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

Let’s pick this up right where the rotor’s spin is being maintained. We’ve got the rotor spinning, and now we’re resolving the forces acting on it. There’s a force that can be split into two components: component ‘Y’, which lies in the plane of rotation and keeps the rotor spinning, and component ‘Z’, which acts at 90° to that plane of rotation. That’s the key geometry here — one component sustains the spin, the other pushes perpendicular to it. Now, because this is a gyro, that component ‘Z’ doesn’t just push the rotor sideways. It causes precession. The rotor behaves as if the force had been applied at a point 90° around the circumference of the rotor, in the direction of its spin. So the effect is as though a force ‘Q’ is operating to re-erect the rotor, bringing its axis back into the yawing plane. Here’s the important limitation: if the heading is such that the rotor axis is aligned with the longitudinal axis of the aircraft, then applying bank alone — with no turn — will not displace the rotor axis from the yawing plane. That’s a crucial point, and I’ll come back to it when we discuss limitations. Now, there’s a second control system, usually combined with the first. Here’s how it works: the jet of air spins the rotor, then flows around 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 — those are labelled R1 and R2. But as soon as the rotor axis is displaced from the yawing plane, the streams become unbalanced. The reactions on the outer gimbal at the wedge plate become unequal too. 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 from the outer gimbal to the inner gimbal, and it’s represented by force ‘F’. That force makes the rotor and inner gimbal precess. So the whole loop is: displacement of the rotor axis → unbalanced air streams at the wedge plate → unequal reactions → torque on the outer gimbal about the vertical axis → transmitted to the inner gimbal → precession that re-erects the rotor. That’s the self-erecting mechanism of the directional gyro indicator.