Gyroscopic Instruments Questions and Answers

Answer: A

Centrifugal force in the turn displaces the pendulous sensor, so the erection system precesses the gyro slightly away from true vertical. The residual error decays as the erection system brings the axis back once the turn ends.

Answer: B

Correct answer: Transport wander partly offsets earth rate on a westerly heading

Both effects carry the sine of the latitude, but their signs depend on direction. Earth rate always acts in one sense in the northern hemisphere; transport wander acts with it flying east and against it flying west.

Flying west therefore subtracts, and the net drift is \( (15-5)\sin 55^\circ \) rather than the sum. Earth rate is certainly not zero at 55 degrees, where the sine is 0.82, and transport wander is greatest on east-west tracks rather than absent.

Answer: C

At very low rates the counter-rotating beams couple and their frequencies lock together, so small rotations produce no beat. Mechanically dithering the block keeps the instantaneous rate outside that dead band while averaging to zero.

Answer: D

Correct answer: rigidity in space

Rigidity in space is the tendency of a spinning rotor to maintain its axis fixed relative to inertial space unless an external torque acts.

It is what makes a gyroscope useful as a reference, and it follows from the conservation of angular momentum. Precession is the response to a torque rather than the resistance to one; erection is the system that holds a gyro to the local vertical; and nutation is a small oscillation superimposed on precession.

Answer: A

Correct answer: 90 degrees

Precession appears 90 degrees from the point of application, measured in the direction of rotor rotation. Push the rim at one point and the gyro responds as though pushed a quarter turn later.

The rule follows from the torque being a vector perpendicular to both the force and the spin axis. It is what makes gyroscopic behaviour counter-intuitive, and it is exactly what an instrument designer exploits: applying a torque at the right place produces the wanted movement about a different axis.

Answer: B

Correct answer: sine

Earth rate apparent drift is \( 15\sin\phi \) degrees per hour, so it is zero at the equator and 15 at the poles.

Physically, at the equator the local vertical is perpendicular to the earth's axis and the rotation produces no apparent movement in azimuth; at the pole the vertical is parallel to it and the full rate appears. The same sine of latitude governs transport wander, which is why the two combine simply on any given track.

Answer: C

That rate completes a full circle in two minutes and a half circle in one, which is why instrument procedures are built around it. Half standard rate is used at higher speeds where the required bank would otherwise be excessive.

Answer: D

Correct answer: lock-in

At very low rotation rates the two counter-propagating beams in a ring laser gyro couple through backscatter and lock to a common frequency, so the instrument reads zero across a small dead band.

Dithering - oscillating the whole assembly rapidly through a small angle - keeps the rate outside that band almost all the time, and the known dither motion is subtracted from the output. It is the one moving part in an otherwise solid state device.

Answer: A

Correct answer: At high latitude, where apparent drift is greatest

Read the drift column: it rises with the sine of latitude from zero at the equator to 15 degrees per hour at the pole.

A directional gyro therefore needs realignment far more often at high latitude, and the error accumulates fastest exactly where magnetic compass reference is also least reliable. That combination is why polar navigation traditionally used grid heading and, later, inertial systems that compute the drift instead of suffering it.

Answer: B

Correct answer: So a single failure cannot remove every attitude reference

Read the third column. A vacuum failure takes the attitude indicator and the directional gyro together, but the turn coordinator is electrical and survives it, and the pressure instruments need no power at all.

The pilot therefore retains a rate-of-turn reference and can fly partial panel. Splitting the drive is a deliberate redundancy measure rather than an economy, and electrical gyros show attitude perfectly well - larger aircraft use them throughout.