The Turn and Slip Indicator — Page 192, Lesson 226

Let’s pick up right where we left off. We’ve already covered the basic idea of the Turn and Slip Indicator — how it measures rate of turn using a gyroscope. Now I want to walk you through the calibration and the constructional details, because that’s where the real engineering lives. First, the calibration. The instrument has marks for different rates of turn. We already know rate 1 is 3° per second. Now, a second mark for rate 2 corresponds to 6° per second. And there may be further graduations for higher rates of turn beyond that. So you’ve got a scale that lets you read off how fast you’re turning. Now, here’s a key point about how that scale is set up. The calibration of the correct rate of turn — that is, the spring tension — is optimized for a design TAS, a design true airspeed. The spring tension is what resists the gyro’s precession, and it’s tuned so that the instrument reads correctly at one specific speed. But here’s the good news: only a small amount of error is introduced, even for quite large departures from that design TAS. 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 even if you’re flying well away from 260 knots, you’re still within 5% — that’s the practical tolerance you can expect. Now let’s get into the constructional details. There are two types available: suction-driven and electrically-driven. With the suction-driven type, an engine-driven pump or a venturi tube is used to apply 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. So the air jet hits those buckets and spins the rotor. The rotor rpm are low compared with those of the DGI and the artificial horizon. Why? Because the gyroscopic property of precession is used to measure rate of turn, so a high gyroscopic rigidity is undesirable. You don’t want a stiff, rigid gyro here — you want it to precess easily so it can respond to the turn. There’s also a damping system fitted to the gimbal that reduces oscillation. This may be the piston-in-cylinder type or an electromagnetic device. And there are stops that limit the movement of the gimbal to a tilt corresponding to a turn of about 20° per second. So the gimbal can only tilt so far. And here’s a nice note: because there is only one gimbal, the gyro will not ‘topple’ when it comes against the stops. That’s a real advantage of this single-gimbal design. Now, the effect of varying rotor speed. This is important for understanding errors. If the suction is inadequate — say at high altitude, or with a choked filter, or a leaking suction tube — with an air-driven instrument, gyro rigidity will be lowered as the gyro is “underspeeding”. So the rotor spins slower, and the gyro is less rigid. 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 you’re turning more than the instrument shows. Conversely, 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 too fast a rotor, and the instrument reads high. 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. But here’s the catch: 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 in a steep turn, you can get a false high reading, and the needle can even hit the stop. So to tie it all together: the instrument is calibrated for a design TAS with a small error band, it uses a low-rigidity gyro with a damping system and tilt stops, and its accuracy depends on rotor speed — underspeed under-reads, overspeed over-reads — and steep turns can introduce looping-plane errors that make it over-read. That’s the full picture of how this instrument behaves and where its limitations lie.