled from the cockpit by a single slat/flap handle, and when not in use, flap fits smoothly into the lower surface of each wing and slats retract to the initial position at the leading edge of the wing. The use of slats/flaps together increases the camber of a wing greatly and therefore the lift of the wing, making it possible for the speed of the aircraft to be decreased greatly without stalling. This also permits a steeper gliding angle to be obtained as in the landing ap-proach. Flaps are primarily used during takeoff and landing. power to operate the Flap/Slat system. The system normally has the following components:
a) Cockpit controls: Flap/Slat handle on the pedestal
b) Hydraulic motors, gearbox, drive shafts
c) Control valve/ PCU (Power control unit)
d) Flap/Slat computers (in fly-by-wire aircraft)
e) Mechanical linkages to connect cockpit slat/flap control handle to control valve
f) Electrical wiring to carry manual selection signal generated by a transducer generated by the slat/flap handle movement. The signal is received by the slat/flap computer. The computer signals the control valve/PCU to select hydraulic pressure to rotate hydraulic motor in required direction. Gearbox and shaft mechanism translates the flap/slat to extend or to retract.
Typical slat/flap system: Figure 5.7 illustrates Slats/Flaps on a typical aircraft.
Each wing has three retractable slats plus one Kruger and two flap sections. A single control lever located on the center pedestal of the cockpit permits slats and flaps control. The lever has five gated positions.
Control system has electrical/electronic, hydraulic and mechanical systems to actuate the whole system in this typical aircraft.
Electrical and electronic part (fly-by-wire system) has transducers at the lower end of the slat/flap handle, electrical wiring and the SFCC (Slats Flaps Control Computers). The SFFC which are digital computers has slats control channel and flaps control channel.
Hydraulic section has hydraulic power control units (PCUs) consisting of hydraulic control valve ( a solenoid valve) and a hydraulic motor. The system receives hydraulic pressurized fluid from the aircraft hydraulic system. Solenoid valves receive electrical signal from the channels of SFCC.
Mechanical part consists of gearbox, shafts, screw jacks, slat/flap handle. Also there is a transducer which is an electro-mechanical component generating electrical signal from the mechanical rotation of the slat/flap handle.

Figure 5.8 represents simplified schematic of the slat control system. Similar is the control for flap control operation.