Directional Gyro Indicator (DGI) — Page 153, Lesson 186

I want to walk you through the Directional Gyro Indicator — the DGI — and specifically how it erects itself. This is the heart of how the instrument keeps its rotor axis locked in the yawing plane, so let's build it up carefully. We're starting mid-explanation, with a force that's acting on the gyro rotor. That force gets split into two components. Component 'Y' lies in the plane of rotation — it just maintains the spin of the rotor, keeping it turning. Component 'Z' acts at 90° to the plane of rotation. Now here's the key gyroscopic behaviour: because this is a gyro, component 'Z' will precess the rotor as if the force had been applied at a point 90° around the circumference of the rotor, in the direction of its spin. That's the fundamental rule of gyroscopic precession — the applied force acts 90° ahead in the direction of rotation. The result of that precession is as though a force 'Q' is operating to re-erect the rotor, pulling its axis back into the yawing plane. Now, there's an important special case here. If the heading is such that the rotor axis is aligned with the longitudinal axis of the aircraft — that is, the aircraft's fore-and-aft axis — then applying bank alone, with no turn, will not displace the rotor axis from the yawing plane. Bank without turn, no displacement. I'll come back to that in the limitations paragraph, but keep it in mind. Now, there's a second control system, usually combined with the first, and this one is mechanical and elegant. The jet of air spins the rotor, and then the air flows round the outside of the rotor, inside a metal case. The air leaving the case is directed at a wedge plate, which is fixed to the outer gimbal. When the gyro is correctly erected, this exhaust jet is divided by the wedge plate into two equal streams, producing equal reactions on the outer gimbal — call them R1 and R2. Equal streams, equal reactions, balanced. But as soon as the rotor axis is displaced from the yawing plane, the streams become unbalanced, and so the reactions on the outer gimbal at the wedge plate become unequal. The resultant of these unequal reactions applies a torque to the outer gimbal about the vertical axis of the gyro. That torque is instantaneously transmitted by the outer gimbal to the inner gimbal, and it's represented by force 'F'. And that force 'F' makes the rotor and inner gimbal precess — which is exactly what re-erects the rotor back into the yawing plane. So the whole loop is: displacement of the rotor axis unbalances the air streams, the wedge plate converts that into a torque on the outer gimbal, the torque passes to the inner gimbal as force 'F', and precession corrects the displacement. That's the self-erecting mechanism of the DGI. Let me make sure the gimbal picture is clear, because it's easy to get lost. The rotor spins inside the metal case. The case is mounted in the inner gimbal, and the inner gimbal is mounted in the outer gimbal. The wedge plate is fixed to the outer gimbal, so it's the outer gimbal that feels the air reaction. When the rotor axis tilts out of the yawing plane, the exhaust stream hits the wedge plate unevenly, and that imbalance is what drives the correction. One thing I want to stress: the torque about the vertical axis is transmitted instantaneously from the outer gimbal to the inner gimbal. There's no lag in that transfer — it's a rigid mechanical connection. The precession that follows is the gyro's response to force 'F'. And remember that special case from earlier — rotor axis aligned with the longitudinal axis, bank without turn, no displacement. That's the limitation I flagged. When the aircraft banks without turning, and the rotor axis is aligned fore-and-aft, the bank alone won't push the rotor out of the yawing plane. That's a limitation of the instrument, and it's why the DGI has to be handled with care in certain manoeuvres. So to tie it together: the DGI uses two combined control systems to keep the rotor axis in the yawing plane. The first, through components 'Y' and 'Z' and force 'Q', handles the precession-based re-erection. The second, through the air jet, the wedge plate, the unequal reactions R1 and R2, and force 'F', provides the mechanical correction. Both work together to keep your heading reference stable.