13.1(a) — Aeroplane Aerodynamics and Flight Controls

This section covers the fundamental principles governing how fixed-wing aircraft generate lift, produce drag, and are controlled in flight. Although B2 avionics engineers work primarily with electronic systems, understanding aerodynamic principles is essential for correctly interpreting flight data, troubleshooting sensor inputs, and appreciating why avionics systems behave as they do.

The practical reason this matters is that almost every avionics box on a modern aeroplane is either measuring an aerodynamic quantity, computing one, or acting on one. An air data computer turns two pressures into altitude, airspeed and Mach number using an assumed atmospheric model. A stall warning computer compares a measured angle of attack against a threshold that itself moves with flap position. A yaw damper exists only because a swept-wing aeroplane has a particular unstable oscillation mode. When one of those systems misbehaves, the engineer who understands the underlying aerodynamics can tell the difference between a sensor fault, a rigging fault and an aircraft that is simply doing what physics requires.

The Atmosphere and International Standard Atmosphere (ISA)

The atmosphere is composed of approximately 78% nitrogen, 21% oxygen, and 1% other gases (argon, CO₂, water vapour). Its properties — pressure, temperature, and density — decrease with altitude, directly affecting aircraft performance and the readings of pressure-based instruments.

In the round figures the examiner tends to use, air is about one-fifth oxygen and four-fifths nitrogen. Those proportions stay essentially constant with height throughout the region in which aeroplanes operate, because the atmosphere is continually mixed by convection and wind. What changes with altitude is not the mixture but the amount of it in a given volume, which is why a cabin at 40 000 ft still contains 21% oxygen yet cannot sustain an unpressurised occupant: the partial pressure of that oxygen has collapsed along with the total pressure.

Structure of the Atmosphere

The lowest layer, the troposphere, contains almost all the water vapour and weather, and is the layer in which temperature falls steadily with height. Above it lies the tropopause, the boundary at which the temperature stops falling, and then the stratosphere, in which temperature is initially constant with height. The tropopause is not at a fixed altitude in the real atmosphere: it sits at roughly 55 000 ft over the equator and can be below 30 000 ft over the poles, so a transport aircraft in cruise may be in the troposphere on one sector and the stratosphere on the next. The ISA model fixes it at 36 089 ft as a compromise so that performance charts and instrument calibrations have one agreed reference.

The word lapse rate has a precise meaning that the exam tests directly: it is the rate at which temperature changes with altitude. Pressure and density also fall with height, but those gradients have their own names and are not called lapse rates. The ISA figure of −1.98 °C per 1 000 ft is usually rounded to 2 °C per 1 000 ft for mental arithmetic, and it applies only up to the tropopause — above that height the model holds temperature constant at −56.5 °C.

The ISA is a model, not a measurement of today's air. It was built from mean conditions measured at 45° North latitude, deliberately chosen as a mid-latitude average rather than equatorial or polar data, so that it represents the temperate regions where most aviation takes place. Because every altimeter, air data computer and performance chart in the world is calibrated to the same model, two aircraft flying the same indicated flight level are vertically separated even when neither is at the true geometric height the model implies.

How Pressure Falls With Height

Pressure at any level is simply the weight of the air column above that level, so it falls approximately exponentially rather than linearly. A useful rule the exam expects you to know is that pressure halves for roughly every 18 000 ft of climb: from 1013 hPa at sea level to about 500 hPa near 18 000 ft, and to about a quarter of sea-level pressure near 36 000 ft. This is also why the pressure change per 1 000 ft is roughly 1 hPa per 27 ft near sea level but far less per foot at altitude — the altimeter's capsule is working against a much weaker pressure gradient up high, which is one reason altimetry tolerances widen with altitude.

Density depends on both pressure and temperature through the gas law \( \rho = P / (R T) \). Climbing reduces pressure, which reduces density; but a hot day reduces density at constant pressure. The two combine into density altitude — the ISA altitude at which the prevailing density would be found. High, hot and humid conditions all reduce density, and because lift, drag and engine thrust all depend on \( \rho \), density altitude is the single number that predicts degraded take-off performance. Humidity lowers density because a water molecule is lighter than the nitrogen or oxygen molecule it displaces, which is the opposite of most people's intuition.

Bernoulli's Theorem and Subsonic Airflow

Bernoulli's theorem states that in a steady, incompressible airflow with no energy added or removed, the total energy (the sum of static pressure, dynamic pressure, and potential energy) remains constant along a streamline:

Bernoulli's theorem cannot act alone — it needs the continuity equation to tell it where the air speeds up. Continuity says that for a steady flow through a duct or streamtube, mass flow is constant, so \( \rho A V \) is the same at every station. In subsonic flow density is nearly constant, so a reduction in area forces an increase in velocity. That is the venturi: the throat is narrow, so the air accelerates, and Bernoulli then demands that its static pressure fall. A cambered aerofoil works as half a venturi, with the curved upper surface and the free stream above it forming the constriction.

