Stability & control

081 Principles of FlightChapter 5

Stability basics

Key fact

Positive static stability required before dynamic stability possible. Static = initial tendency to return; dynamic = returns over time with damping. Less stability = more manoeuvrability. Transport aircraft limit load factor: +2.5 g (flaps extended +2.0 g).

Longitudinal stability (lateral axis)

Key fact

Tailplane gives greatest positive contribution. Fuselage, wing downwash — negative. Engine nacelles aft of CG — positive. Forward CG → increased stability, higher stick force per g, higher VS. Aft CG limit set by minimum acceptable longitudinal stability. Neutral point: CG beyond this → longitudinally unstable. Static margin = distance CG to neutral point (+ve = stable, typically 10–30% MAC).

Key fact

Phugoid: slow speed/altitude oscillation (20–100 s), easily controlled. Short-period: AoA changes, speed constant, must be heavily damped. Stick force stability: above trim speed requires push; below requires pull. Stick position stability always constant regardless of trim.

Directional stability (normal axis)

Key fact

Mainly the fin. Sideslip: angle between speed vector and plane of symmetry. Positive sideslip = RAF from right. Contributors: fin (+), sweepback (+), dorsal fin (+), ventral fin (+ directional, − lateral). Straight high-AR wings: negligible directional effect.

Lateral stability (longitudinal axis)

Key fact

Increased by: dihedral, sweepback, high wing, higher aspect ratio. Decreased by: anhedral, forward sweep, flap extension, low wing, ventral fin. Excessive lateral stability → excessive aileron demand in sideslip/crosswind landing.

Exam trap

Constant IAS, increasing altitude: lateral stability increases, directional decreases. Constant Mach: lateral same, directional decreases. Dutch roll: lateral greater than directional — yaw damper failure: reduce altitude and Mach.

Dutch roll & spiral instability

Key fact

Dutch roll: combined lateral + directional oscillatory motion; sensitivity ↑ with lateral stability; reduced by anhedral. Spiral dive: directional greater than lateral, aperiodic — bank steadily increases in level turn.

Primary controls

Key fact

Elevator: pitch; CC rearward = elevator up = increased tail downforce = pitch up. Rudder: yaw primary, roll secondary (same direction). Ailerons: roll primary, adverse yaw secondary. Outboard ailerons: slow flight only (locked out cruise). Inboard ailerons + spoilers: cruise — reduce wing twist at high speed.

Key fact

Adverse yaw: upgoing wing more induced drag → yaw opposite turn. Corrected by differential, Frise, or rudder coupling. Rate of roll increases with altitude (less aerodynamic damping). Manoeuvrability decreases as IAS decreases for given deflection.

Aerodynamic balance & trimming

Key fact

Inset hinge, horn balance, internal balance: reduce hinge moment/stick forces. Balance tab: moves opposite control surface. Anti-balance tab: same direction — increases effectiveness. Servo tab: tab moved first; at low IAS may be insufficient; elevator jam → inverted controls possible.

Key fact

Trim tab: opposite to control surface; trim wheel forward = nose down; reduces pitch authority in trimmed direction. THS (trimmable stabilizer): less trim drag, larger CG range, more serious if runaway/jammed. Forward CG → stabilizer leading edge lower (more negative tail lift).

Speed stability

Key fact

Below VMD: speed decrease → drag increase → further slowdown (unstable). Above VMD: speed decrease → drag decrease → return to original (stable). V1 typically below VMD (unstable region on take-off roll).

Asymmetric flight & VMC

Definition — VMCA

Min control in air, take-off config, max thrust, 5° bank toward live engine, rudder only for directional equilibrium. Not reduced at bank angles above 5°. Decreases with altitude and temperature.

Definition — VMCG

Min control on ground; no nose-wheel steering; CG fully aft; max 30 ft lateral deviation. Decreases with field elevation and temperature.

Definition — VMCL

Landing/approach config; limited by max roll rate. Not for take-off; not affected by altitude (flight idle thrust).

Key fact

OEI controllability worse: low temperature, aft CG, low altitude. Better: high temperature, forward CG, high altitude. Critical engine (clockwise props): left. With crosswind: outboard engine on upwind side is critical.

Flight envelope

Formula — Manoeuvring speed

VA ≥ VS × √(limit load factor)
VA new = VA old × √(W new / W old)

Key fact

VA: above this, full elevator deflection can cause structural damage. VMO: max operating IAS. MMO: max Mach — can be exceeded climbing at constant IAS. VB: design speed for max gust intensity. Gust load factor greater than manoeuvring (affects small area). Flaps reduce VS but reduce margins to structural limits.