The Earth itself acts as a gyro — Page 139, Lesson 164

Let’s start with the very basics of what a gyroscope actually is, because everything in this chapter builds on that. The simplest form of a gyroscope—we call it a gyro for short—is a rapidly spinning disc, and that disc is called the rotor. Now, here’s a key point: any rapidly spinning symmetrical rotor exhibits gyroscopic properties, even if it was never designed to be a gyro. The Earth itself acts as a gyro. Spinning tops act as gyros. Bicycle wheels act as gyros. So this is a fundamental property of spinning mass, not something we invented. For aircraft, most gyros are discs between 2 and 5 centimetres in diameter, and they spin at speeds between 4000 and 55,000 rpm—revolutions per minute—depending on their design. So you can see the range is huge: some spin relatively slowly, some spin extremely fast. Now, the shaft about which the rotor spins is called the axis. And this is an important definition: gyros are defined in their orientation as either horizontal or vertical by reference to the spin axis, not by the rotor itself. So when we say a gyro is horizontal or vertical, we’re talking about the direction of that spin axis, not the orientation of the disc. Now, why do we care? Because gyros have two basic properties that make them the foundation of aircraft attitude and direction instruments. Those two properties are rigidity and precession. Let’s take them one at a time. Rigidity is the gyro’s property of maintaining its axis in a fixed direction in space, unless it is subjected to an external force. The cause of rigidity is the inertia of the spinning mass. Inertia is the tendency of a mass to keep doing what it’s already doing—in this case, to keep the spin axis pointing the same way in space. But here’s the catch: for a gyro to actually maintain that fixed direction, it has to be attached to the airframe in such a way that the aircraft can manoeuvre freely without disturbing the gyro’s orientation. If you bolted the gyro rigidly to the aircraft, every turn of the aircraft would force the gyro to move with it, and you’d lose the fixed reference. So we use suspension devices that allow this freedom, and those devices are called gimbals. A gyro may have one or two gimbals. Let me explain what that means in terms of freedom. With one gimbal, the gyro has one degree of freedom. That means the gyro can rotate about one axis relative to the airframe, but it’s constrained in the other directions. So it’s free to move in one plane only. With two gimbals, you get two degrees of freedom—the gyro can rotate about two axes, giving it much more freedom to maintain its orientation while the aircraft manoeuvres around it. So the picture you should hold in your mind: the spinning rotor gives rigidity, the gimbals give the rotor the freedom to keep pointing in a fixed direction in space while the aircraft moves around it. That combination is what makes the gyro useful as a reference for attitude and direction. That’s the foundation. Next we’ll look at the second property, precession, and then how rigidity and precession relate to each other.