Gyroscopic Instruments Questions and Answers

Answer: A

The angular momentum vector stays fixed unless a torque acts, which is what lets a gyro provide an attitude or heading reference. Precession is the separate property describing what happens when a torque is in fact applied.

Answer: B

The rule is to move the point of applied force 90 degrees around the rotor in the direction of rotation and apply it there. It is the mechanism erection systems use deliberately and the mechanism bearing friction exploits as an unwanted drift.

Answer: C

The relation \( \Omega = T/(I\omega) \) shows why a fast heavy rotor is a stable reference: the same disturbing torque produces a far smaller drift. It also explains why a gyro running down becomes progressively less trustworthy.

Answer: D

Correct answer: Bearing friction and mechanical imperfections

Real drift is an actual movement of the spin axis, caused by unwanted torques - friction in the gimbal bearings, rotor imbalance, and manufacturing imperfections that apply a small couple.

Apparent drift is quite different: the axis stays fixed in space while the observer's frame rotates beneath it, from the earth's rotation and from the aircraft's movement over the surface. Real drift can be reduced by better bearings; apparent drift is a matter of geometry and must be compensated instead.

Answer: A

The drift is \( 15\sin(\mathrm{latitude}) \) degrees per hour, and the sine reaches one only at the pole. There the earth's axis coincides with the local vertical, so a full rotation of the reference frame appears in azimuth over a sidereal day.

Answer: B

Flying east or west changes the local meridian, and flying north or south changes the local vertical, so a space-fixed axis appears to move relative to the aircraft. Like earth rate it is an apparent effect rather than a real wander of the axis.

Answer: C

A vertically mounted rotor with two gimbals gives a stable horizontal reference against which pitch and roll are read. An erection system continually corrects it towards the true vertical, since otherwise drift would accumulate without limit.

Answer: D

Pendulous vanes or an equivalent sensor detect departure from vertical and apply a torque, which precesses the axis back. Because the sensor cannot distinguish gravity from acceleration, the same mechanism is the source of the instrument's turn and acceleration errors.

Answer: A

The pendulous erection sensor is displaced rearward by the acceleration and reads it as a tilt, so the erection system precesses the gyro accordingly. The error is transient and disappears once the acceleration ceases.

Answer: B

A gyro holds a direction but does not find one, so its indication slowly departs from magnetic heading. The compass by contrast is north-seeking but unusable in turns and accelerations, so each instrument covers the other's weakness.