Where Module 11 starts
Module 8 gave you the physics of flight. Module 11 turns that physics into hardware: the surfaces, linkages and devices that let a crew control a transport aeroplane, and the structures and systems that keep it flying. This first chapter is the bridge — a short recap of the aerodynamic language, then a tour of every control surface the syllabus lists, what it does to the aeroplane and why designers chose it.
A wing makes lift because it turns the air that flows past it downwards. The useful working form is the lift equation:
L = ½ ρ V² S CL — lift depends on air density ρ, the square of the true airspeed V, the wing area S and the lift coefficient CL. CL is set by the shape of the section (camber) and the angle of attack. Every device in this chapter works by changing one of these terms: flaps change camber and area, slats change the stalling angle, spoilers destroy CL.
- Chord line — straight line from the leading edge to the trailing edge. Camber is the curvature of the mean line between the upper and lower surfaces.
- Angle of attack (α) — angle between the chord line and the relative airflow. Do not confuse it with the angle of incidence, the fixed rigging angle between the chord line and the aircraft's longitudinal axis.
- Lift acts at 90° to the relative airflow, drag parallel to it; both act through the centre of pressure, which moves forward as α increases (on a conventional cambered section) until the stall.
- Stall — beyond the critical angle of attack (typically about 15–16° for a clean wing) the boundary layer separates from the upper surface, lift falls sharply and drag rises. A wing always stalls at the same angle of attack, whatever the speed or weight.
Three axes, three primary controls
An aeroplane rotates about three axes that all pass through its centre of gravity (CG). Each has a primary control surface. The primary controls are the ailerons, elevator (or stabilator) and rudder; everything else — flaps, slats, spoilers, tabs — is a secondary (auxiliary) control, although on jets the spoilers also share the roll job.
| Motion | Axis | Primary surface | Cockpit control and sense | Stability about that axis |
|---|---|---|---|---|
| Roll | Longitudinal (nose to tail) | Ailerons (+ roll spoilers) | Wheel/stick right → right aileron up, left aileron down → roll right | Lateral |
| Pitch | Lateral (wing tip to wing tip) | Elevator, stabilator or trimmable stabiliser | Column back → elevator trailing edge up → tail down → nose up | Longitudinal |
| Yaw | Normal / vertical | Rudder | Right pedal forward → rudder trailing edge right → nose yaws right | Directional |
Movement about the longitudinal axis is roll, but stability about that axis is called lateral stability. Movement about the lateral axis is pitch, and the stability is longitudinal. Rudder and yaw go with directional stability about the normal axis.
Roll control: ailerons and spoilers
Ailerons
Ailerons are hinged to the outboard trailing edge of each wing and move in opposite directions. The down-going aileron increases camber and lift on its wing; the up-going aileron reduces lift on the other wing, so the aeroplane rolls towards the raised aileron. On light aircraft cables or push-pull rods move them; on transport aircraft hydraulic power control units (PCUs) do, signalled mechanically or, on fly-by-wire types, electrically.
Adverse aileron yaw and its cures
The wing with the down-going aileron makes more lift, so it also makes more induced drag. That extra drag pulls the rising wing back and yaws the nose away from the intended turn — adverse yaw. Designers fight it in several ways:
- Differential ailerons — the up-going aileron moves through a larger angle than the down-going one, adding profile drag on the side of the descending wing.
- Frise ailerons — the hinge is set back so that the nose of the up-going aileron projects below the wing's lower surface into the airflow, adding drag on the descending-wing side (and helping to balance the control forces).
- Aileron–rudder interconnect or simply coordinated rudder by the pilot (and on jets the yaw damper).
- Roll spoilers — they rise only on the wing whose aileron goes up. They kill lift on the descending wing and their drag yaws the nose into the turn, which cancels adverse yaw instead of adding to it.
Inboard and outboard ailerons, and aileron reversal
Many large jets (for example the B747 and B767) carry two ailerons per wing. The outboard (low-speed) ailerons are powerful but act on the flexible outer wing; at high speed their air load can twist the wing so that the twist cancels — or even reverses — the intended roll. This is aileron reversal. To prevent it, and to keep wing loads down, the outboard ailerons are locked out (held faired) once the flaps are retracted or above a set speed, and roll is then provided by the inboard (all-speed) ailerons, which sit on the stiff wing root, together with the roll spoilers. Single-aileron designs such as the A320 rely on aileron plus roll spoilers at all speeds.