An aerofoil is shaped so air accelerates over the upper (cambered) surface. As velocity increases, static pressure decreases. The pressure difference between the lower and upper surfaces produces an upward net force — lift. This is also supported by Newton's Third Law: the wing deflects air downward (downwash), and the equal and opposite reaction acts upward on the wing.

These two explanations are not rivals; they are two accounts of the same event. The pressure field that Bernoulli describes is exactly what turns the air downward, and the momentum change that Newton describes is exactly what the pressure field must produce. Ahead of the wing the air is turned upwards (upwash) before being turned down behind it, and at the leading edge there is a stagnation point where the flow is brought completely to rest and static pressure reaches its maximum — the total pressure. The stagnation point is not fixed: as angle of attack increases it migrates rearwards and downwards along the lower surface, which is precisely what a leading-edge stagnation-point stall sensor is built to detect.

Boundary Layer

The boundary layer is the thin layer of air in direct contact with the wing surface where friction slows the airflow from the free-stream velocity to zero at the surface itself.

Air has viscosity, so the molecules immediately touching the skin do not move relative to it at all — the "no-slip" condition. Each layer above is dragged along by the one below through shear, and the velocity climbs from zero at the surface to essentially free-stream value within a layer that is typically only a few millimetres thick near the leading edge, growing to a few centimetres near the trailing edge of a large wing. Thin as it is, the boundary layer governs skin-friction drag, decides whether and where the wing stalls, and is the reason surface finish is an airworthiness matter rather than a cosmetic one.

Type	Characteristics	Skin Friction
Laminar	Smooth, orderly layers; found near the leading edge	Low
Turbulent	Irregular, mixed flow; better energy exchange with free-stream	Higher
Separated	Flow has left the surface; reversed flow and eddies beneath	Low friction but very high form drag

The point where the laminar boundary layer transitions to turbulent is called the transition point. A turbulent boundary layer resists separation better but produces more skin-friction drag. The point where the boundary layer separates from the surface is the separation point — when this moves forward significantly, the wing stalls.

Why the Flow Separates

Over the front of the wing the pressure is falling in the direction of flow — a favourable gradient that accelerates the boundary layer and helps hold it on. Behind the point of minimum pressure the pressure rises again towards the trailing edge, and this adverse pressure gradient works against the flow. The air in the boundary layer has already lost energy to friction, so it is the least able to climb that pressure hill. Where its remaining momentum is exhausted the flow stops, reverses, and lifts off the surface: separation. Increasing angle of attack steepens the adverse gradient and drags the separation point forward, which is the direct mechanism of the stall.

A turbulent boundary layer mixes high-energy free-stream air down towards the surface, so it carries more momentum near the skin and can fight further up the pressure hill before it gives up. That is the whole logic behind boundary layer control: designers frequently accept a small penalty in skin-friction drag in exchange for a much later separation. Vortex generators — the small rows of upright vanes seen ahead of the ailerons and on engine nacelles — deliberately trip the flow into turbulence and stir high-energy air downwards. Slots and slotted slats duct high-pressure air from beneath the wing onto the upper surface for the same reason: to reinforce and re-energise the boundary layer so that it stays attached to a higher angle of attack, raising the stalling angle.

Generation of Lift

Angle of attack (AoA) is the angle between the chord line and the relative airflow. As AoA increases, \( C_L \) increases — up to a critical angle (typically 15–18°) beyond which the flow separates from the upper surface and lift drops abruptly: this is a stall.

Note carefully what angle of attack is measured against. It is the relative airflow — the direction of the air actually meeting the aerofoil — and not the horizon, not the aircraft's longitudinal axis, and not any fixed geometric reference. The angle between the chord line and the longitudinal axis is the fixed angle of incidence set at build; the angle between the longitudinal axis and the horizon is pitch attitude. All three can differ at once: an aeroplane descending steeply with a level attitude has a substantial positive angle of attack, and an aeroplane in a ballistic pushover can hold a nose-high attitude at zero or negative angle of attack. This distinction is exactly why a stall warning system must sense angle of attack directly with a vane or probe rather than infer it from pitch attitude.

