What "other aerodynamic devices" means
Chapter 1 covered the surfaces that control the aeroplane. This chapter covers the devices that make those surfaces easy, safe and effective to use: tabs and balances that set how heavy the controls feel, mass balance that stops them fluttering, and small fixed devices — fences, saw-teeth, vortex generators and stall wedges — that keep the airflow attached where the controls need it. Syllabus 11.1(b) is examined at level 2 for B1.1. At the end there is a revision of high-speed flight, which is now examined in Module 8.4 but underpins Mach trim and other systems in Chapter 12.
Hinge moments: why control forces need managing
Any deflected control surface carries an air load. Acting behind the hinge line, that load makes a hinge moment that tries to return the surface to neutral, and the pilot (or the actuator) must overcome it. The hinge moment grows with the size of the surface and with the square of the airspeed, so a large, fast aeroplane needs help — either power (hydraulic actuators) or aerodynamic balancing. Too much balance is just as bad: an overbalanced control snatches away from the pilot. Designers aim for forces that are light enough to fly but heavy enough to give feel and to protect the structure.
Balance, anti-balance and spring tabs
- Balance (geared, lagging) tab — linked to the fixed structure so that whenever the main surface moves, the tab automatically moves the opposite way. Its air load helps the surface along, reducing the pilot's effort. If the link's length can be adjusted from the cockpit, the same tab doubles as a trim tab.
- Anti-balance (anti-servo, leading) tab — geared to move in the same direction as the surface. Its air load opposes the movement, making the control heavier and less sensitive. It is standard on stabilators, which would otherwise be far too light and twitchy, and it often doubles as the trim tab.
- Spring tab — the pilot's input reaches the control surface through a spring, and the tab is linked so that spring deflection drives it. At low speed the air loads are small, the spring hardly yields and the surface moves almost directly. At high speed the air loads compress the spring, and that movement deflects the tab, which then acts as a servo tab and does much of the work. It gives near-direct control at low speed and assistance where forces are highest.
| Tab | Moved by | Direction relative to the surface | Effect |
|---|---|---|---|
| Trim tab | Separate cockpit control | Opposite | Holds the surface; control force trimmed to zero |
| Balance tab | Linkage to the fixed structure | Opposite | Reduces control force |
| Anti-balance tab | Linkage to the fixed structure | Same | Increases control force / feel |
| Servo tab | The pilot's control linkage | Opposite | Flies the free-floating surface |
| Spring tab | Pilot linkage via a spring | Opposite (once the spring yields) | Assists only when forces are high |
Aerodynamic balance: horns, inset hinges and balance panels
Aerodynamic balance reduces the hinge moment by putting part of the surface's area — and therefore part of its air load — ahead of the hinge line, where it pushes in the helping direction.
- Horn balance — an extension of the control surface, usually at the tip, that reaches forward of the hinge line into a cut-out in the fixed surface. It is often where the mass balance weight is housed too.
- Inset (set-back) hinge — the hinge line is moved back from the leading edge of the control surface, so the nose of the surface projects into the airflow on the opposite side when it deflects. Moving the hinge too far back would overbalance the control.
- Internal balance (balance panel) — used on many large-aircraft ailerons and elevators. A panel attached to the leading edge of the control surface projects forward into a cove in the trailing edge of the wing or stabiliser. A flexible seal joins the panel to the fixed structure, and vent gaps admit upper-surface pressure on one side of the panel and lower-surface pressure on the other. When the surface deflects, the pressure difference across the panel pushes it in the direction that assists the movement. Balance panels are typically aluminium-skinned frames or honeycomb, and the seal must be intact for the balance to work.
Mass balance and flutter
Flutter is a rapidly building oscillation in which the aerodynamic forces on a surface couple with its own elastic and inertia properties. A control surface whose centre of gravity lies behind its hinge line lags behind any up-and-down flexing of the wing or tail, and that lag can feed energy into the oscillation until the structure fails — sometimes within seconds.
- Mass balance weights (lead, tungsten or depleted-uranium on older types) are fitted ahead of the hinge line — inside the leading edge of the surface, in a horn, or on an arm — so that the surface's centre of gravity sits on or ahead of the hinge.
- Balance is specified as an allowable static moment about the hinge. After repair, repainting or replacing parts of a control surface, the manufacturer's structural repair manual normally requires the surface to be re-balanced and its static balance checked, because extra paint or a patch aft of the hinge adds a nose-up (tail-heavy) moment.
- Worn hinges, slack in the actuating linkage and loose tab attachments also lower flutter margins; free play limits for tabs and surfaces are checked during maintenance.
Aerodynamic balance (horn, inset hinge, balance panel, balance tab) reduces the control force. Mass balance prevents flutter; it may slightly reduce stick forces too, but that is not its purpose.
The boundary layer and why it separates
Because air is viscous, the layer touching the skin is brought to rest, and a thin boundary layer forms in which the speed rises from zero to the local free-stream value. Near the leading edge it is laminar (smooth layers, low skin friction); at the transition point it becomes turbulent (thicker, more friction, but more energetic near the surface). Over the rear of the upper surface the pressure rises again (an adverse pressure gradient), and the slow air near the skin is pushed back. When it runs out of energy it stops following the surface — separation. As the angle of attack increases, the separation point creeps forward until the wing stalls.
