Gyroscopes — Page 148, Lesson 179

Let’s pick up right where the gyro story gets practical. We’ve already seen how a gyroscope holds its axis rigid in space, and how the gimbals give it freedom to move. Now I want to walk you through what happens when that freedom runs out, and why we bother with all this in the first place. First, a quick note on power. On non-electronic training aircraft, the main gyro instruments are usually electric powered for greater accuracy, while the standby instruments are often air driven so they’re still available after a power failure. Both types can fail, though. The electric type will have some kind of ‘flag’ indicator to show the pilot if power is lost, so he can select standby power or switch to the standby instrument. Pneumatically driven gyroscopes usually have an air pressure indicator — often called the ‘suction’ gauge — to show failure of the vacuum pump. Some air driven gyroscopes have an alternate power source that is manually selected. Now, the big one: gimbal lock. This is the ultimate case of gimbal interaction. It occurs if the aircraft continues its bank to 90°. In that case, the inner and the outer gimbal take up the same orientation. The gimbals still give the freedom to continue any roll, but one degree of freedom has been lost. Here’s the danger: if the pilot were now to pull back on the control column — that is, rotate in the looping plane — the gyro would be forced out of its orientation. That results in precession of the gyro, probably violently, usually described as ‘toppling’. Let me make sure that’s clear. Normally the gimbals let the aircraft move around the gyro without disturbing the spinning rotor’s axis. But at 90° of bank, two of the gimbal axes line up, so the gyro can no longer absorb motion in one plane. Pulling back on the column then physically pushes the gyro axis, and instead of the gimbal absorbing it, the gyro precesses — it gets knocked out of its orientation. That’s toppling. For most unsophisticated aircraft, gimbal lock is a problem and will result in toppling — a temporary loss of the use of the gyro until it can be re-erected. For more complex aircraft, especially those with aerobatic manoeuvrability, it can be avoided, either by the use of a fourth gimbal or by a gimbal flip mechanism. Let’s look at gimbal flip. It incorporates a powerful torque motor. When the inner and outer gimbals are nearing a locked situation, the motor is triggered to rapidly flip the outer gimbal round by 180°, thereby restoring freedom. So instead of letting the gimbals line up and lock, the mechanism snaps the outer gimbal halfway around, buying back the lost degree of freedom. Now, why do we care about all this? Here’s the motivation. One of the biggest limitations of Direct Indicating Magnetic Compasses is their susceptibility to turning and acceleration errors. Using a more stable datum than a light compass needle or magnetic assembly would reduce or eliminate those errors. What’s required is a datum which has rigidity in space — the property of pointing in a specific direction, once set up — and which is stable enough not to be disturbed by the effects of aircraft manoeuvres. A gyroscope provides exactly that. So the whole point of the gyro in instrumentation is to give us that rigid, stable reference in space that a compass needle can’t. The gimbal system protects that reference during manoeuvres — until you hit gimbal lock, and then you need a fourth gimbal or a flip mechanism to keep the datum alive.