A rising gust makes the same point in reverse. If the air ahead of the leading edge is moving upwards, the relative airflow is tilted upwards even though the aeroplane has not moved, so the angle of attack increases. Combine that with a lowered trailing-edge flap — which already increases the effective camber and effective angle of attack of the section — and the wing can be pushed appreciably closer to the stall by turbulence alone. This is the mechanism behind gust-induced stall warnings during a flapped approach in gusty conditions, and it is why the manoeuvre and gust envelopes are drawn as they are.

Reading the Lift Equation

In steady level flight lift equals weight, so the equation is really a constraint: if \( \rho \) falls with altitude, or \( V \) falls in the approach, then \( C_L \) must rise to compensate, and the only way the pilot can raise \( C_L \) is by increasing angle of attack or changing configuration. That is why an approach is flown at a higher angle of attack than a cruise, and why a high-altitude cruise is flown closer to the buffet boundary than a low-level one. It also explains why the stall always occurs at the same angle but at very different speeds.

Wing loading is the aircraft's weight divided by its wing area, \( W/S \). Because both quantities are properties of the aircraft itself, wing loading is independent of altitude and of air density — climbing does not change the weight carried per square metre of wing. Density changes how much lift a given speed produces, but it cannot change a ratio built from weight and area. High wing loading gives a smoother ride in turbulence and a higher stalling speed; low wing loading gives better field performance and a gustier ride.

Two more terms are needed to read an aerofoil drawing. The chord line is the straight line from leading edge to trailing edge; camber is the curvature of the mean line away from that chord. The centre of pressure (CP) is the point through which the resultant aerodynamic force acts. On a conventional cambered aerofoil the CP moves forward as angle of attack increases up to the stall, and then jumps sharply rearward at the stall — a rearward CP movement that produces the nose-down pitch many aircraft show at the break.

Aerofoil Types

Type	Features	Typical Use
Symmetrical	No camber; zero lift at zero AoA	Tail surfaces, aerobatic aircraft
Cambered (conventional)	Upper surface more curved; lift at zero AoA	General aviation, transport
Supercritical	Flat upper surface, reflex camber at trailing edge; delays shock wave	High-subsonic transport aircraft
Laminar flow	Maximum thickness far aft; low drag	High-performance gliders
Thin, sharp-nosed	Very low thickness/chord ratio and a sharp leading edge; minimises wave drag but stalls abruptly at low speed	Supersonic aircraft, missile fins

Drag

Total drag is the sum of parasite drag and induced drag.

Drag Type	Cause	Varies With Speed
Parasite drag	Form (shape), skin friction, interference	Increases with V²
Induced drag	By-product of lift; caused by wingtip vortices deflecting airflow downward	Decreases with V² (highest at low speed/high AoA)
Wave drag	Energy lost across shock waves once local flow reaches M 1.0	Zero below Mcrit, then rises steeply with Mach number

The lift/drag (L/D) ratio peaks at the speed where parasite drag equals induced drag. This is the most aerodynamically efficient speed and determines best glide range.

The Three Components of Parasite Drag

• Form (pressure) drag — caused by the wake behind a bluff body, where the pressure never fully recovers. Streamlining and fairings attack this component directly; an unfaired landing gear leg or a missing gap seal can cost far more drag than its size suggests.
• Skin friction drag — the shear stress of the boundary layer acting over the whole wetted area. It depends on surface finish and on how much of the boundary layer is turbulent.
• Interference drag — extra drag generated where two flow fields meet, such as the wing-root, pylon and empennage junctions. This is why blended fillets exist and why a damaged or missing fillet fairing is not a purely cosmetic defect.

Induced drag is a direct and unavoidable consequence of producing lift with a finite wing. Higher pressure beneath the wing spills around the tip into the lower pressure above it, creating a trailing vortex at each tip. Those vortices tilt the local relative airflow downwards over the wing (downwash), which tilts the lift vector rearwards, and the rearward component of that vector is induced drag. Because it depends on how hard the wing is working, induced drag varies with \( C_L^{2} \) and therefore falls rapidly as speed increases. It is inversely proportional to aspect ratio, which is why gliders have long slender wings, and it is what winglets and raked tips are fitted to reduce by weakening the tip vortex.

Plotting the two components against speed gives the familiar U-shaped total drag curve. Its minimum is \( V_{MD} \), the minimum-drag speed, which is also the maximum L/D speed. Flight slower than \( V_{MD} \) lies on the back of the drag curve, where reducing speed increases drag and therefore increases the thrust required — a speed-unstable region that requires deliberate power management on approach and is a recognised contributor to low-energy approach accidents.

