Gyroscopes — Page 148, Lesson 179

Let’s pick this up right where the gyro instruments are powered and think about what happens when things go wrong — and then the big one, gimbal lock. 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. So you’ve got two power families: electric and pneumatic. Both types can suffer from failure of their power source. The electric type will have some kind of ‘flag’ indicator to show the pilot if this happens, 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 which is manually selected. Now, the ultimate case of gimbal interaction is gimbal lock. It occurs if the aircraft continues its bank to 90°. In that case, the inner and the outer gimbal take up the same orientation, as shown in Figure 11.14. The gimbals still give the freedom to continue any roll, but one degree of freedom has been lost. 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’. For most unsophisticated aircraft, gimbal lock is a problem and will result in toppling — that is, 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. Gimbal flip 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. Now, 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 these. What is 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 this. So let me tie that together. We’ve got two power sources for gyros — electric with a flag indicator, pneumatic with a suction gauge. We’ve got gimbal lock at 90° of bank, where one degree of freedom is lost and pulling back on the column causes violent precession, or toppling. For simple aircraft, that’s a temporary loss until re-erection; for aerobatic aircraft, we avoid it with a fourth gimbal or a gimbal flip mechanism that uses a torque motor to flip the outer gimbal 180°. And finally, the whole reason we use gyros as a datum is that they have rigidity in space — they point in a specific direction once set up and aren’t disturbed by aircraft manoeuvres, which is exactly what a magnetic compass lacks.