Aerodynamics 6: Hydraulic Powered Flight Controls: Fly-by-Wire A6

Hydraulic Powered Flight Controls: Fly-by-Wire

• Fly-by-wire means that the pilot or autopilot control inputs are fed via electrical signals to the flight computer.
• Mechanical levers or cables going from the flight deck to the flight control surfaces or actuators are not present.
• Computers send all output actions to electrically controlled hydraulic valves inside actuators or to electric motors.
• Feedback systems from the input device and control surfaces and/or actuators signal the computers when to stop the control surface movement.
• The Avro Canada CF-105 Arrow was the first non-experimental aircraft flown with a fly-by-wire control system on March 25, 1958.

Fly-by-Wire Benefits

• Reduction of weight.
• Better fuel efficiency.
• Improved controls for flight crew.
• Automated computerized aerodynamic protection systems to not exceed aircraft design limits.
• Fail-safe backup by design to back up computer and or flight control failures.
• The fly-by-wire system was designed and certified to render the new generation of aircraft even safer, cost-effective, and pleasant to fly.

Fly-by-Wire Basic Operation

• Flight control surfaces are electrically controlled and hydraulically activated.
• The horizontal stabilizer and rudder can also be mechanically controlled for additional redundancy or backup.
• The computer continuously communicates back and forth between input and output/actuator to obtain desired positioning.
• The computer processes signals from input and determines the required control surface movements.
• The computer transmits this using electrical signals (through wires, hence the term fly-by-wire) to the appropriate control surface actuators.
• Actuators contain control valves that are electrically operated to direct fluid flow to extend or retract the actuator.
• Actuators provide feedback to the computer relating to both the internal valve position and the actuator piston position, which is also the flight control position.

Fly-by-Wire Control Input

• Multiple variable signals are sent from the control handle, referred to as the side stick or control column, using transducers known as LVDTs or RVDTS.
• LVDT, or Linear Variable Differential Transducer, varies an electrical signal based on linear movement to communicate its position to a computer.
• RVDT, or Rotary Variable Differential Transducer, varies an electrical signal based upon rotating movement to tell a computer its position.
• The control system also includes an artificial feel mechanism that provides simulated feedback to the pilot, reflecting the amount of movement and the control loads being exerted.

FBW PCU Components

• EHSV is a four-way sliding spool valve that contains a torque motor and an LVDT for spool valve position feedback to the SECU.
• When energized, the solenoid bypass valve arms the PCU hydraulically by directing pressure to the pressure-operated bypass valve.
• The bypass valve is a pilot-pressure-operated valve, spring-loaded to the bypass position.
• With the solenoid valve energized, if the hydraulic pressure falls below 170-180 psi, the bypass valve spring brings the system into bypass mode.
• The anti-cavitation check valve is used to prevent cavitation due to differing actuator piston surface areas potentially leading to sudden assisted piston movement.
• The cylinder/manifold sub-assembly contains an unbalanced area piston, a cylinder, the manifold body, a one-piece gland, the clevis end, and the piston operating LVDT.
• The LVDT is used for surface position feedback to the SECUs.
• The torque motor which receives a signal from the SECU positions the EHSV, directing hydraulic pressure through the bypass valve to the appropriate side of the RAM.

Electro-Hydraulic Servo Operation

• A hydraulically actuated and electrically controlled device, responding to computer inputs, serves as a damper controlling fluid cycling.
• An EHSV (electro-hydraulic shut-off or servo valve) controls fluid flow to extend, retract, dampen, or neutralize control.
• LVDTs (Linear Variable Differential Transducers) communicate the EHSV and actuator positions to the computer.
• Hydraulic actuators include a piston with an actuation rod inside a cylinder, converting fluid movement into mechanical movement.

Boeing 787 Dreamliner Flight Control Surfaces

• The main instrument panels in the flight deck contain five 12" x 9" flat-panel LCDs.
• The LCD panels are identified as: P1 Left forward panel, P2 Center forward panel, P3 Right forward panel, P5 Overhead panel, P7 Glareshield panels, P55 Glareshield center panel, P13 Left sidewall panel, P14 Right sidewall panel, P9 Forward aisle stand, P10 Control stand, and P8 Aft aisle stand.
• There are also two head-up display (HUD) combiners.
• The two outboard LCDs typically display primary flight information, while the two inboard and one lower LCD are multi-function displays.
• Multi-function displays can show engine indicating and crew alerting system (EICAS) data, navigation, secondary engine, status, synoptic, checklist, communication, and maintenance displays.