A swept wing adds a further problem: the boundary layer drifts outwards along the span, thickening towards the tips. The tips therefore tend to stall first. That loses aileron control just when it is needed, and because the tips of a swept wing are behind the centre of gravity, losing their lift makes the nose pitch up — deeper into the stall.
Boundary-layer control devices
| Device | What it is | How it helps |
|---|---|---|
| Wing (stall) fence | A thin chordwise plate on the upper surface, sometimes wrapping round the leading edge | Blocks the spanwise drift of the boundary layer, so the outer wing keeps attached chordwise flow |
| Saw-tooth (dog-tooth) leading edge | A step in the leading edge where the outer wing's chord is extended forward | The step sheds a strong vortex over the upper surface that acts like a fence and re-energises the flow over the outer wing and ailerons |
| Vortex generators (VGs) | Rows of small, low-aspect-ratio vanes set at an angle to the flow, on wings, tails, nacelles or ahead of control surfaces | Each vane's tip vortex mixes fast free-stream air down into the boundary layer, delaying separation at high angles of attack and behind shock waves at high Mach numbers |
| Stall wedge / stall strip | A small sharp-edged strip on the inboard leading edge | At high angle of attack it trips the flow so the root stalls first; the ailerons keep working and the buffet warns the pilot |
| Leading-edge devices | Slots and slats (Chapter 1), sometimes only on the outer wing | Feed high-energy air over the upper surface, raising the stalling angle where they are fitted |
These small parts matter. Missing or damaged vortex generators and seals are typically controlled through the Configuration Deviation List (CDL), which may allow dispatch with a few missing under performance penalties. Stall strips must be fitted exactly where the manufacturer specifies — moving one changes the stall behaviour of the aeroplane.
Wing-tip vortices and winglets
High-pressure air below the wing spills round the tip towards the low-pressure upper surface, forming the tip vortices that are the visible side of induced drag and the source of wake turbulence. Winglets and other wing-tip devices (blended winglets, sharklets, wing-tip fences, raked tips) weaken the vortex and recover some of its energy, reducing induced drag and fuel burn. Their attachment structure, lighting and lightning protection are maintenance items like any other part of the wing.
High-speed flight — revision (Module 8.4)
Transport jets cruise at around Mach 0.78–0.85, where compressibility shapes the design of wings, tails and flight-control systems.
- Speed of sound — depends only on air temperature: about 340 m/s (661 kt) at 15 °C (ISA sea level), falling to about 295 m/s (573 kt) at −56.5 °C in the stratosphere.
- Mach number — true airspeed divided by the local speed of sound. Approximate regimes: subsonic below M 0.75, transonic M 0.75–1.2 (mixed subsonic and supersonic flow), supersonic M 1.2–5, hypersonic above M 5.
- Critical Mach number (Mcrit) — the free-stream Mach number at which the flow somewhere on the aircraft (usually over the wing's upper surface) first reaches Mach 1.
- Shock waves — a normal shock stands at right angles to the flow; behind it the flow is subsonic and the static pressure, density and temperature jump up while total pressure (energy) is lost. An oblique shock forms at an angle on sharp edges in supersonic flight; the flow behind it slows but stays supersonic. The energy lost in shocks appears as wave drag, which rises steeply just above Mcrit (the drag-divergence Mach number).
- Compressibility (high-speed) buffet — the boundary layer separates behind the shock and the turbulent wake shakes the structure and the tail. At high altitude the high-speed buffet speed and the low-speed stall speed close together — the "coffin corner".
- Mach tuck — as the shock moves aft on a swept wing, the centre of pressure moves aft and the nose tends to pitch down. A Mach trim system compensates automatically (Chapter 12).
- Aerodynamic heating — air brought to rest against the skin is heated by compression and friction. At airliner speeds this "ram rise" is why the probe-measured total air temperature (TAT) is higher than the static (outside) air temperature (SAT); on Concorde at Mach 2 the nose reached about 127 °C and the airframe grew measurably in length.
- Area rule — transonic wave drag is lowest when the aircraft's total cross-sectional area changes smoothly from nose to tail. Designers "waist" the fuselage where the wing joins (the coke-bottle shape) and add fairings, such as flap-track fairings, to smooth the area distribution.
- Engine intakes — a turbine engine needs subsonic air, roughly Mach 0.4–0.5, at the fan or compressor face whatever the flight speed. A subsonic (pitot) intake is a divergent duct that slows the air and raises its pressure. A supersonic intake uses one or more oblique shocks, then a weak normal shock, often with variable ramps or a movable centre-body and spill/bleed doors, to slow the air with the least loss.
- Sweepback raises Mcrit because only the velocity component normal to the leading edge (V cos Λ) governs the pressure distribution. Thin sections and supercritical aerofoils (flat-topped, with aft camber that keeps the shock weak and well aft) raise it further. The costs of sweep are a tendency to tip stall and pitch-up, a lower CL max (hence powerful high-lift devices) and Dutch roll (hence yaw dampers).