Spoilers in roll
Spoilers are hinged panels on the upper wing surface, ahead of the flaps, usually of honeycomb construction. Used asymmetrically with the ailerons they provide roll; used symmetrically they are speed brakes in flight and lift dumpers on the ground (see below). A spoiler mixer adds the roll command to the speed-brake command so that a panel can do both jobs at once.
Pitch control: elevators, stabilisers, stabilators and canards
- Elevator — hinged to the trailing edge of the fixed or trimmable horizontal stabiliser. Pulling the column raises the elevator trailing edge, increases the tail's down-load and pitches the nose up.
- Variable-incidence (trimmable) horizontal stabiliser — THS — the whole stabiliser pivots at its rear spar and its leading edge is driven up or down by a screwjack (ball screw) turned by electric or hydraulic trim motors, or by a cable-driven manual trim wheel. Changing the incidence of the whole surface gives a very large trim range across the full CG and speed envelope while the elevator stays close to faired, which means less trim drag and a smaller elevator. The elevator is then left for manoeuvring. Almost every jet transport trims this way.
- Stabilator (all-moving tailplane) — the entire horizontal tail moves for pitch control, doing the job of stabiliser and elevator together. It is very powerful, so it is normally fitted with an anti-servo (anti-balance) tab to give the pilot feel (Chapter 2).
- Canard — a foreplane ahead of the main wing. Where a conventional tail usually pushes down to balance the aeroplane, a canard lifts up, so both surfaces carry weight. It is normally designed to stall before the main wing, so the nose drops and the main wing never reaches its own stall.
On a jet with a trimmable stabiliser, the stabiliser sets the "hands-off" pitch attitude for the current speed and CG, and the elevator only makes the changes from that datum. A runaway stabiliser therefore has huge authority — which is why stabiliser trim systems have brakes, cut-out switches and stop limits (Chapter 12).
Yaw control: the rudder and rudder limiters
The rudder is hinged to the fin (vertical stabiliser). It is used to coordinate turns, to correct adverse yaw, to hold the aeroplane straight in a crosswind take-off or landing and — most demandingly — to counter the asymmetric thrust after an engine failure at low speed. On many aircraft the pedals also steer the nose wheel for taxiing. Some large aircraft (for example the B747) have a split rudder: upper and lower sections, each powered by different hydraulic systems, so that one runaway or one hydraulic failure cannot take all yaw control away.
Rudder limiters
The load that a given rudder angle puts on the fin rises with the square of the airspeed. Full rudder is needed at low speed (engine failure, crosswind), but the same deflection at cruise speed could overload the fin and rear fuselage. A rudder limiter therefore reduces the rudder travel available as airspeed increases — for example from about 30° each way at take-off and landing speeds down to only a few degrees at cruise. Systems include a speed-scheduled variable stop (rudder travel limiter), a ratio changer that reduces the rudder angle per unit of pedal travel, and a pressure reducer that limits the force the actuator can develop. Limiting uncommanded rudder movement is the job of the yaw damper and actuator design, covered in Chapter 12.
Mixed surfaces: elevons, ruddervators and flaperons
| Surface | Combines | Found on | How it works |
|---|---|---|---|
| Elevons | Elevator + aileron | Tailless and delta-wing aircraft (Concorde, flying wings) | Both up or both down together = pitch; opposite directions = roll; a mixer combines the two inputs |
| Ruddervators | Rudder + elevator | V-tail aircraft | Both trailing edges up together = nose up; moving in opposite directions = yaw |
| Flaperons | Flap + aileron | Many light and some transport aircraft (e.g. the inboard flaperons of the B777 and B787) | Droop together as flaps; move differentially as ailerons, with the roll command added by a mixer |
High-lift devices
A wing optimised for cruise is too small and too thin-cambered to lift the aeroplane at take-off and landing speeds. Rearranging the lift equation, the minimum (stalling) speed falls as CL max rises, so high-lift devices raise CL max to give lower take-off and landing speeds and shorter runs. They come in two families: trailing-edge flaps, which mainly add camber (and sometimes area), and leading-edge devices, which mainly delay the stall.
