A theoretical analysis of the stability of an aeroplane on northerly headings when controlled by an aileron-steering autopilot monitored from a gyro-magnetic compass

Abstract

Long-period lateral oscillations of an aeroplane under automatic control on Northerly headings are shown to be due to high speed in a region of large magnetic-dip angle accompanied by strong monitoring of the gyromagnetic compass and autopilot. Aerodynamic characteristics are not important, and speed is the predominant aircraft parameter. An approximate analysis assuming that the motion consists of co-ordinated turns is found reliable for many purposes. The requirement of weak monitoring for ensuring stability conflicts with the need for strong monitoring to restrict datum errors caused by random precession torques on the azimuth gyro and by drifts in the autopilot. Modifications to the compass precession signal may stabilize the motion, but usually at the expense of a deterioration in datum errors, or with insufficient reduction in oscillation amplitude on account of thresholds of rate gyros. Of the five modifications studied two seem marginally acceptable, but the best way of preventing Northerly-heading oscillations is to employ a good azimuth gyro so that a weak monitor is feasible, or alternatively to monitor the gyro from a sensor that is little affected by aircraft banked turns. The latter requires very good stabilization of the sensor if it is a magnetic detector.