Gyro Principles

Each of the module's classic instruments contains the same heart: a rotor spun at high speed, mounted so that its axis can hold an attitude the aircraft cannot disturb. This lesson builds the vocabulary the instrument lessons rely on: what a gyro is, the two properties spinning gives it, the freedoms its mounting allows, the three ways its reference decays, and the two ways the rotor is driven.

What a gyro is, and why spinning matters

A gyro is a rotor, a wheel with its mass concentrated in the rim, spinning at high speed about its spin axis, mounted so the axis can be used as a reference. The physics underneath is angular momentum: a spinning mass stores momentum along its axis in proportion to its mass, how far that mass sits from the axis, and how fast it turns — which is why gyro rotors are rim-heavy and fast. A large stored momentum resists change, and everything a gyro does for an instrument follows from that resistance, expressed as two properties.

Rigidity — the spin axis holds its direction in space unless a force acts on it: the faster and rim-heavier the rotor, the more stubbornly the axis stays put. Rigidity is the property the attitude and direction instruments use: the gyro remembers a reference while the aircraft moves around it.
Precession — when a force IS applied, the axis does not move away from the push: it moves as if the force had been applied ninety degrees later in the direction of rotation. Precession is what the rate instruments use deliberately, and what every disturbing force produces whether wanted or not.

Wander: how the reference decays

A gyro's reference is only as good as its stillness, and the syllabus names the ways it moves. Wander is any movement of the spin axis away from its set direction:

movement in the horizontal plane is drift.
movement in the vertical plane is topple.

Three causes produce it:

Real wander — the axis genuinely moves, pushed by the gyro's own imperfections: bearing friction, imbalance, wear. Reduced by manufacture, never to zero.
Apparent wander — the axis holds perfectly still in space while the Earth rotates underneath it: to an observer turning with the Earth, the axis appears to drift. Its drift component depends on latitude, greatest at the poles and zero at the equator; the vertical (topple) component behaves the opposite way, largest over the equator.
Transport wander — the aircraft carries the gyro across the Earth's curved surface: local vertical and local north change along the way, and an axis that held the old ones appears to wander against the new. The faster and further the flight, the larger it grows.

Driving the rotor

Two power sources spin the rotors, each with its own tell-tale:

Air-driven (vacuum) gyros — a pump (or, on the simplest aircraft, a venturi: a constricted tube in the slipstream whose accelerated flow provides suction) draws air through the instrument, and the stream hits buckets cut into the rotor's rim, spinning it like a water wheel. The associated indication is the suction gauge: too little suction means a slow rotor, and a slow rotor means a lazy, toppling gyro long before the instrument dies outright.
Electrically driven gyros — the rotor is the rotating part of an electric motor, immune to altitude (thin air weakens the air-driven rotor's stream) and running at tighter speed control. Their indication is a power-failure flag on the instrument face: power gone, flag in view, and the display is not to be trusted while the rotor coasts down, still gyroscopic but slowly surrendering its rigidity.

What a gyro is, and why spinning matters

Check you understood

Degrees of freedom

Wander: how the reference decays

Driving the rotor

What the exam asks