12.2 Aerofoils, Stability & Load Factor

12.2 Aerofoils, Stability & Load Factor

Quick Answer: Angle of attack (AoA)—the angle between the chord and relative airflow—drives lift, drag, and stall. Stability (longitudinal, lateral, directional) and CG decide whether the aircraft restores after a disturbance. Know axes/controls/trim, high-lift and drag devices (flaps, spoilers), and load factor in turns and gusts so you respect structural and power limits.

Section 12.1 established forces. This section explains how those forces change with geometry and manoeuvre—and why an RPAS can suddenly stop flying efficiently (stall), wander after a gust (stability), or overstress in a steep turn (load factor). Fixed-wing vocabulary appears heavily in TP 15263; apply the same AoA and load-factor logic to propeller/rotor blade elements and to multirotor structural loads.

Relative airflow and angle of attack

Relative airflow (relative wind) is the airflow direction felt by the aerofoil—opposite the aircraft’s (or blade’s) velocity through the air, modified by local induced flow.

Angle of attack (AoA) is the angle between the chord line of the aerofoil and the relative airflow.

Critical distinctions:

On a propeller or rotor blade, AoA is a blade-element concept: rotational speed, inflow, and pitch angle combine. Tip regions see higher dynamic pressure; root regions may have different local AoA.

Lift and drag versus AoA

As AoA increases from low values (below stall):

1. Lift coefficient generally increases (more flow turning / pressure difference).
2. Drag coefficient also increases—slowly at first, then more rapidly as separation grows.
3. Beyond a critical AoA, lift falls sharply or plateaus poorly—stall.
4. The lift-to-drag ratio (L/D) has a maximum at a moderate AoA used for efficient cruise/glide design points.

Aerodynamic stall

An aerodynamic stall is loss of smooth attached flow over the upper surface (or critical portion of a rotor disc/blade) when critical AoA is exceeded, causing a large loss of lift and rise in drag/buffet.

Key exam facts:

• Stall is about AoA, not “not enough speed” alone—though low speed usually means high AoA is needed to support weight, so they correlate in level flight.
• A fixed-wing can stall in any attitude if AoA is critical (including high-speed accelerated stalls in steep turns).
• Multirotor props can experience blade stall or inefficient high-AoA operation under extreme pitch/low RPM, contributing to loss of thrust margin.
• Recovery for fixed-wing teaching: reduce AoA (lower nose / ease back-pressure), add power as appropriate, wings level—then re-establish climb. For multirotors, reduce demand, restore attitude, ensure power available; do not yank collective-equivalent throttle into a deeper blade stall without understanding inflow.

Stability: longitudinal, lateral, directional

Stability is the tendency to return toward (or diverge from) an equilibrium condition after a disturbance.

Static vs dynamic (exam level)

• Positive static stability: initial tendency to return toward equilibrium.
• Negative static stability: initial tendency to diverge.
• Dynamic stability: whether oscillations damp out over time (positive dynamic) or grow.

Inherent stability

Inherent stability means the aircraft’s basic design (geometry, mass distribution, aero surfaces) provides stability without continuous pilot or autopilot input. Many toys are unstable and rely entirely on electronic stabilization. Professional RPAS still use flight controllers, but airframe layout (prop spacing, vertical area, wing dihedral on fixed-wing) contributes inherent qualities the autopilot then augments.

Exam posture: electronic stabilization is not a license to ignore CG and configuration; an inherently poor layout fights the autopilot and reduces margins when a sensor fails (ATTI mode).

Centre of gravity (CG) effects on stability and control

The CG is where weight acts. Its position relative to the aerodynamic centre / neutral point sets stability margins.

Payloads, dual batteries, and side-mounted sensors move CG. Theory of Flight meets Weight & Balance (flight-ops chapter): out-of-envelope CG is both a performance and a stability failure.

Wing planform, flaps, and spoilers

Planform

Planform is the wing’s shape viewed from above (span, chord, taper, sweep, aspect ratio).

