Flight Control Systems: An Overview

Introduction to Flight Control Systems

• Early flight control involved wing warping and control surfaces actuated by wire-and-pulley systems.
• As aircraft speeds increased, especially in the transonic region, control loads became too high for purely mechanical actuation, leading to the transition to hydraulic systems.
• Hydraulically powered actuators were introduced to boost pilot effort.
• This introduced two concerns: artificial feel systems to provide pilots with realistic force feedback and mach trim systems to automatically adjust trim at high speeds.
• High-performance aircraft experienced yaw/roll oscillations, called the Dutch roll problem, requiring yaw dampers for stabilization and electronic control systems for auto-stabilization.
• Category III Autoland systems were fitted to Trident and Belfast aircraft to guard against equipment failures.
• A Control and Stability Augmentation System (CSAS) is an electronic means of controlling an aircraft without a mechanical link between the pilot and flight control actuators.
• Fly-By-Wire (FBW) removes mechanical linkages between cockpit controls and control surfaces and allows for weight savings and handling characteristic improvements.
• Fly-By-Wire (FBW) uses multiple electronic channels with failure logic for redundancy and was first used in the Airbus A320 and later adopted in the Boeing 777.

Principles of Flight Control

• Aircraft motion consists of translational motion (direction of flight and/or the direction of pointing) and rotational motion (movement around the three axes).
• Pitch is movement about the lateral axis (Y-axis), resulting in nose up/down movement.
• Roll is movement about the longitudinal axis (X-axis), resulting in wing up/down movement.
• Yaw is movement about the vertical axis (Z-axis), resulting in nose left/right movement.
• Flight control systems enable stable and controlled flight across the entire flight envelope.
• Different requirements of flight (role, range, agility...) mean different flight control surfaces.

Flight Control Surfaces

• Primary flight controls include elevators or canards (horizontal stabilizers) for pitch control, ailerons (outboard wing trailing edge) for roll control, and a rudder (vertical stabilizer) for yaw control.
• Secondary flight controls include high-lift devices like flaps, which increase lift and camber (trailing edge), and slats, which delay stall (leading edge).
• Spoilers reduce lift (on top of wings), and speed brakes increase drag.
• Fighter jets have large control surfaces for high agility (maneuverability at any cost).
• Commercial airliners have smaller surfaces, optimized for fuel efficiency and stability (passenger comfort and safety).
• A canard design features two lifting surfaces, with the canard acting as a forward horizontal stabilizer.
• Unlike conventional aft-tail designs, the canard contributes to lift rather than detracting from it, allowing for a smaller main wing, reducing weight and drag.
• The elevator is a hinged, movable section at the rear of the stabilizer that controls an aircraft's pitch by adjusting the force generated by the tail.
• Each side of the fuselage has an elevator, and they operate in unison, both moving up or down together.
• Ailerons are hinged flight control surfaces on the trailing edge of each wing that control an aircraft's roll around its longitudinal axis.
• Ailerons work in pairs to tilt the lift vector, changing the flight path, but their use can cause adverse yaw, where the aircraft yaws opposite to the intended roll direction.
• The rudder is a directional control surface, typically attached to the vertical stabilizer, that controls yaw around the vertical axis.
• The rudder allows a pilot to adjust an aircraft's horizontal direction, working alongside the elevator for pitch control and ailerons for roll control.
• Flaps are high-lift devices mounted on the trailing edge of an aircraft's wings to increase lift at lower airspeeds, improving takeoff and landing performance.
• A flaperon on an aircraft's wing is a type of control surface that combines the functions of both flaps and ailerons.
• A spoiler is a device on the top surface of an aircraft's wing that reduces lift by disrupting airflow.
• When deployed, spoilers create a controlled stall over the affected wing section, decreasing lift and aiding in descent, braking, or roll control.
• deployed Slats are aerodynamic surfaces on the leading edge of an aircraft's wings that allow for a higher angle of attack.
• This increases lift, enabling lower-speed flight and shorter takeoff and landing distances.
• On the BAE 146 a/c, trailing edge flap control is achieved with Fowler flaps, which extend across 78% of the inner wing trailing edge.
• The Boeing 747-400 has 28 leading-edge flaps (14 per wing), divided into Group A (six sections outside the outboard engines) and Group B (five between inboard and outboard engines, plus three inside the inboard engines).
• The inboard flaps on Boeing 747-400 are Krueger flaps, which remain flat when extended, while the rest have variable camber for aerodynamic efficiency.