Stalling

A stall occurs when the critical angle of attack is exceeded — the boundary layer separates from the upper surface and lift decreases sharply. The stall speed depends on weight, load factor, configuration (flaps/slats), and bank angle:

The single most important idea is that a wing stalls at an angle, not at a speed. For a given aerofoil and configuration the critical angle of attack is fixed, and an aeroplane can be stalled at any airspeed and any attitude if it is manoeuvred hard enough to reach that angle. The published stall speed is simply the speed at which, in one specific condition of weight, configuration and 1 g flight, the wing happens to arrive at that angle. Change the weight, the load factor or the flap setting and the stall speed changes while the stall angle does not.

Stall Warning

A stall warning must arrive early enough to be useful, so it is deliberately set to trigger at a speed higher than the actual stall speed — equivalently, at an angle of attack lower than the critical angle. A warning at, or below, the stall speed would be an announcement rather than a warning, arriving after the point at which recovery action could prevent the stall. Certification requires a clear and distinctive warning with an adequate margin in every configuration, and because the stalling angle changes with flap and slat position, the warning threshold must be scheduled against configuration. That scheduling is the reason a stall warning computer takes flap and slat position inputs at all — and the reason a mis-rigged flap position sensor can produce a spurious or absent stick shaker.

• Angle of attack vane or probe — a small external vane that aligns itself with the relative airflow and drives a resolver or synchro; the primary sensing element on transport aircraft.
• Leading-edge lift transducer (reed or flapper switch) — a spring-loaded flap in the leading edge that is sucked upwards as the stagnation point migrates down and aft at high angle of attack, closing a switch.
• Stick shaker — an eccentric motor on the control column giving an unmistakable tactile and audible warning, the standard transport-category warning device.
• Stick pusher — fitted where the natural stall characteristics are unacceptable (notably rear-engined T-tail aircraft prone to deep stall); it applies an automatic nose-down input rather than merely warning.
• Aerodynamic pre-stall buffet — turbulent wake from the separating inboard wing striking the tailplane. It is a natural warning, but it can be masked by ice or by a very clean modern wing, which is exactly why artificial warning is required.

Designing Where the Wing Stalls First

A wing that stalled at the tip first would lose the ailerons at the moment they are most needed, and the loss of lift outboard would generate a violent rolling moment. Designers therefore force the root to stall first, using washout (a built-in reduction of incidence towards the tip so the tip is at a lower angle of attack than the root), stall strips on the inboard leading edge to trip separation early, and root aerofoil sections with less gentle stalling behaviour than the tip sections. Sweptback wings fight this tendency, because spanwise flow thickens the boundary layer outboard, and so they commonly need wing fences, vortilons or saw-tooth leading edges as well.

Two special stalls are worth naming. A deep stall (superstall) affects T-tail aircraft when the stalled wing wake blankets the tailplane, removing elevator authority and locking the aircraft into the stall — the reason those types carry stick pushers. A shock stall, by contrast, occurs at high speed: the local airflow over the upper surface reaches M 1.0, a shock wave forms, and the boundary layer separates immediately behind it. Both are separations, but one is produced by exceeding the critical angle at low speed and the other by exceeding the critical Mach number at high speed. Confusing the two is a classic examination trap.

Aerofoil Contamination — Ice, Frost, Snow and Rain

The callout above states the outcome; this section supplies the mechanism behind it and what the engineer is expected to do with that knowledge. Contamination is treated separately in the syllabus because it attacks the wing in a way no other condition does: it degrades performance and it defeats the warning system at the same time. Every stall warning threshold, every published stall speed and every certified performance figure assumes a clean aerofoil. Contamination invalidates that assumption without producing any cockpit indication that the assumption has been invalidated. That is what makes the quoted lift loss so dangerous — not its size, but the fact that no instrument reports it and no protection compensates for it.

Mechanism

Roughness is the primary mechanism, and it does its damage where the flow is most delicate — on the upper surface just aft of the leading edge, where the boundary layer is thin, laminar and about to meet the adverse pressure gradient. Surface texture no coarser than medium sandpaper is enough to trip the boundary layer prematurely and rob it of the momentum it needs to stay attached. The separation point moves forward, so the wing reaches its maximum lift coefficient at a lower angle of attack than the clean wing does. The consequences follow directly:

• The speed schedule stops being valid — every V-speed on the crew's card was computed from clean-wing data, so a rotation or approach speed that carried a healthy margin can end up at or below the speed at which the contaminated wing now stalls. The margin is consumed silently, before the take-off roll begins.
• The protection that assumes a clean wing is the protection that fails first — the vane or reed switch still triggers at its fixed, clean-wing angle, but the wing now reaches maximum lift below that angle. The shaker therefore fires late or does not fire at all, and the artificial warning fitted precisely because natural buffet is unreliable is itself disabled by the same contamination.
• The drag penalty arrives at the worst possible moment — roughness drag and premature separation act together while the aircraft is heavy, slow and low, cutting the second-segment climb gradient in the one phase of flight where there is nothing in reserve to trade.
• Handling degrades — asymmetric contamination produces asymmetric stall onset and a roll at the break, and ice on the tailplane can produce a tailplane stall with an abrupt nose-down pitch, often on flap extension.
• Weight increases and control surfaces can jam, with ice in hinge gaps, tab linkages and balance-weight recesses restricting free movement of the surface.

