13.1–13.2 Theory of flight and structures — general concepts (B2)

Scope and exam format

Module 13 is the large B2 systems module (EASA: 188 questions, may be split into two sessions). For B2L, 13.1, 13.2, 13.5 and 13.9 form the "basic" block taken by every B2L holder.

• 13.1(a) aeroplane aerodynamics and flight controls — level 1.
• 13.1(b) rotary-wing aerodynamics: terminology; effect of cyclic, collective and anti-torque controls — level 1.
• 13.2(a) general concepts; zonal and station identification; electrical bonding; lightning strike protection — level 2.
• 13.2(b) fundamentals of structural systems — level 1.

Aeroplane flight controls — operation and effect

Axis	Movement	Controls
Longitudinal	Roll	Ailerons, roll spoilers
Lateral	Pitch	Elevators, stabilator (all-moving tailplane), variable-incidence (trimmable) horizontal stabiliser, canards
Normal (vertical)	Yaw	Rudder (with rudder limiter reducing travel at high speed)

Combined surfaces:

• Elevons — elevator + aileron (delta/tailless aircraft).
• Ruddervators — rudder + elevator on a V-tail.
• Flaperons — flap + aileron.

High-lift devices increase maximum lift coefficient for low-speed flight:

• Trailing-edge flaps (plain, split, slotted, Fowler) — increase camber (Fowler also increases wing area).
• Leading-edge slats — open a slot that re-energises the boundary layer, delaying the stall to a higher angle of attack.
• Fixed slots, Krueger flaps.

Drag-inducing devices:

• Spoilers / speed brakes in flight — reduce lift and increase drag, can assist roll (roll spoilers on the down-going wing).
• Lift dumpers / ground spoilers — fully deployed on landing to put weight on the wheels for braking.

Tabs and bias:

• Trim tab — set by the pilot to remove a steady control force.
• Servo tab — moved by the pilot's control; its aerodynamic force moves the main surface.
• Balance tab — moves opposite to the surface automatically, reducing control force.
• Anti-balance tab — moves in the same direction, increasing feel.
• Control surface bias — a surface deliberately rigged slightly away from neutral (e.g. aileron droop / bias) to compensate for a known effect.

Rotary-wing basics for B2

Control	Changes	Effect
Collective	Pitch of all main-rotor blades equally	Total rotor thrust: climb/descend; increased torque must be balanced by the pedals
Cyclic	Blade pitch cyclically around the disc via the swashplate	Tilts the rotor disc — pitch/roll and direction of flight
Anti-torque pedals	Tail-rotor pitch (or fenestron/NOTAR)	Yaw/heading; counter main-rotor torque

Terminology to recognise: rotor disc, tip-path plane, coning, flapping, feathering, lead/lag, dissymmetry of lift, translating tendency, autorotation, ground effect (see Module 12 notes for detail).

Structural concepts

• Primary structure — failure would endanger the aircraft (wing spars, fuselage frames, pressure bulkhead); secondary — less critical; tertiary — lightly loaded.
• Safe-life (retire before fatigue failure), fail-safe (alternative load paths), damage tolerance (cracks found by inspection before critical size).
• Stresses: tension, compression, shear, bending, torsion, hoop stress in pressurised fuselages; fatigue from repeated pressurisation cycles.
• Monocoque (skin carries load) and semi-monocoque (skin + frames + stringers + longerons) fuselages; wings with spars, ribs, stringers and skins forming a torsion box.

Zonal and station identification

Reference	Direction
Fuselage station (FS/STA)	Longitudinal distance from a datum
Waterline (WL)	Vertical height from a reference below the fuselage
Buttock line (BL)	Lateral distance left/right of the centreline
Wing station (WS)	Spanwise along the wing

Zones (ATA iSpec 2200): major zones in hundreds — 100 lower fuselage, 200 upper fuselage, 300 empennage, 400 power plants/nacelles, 500 left wing, 600 right wing, 700 landing gear, 800 doors. Access panels are numbered from these zones, which helps the avionics engineer locate equipment bays (e.g. electrical/electronic bay in zone 100).

Electrical bonding

Bonding gives a low-resistance electrical path between metallic parts so that:

• the airframe is at one potential — no arcing between parts (fire/fuel ignition risk, radio interference);
• lightning and static currents flow safely through structure;
• a good return (earth) path exists for single-wire DC systems;
• radio and antenna performance (ground plane) is maintained and interference is minimised.

Bonding jumpers/straps (braided copper or aluminium) cross hinges, shock-mounts and flexible joints. Resistances are measured with a low-resistance ohmmeter (bonding meter) — values are typically a few milliohms, as given by the manufacturer. Surfaces are cleaned to bare metal, joined, then re-protected against corrosion; use compatible metals/washers to avoid dissimilar-metal corrosion.

Lightning strike protection

• Strikes usually attach at extremities (nose radome, wing tips, engine nacelles, tail) and exit at another extremity, "sweeping" along the surface.
• Metal skins conduct the current; composite structures need expanded metal foil/mesh, conductive coatings or diverter strips (e.g. on radomes).
• Fuel tanks are designed to avoid sparks (fastener sealing, bonding of pipes, no ignition sources).
• Static dischargers (wicks) on trailing edges bleed precipitation static to reduce radio noise (they are not lightning protection).
• After a reported strike: inspect entry/exit points (burn marks, pitting), bonding straps, compass calibration and avionics systems as the manual requires.

Fundamentals of structural systems (13.2(b))

At level 1, recognise the major assemblies (fuselage, wings, empennage, flight-control surfaces, nacelles/pylons, doors and windows, landing gear attachments) and know where avionic equipment is installed: electronic/equipment bays, radome (weather radar), antennas on fuselage upper and lower surfaces with doublers reinforcing the skin cut-outs.

Exam traps

• Stabilator = all-moving tailplane; trimmable stabiliser changes incidence for trim.
• Servo tab is driven by the pilot; balance tab is driven by surface movement.
• Static wicks reduce radio interference, not lightning damage.
• Waterlines are vertical, buttock lines lateral.