• High aspect ratio (long, slender): generally better L/D, lower induced drag—common on efficient fixed-wing mappers.
• Low aspect ratio / delta: different stall and tip behaviour; more structural compactness.
• Taper and washout help manage spanwise lift and tip-stall characteristics.

Flaps

Flaps are high-lift devices on the trailing edge that increase camber (and sometimes area):

• Raise maximum lift coefficient → lower stall speed for landing/launch.
• Increase drag (especially large deflections) → steeper descent path without huge speed increase.
• Change pitching moment; manufacturer procedures define when to deploy.

Spoilers

Spoilers disrupt upper-surface flow to dump lift and increase drag:

• Used to steepen descent, assist roll on some designs, or reduce float in landing.
• Not common on small multirotors; know the definition for fixed-wing exam items.

Aeroplane axes, control surfaces, and trim

Three axes

Moments about these axes are produced by control surfaces (or by differential thrust on multirotors).

Control surfaces (fixed-wing)

• Ailerons: differential lift on wings → roll.
• Elevator / stabilator: tail force → pitch.
• Rudder: side force on fin → yaw (also coordinates turns, counters adverse yaw).
• Combined surfaces (elevons, ruddervators, flaperons) appear on some UAVs—same axes, shared hardware.

Trim

Trim reduces continuous control force/stick input needed to hold an attitude or speed. Fixed-wing may use trim tabs or movable stabilizers; multirotors use software attitude trim / hover throttle hover-point. Improper trim causes constant pilot workload and can mask progressive CG or wind issues.

Multirotor mapping of the same axes

Same physics of moments about the CG; different actuators.

Load factor in turns

Load factor (n) is the ratio of lift (or total aerodynamic/structural load) to weight. In straight-and-level flight, n ≈ 1. In a coordinated level turn, lift must equal weight plus provide centripetal force:

n ≈ 1 / cos(φ) where φ is bank angle.

Consequences:

1. Stall speed increases with √n in accelerated flight—steep turns stall “earlier” than wings-level slow flight intuition suggests.
2. Structural loads on airframe, arms, and mounts increase.
3. Power required rises (multirotor must support n×weight equivalent demand).
4. Mapping orbits that demand high bank at high speed stack turn radius physics with load factor—plan gentler geometry.

Gust loads

Gusts impose sudden changes in AoA and dynamic pressure, producing gust load factors that can exceed the pilot’s intended manoeuvre load. Effects:

• Transient high n on wings, arms, and payload mounts
• Prop/rotor RPM and current spikes as the controller fights attitude
• Risk of tip-over on ground or during low hover in rotor wash / building corner gusts
• Multirotor “toilet bowl” or oscillation if gains and wind interact poorly

Site survey and wind limits (AFM) are operational controls against gust-driven overstress. Theory point for the exam: gusts add load independently of intentional bank.

Structural and power limits

Structural limits

Manufacturer limit load / maximum load factor, maximum speed (Vne equivalent), and maximum mass define the airframe envelope. Exceeding them risks permanent deformation, arm cracks, or catastrophic failure. Advanced operations near people raise the consequence of structural failure—so envelope discipline is a safety control, not a suggestion.

Power limits

Even if the structure could take a manoeuvre, motors, ESCs, and batteries may not:

• Current limits and thermal soak
• Voltage sag at high load factor hover/climb
• Propeller RPM ceilings
• Reduced excess power at high density altitude

A steep turn into wind on a hot day with a heavy camera can hit power limit before structural limit—resulting in altitude loss, not a dramatic wing snap. Both limits matter; the first one you hit is the operational ceiling for that manoeuvre.

Integrated exam scenarios

Typical combined stem: “Fixed-wing mapper, aft CG, steep turn in gusty ridge lift.” Correct reasoning chain: aft CG reduces pitch stability → steep turn raises load factor and stall speed → gust spikes AoA → stall or structural/power exceedance risk → reduce bank, respect CG envelope, avoid ridge gust cores.

Bottom line: AoA rules lift and stall; stability and CG rule whether the aircraft helps or fights you; flaps/spoilers and planform tune the polar; axes/controls/trim organize inputs; load factor and gusts multiply forces until structure or power says stop.