Types of Contamination

Contaminant	How It Forms	Principal Effect
Hoar frost	Water vapour freezing directly onto a cold surface, typically overnight on the ground	Roughness with negligible weight or shape change; still severely reduces maximum lift
Cold-soaked fuel frost	Moisture freezing onto upper wing skin chilled by cold fuel after a long sector, even in above-freezing ambient	Same roughness effect, but forms in weather no one expects icing in
Rime ice	Small supercooled droplets freezing on impact, trapping air	Opaque, brittle, follows the leading-edge shape; mainly a roughness and shape penalty
Clear (glaze) ice	Large supercooled droplets running back before freezing	Heavy, tenacious, can build behind the protected area as runback ice and form a lift-destroying horn
Snow and slush	Precipitation lying on the surface, or thrown up by the wheels on a contaminated runway	Bulk roughness and weight; slush also adds retardation drag during the take-off roll
Heavy rain	A water film with surface waves, plus droplet impact momentum	Effective surface roughening reduces maximum lift and adds drag while it persists

Avionics and Maintenance Consequences

For the B2 engineer the contamination case is where several systems intersect. Ice detectors — vibrating-probe types that sense the frequency shift caused by accreted ice, or optical types — provide the advisory or automatic anti-ice activation. Heated probes protect the pitot, static and total air temperature sensors and the angle of attack vanes; a failed vane heater in icing conditions is a serious defect precisely because a frozen vane can report a plausible but false angle of attack to the stall warning and, on fly-by-wire types, to the flight control laws. Blocked or partially blocked pitot and static ports produce erroneous airspeed and altitude, and any comparison monitor that then declares a disagreement is reporting a symptom, not the cause.

Stability

Static stability — the initial tendency to return to the original state after a disturbance. Dynamic stability — whether the oscillations following a disturbance decrease (stable), remain constant (neutral), or increase (unstable) over time.

The two are independent, and the distinction matters. An aeroplane can be statically stable but dynamically unstable: each disturbance produces a restoring moment, but the aircraft overshoots by more each time and the oscillation diverges. Static stability is a prerequisite for dynamic stability, but it is not a guarantee of it. Note also that stability and controllability pull in opposite directions — the more strongly an aeroplane resists being disturbed, the more control force is needed to manoeuvre it deliberately, which is why designers aim for adequate rather than maximum stability.

Axis	Stability Type	Primary Contributor
Longitudinal (pitch)	Most important	Horizontal stabiliser, CG position
Lateral (roll)	Dihedral effect	Wing dihedral, sweepback, high wing
Directional (yaw)	Weathercock stability	Vertical fin (area and moment arm)

Longitudinal Stability and the CG

Longitudinal stability is dominated by the position of the centre of gravity relative to the neutral point, the CG position at which the aircraft would be neutrally stable. The distance between them, expressed as a fraction of the mean aerodynamic chord, is the static margin. A forward CG gives a large static margin: strongly stable, heavy in pitch, a higher stall speed because the tailplane carries more download, and a longer take-off run. An aft CG gives a small static margin: light and responsive in pitch, lower trim drag and better cruise economy, but reduced stall recovery margin and, beyond the aft limit, no stability at all. This is the entire reason mass and balance limits exist, and why a loading error is an airworthiness matter rather than a performance nicety.

Dynamic Modes

• Phugoid — a long-period, lightly damped exchange of height and speed at roughly constant angle of attack. Slow enough that the pilot or autopilot corrects it without difficulty.
• Short-period pitch oscillation — a rapid oscillation in angle of attack, normally heavily damped. If it becomes lightly damped it is dangerous, because it is fast enough for pilot inputs to reinforce it.
• Spiral mode — a slow divergence in which a small bank is allowed to steepen. Common and usually acceptable, since it develops slowly enough to correct.
• Dutch roll — the coupled lateral-directional oscillation described below.

