Gyroscopes — Page 148, Lesson 177

Let’s start with a clean definition, because it anchors everything else. An earth gyro is one that is maintained vertical or horizontal with respect to local gravity. The Artificial Horizon is the classic example. Now, here’s the key relationship: earth gyros are a subset of tied gyros. So every earth gyro is a tied gyro, but not every tied gyro is an earth gyro. A tied gyro is simply one whose axis is tied, or referenced, to something — in the earth gyro’s case, that something is local gravity. Now let’s look at the types of gyro by construction. There are three. The first is the tuned rotor gyro. This is the traditional “spinning disc” type. It’s fitted to all elementary and most intermediate training aircraft. The basic DGI, the Artificial Horizon, and the Turn Meter — which we’ll cover in the next three chapters — are all tuned rotors. So when you fly a typical training aircraft, you’re almost certainly looking at tuned rotor instruments. The second type is the ring laser gyro, or RLG. These emerged in the 1980s and are now used in nearly all modern airliners. They work by comparing two light paths round a glass prism. The advantage is greater reliability and accuracy, but the cost is higher. The third type is the fibre optic gyro, or FOG. It’s an extension of the RLG principle. Fibre optic gyros have only recently become accurate enough for aircraft applications, and the Airbus A380 is the first commercial aircraft to use them. Now, there’s a second way to classify gyros — by power source. Tuned rotor gyros may be either air driven, which is suction, or electric powered. Let’s take the air driven type first. An engine-driven vacuum pump, or a carburettor venturi pressure — or a venturi tube on some light aircraft — reduces the pressure inside the instrument case. That pressure reduction means filtered replacement air is sucked in and led through a jet. That jet blows onto “buckets” cut in the periphery of the rotor, making it spin. It’s the same principle as a water wheel — the air acts on the buckets to turn the rotor. In electric gyros, the rotor is simply part of an electric motor. Now, the advantages and disadvantages. Suction gyros are independent of electric power, so they are not affected by electrical failure. That’s a real benefit. But there are drawbacks. Moisture, dust, oil, and grit in the airflow can block the filter, which gives variable rotor rpm. At high altitude, the engine manifold pressure available may be insufficient to maintain rotor speed. And any atmospheric impurities that penetrate the filter can reduce bearing life and unbalance the gimbals, which impairs accuracy. Electric gyros, on the other hand, are generally more expensive and heavier than the air driven type, and they require power supplies. But they can be faster and have more moment of inertia. The rotor rpm can be achieved more rapidly and then maintained more accurately. So, to sum up the trade-off: suction gyros survive electrical failure but suffer from contamination and altitude problems; electric gyros cost more and weigh more, but they spin up faster and hold their speed more precisely. That’s the core of the power-source comparison.