Flight Control Linkage Systems

• Pilots provide manual inputs through the cockpit control column and rudder pedals.
• Mechanical linkages include push-pull rod systems for direct mechanical connection and cable and pulley systems (used in older aircraft) for flexibility.
• Trim systems include: pitch trim (adjusts tailplane angle for level flight) and roll trim (adjusts aileron bias).

Trim Systems

• Yaw trim adjusts rudder bias.

Feel Systems

• Feel systems include spring feel, which simulates aerodynamic resistance, and 'Q' feel, which adjusts force feedback based on airspeed.

Trailing & Leading Edge Flaps

• BAE 146: Fowler flaps cover 78% of the inner wing trailing edge.
• Boeing 747-400: 28 leading edge flaps (14 per wing) divided into Group A (6 sections outside outboard engines) and Group B (5 sections between inboard & outboard engines + 3 inside inboard engines).
• Boeing 747-400 inboard flaps are Krueger flaps (flat in extended position); others have variable camber.

Primary Flight Controls

• Pitch Control: Achieved with the control column (fore/aft for nose down/up)
• Roll Control: Achieved with the control column (side-to-side, wing drops accordingly).
• Yaw Control: Achieved with the rudder pedals (left pedal yaws aircraft left, right pedal yaws right).

Flight Control Linkage Systems

• Push-Pull Rod System: Used in Hawk 200 for roll, pitch, and yaw.
• Cable & Pulley System: Used to transmit manual control inputs to aircraft surfaces, including trim actuators, spring feel units, 'Q' feel unit, and power control units (PCUs).

Trim Systems

• Adjusts forces required for level flight
• Pitch trim: Alters tailplane incidence angle.
• Roll trim: Adjusts aileron position.
• Yaw trim: Adjusts rudder position.

Feel Systems

• Spring Feel: Resistance proportional to pilot input.
• 'Q' Feel: Adjusts control stiffness based on airspeed.

Flight Control Actuation

• Simple Mechanical/Hydraulic Actuation: A hydraulic Servo Valve (SV) directs fluid for ram movement.
• Autopilot Integration: Electrical input overrides pilot input when autopilot is engaged.
• Multiple Redundancy Actuation: Quadrupled lanes for redundancy, using servo valves and feedback loops.
• Mechanical Screwjack Actuator: Used for slow but high-force movements (e.g., tailplane control).
• Integrated Actuator Package (IAP): Uses an electric motor to drive a variable displacement hydraulic pump.
• Direct Drive Actuation: Torque motors directly drive hydraulic spools.

Fly-By-Wire (FBW) Systems

• Modes of Operation: Full FBW Mode (normal operation with control augmentation), Direct Electrical Link Mode (basic control retained after FBW failure), and Mechanical Reversion Mode (rudimentary control using trim and rudder).
• Example Aircraft: Airbus A320, A380, Boeing 787.
• FBW Control Laws: Normal Laws (full protections & augmentation), Alternate Laws (limited protections), Direct Laws (manual trimming required), and Mechanical Reversion (basic manual control via trim and rudder pedals).

Advanced Actuation Technologies

• Electro-Hydrostatic Actuator (EHA): Uses an onboard hydraulic system for localized actuation.
• Electro-Mechanical Actuator (EMA): Uses an electric motor and gearbox, common in trim and door actuation.
• Electro-Backup Hydraulic Actuator (EBHA): Can switch between normal hydraulic and EHA backup mode.

Flight Control, Guidance & Management

• Inner Loop (FBW System): Controls aircraft attitude.
• Middle Loop (Autopilot & Flight Director System - AFDS): Manages trajectory.
• Outer Loop (Flight Management System - FMS): Controls route and waypoints.