Dutch roll affects yaw and roll simultaneously, not pitch. It arises when the dihedral effect (lateral stability) is strong relative to the weathercock effect (directional stability) — precisely the combination produced by a swept, high-aspect-ratio wing on a modern jet. A small yaw makes the advancing wing produce more lift, so the aircraft rolls; the roll produces sideslip, which yaws the aircraft the other way; and the motion repeats as a wallowing, alternating yaw-and-roll. Its frequency is low — a period of several seconds — but because it is only lightly damped and worsens with altitude, it is uncomfortable and can be fatiguing to fly manually. The yaw damper senses the yaw rate with a rate gyro and applies small, rapid, opposing rudder inputs that are transparent to the pilot's own pedal inputs. Loss of the yaw damper is normally accompanied by an altitude or speed restriction for exactly this reason.

Flight Controls

Primary Controls

Control	Axis	Cockpit Input	Movement
Ailerons	Lateral (roll)	Control wheel / sidestick lateral	Differential — one up, one down
Elevator	Longitudinal (pitch)	Control column fore/aft	Up for nose up, down for nose down
Rudder	Directional (yaw)	Rudder pedals	Deflects to yaw aircraft
Elevons	Pitch and roll combined	Column and wheel, mixed mechanically or electronically	Together for pitch, differentially for roll
Ruddervators	Pitch and yaw combined	Column and pedals, mixed	Together for pitch, differentially for yaw
Stabilator (all-moving tailplane)	Longitudinal (pitch)	Control column fore/aft	Whole surface rotates; usually with an anti-balance tab

The two combined-function surfaces are frequently examined. Elevons are fitted to delta-wing and tailless aircraft, which have no separate tailplane; moved together they act as elevators and control pitch, moved differentially they act as ailerons and control roll — so a delta wing's pitch and roll both come from the elevons. Ruddervators are the two surfaces of a V-tail (butterfly tail); moved together they act as an elevator and control pitch, moved differentially they act as a rudder and control yaw. Learn the pairs by what the configuration lacks: a delta has no tailplane, so its trailing-edge surfaces must do pitch and roll; a V-tail has no separate fin or tailplane, so its surfaces must do pitch and yaw.

What the Cockpit Input Actually Commands

Pulling the control column back raises the elevator, which produces a downward force on the tail, pitches the nose up and increases angle of attack. Moving the wheel or stick left rolls left, which requires the left aileron up and the right aileron down — the down-going aileron increases lift on the right wing, and the up-going aileron reduces it on the left. So a column pulled back and to the left gives elevator up with the right aileron down. Confirming this by hand during a control-and-rigging check, with the surfaces observed directly rather than assumed from an indicator, is a standard and non-negotiable duplicate-inspection item after any work on the flying control system.

The rudder has a secondary rolling effect that follows from geometry. Pushing the left rudder pedal deflects the rudder and yaws the nose left; the outer (right) wing then travels faster through the air than the inner wing, generates more lift, and rises, so the aircraft rolls left as it yaws left. This roll-with-yaw coupling is the same effect that drives Dutch roll, and it is what makes rudder alone a usable, if crude, roll control after a lateral control failure.

Secondary Controls

• Trim tabs — small surfaces on primary controls to relieve stick forces in steady flight
• Flaps — increase camber and wing area; lower stall speed for take-off and landing
• Slats — extend from the leading edge to energise boundary layer and increase stall angle
• Spoilers — rise from wing upper surface to destroy lift (in flight: speed brakes; on ground: lift dumpers)
• Speed brakes — increase drag without significantly changing pitch
• Slots — fixed or automatic gaps that duct high-pressure air from below onto the upper surface to reinforce the boundary layer and raise the stalling angle
• Vortex generators — small vanes that energise the boundary layer locally to delay separation over ailerons, nacelles and the tailplane

Tabs — Four Types, Four Behaviours

Tab	Moves	Purpose
Trim tab	Set independently by the pilot; then fixed relative to the surface	Holds the surface where the aerodynamic loads balance, so the pilot need hold no residual force — it eases control loading
Balance tab	In the opposite direction to the control surface, proportionally to its movement	Aerodynamic force on the tab assists the surface, reducing hinge moment and stick force
Anti-balance tab	In the same direction as the control surface, proportionally	Deliberately increases stick force to restore feel on an over-light surface, typically an all-moving stabilator
Servo tab	Driven directly by the pilot's control run; the main surface follows aerodynamically	Pilot moves only the small tab, which flies the large surface — used on large unpowered controls
Spring tab	Acts as a servo tab only once spring pre-load is overcome at higher airspeed	Gives assistance in proportion to airspeed, so stick force stays consistent across the speed range

The two examinable points are that a trim tab eases control loading for the pilot — it does not permit the centre of gravity to be flown outside its certified limits, and it does not make the controls finer or more precise — and that a balance tab moves in the opposite direction to the surface it is attached to, proportionally to that surface's movement. An anti-balance tab is its mirror image in both motion and purpose, so a tab linkage assembled one hole out, or reversed, can convert an assistance device into a resistance device or vice versa. Tab rigging and free-play checks exist for exactly this reason, and control-surface tabs are also a classic flutter source when their linkage develops wear.