Boeing 787 Dreamliner Flight Control Systems

• The primary flight control function (PFCF) is a modern, three-axis, fly-by-wire system is designed for efficient structural design.
• Some benefits of this design are increased fuel economy, smaller vertical fin, smaller horizontal stabilzer.
• The airplane meets strict safety requirements with decreased weight and supplies improved control and protection.
• The PFCF supplies manual (Pilot Input) and automatic (Autopilot Input) airplane control and envelope protection in all three axes.
• The PFCF calculates commands to move the control surfaces with sensor inputs from components such as: Control wheels, Control column, Rudder pedals, Speed brake lever, Pitch trim switches, and Rudder trim selector.
• Roll control surfaces: Ailerons, flaperons, and fourteen spoilers exist.
• Pitch control surfaces: Two elevators and a moveable horizontal stabilizer exist.
• Yaw control is produced by a single piece rudder.
• The high lift control system (HLF) supplies increased lift at lower speeds for takeoff and landing.
• High lift surfaces include one inboard and one outboard trailing edge flap on each wing.
• There are six leading edge slats and one Krueger flap on each wing.

Boeing 787 Dreamliner General Configuration

• Position transducers change the flight crew commands of the control wheels, control columns, rudder pedals, rudder trim switch, pitch trim switches, and speed brake lever to electrical signals.
• These signals go to the primary flight control function (PFCF) in the flight control electronics (FCE) cabinets.
• The PFCFs communicate with the airplane systems through the common data network (CDN).
• In addition to command signals from the position transducers, the PFCFs also receive sensor inputs: Air data, Inertial reference data, and Navigation receiver data.
• The PFCFs calculate the flight control commands based on control laws, stability augmentation, ride quality, envelope protection, and load alleviation.
• Digital command signals from the PFCFs go to: Remote Electronic Units (REU), Spoiler REUS, Electric motor control units (EMCU), Empennage door actuation system (EDAS) controller (787-9) and Power drive units (PDU) for flaps and slats.
• The REUs are integrated with hydraulic power control units (PCU) to operate the rudder, elevators, ailerons, and flaperons.
• The spoiler REUs control five pairs of hydraulically operated spoilers. There are two pairs of spoilers controlled by EMCUs.

Boeing 787 Dreamliner Flight Control Computers

• There are four flight control electronics (FCE) cabinets, with three in the forward electronic equipment (EE) bay and one in the aft EE bay.
• Each cabinet has three equipment slots used for line replaceable modules (LRM): Power conditioning module (PCM), Actuator control electronics (ACE) and a Flight control module (FCM).
• The C2 cabinet does not have a FCM.
• The PCM supplies power to the other components in the cabinet, receiving primary power from variable frequency starter generator (VFSG), permanent magnet generator (PMG), Left and right DC buses, Captain's instrument bus and the First officer's instrument bus.
• There are two FCE batteries that provide backup power for the left and right FCE cabinets.
• The hot battery bus provides backup power for FCE cabinets C1 and C2.

Boeing 787 Dreamliner Roll Control System

• The ailerons, flaperons, and spoilers control the roll attitude of the airplane.
• They provide these functions: Maneuver load alleviation, enhanced ride quality, and improved high lift performance, and also function as speedbrakes.
• The control wheel movement is transmitted through a linkage to a shaft at the bottom of the control column.
• A mechanical feel and centering mechanism supplies some of the feel forces to the control wheels.
• Each control wheel moves a position transducer, whose signals go to the actuator control electronics (ACE) and then to the primary flight control functions (PFCF) in the flight control electronics (FCE) cabinets.
• The PFCF calculates the control surface command and sends it to the remote electronic units (REU) via the ACEs and the FCE intermodule buses.
• The REUs control the power control units (PCU) to hydraulically move the control surfaces.
• Two actuators operate each aileron and flaperon.
• The aileron and flaperon PCUs operate in these modes: Active, Damped, Damped/blocked.
• The PFCF automatically calculates aileron trim without the need for pilot input.
• When the flaps extend, the ailerons, flaperons and spoilers move down (droop) to increase lift.
• When drooped, the ailerons and flaperons continue to supply roll control.
• During high speed flight, the PFCFs lock out the aileron operation, and at low speed, the PFCFs unlock the ailerons and command their operation.
• Ailerons and flaperons are used symmetrically to provide maneuver load alleviation for the wing.
• The auto-drag function deflects the ailerons downwards and the outboard spoiler upwards during some approaches.
• On the ground, the ailerons and flaperons are deflected up to assist the speed brake function.

Fly-by-Light Technology

• Fly-by-optics is sometimes used instead of fly-by-wire because it offers a higher data transfer rate, immunity to electromagnetic interference, and lighter weight.
• In most cases, the cables are just changed from electrical to optical fiber cables.
• Sometimes it is referred to as "fly-by-light" due to its use of fiber optics.
• The data generated by the software and interpreted by the controller remain the same.
• Fly-by-light has the effect of decreasing electro-magnetic disturbances to sensors in comparison to more common fly-by-wire control systems.
• The Kawasaki P-1 is the first production aircraft in the world to be equipped with such a flight control system.