| Device | How it works | Typical CL max gain* |
|---|---|---|
| Plain flap | Hinged rear section of the wing deflects down — more camber | ≈ 50% |
| Split flap | Plate on the lower surface hinges down; the upper surface stays put — lift plus high drag | ≈ 60% |
| Slotted flap | Opens a gap so high-pressure air from below re-energises the boundary layer over the flap; double- and triple-slotted versions on jets | ≈ 65–70% |
| Fowler flap | Moves aft on tracks (more area) and then down (more camber) — the most effective; small settings add lift with little drag | ≈ 90% |
| Slat | Leading-edge section moves forward and down, opening a slot; on airliners powered, on some light/older aircraft deployed automatically by aerodynamic load | raises the stalling angle |
| Fixed slot | Permanent duct near the leading edge (often outboard, ahead of the ailerons) — good slow-speed handling at the cost of cruise drag | raises the stalling angle |
| Krueger flap | Panel hinged at the lower leading edge swings forward and down, often with a folding bull-nose; common on inboard leading edges (B737, B747) | adds camber at the nose |
| Droop nose | The whole leading edge droops, increasing camber at the nose | adds camber at the nose |
*Approximate figures quoted in training texts for a single device; they vary with design.
- Trailing-edge flaps increase CL at every angle of attack, so the aeroplane can fly slower or more nose-down, but the wing stalls at a lower angle of attack. They also add drag — modest at take-off settings, large at landing settings, which helps steepen the approach.
- Leading-edge slats and slots carry the lift line on to a higher stalling angle of attack — perhaps 20–25° instead of 15–16° — by feeding high-energy air over the upper surface.
- Lowering trailing-edge flaps moves the centre of pressure aft (a nose-down tendency) while increasing the downwash over the tailplane (a nose-up tendency); which effect wins depends on the design.
- On most jets the slats extend before the trailing-edge flaps and retract after them, so the wing never has the extra camber of the flaps without the stall protection of the slats. Monitoring for flap or slat asymmetry is covered in Chapter 12.
Drag devices: spoilers, lift dumpers and speed brakes
- Flight spoilers — operate in flight: asymmetrically for roll, symmetrically as speed brakes when the crew pulls the speed-brake lever. They let the aeroplane descend steeply or slow down without gaining speed.
- Ground spoilers / lift dumpers — operate only on the ground. Together with the flight spoilers they deploy fully at touchdown, or during a rejected take-off, to destroy lift, put the aeroplane's weight onto the wheels so the wheel brakes work, and add aerodynamic drag. They are usually armed before landing and deploy automatically when the system senses touchdown (main-gear compression or wheel spin-up) with the thrust levers at idle, or when reverse thrust is selected.
- Speed brakes (air brakes) — panels whose only job is drag, such as fuselage side panels or the clamshell tail cone of the BAe 146 / Avro RJ. Unlike roll spoilers they do not work with the ailerons.
Spoilers reduce CL max, so with speed brakes deployed the aeroplane needs a higher angle of attack for the same lift and its stalling speed rises. That is why speed brake use is restricted with flaps extended on many types.
Trim tabs, servo tabs and control surface bias
A tab is a small hinged surface at the trailing edge of a control surface. Because it sits a long way behind the main surface's hinge, a small tab air load makes a large hinge moment. Every tab below moves the main surface the opposite way to its own deflection.
- Trim tab — set from the cockpit (trim wheel or switch) through its own linkage. Deflected against the surface, it holds the surface in the required position so that the pilot's control force becomes zero ("hands-off"). Elevator tabs trim pitch for the selected speed, rudder tabs hold heading against asymmetric thrust, aileron tabs keep the wings level.
- Fixed (ground-adjustable) trim tab — a small metal plate, usually on the rudder or aileron of light aircraft, bent on the ground to correct a steady out-of-trim condition. Only a test flight shows whether the bend is right, and a small bend goes a long way.
- Servo tab — the pilot's controls are connected only to the tab; the main surface is free on its hinge and is flown into position by the tab's air load. It lets large surfaces be moved without power (for example the elevators of the DC-9 / MD-80 family) and is also used as a manual back-up when hydraulic power is lost.
- Control surface bias — a surface deliberately rigged slightly away from the faired position so that it applies a constant small force to counter a built-in asymmetry of the design (for example engine torque or a wing that is slightly heavy). In straight and level flight the bias is working, yet the cockpit trim indicators read zero.
Tabs move opposite — trim, balance and servo tabs all deflect against their surface. The one exception, the anti-balance (anti-servo) tab, moves the same way to make the controls heavier. Balance, anti-balance and spring tabs, mass balance and balance panels are the "other aerodynamic devices" of Chapter 2.