Flaps and the Pitch Change

Flaps increase the camber of the rear of the wing, and the larger types (Fowler and slotted-Fowler) also translate rearwards to increase wing area. Both effects raise the maximum lift coefficient, so the wing can support the weight at a lower speed. Because a landing flap setting is a large deflection chosen for maximum lift and maximum drag, its principal benefit is a lower landing speed together with a steeper approach path and better speed control. A take-off flap setting is a smaller deflection, chosen to buy lift without the drag penalty that would spoil the climb gradient — so the full landing setting is not what reduces take-off speed, and selecting it for take-off would be actively harmful. Whatever the setting, lowering flap increases both lift and drag together: the same increase in camber and frontal area that generates the extra lift also generates the extra drag, and this is precisely what makes short-field landings possible.

Extending a trailing-edge flap loads the rear of the aerofoil more heavily, so the centre of pressure moves rearward. A rearward CP acting behind the centre of gravity produces a nose-down pitching moment on most configurations. Note two things the exam probes here: the movement is rearward, not forward, and the aircraft's centre of gravity does not change at all — moving a control surface redistributes aerodynamic pressure, it does not move mass in a way that shifts the CG. On a low-set tailplane the flap downwash may reach the tail and partly offset the pitch change; the net trim change is therefore type-specific, but the CP movement itself is not.

Flap Type	Action	Relative Lift / Drag
Plain	Rear portion of the aerofoil hinges down	Modest lift increase, large drag increase
Split	Lower surface only hinges down, upper surface unchanged	Similar lift to plain, more drag
Slotted	A gap ducts high-energy air over the flap upper surface	Better lift than plain for less drag, because separation is delayed
Fowler	Translates aft on tracks before deflecting, increasing area and camber	Greatest lift increase; drag stays low at small settings and rises steeply at large ones
Krueger (leading edge)	Hinges forward and down from the lower leading edge	Raises stalling angle; commonly fitted inboard alongside outboard slats

Spoilers, Speed Brakes and Lift Dumpers

Flaps and tabs add lift or trim; spoilers do the opposite, and they are the only aerodynamic device on the aeroplane whose purpose is to make the wing work worse. A spoiler is an upper-surface panel hinged at its leading edge that rises into the airflow. It does not simply add an obstruction: it forces the boundary layer to separate over the whole of the wing behind it, so the suction peak on the upper surface collapses. Lift is destroyed over that part of the span, and form drag rises as a consequence. That order matters for the exam — a spoiler is a lift-destroying device that also produces drag, whereas a true speed brake is a drag-producing device designed to change lift and pitch as little as possible.

On most transport aircraft the same row of wing panels performs all three roles, and the flight control computers decide which one is active from the aircraft's state. The distinction the engineer must hold is functional, not physical.

Function	How the Panels Are Driven	What It Achieves
Speed brake (in flight)	Both wings together, partial travel, from the speed brake lever; travel is usually limited with flap out and blown down by airload at high speed	Increases drag so the aircraft can descend steeply or decelerate without changing thrust; the lift lost is made up by a slightly higher angle of attack
Roll spoiler (roll assist)	Asymmetrically, on the down-going wing only, mixed with the aileron command	Rolls the aircraft by spoiling lift on the inside wing; the extra drag is on the inside of the turn, so it yaws into the turn instead of producing adverse yaw
Lift dumper (ground spoiler)	All panels, full travel, automatically on touchdown once the arming, weight-on-wheels, wheel spin-up or thrust lever conditions are met	Kills residual lift so the weight transfers from the wings onto the wheels, which is what makes wheel braking and anti-skid effective; also adds a large drag increment

Two consequences follow that are easy to miss. First, the roll-spoiler function is why high-speed roll control can be handed to the spoilers: a spoiler cannot suffer the aileron reversal described later under high-speed flight, because it works by separating flow rather than by twisting the wing with a trailing-edge load. Second, the lift-dumper function is a braking system as much as an aerodynamic one — an aircraft that touches down with residual wing lift has little weight on its tyres, so the brakes and anti-skid have almost nothing to work with. This is the direct reason a failed weight-on-wheels switch, a wheel that does not spin up, or an unarmed speed brake lever produces a long landing roll that feels like a brake fault but is not one.