Power-by-Wire

• After eliminating the mechanical transmission circuits in fly-by-wire flight control systems, the next step is to eliminate the bulky and heavy hydraulic circuits.
• The hydraulic circuit is replaced by an electrical power circuit.
• The power circuits power electrical or self-contained electrohydraulic actuators that are controlled by the digital flight control computers.
• All benefits of digital fly-by-wire are retained, and the absence of hydraulics greatly reduces maintenance costs.
• The biggest benefits are weight savings, the possibility of redundant power circuits, and tighter integration between the aircraft flight control systems and its avionics systems.
• Current applications include Airbus A380 & A350 and Boeing 787 back-up flight control systems.

Future Flight Control Systems

• Fly-by-Wireless will use wireless technology within the aircraft to communicate between the control input and the actuating device at the flight control surface, replacing electrical wire bundles or fiber optics to reduce weight and maintenance costs
• Intelligent Flight Control Systems will have preprogrammed automated responses using Engine & Avionics systems to back-up to abnormal occurrences or emergencies such as a flight control system or engine failure.
• This already exists in its preliminary form with the redundancy functions switching between computers in the event of a failure.

Autopilots

• An Autopilot is a system used to guide an aircraft with minimum assistance from a pilot. The function of the autopilot is stabilization of flight path.
• Aircraft required the continuous attention of a pilot, and this constant attention led to serious pilot fatigue. Therefore, an autopilot is designed to perform some of the pilot’s tasks.

Autopilot Basic System Types

• Single-axis Autopilot manages both the ailerons (ROLL MODE).
• Two-axis Autopilot manages elevators and ailerons (ROLL and PITCH MODE).
• Three-axis Autopilot manages ailerons, elevators and rudder (ROLL, PITCH and YAW MODE).
• Four-axis Autopilot Helicopters manages pitch, roll, and yaw but also has auto-pilot hover capabilities by controlling collective pitch and power.
• Autonomous Autopilot Modern Fixed Wing manages pitch, roll, yaw, and auto-throttle for fully automated flight and landing capabilities.

The Modern Autopilot

• Information is gathered from sensors located at various points on the aircraft, in addition to other airplane systems and equipment.
• Course vectors set by the pilot will command the aircraft to follow the commanded action without physically auctioning of the controls.
• The auto-pilot hydraulic or electrical servo unit transforms the auto-pilot computer electrical commands into hydraulic and mechanical commands, operating the flight control linkage to maintain aircraft attitude and heading.

Autopilot Basic Modes

• HDG MODE: Selected heading acquisition and hold.
• NAV MODE: Navigation course capture and tracking.
• ILS MODE: Landing Guidance, Localizer and Glideslope.
• Heading Hold Control, VOR radial capture and tracking, and ILS capture and tracking are available.
• Altitude Hold Mode maintains altitude by manipulating pitch servos or cables.

Basic Autopilot Operation

• An autopilot uses the "negative feedback loop".
• Control systems apply an action based on measurement and almost always have an impact on the value they're measuring.
• Reactive Control Systems utilize similar principles; e.g. achieving a desired room temperature with HVAC.
• Even with smooth air, a wing will eventually dip, and gyroscopes (or other position sensors) on the wing will detect this and send signals to the autopilot computer.
• The autopilot computer processes position data and determines when the wings are no longer level, sending a specific command for a small electric motor to adjust the control surfaces accordingly.

Additional Auto-Flight Devices

• A yaw damper pulls aircraft movement information from a series of accelerometers or rate sensors in the rudder and translates it into the proper amount of calming rudder inputs.
• In single-engine airplanes, the yaw damper smooths out the left-right movements of the stabilizer, creating a more comfortable ride.
• The yaw damper inhibits the Dutch roll tendency, a wallowing combination of yawing and rolling motions, on a swept-wing aircraft, especially one with a T-tail.
• When a Dutch roll occurs on an aircraft without a damper, any yawing motion will lead to corkscrew-like oscillations that continue until they either die out naturally or escalate.

Mach Trim

• As an aircraft approaches Transonic Flight and into supersonic flight, the Center of Pressure moves rearward along the cord line of the wing.
• This will cause the Center of Gravity to be forward of the C of P, hence pitching the nose downward.
• Mach Tuck is a device that varies the pitch trim automatically as a function of Mach number, opposing Mach tuck and maintaining level flight.
• This system is generally installed only in aircraft with hydro-mechanical flight control; fly-by-wire systems generally compensate via their "control laws".