Flight Control Systems Introduction Dr Slide 1
Flight Control System A Flight Control System (FCS) consists of the flight control surfaces, the respective cockpit controls, connecting linkage, and necessary operating mechanisms to control aircraft in flight. Slide 2
Control Surfaces Control Surface Primary control surfaces Elevator Aileron Rudder Secondary control surfaces Flaps Slats spoilers Alternative primary control surfaces Ruddervator Elevon Flaperon Taileron Canard S P S S P P Slide 3
Primary Control Surfaces Elevators are used to control the aircraft in pitch. Ailerons are used to control the aircraft in roll. The two ailerons are typically interconnected so that one goes down when the other goes up Rudder is used to control the aircraft in yaw. Slide 4
Secondary Control Surfaces-Flaps Flaps are high-lift devices hinged on the trailing edge of the wings. Flaps occupy 25-30% of the wing trailing edge inboard of the ailerons As flaps are extended, the stalling speed of the aircraft is reduced. Flaps reduce the stalling speed by increasing the camber of the wing and thereby increasing the maximum lift coefficient. Some flaps also increase the area of the wing. A supplementary function is to increase drag during landing Slide 5
Secondary Control Surfaces-Flaps (cont.) Plain flap - rotates on a simple hinge. Split flap - upper and lower surfaces are separate, the lower surface operates like a plain flap. Fowler flap - slides backwards before hinging downwards, thereby increasing both camber and chord, creating a larger wing surface. Slotted flap - a gap between the flap and the wing enables high pressure air from below the wing to re-energise the boundary layer over the flap to help the airflow stays attached to the flap, delaying the stall. Slide 6
Secondary Control Surfaces-Spoilers Spoilers are used to disrupt airflow over the wing and greatly reduce the amount of lift. This allows to lose altitude without gaining excessive airspeed wing load alleviation Some spoilers, termed spoilerons, may be used to roll an aircraft by reducing the lift of one wing but unlike ailerons not increasing the lift of the other wing. A raised spoileron also increases the drag on one wing which causes the aircraft to yaw. This can be compensated with the rudder. Slide 7
Secondary Control Surfaces-Slats Slats are aerodynamic surfaces on the leading edge of the wings of which, when deployed, allow the wing to operate at a higher angle of attack. Slats are very powerful devices to increase the maximum lift. By deploying slats an aircraft can fly slower or take off and land in a shorter distance. They are usually used while landing or performing manoeuvres which take the aircraft close to the stall, but are usually retracted in normal flight to minimise drag. Slide 8
Alternate Primary Control Surfaces: Ruddervator, Elevon, Flaperon, Taileron and Canard Some aircraft configurations have non-standard primary controls. Slide 9
Alternate Primary Control Surfaces Ruddervator a tail in the shape of a V, with moving parts at the back combining the functions of elevators and rudder Elevons combines the functions of the elevator (used for pitch control) and the aileron (used for roll control) The inputs of the two controls are mixed either mechanically or electronically to provide the appropriate position for each elevon. When moved in the same direction (up or down) they will cause a pitching force (nose up or nose down) to be applied to the airframe. When moved differentially, (one up, one down) they will cause a rolling force. These forces may be applied simultaneously by appropriate positioning of the elevons e.g. one wing's elevons completely down and the other wing's elevons partly down. Flaperon is a type of control surface that combines aspects of both flaps and ailerons. In addition to controlling the roll or bank of an aircraft like conventional ailerons, both flaperons can be lowered together to function much the same as a dedicated set of flaps would Taileron Instead of elevators at the back of the stabilisers, the entire tailplane changes angle Canard (foreplane) A stabilator mounted in front of the main wing Slide 10
Secondary effects of controls-effect of ailerons Ailerons deflection to roll may cause adverse yaw. When moving the stick to the left to bank the wings, adverse yaw moves the nose of the aircraft to the right. Whenever lift is increased, induced drag is also increased. When the stick is moved left to bank the aircraft to the left: the right aileron is lowered which increases lift on the right wing and therefore increases drag on the right wing. Adverse yaw is more pronounced for light aircraft with long wings, such as gliders. It is counteracted by the pilot with the rudder. Frise and Differential ailerons are ailerons which produces less adverse yaw. Slide 11
Secondary effects of controls-effect of ailerons (cont). Differential ailerons, have been rigged such that the down-going aileron deflects less than the upward-moving one, reducing the adverse yaw. Frise ailerons achieve the same effect by opposing the airflow beneath the wing and producing drag of an upward-deflected aileron. Ailerons may also use a combination of these methods. Slide 12
Secondary effects of controls-effect of rudder Using the rudder causes one wing to move forward faster than the other. Increased speed means increased lift, and hence rudder use causes a roll effect. Also, since rudders generally extend above the aircraft centre of gravity, a torque is imparted to the aircraft resulting in an adverse bank. Pushing the rudder to the right not only pulls the tail to the left and the nose to the right, but it also spins the aircraft as if a left turn were going to be made. Out of all the control inputs, rudder input creates the greatest amount of adverse effect. For this reason ailerons and rudder are generally used together on light aircraft. Slide 13
Flight Control Systems FCS types w.r.t. actuation power Fully manual Boosted Fully powered Manual Boosted Fully powered Slide 14
Fully Manual FCS Mechanical link connecting the control column and pedals to the control surface Push-pull rod system Cable-pulley system Aerodynamic forces on the surfaces feedback directly to the pilot. Fully manual control may also be referred to as a reversible system Applicable to small aircraft As the aircraft size or speed increases, the aerodynamic load on a deployed control surface increases demanding larger hinge moments There are some hinge moment reducing features such as surface overhangs, horns and tabs. Small number of control surfaces Slide 15
Fully Manual Control System: Cable-Pulley System Slide 16
Boosted FCS When the pilot s action is not directly sufficient for deploying a control surface, the main option is a powered system that assists the pilot. In case of hydraulic system failure, reversion of the control to a manual system occurs (Manual reversion) Slide 17
Fully Powered FCS Fully powered is (normally) a hydraulically powered system with no other means of actuating the surfaces It is a fully irreversible system with no aerodynamic load feedback to pilot The feel is artificially achieved through springs connected to the stick and pedals (the resisting force is produced by spring and is proportional to the stick deflection) Q Feel system (the resisting force is generated by a hydraulic actuator and is proportional to the aerodynamic force on the control surface) The control surface response must be proportional to the pilot s demand Servo-valves Slide 18
Fully Powered FCS: Artificial Feel Systems Slide 19
Fully Powered FCS: Hydraulic Servomechanism Slide 20
Fully Powered Commanding actuators in a fully powered FCS Mechanical linkage Fly by wire (FBW) Analogue Digital (FBD) FBW with mechanical backup C C Slide 21
Fly by wire FBW Data used by the system pitch, roll, yaw rate and linear accelerations; angle of attack and sideslip; airspeed/mach number and pressure altitude; stick and pedal demands; other cabin commands such as landing gear condition, thrust lever position, etc. Slide 22
Fly by wire FBW Some of the most important benefits of a FBW system: flight envelope protection (the computers will reject and tune pilot s demands that might exceed the airframe load factors); increase of stability and handling qualities across the full flight envelope, including the possibility of flying unstable vehicles; turbulence suppression and wing load alleviation and consequent decrease of fatigue loads and increase of passenger comfort; use of thrust vectoring to augment or replace lift aerodynamic control, then extending the aircraft flight envelope; drag reduction by an optimised trim setting; higher stability during release of tanks and weapons; easier interfacing to auto-pilot and other automatic flight control systems; weight reduction (mechanical linkages are substituted by wirings); maintenance reduction; Slide 23