The Turn and Slip Indicator — Page 184, Lesson 224

I want to walk you through the Turn and Slip Indicator. This is one instrument face, but it actually houses two separate measuring devices. Let's take them one at a time. First, the rate of turn indicator — commonly shortened to just the "turn" indicator. This uses a rate gyro to measure the rate of turn about the vertical axis. The second device is the slip indicator, which is a very simple pendulous device. Its job is to show whether or not the turn is balanced — in other words, whether the angle of bank is correct for the true airspeed and the rate of turn. If it's not balanced, it shows the extent of slip or skid. Now let's get into the rate gyro itself, because this is the heart of the turn indicator. It's a horizontal-axis rate gyro with only one gimbal, and therefore only one degree of freedom. That's the key. Think about what happens in two different aircraft motions. If the aircraft banks without turning, the gyro axis has the freedom to remain horizontal — the gimbal lets it stay put. 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, so it has no freedom there. That force is labelled the primary torque. Because the gyro can't move in that direction, this produces precession, which causes 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 becomes the measure of the rate of turn. Let me walk you through the operation step by step, because the sequence matters. 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. The direction of this precession is as if the applied force were moved 90° in the direction of rotor spin. As the rotor tilts about the YY axis, it extends a spring between the gimbal and the frame. The resultant spring tension then subjects the rotor to a secondary torque, also acting about the YY axis. This secondary torque, combined with the precession, continues until the gimbal has tilted just the right amount — the amount that gives the spring tension required to generate a rate of secondary precession equal to the rate of turn of the aircraft. That's the equilibrium point. And I want to emphasize: this whole chain of events is virtually instantaneous. As the aircraft goes into a turn, the gimbal takes up the appropriate angle of tilt right away. Now, if the rate of turn changes, the tilt of the gimbal changes too, 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 that tilt on a scale on the face of the instrument. The scale is calibrated to indicate rates of turn on either side of a centre zero. The first graduation corresponds to a Rate 1 turn — that's the aircraft turning at 3° per second. So to tie it together: the turn indicator gives you the rate of turn via that spring-restrained rate gyro, and the slip indicator — that simple pendulous device — tells you whether the turn is balanced. Together they're the Turn and Slip Indicator.