Gyro instruments

022 InstrumentationChapter 2

Schematic

Precession: reaction occurs 90 degrees ahead in the plane of rotation.

Gyroscopic principles

Definition — Rigidity in space

Tendency to remain in axis of rotation. Rigidity ∝ mass × radius² × RPM² (inertia × speed). Rigidity — remain fixed; precession — 90° reaction to applied force (bearing friction, force on spinning rim).

Definition — Precession

Change of direction (angular change) of rotor plane caused by an applied force. Erection uses precession to align the gyro.

Gyro types

Type	Gimbals	DOF	Examples
Displacement	2	2	AI (earth-tied vertical), DI (horizontal)
Rate	1	1	Turn indicator, turn coordinator

Key fact

Space gyros — free orientation, not aligned to datum (not useful for aviation). Tied gyros — fixed to earth reference (AI, DI).

Air-driven vs electrical gyros

Feature	Air-driven	Electrical
Power	1 source (engine)	2 sources (engine + gen)
RPM	9000–12000	22000 (more rigidity)
Start-up	Longer	Faster
Complexity	Higher	Simpler

Key fact

Both types used for redundancy. Vacuum gyro uses pendulous unit for automatic erection (air-flow reaction).

Erection methods

Key fact

Erection (AI): maintains rotor axis vertical — jet method, pendulous vane, or torque motor with mercury switches. Levelling (DI): maintains rotor axis horizontal. Caging knob locks gimbals at 90° for initial erection.

Key fact

Fast erection: 60–120°/min (max 30–60 s due to heat). Normal erection: 2–40°/min. Normal AI gimbal limits (old): ±110° roll, ±60° pitch. New designs: 360° roll, ±85° pitch.

Attitude indicator errors

Key fact

Acceleration error (anticlockwise gyro): pitch up and roll right (~2–3°). Deceleration: pitch down, roll left. Caused by lag in lateral vanes and weighted base inertia.

Key fact

Turning error: centripetal force on vanes/weighted base causes pitch and roll errors — magnitude varies with speed, rate and duration of turn. Steep turns: mercury pools at ends — may prevent erection; secondary contacts for fast erect.

Exam trap

Electrical AI — fewer errors (no heavy base, high rotor speed, cut-out switches). Low bank turns with torque-motor AI should be avoided — overcorrection.

Turn and balance indicator

Key fact

Turn indicator: horizontally tied rate gyro, one gimball, denied yaw freedom. Accurate yaw rate at low bank angles. Rate one turn = 180°/min = 30°/s.

Formula — Rate-one bank angle

Bank (°) = TAS ÷ 10 + 7

Also: 15% of IAS. Turn indicator calibrated to fixed IAS — different TAS causes under-read.

Key fact

Turn coordinator: gimball tilted 30–40° upward — sensitive to yaw and roll. Balance (slip/skid): step on the ball — slip: ball inside turn; skid: ball outside turn.

Directional gyro wander

Formula — Total drift

Total drift = RD ± ER ± LN ± TD

Earth rate (°/hr) = ±15.04 × sin(lat)     [subtract NH, add SH]
Latitude nut (°/hr) = ±15.04 × sin(lat)   [opposite sign to ER]
Transport drift (°/hr) = ±GS ÷ 60 × tan(lat)

Key fact

Real wander (drift/topple): friction, imperfections, turbulence, hard manoeuvres, latitude nut (intentional). Apparent wander: earth rotation, transport wander. Horizontal wander = drift · vertical = topple. Max earth drift at poles, zero at equator.

Exam trap

Longitudinal acceleration does not affect the DG — travelling with meridians. Gimbal error: least disturbance when heading aligned with gimbals (90° to aircraft heading).

AHRS

Key fact

Attitude and Heading Reference System — lower weight, size, cost; higher accuracy. MEMS, piezo-electric gyros; three gyros; provides horizontal and vertical magnetic field components.