Definition
Gyroscopes mounted so the spin axis is held in a fixed position relative to the aircraft, with limited freedom of movement in only one or two axes. Because the gyro is partly tied to the airframe, it senses motion of the aircraft about the axis (or axes) in which it is restricted, rather than maintaining a free reference in space. Turn indicators and turn coordinators use this type of mounting.
Plain English
A spinning wheel that is partly locked to the aircraft instead of being free to point any direction. Because it can only move a little, when the aircraft turns or tilts, the gyro feels that movement and the instrument shows it.
Context Anchor
Seen in discussions of gyroscopic flight instruments, especially when explaining how an instrument’s gyro is mounted inside the case.
Derivation
Semi means partly or half, from Latin. Rigid means stiff or fixed. So semi-rigidly mounted simply means partly fixed in place — free to move in some directions but held in others.
Why Pilots Care
This mounting lets the instrument use precession to show rate of turn or attitude change while remaining simple and rugged enough for cockpit use.
Grounding Statement
Picture a spinning wheel inside an instrument case: it is supported so it can move in the needed direction, but its support stops it from moving freely every way.
Intuition Check
Semi-rigidly mounted does not mean the gyroscope is loose or poorly attached. It means the mounting is deliberately limited: partly free, partly restrained.
Example Sentence 1
The turn coordinator uses a semi-rigidly mounted gyroscope, so it senses both roll and yaw as the aircraft enters a turn.
Example Sentence 2
Early attitude indicators relied on semi-rigid mounting to keep the mechanism compact while still sensing pitch and roll through controlled precession.