Definition
Gyroscopes mounted so they are free to rotate, or tilt, in only one plane rather than freely in any direction. The mounting deliberately limits the gyro's freedom of movement to a single axis, which causes the gyro to react to changes around that axis through precession. This type of mounting is used in turn indicators and turn coordinators, where the instrument must sense and display rotation about a specific aircraft axis.
Plain English
A spinning wheel held in a frame that only lets it tilt or pivot one way, instead of swinging freely in every direction. By limiting how it can move, the instrument can measure motion around just that one axis.
Context Anchor
Seen when studying gyroscopic flight instruments, especially turn indicators and turn coordinators.
Derivation
Restricted means held back or limited. Semi-rigid means partly stiff -- not fully locked in place, but not fully free either. The phrase tells you the gyro is partly free to move, but only in a controlled way.
Why Pilots Care
These mounts make the gyro show how fast the airplane is yawing rather than its exact nose attitude, which is essential for coordinated turns in instrument flight.
Analogy
Think of a spinning bicycle wheel held in a bracket. If the bracket lets it move only in certain ways, the wheel’s reaction becomes controlled and useful instead of completely free.
Grounding Statement
Picture a fast-spinning wheel inside an instrument case, held so it can respond to a turn but cannot flop freely in every direction.
Intuition Check
Restricted does not mean the gyro is damaged or jammed. Here it means the gyro’s movement is intentionally limited so the instrument can measure a specific kind of aircraft motion.
Example Sentence 1
The turn coordinator uses a restricted, or semi-rigidly mounted, gyroscope so that it responds to roll and yaw but not to pitch.
Example Sentence 2
Because it is semi-rigidly mounted, the gyro in the turn and slip indicator does not precess in response to pitch or roll.