The dedicated speed brake of the original definition is the fuselage- or tail-mounted airbrake found on some types, and its "without significantly changing pitch" characteristic is a deliberate design result: the panels are placed and sized so that the drag line acts close to the centre of gravity and the wake misses the tailplane, so the aircraft slows without a trim change the pilot must fight. Wing-mounted spoilers used as speed brakes cannot achieve that as cleanly — their wake washes over the tail and their lift loss is well away from the CG, which is why deploying them typically produces buffet and a noticeable pitch and trim change that the type's flight manual describes.

High-Speed Flight

As an aircraft approaches the speed of sound (\( a = \sqrt{\gamma R T} \approx 340\,\text{m/s at sea level} \)), compressibility effects become significant. The Mach number is the ratio of TAS to the local speed of sound:

The formula \( a = \sqrt{\gamma R T} \) contains one variable that matters operationally: temperature. The speed of sound depends on temperature alone, not on pressure or density, because sound is transmitted by molecular collisions and molecular speed is set by temperature. As an aircraft climbs into colder air, the local speed of sound falls — from about 340 m/s at ISA sea level to roughly 295 m/s at the tropopause. A constant true airspeed therefore represents a steadily increasing Mach number in the climb. This is why the climb profile changes over from a constant indicated airspeed to a constant Mach number at the crossover altitude, and why the airspeed indicator carries a moving barber-pole \( V_{MO}/M_{MO} \) pointer driven by the air data computer rather than a fixed red line.

Regime	Mach Range	Characteristics
Subsonic	M < 0.75	No shock waves
Transonic	0.75 – 1.2	Mixed sub/supersonic flow; shock waves form
Supersonic	1.2 – 5.0	Entire flow supersonic; bow and oblique shocks
Hypersonic	Above M 5.0	Severe kinetic heating; chemical changes in the airflow

Note the naming convention the exam uses: speeds above the speed of sound but not exceeding roughly four times it are supersonic, and only around M 5 and above does the term hypersonic apply. "Hyposonic" is not a recognised regime at all. Below the transonic band the flow is subsonic everywhere on the airframe; the transonic band is defined by the coexistence of subsonic and supersonic flow over the same aircraft, which is what makes it the most aerodynamically awkward region to fly in.

Mcrit is the free-stream Mach number at which local airflow first reaches M = 1.0 (typically on the upper wing surface). Beyond M_crit, shock waves produce wave drag and can cause shock-induced separation (Mach buffet), pitch changes (Mach tuck), and aileron reversal. Wing sweepback, thin aerofoils, and supercritical wing sections are all design features that raise M_crit.

The mechanism behind each of those effects is worth having straight. A shock wave is a near-instantaneous compression across which the flow decelerates from supersonic to subsonic, with an abrupt rise in static pressure and an unrecoverable loss of energy. That energy loss appears as wave drag, so as the aircraft accelerates past M_crit the total drag increases sharply — the drag-divergence rise — on top of the parasite and induced drag already present. Thrust is set by the engine and is not what changes here, and lift can still be maintained; the dominant aerodynamic change on approaching supersonic flow is that large increase in total drag.

• Shock-induced separation and Mach buffet — the steep adverse pressure gradient behind the shock separates the boundary layer, giving high-speed buffet. This is the shock stall: a separation stall that occurs at high speed rather than high angle of attack.
• Mach tuck — the shock forms first near mid-chord and moves aft with increasing Mach number, shifting the centre of pressure rearward and producing a nose-down pitching moment that steepens the dive and increases Mach further. A Mach trim system senses Mach number and applies a compensating nose-up trim automatically.
• Aileron buzz and control reversal — a shock sitting over a control surface makes its hinge moment erratic, and on a flexible wing a down-aileron can twist the outer wing leading-edge-down enough to reverse the intended roll. High-speed roll control is therefore commonly given to inboard ailerons and roll spoilers.
• Reduced control effectiveness — pressure changes cannot propagate forward through a shock, so a trailing-edge surface behind a shock loses much of its influence on the wing ahead of it.

Sweepback is the principal design answer, because only the component of the airflow perpendicular to the wing leading edge determines the local Mach number over the section; sweeping the wing reduces that component and so raises M_crit. The costs are real — reduced maximum lift coefficient, a tip-stall tendency from spanwise flow, and a stronger dihedral effect that brings on Dutch roll. Thin aerofoil sections and supercritical sections attack the same problem differently: the supercritical section's flattened upper surface keeps local accelerations low, holds the inevitable shock weak and far aft, and recovers lift with reflex camber at the trailing edge.