NATURAL ERRORS I
spin-axle which displaces the settling point from the normal equilibrium position. The maximum deflection of the spin-axle from the equilibrium position due to this cause is called the maximum ballistic damping error. In the case of any gyro-compass, the damping of the vibration of the gyro-axle produces an error after the ship has completed a change in course or speed. This ballistic damping error is most marked after the ship, steaming at full speed, has completed a turn of 90 degrees or more. It attains a maximum value in about 20 minutes after the velocity of the ship has become constant. The magnitude of the damping error increases with increase in the acceleration of the ship. Much greater accelerations are produced by sudden turns than by any possible change in the speed of the ship. Since this error is small while the ship is on a straight course but may be large when the ship is making a turn, it is also called the ballistic turning error. It is also called the damping acceleration error.
The ballistic damping error and the accompanying oscillation of the spin-axle can be prevented by stopping the operation of the damping device during the turning of the ship.
115. The Compass Error Due to Rolling of a Ship When on an Intercardinal Course. The Quadrantal or Rolling Error. - A pendulous gyro-compass on a ship that is rolling or pitching is acted upon by a force that has a maximum value at the end of a roll or pitch, and another force that has a maximum value when the pendulous system is passing through its equilibrium position. The first of these forces will be considered in the present Article. The second will be considered in the following Article.
A ship's compass is supported in a Cardan gimbal mounting consisting of two horizontal rings, one capable of rotation about an axis parallel to the keel of the ship and the other about a transverse axis. In so far as freedom to turn in any direction is concerned, a gyro-compass on board ship is equivalent to a gyro-wheel mounted in five rings. For simplicity of representation, in the present Article, the pendulous gyro-compass will be represented by a gyro-wheel in a casing free to turn about any axis through its center and carrying an additional mass attached to the lower side of the casing. This mass that causes the gyro and casing to be pendulous we shall call " the pendulous mass."
Consider the effect on the direction of a gyro-compass produced by rolling or pitching of a ship about an axis that is perpendicular to the spin-axle. In Fig. 147, the keel of the ship is east and