The Turn and Slip Indicator — Page 184, Lesson 224

Let’s start with the instrument itself. The Turn and Slip Indicator is actually two separate measuring devices sharing one instrument face. One is the rate of turn indicator — commonly shortened to just the ‘turn’ indicator — and it uses a rate gyro to measure the rate of turn about the vertical axis. The other is the slip indicator, a very simple pendulous device, and its job is mainly to show whether a turn is balanced — that is, whether the angle of bank is correct for the true airspeed and the rate of turn — and if it isn’t balanced, to show the extent of slip or skid. Now let’s dig into the rate gyro itself, because that’s the heart of the turn indicator. The turn indicator is based on a horizontal-axis rate gyro, and the key feature is that it has only one gimbal, which means it has only one degree of freedom. Let me unpack that. A gimbal is the ring that supports the gyro rotor, and it allows the rotor to tilt about one axis. With only one gimbal, the gyro can only move about one axis — that’s its single degree of freedom. Here’s the behaviour. If the aircraft banks without turning, the gyro axis has the freedom to remain horizontal — the gimbal lets it stay level. But if the aircraft yaws, the frame — which is fixed to the airframe — applies a force to the gyro in a direction where the gyro is not gimballed, meaning it has no freedom to move in that direction. That force is labelled the primary torque. Because the gyro can’t move in that direction, this results in precession, which will cause the gyro to depart from the horizontal. A spring system prevents the gyro from turning all the way to the vertical, and the amount of spring stretch is a measure of the rate of turn. Let me walk you through the operation step by step, because this is the classic rate gyro behaviour. When the aircraft turns, the rotor is subjected to a primary torque acting about the ZZ axis. That primary torque produces a primary precession about the YY axis, and the direction of this precession is as if the applied force were moved 90° in the direction of rotor spin. That’s the fundamental gyroscopic rule — precession acts 90° from the applied force, in the direction of spin. As the rotor tilts about the YY axis, it causes a spring between the gimbal and the frame to be extended. The resultant spring tension then subjects the rotor to a secondary torque acting about the YY axis. Now here’s the clever part. This secondary torque, combined with the precession, will continue until the gimbal has tilted just the right amount to give the spring tension required to generate a rate of secondary precession equal to the rate of turn of the aircraft. That gives equilibrium. So the system settles when the secondary precession rate matches the aircraft’s rate of turn. And I want to emphasise — the chain of events is virtually instantaneous. As the aircraft goes into a turn, the gimbal takes up the appropriate angle of tilt almost immediately. If the rate of turn changes, the tilt of the gimbal will also change, to re-establish the balance of torques on the gyro. So the angle of tilt is a measure of the rate of turn. A pointer, fixed to or linked with the gimbal, indicates the tilt on a scale on the face of the instrument. And that scale is calibrated to indicate rates of turn either side of the centre zero, so that the first graduation corresponds to a Rate 1 turn — that’s the aircraft turning 3° per second. So to tie it all together: the turn indicator measures rate of turn about the vertical axis using a single-gimbal rate gyro, the spring provides the restoring force that makes the tilt proportional to the rate of turn, and the scale is set so the first mark is a Rate 1 turn at 3° per second. The slip indicator alongside it tells you whether that turn is balanced — whether the bank angle matches the true airspeed and rate of turn — and if not, whether you’re slipping or skidding.