Chapter 9 Understand the Principles of Flight Control PDF

ME3531 Aircraft Systems Hydraulic and Pneumatic Power Systems Hydraulic Power Flight Control System Primary Control System Operation Methods The types and complexity of control mechanisms used depend on the aircraft size, the speed it flies and the mission of the aircraft. A...

ME3531 Aircraft Systems Hydraulic and Pneumatic Power Systems Hydraulic Power Flight Control System Primary Control System Operation Methods The types and complexity of control mechanisms used depend on the aircraft size, the speed it flies and the mission of the aircraft. A small or low speed aircraft may have cockpit controls connected directly to the control surface by cables or push-rods. Some aircraft have both cable and a pushrod system. The force exerted by the pilot is transferred through them to the control surfaces On large or high performance aircraft, the control surfaces have high pressure exerted on them by the airflow hence it is difficult for the pilot to move the control manually. The hydraulic actuators are used within the linkages to assist the pilot in moving the control surface to reduce the pilot from fatigue and improve system performance. Page  2 Hydraulic Power Flight Control System Basic Control System Components Cables These are generally fabricated from carbon fibre or corrosion resistant steel wire used fro aircraft control. All steel cables used in aircraft mechanical transmission are constructed with 7 strands which are helically twisted and each strand consists of a number of wires. The size of the cables is based on the circumscribed circle, the number of strand and number of wires per strand. Page  3 Hydraulic Power Flight Control System Basic Control System Components Pulleys They are used in aircraft control to change the direction of a cable. Page  4 Hydraulic Power Flight Control System Basic Control System Components Turnbuckles They are commonly used for adjusting the tension of the control cables. Page  5 Hydraulic Power Flight Control System Basic Control System Components Bell Crank It is used to transmit and permit change in the direction of the force. Bell crank Page  6 Hydraulic Power Flight Control System Basic Control System Components Push pull rod It used between bell crank to torque arm to transmit the force and motion from one member to the other. Push pull rods are also called control rods. Page  7 Hydraulic Power Flight Control System Basic Control System Components Quadrant It is used at the base of a control column or control stick to impart force and motion to a cable system. Quadrant Page  8 Hydraulic Power Flight Control System Basic Control System Components Torque tube It is a hollow shaft by which the linear motion of a cable or push-pull tube is changed to rotary motion. A torque arm, or horn, is attached to the tube by welding or bolting and imparts a twisting motion to the tube as the arm is moved back and forth. Page  9 Hydraulic Power Flight Control System Basic Control System Components Cable guards or guard pins They are installed in the flanges of pulley brackets to prevent the cable from jumping out of the pulley. Page  10 Hydraulic Power Flight Control System Basic Control System Components Fairlead These are guide control cables where they pass through a structure member. They also dampen vibration of cable, maintain cable alignment and to seal openings in bulkheads. Page  11 Hydraulic Power Flight Control System Basic Control System Components Cable Tension regulator It will ensures that tension on the cable is kept constant at different temperatures. The tension of a cable can be checked using a tensionmeter. Page  12  Hydraulic Power Flight Control System Types of mechanical control system There are 3 types of mechanical control system used in aircraft. Push-pull rod system Cable and pulley systems Chain and sprocket system Page  13 Hydraulic Power Flight Control System Direct cable control system In a direct cable control system, the cockpit controls are connected to the control surfaces with high-strength steel cable. The operation of the control column places tension on the cable. As the cable passes through the fuselage, it is supported by the pulleys. The pulleys enable the direction of the control cable to be changed. The tension of the control cable system is considered critical and this control is generally used in low speed aircraft. The force the pilot feels on the control column while steering the aircraft in flight is in direction to the airspeed. Note: The higher the airspeed, the greater the force input by the pilot at the control column Page  14 Hydraulic Power Flight Control System Push pull rod control system The push pull rod system generally used bell cranks, levers and torque tubes. On the system, the cockpit control is connected to the device to be operated with a hollow aluminium tube whose ends are fitted with threaded inserts and a clevis or a rod- bearing. Page  15 Hydraulic Power Flight Control System Cable and pulley Vs Push-pull rod Systems 1. Cables are lighter than push-pull rod. 2. Cables can only transmit pull forces, while push-pull rod can transmit both push and pull forces. 3. Cable system has less connection, thus less mechanical play and more accurate transmission compare to push-pull rod system. 4. Cables show warning signs before breaking (e.g. stretching & breaking of some wires), whereas the push-pull rod breaks suddenly without warning. So, it is a safer and more reliable transmission system than the push pull rod system. 