We're starting a brand-new chapter, Chapter 11, on Gyroscopes — Page 139, Lesson 164

We're starting a brand-new chapter, Chapter 11, on Gyroscopes. This is the heart of your attitude and direction instruments, so let's build it properly from the ground up. The simplest form of a gyroscope, which we call a gyro for short, is a rapidly spinning disc. That disc has a proper name: the rotor. Now, here's a key point I want you to hold onto: any rapidly spinning symmetrical rotor exhibits gyroscopic properties, even if it was never designed as a gyro. The earth itself acts as a gyro. Spinning tops act as gyros. Bicycle wheels act as gyros. So this isn't some exotic aircraft-only device; it's a fundamental property of spinning mass. In our aircraft, most gyros are discs between 2 and 5 centimetres in diameter, and they spin at speeds between 4000 and 55,000 rpm, depending on their design. That's revolutions per minute. So we're talking about a small disc spinning incredibly fast. The shaft about which the rotor spins is called the axis. And this is a crucial definition for how we classify gyros: gyros are defined in their orientation as either horizontal or vertical by reference to the spin axis, not the rotor. So when we say a gyro is horizontal or vertical, we're talking about the direction of that spin axis in space, not the orientation of the disc itself. 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's subjected to an external force. It's caused by the inertia of the spinning mass. Think of it this way: a spinning mass resists any change to its axis of rotation. That resistance is rigidity. But here's the engineering problem. For a gyro to maintain that fixed direction, it has to be attached to the airframe in such a way that the aircraft has freedom to manoeuvre without disturbing the gyro's orientation. If you bolted the gyro rigidly to the airframe, every turn or bank of the aircraft would force the gyro's axis to move, and you'd lose that fixed reference. So we use suspension devices that allow this freedom. These are called gimbals. A gyro may have one or two gimbals. Let me show you what that looks like. Here's the basic gyro parts and orientation. And shows one gimbal giving one degree of freedom. A single gimbal allows the gyro to tilt in one plane, giving it one degree of freedom. Two gimbals, as we'll see shortly, give two degrees of freedom, allowing the gyro to maintain its orientation while the aircraft manoeuvres in both pitch and roll. So the picture you should have is this: a small, fast-spinning rotor on an axis, mounted in gimbals so the aircraft can move around it freely. The rotor's rigidity keeps that axis fixed in space, and that fixed reference is what your attitude indicator and directional gyro will use. That's the foundation. Next, we'll look at the second property, precession, and how it interacts with rigidity.