Gyroscopes — Page 148, Lesson 177

Let’s start with the big picture. A gyroscope is a spinning wheel, and its whole job is to give us a fixed reference in space. But not every gyro is used the same way. The book introduces a key classification right at the top: the earth gyro. An earth gyro is one which is maintained vertical or horizontal with respect to local gravity. So think about that — it’s not just any spinning wheel. It’s a gyro whose axis is deliberately held in line with the local vertical, or with the local horizontal, meaning it’s referenced to gravity. The Artificial Horizon is the classic example. That instrument keeps its gyro axis aligned with the true vertical, so the pilot can see the attitude of the aircraft relative to the horizon. Now here’s an important logical relationship. Earth gyros are a subset of something called tied gyros. A tied gyro is a gyro whose axis is tied, or constrained, to some reference — in this case, gravity. So the book states it precisely: all earth gyros are tied gyros, but not all tied gyros are earth gyros. In other words, every earth gyro is a tied gyro, but there are tied gyros that are tied to something other than gravity — so they wouldn’t be earth gyros. Keep that distinction clear. Now let’s move to the types of gyro by construction. There are three main families. First, the Tuned Rotor. This is the traditional ‘spinning disc’ type — the classic gyroscope you picture, a heavy disc spinning on an axis. These are fitted to all elementary and most intermediate types of training aircraft. The basic DGI — that’s the Directional Gyro Indicator — the Artificial Horizon, and the Turn Meter are all tuned rotors. You’ll cover those in the next three chapters. Second, the Ring Laser Gyro, abbreviated RLG. These emerged in the 1980s and are now used in nearly all modern airliners. The principle is clever: they work by comparing two light paths round a glass prism. Two beams of laser light travel in opposite directions around the same closed path, and when the gyro rotates, the two paths differ slightly in length — that difference is measured and gives the rotation. RLGs offer greater reliability and accuracy, but they are more expensive. Third, the Fibre Optic Gyro, abbreviated FOG. This is an extension of the RLG principle — instead of a small prism, the light travels through a long coil of optical fibre. FOGs 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. This applies to the tuned rotor type. A tuned rotor gyro may be either air driven, which we call suction, or electric powered. Let’s look at the air driven type first. The power comes from an engine-driven vacuum pump, or from carburettor venturi pressure — on some light aircraft it’s a venturi tube. What happens is this: the pump or venturi reduces the pressure inside the instrument case. That creates a suction, so filtered replacement air is sucked in. That air is led through a jet, and the jet blows onto what the book calls ‘buckets’ — small vanes cut into the periphery, the outer rim, of the rotor. The air striking those buckets makes the rotor spin. The book gives you a lovely analogy — it’s the same principle as a water wheel. Water hits the paddles, the wheel turns; air hits the buckets, the rotor spins. In the electric gyro, the rotor is simply part of an electric motor — the rotor itself is the spinning part of the motor. Now, the book gives you the advantages and disadvantages of each, and this is exam-critical, so let’s be careful. Suction gyros — their big advantage is that they are independent of electric power, so they are not affected by electrical failure. That’s a real safety benefit. But there are several drawbacks. Moisture, dust, oil and grit in the airflow can block the filter, and that gives variable rotor rpm — the rotor speed becomes inconsistent. At high altitude, the engine manifold pressure available may be insufficient to maintain rotor speed — there simply isn’t enough suction up high. And any atmospheric impurities that penetrate the filter can reduce bearing life and unbalance the gimbals, thereby impairing accuracy. So the air-driven gyro is robust against electrical failure, but vulnerable to contamination and altitude. Electric gyros — these are generally more expensive and heavier than the air driven type, and they require power supplies. But they have real advantages: they can be faster, they can have more moment of inertia — that’s the resistance to changes in rotation, which makes them more stable — and the rotor rpm can be more rapidly achieved and then maintained more accurately. So you trade the independence from electrical power for better, more precise performance. Let me just tie that together. You have two classification axes: by construction — tuned rotor, ring laser, fibre optic — and by power — suction or electric, which applies to the tuned rotor family. And remember the opening logic: an earth gyro is tied to local gravity, all earth gyros are tied gyros, but not all tied gyros are earth gyros. That’s the foundation you’ll build on in the next chapters when we look at the DGI, the Artificial Horizon, and the Turn Meter.