5. Cable tension will decrease as the aluminium aircraft fuselage contracts more than steel during cold weather, thus causing the tension to fall whereas push-pull rod has similar material as the aircraft structure. Page  16 Hydraulic Power Flight Control System Hydraulic power flight control system As aircraft increase in size and weight, their control becomes more difficult to operate and systems must be used to aid the pilot. The power boost control system is similar in principle to power steering in an automobile. A hydraulic actuator is in parallel with mechanical operation of the controls, and in addition to moving the control surface, the normal control movement by the pilot also moves a control valve that directs hydraulic fluid to actuator that moves the surface. Hydraulic flight control system has two parts: The mechanical circuit – It links the cockpit controls with the hydraulic circuits. Like the mechanical flight control system, it consists of rods, cables, pulleys, and sometimes chains. The hydraulic circuit – It has hydraulic pumps, reservoirs, filters, pipes, valves and actuators. The actuators are powered by the hydraulic pressure generated by the pumps in the hydraulic circuit. The actuators convert hydraulic pressure into control surface movements. The servo valves control the movement of the actuators. Page  17 Hydraulic Power Flight Control System Hydraulic power flight control system The pilot’s input is transmitted to servo control unit through the mechanical linkages. Servo control unit through the mechanical linkages. Servo control unit moves the control surface in direction and distance proportional to input signal. That is, the servo valve is used to amplify the mechanical input by directing the high pressure hydraulic fluid to move the control surface. Page  18 Hydraulic Power Flight Control System Power-assisted type The direct connection between the steering column and the control surface is in a disconnected mode, and the input coming from the flight deck is only directed to the actuator control valve. In the event of hydraulic system power failure, the hydraulic actuator is bypassed and the input force coming from the steering column is directly to the control surface. Page  19 Hydraulic Power Flight Control System Power-assisted type The pilot’s control is connected to the control surface from the control column to elevator via a control lever. When the pilot moves the control column towards him to increase the pitch, the bottom part of the control lever pivots at ‘X’ will move to the left while the top part of the control lever will move to the right resulting in the elevator control surface to move upwards. Page  20 Hydraulic Power Flight Control System Power-assisted type Page  21 Hydraulic Power Flight Control System Power-assisted type Page  22  Hydraulic Power Flight Control System Power-operated type The pilot’s control is connected to the control lever only, while the servo unit is directly connected to the flight control surface. Thus the effort required by the pilot to move the control column is simply that needed to move the control lever and control valve piston. Page  23 Hydraulic Power Flight Control System Power-operated type Page  24  Hydraulic Power Flight Control System Artificial Feel Unit Power-assisted type Power-operated type In the power-operated type hydraulic transmission system, the pilot does not have direct feel of the effect exerted upon the elevator by the air load. Page  25 Hydraulic Power Flight Control System Artificial Feel Unit In a hydraulic transmission system, the pilot cannot feel any of the air load effect directly. An artificial feel unit is used to simulate air load. Feel unit simulates air load small than actual air load making easier to control aircraft. Two types of feel commonly used in aircraft control systems are spring feel and “Q” feel. The spring feel will dominate at low speed and for high deflection control demands. The “Q” feel will dominate at high speed for low control deflections. Page  26 Hydraulic Power Flight Control System Electrical and Fly-By-Wire (FBW) systems. Modern aircraft use fly-by-wire systems to connect the flight control surfaces to the cockpit controls with electric wires, rather than with steel cables, push-pull tubes, torque tubes or other mechanical methods. The cockpit controls are devices that convert the movement or pressure exerted by the pilot into electric signals which are sent into a computer programmed with all of the flight characteristics of the aircraft. The computer output is directed through more wires to electrohydraulic valves that convert the electric signal into hydraulic fluid flow. This flow changes the position of a main control valve, which directs hydraulic fluid to the appropriate control actuators. Page  27 Hydraulic Power Flight Control System Advantages of the Electrical and Fly-By-Wire (FBW) systems. Save weight The elimination of the conventional connecting rods, cables and pulleys. Reduced maintenance time Maintenance down-time can be reduced though use of replaceable units in FBW. The elimination of rigging or mechanical adjustment which takes a considerable maintenance time. Page  28 Hydraulic Power Flight Control System The main concern with fly-by-wire systems is reliability. While traditional mechanical or hydraulic control systems usually fail gradually, the loss of all flight control computers could result in a total loss of aircraft control without due warning. it may incorporate a backup system that is either mechanically or hydraulically powered or a combination thereof. Page  29