Dynamic requirements

The period of oscillation of a gyrocompass is determined by the requirement that the compass operate usefully in an accelerated vehicle. The pendulosity that produces the north-seeking property responds to vehicle accelerations as well as to gravity. The torques associated with north-south accelerations (east-west accelerations are not important because of the system configuration) would cause a corresponding wander of the gyrocompass about north and would render the instrument useless for navigation. Furthermore, when a vehicle travels north or south it acquires in its motion over the spherical Earth an angular velocity relative to inertial space that is perpendicular to Earth’s daily rotation. This means that the apparent meridian, relative to which stars would appear from a moving vehicle to rise and set, is rotated about a vertical axis from the true meridian, westward for northerly velocity and eastward for southerly motion. The tangent of this angle of rotation is the vehicle’s north-south speed divided by the product of Earth rate (speed of a point on the Equator, which is 1,667 km [1,036 miles] per hour, or 900 knots) and the cosine of the latitude. At ship speeds, this angle is generally less than 4°. A major contribution by Schuler was the discovery that, when the period of oscillation is 2πSquare root of√Earth radius/gravity, the heading precession of the gyroscope spin-axis due to acceleration is exactly the rate of change of the angle between the apparent and true meridians seen on a moving vehicle. The gyrocompass will then read true north at all times if its indicating reference is offset by the angle between these two meridians. The angle, at ship speeds, is a direct function of the north-south speed and is easily set into the system. The need for accurate speed measurement for this offset is the main reason why a gyrocompass is not practical for use in aircraft.

Description of parts

The Sperry unit is a good representative gyrocompass due to its extensive use at sea. The gyro wheel is electrically driven and mounted on spin-axis ball bearings within the rotor case. This case in turn is mounted to tilt on ball bearings about a horizontal and nearly east-west axis in the vertical ring. The vertical ring is pivoted about a vertical axis within the phantom ring, but its weight is borne by strands of steel wire from the phantom head. A follow-up system keeps the phantom ring aligned with the vertical ring, thus preventing torsion in the wires and reducing support friction about the vertical axis to a minimum. The compass card is mounted on the phantom head. The phantom is supported by thrust bearings in the spider (a mounting that connects various parts of the gyroscope), which also carries the follow-up motor. This whole device is mounted with a small pendulosity in the binnacle (a cylindrical pedestal that illuminates the compass face from below) within a covering case. The compass elements are thus protected and also free from the rolling of the ship. The mercury ballistic frame is pivoted to the phantom on horizontal and nearly east-west bearings. The frame connects to the rotor case by a link that makes contact slightly east of the bottom of the case. The frame carries the tanks of mercury and the connecting tubes. The master gyrocompass is usually installed in a compartment that will not be affected by the outside environment. Repeaters of its indication are mounted on the bridge and elsewhere as needed. The course recorder keeps a permanent record of the ship’s heading.