Directional Gyro Indicator (DGI) — Page 153, Lesson 185

Let's start with the Directional Gyro Indicator — the DGI. This is the instrument that gives you a stable heading reference, and the whole trick of it is gyroscopic rigidity. First, look at the geometry. The rotor axis, the inner gimbal axis, and the outer gimbal axis are all mutually at right angles to each other. That's the fundamental arrangement — three axes, each perpendicular to the other two. Now, here's the key behaviour. During a turn, the aircraft and the instrument case rotate on the vertical axis bearings of the outer gimbal. But the gyro rotor, the gimbals, and the indicating scale all remain fixed in azimuth — that is, fixed in direction — because of gyroscopic rigidity. So the case turns with the aircraft, but the gyro itself stays pointing in the same direction in space. That's what gives you a heading reference. Heading is indicated on the scale by a lubber line. That's a fixed reference line painted on a glass window in the instrument case. As the case rotates with the aircraft, the scale stays fixed, and the lubber line shows you your heading against that fixed scale. Some designs use a circular vertical-card indicating scale that's geared to the outer gimbal, instead of the cylindrical scale fixed to the outer gimbal in the earlier type. Now let's talk about how the rotor is driven — the control system for suction gyros. In earlier DGI designs, the rotor is driven by twin jets of air applied from the outer gimbal ring. Suction is applied to the case of the instrument. Replacement air enters the case through a filter, and it's ducted to the jets on the outer gimbal. Those jets act on 'buckets' cut in the rotor — little vanes or pockets on the rotor rim. Here's the clever part. The jets don't just spin the rotor. They also serve to maintain, or tie, the rotor axis in the yawing plane of the aircraft. The yawing plane is the horizontal plane of the aircraft's turn. When the aircraft is straight and level, the rotor axis lies in the yawing plane, and it's at right angles to the outer gimbal axis. In that condition, the full force 'X' of the jets is used to drive the rotor — all the jet force goes into spinning it. But now consider what happens if the aircraft banks. Gyroscopic rigidity keeps the rotor axis fixed in space. So the rotor axis is no longer in the yawing plane — the aircraft has tilted around it, but the rotor hasn't. That means the outer gimbal axis is no longer at right angles to the rotor axis. So the jet driving force 'X' now acts at an angle to the plane of the rotor. Part of that force is still driving the rotor, but part of it is now trying to topple the gyro — to tilt it. That's the beginning of the problem we call toppling, which we'll get into. So the takeaway: the DGI gives you a rigid, fixed heading reference, and the suction-driven jets both spin the rotor and keep it tied to the yawing plane — until a bank disturbs that alignment, and the jet force starts acting at an angle.