The Turn and Slip Indicator — Page 192, Lesson 226

I want to walk you through the turn and slip indicator, and we're going to pick it up right at the calibration details, then move into construction and the errors that affect it. First, let's talk about the rate-of-turn marks. You'll recall the instrument has marks for rate 1 and rate 2 turns. Rate 1 is 3° per second, and the second mark, for rate 2, corresponds to 6° per second. There may be further graduations for higher rates of turn beyond that. Now, the calibration of the correct rate of turn — that's the spring tension — is optimized for a design TAS, a design true airspeed. Here's the key point: even for quite large departures from that design TAS, only a small amount of error is introduced. In practice, the errors produced by TAS deviations are not serious. One manufacturer quotes a maximum error of 5% over a speed range of 85 to 350 knots, with the calibration value being 260 knots. So the instrument is calibrated at 260 knots true airspeed, and across that whole 85-to-350-knot range, the error stays within 5%. Now let's look at the constructional details. There are suction-driven and electrically-driven types available. With the suction-driven type, an engine-driven pump or a venturi tube applies suction to the case. Replacement air enters via a filter and is directed by a jet at the 'buckets' cut in the periphery of the rotor. Those buckets are little vanes or scoops cut into the edge of the rotor wheel that the air jet strikes to spin it. Here's a really important design point: the rotor rpm are low compared with those of the DGI — the directional gyro indicator — and the artificial horizon. Why? Because this instrument uses the gyroscopic property of precession to measure rate of turn, so a high gyroscopic rigidity is undesirable. You want the gyro to be relatively flexible, not rigid, so that precession can do its measuring job. A damping system is fitted to the gimbal to reduce oscillation. This may be the piston-in-cylinder type or an electromagnetic device. Stops limit the movement of the gimbal to a tilt corresponding to a turn of about 20° per second. And there's a note worth remembering: because there is only one gimbal, the gyro will not 'topple' when it comes against the stops. With a single gimbal, there's no second axis to cause toppling. Now, the effect of varying rotor speed. If the suction is inadequate — and that can happen at high altitude, or with a choked filter, or with a leaking suction tube — with an air-driven instrument, gyro rigidity will be lowered as the gyro is "underspeeding." Consequently, the secondary precession needed to equal the aircraft turn can be generated by a smaller secondary torque. That reduced torque will be produced by a smaller angle of gimbal tilt, and this means the instrument will under-read the turn rate. So underspeed gives you an under-read. Alternatively, if the gyro were to "overspeed," by the same token it will over-read the rate of turn that is being achieved by the angle of bank applied. So overspeed gives you an over-read. Finally, let's look at errors in the looping plane. In a gently banked turn, the aircraft is turning mainly in the yawing plane. But in a steep turn, there is more movement in the looping plane. Normally, movement in the looping plane means the aircraft is rotating about the rotor axis, with no effect on the gyro. However, if the gimbal is tilted before movement in the looping plane commences — as happens with a yaw — then the movement in the looping plane will cause additional precession of the rotor. The usual positive movement in the looping plane in a steep turn will increase the gimbal tilt, causing the indicator to over-read, sometimes coming against the stops. So to tie it together: the instrument reads rate of turn via precession, it's calibrated at 260 knots, it under-reads when underspeeding and over-reads when overspeeding, and in steep turns, looping-plane movement can push it to over-read, even to the stops.