Systems

A gyrocompass uses all the properties of gyroscopes to create an inertial compass (as opposed to a magnetic compass).

An ordinary gyroscope, with a single spinning wheel, is only stable in one axis. By mounting three wheel assemblies in a gimballed frame, one can create a platform that is gyroscopically stable in roll, pitch and yaw (see illustration). By adding scales and pointers, it could be used to measure changes in any direction.

It’s easy to imagine how this assembly could be the basis of a three-dimensional stabiliser, able to hold an aircraft's nose and wings level by taking measurements from the gimbal bearings. However, it would only be able to measure relative movement from when its gyros started spinning.

Navigation requires an absolute reference, relative to Earth. To do that, gyrocompasses make use of precession, and the fact that the Earth is a great mass, spinning around its own axis. If the

spin-axis of a gyroscope is held so it’s always horizontal, relative to the Earth’s surface, it will gradually precess until it points to true North, then stop. Various methods have been used to tune the effect, including carefully-adjusted friction bearings, viscous fluids, tension springs and weights. Weights require the least maintenance or adjustment.

The advantage of gyrocompasses is that they point to true North, they react quickly, and they’re not affected by changes in the Earth’s magnetic field, magnetic objects, or power cables. They don’t even need to be made from metal.

The disadvantage is that they need enough rotational inertia to overcome gimbal friction, which means flywheels have to have enough mass and motors to drive them to very high r.p.m. They also need time to erect, which means to spin-up and face North. Light weight, portable compasses are still magnetic.