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Upon completion of this chapter you should be able to:

  • Describe the characteristics of a gyroscopes.
  • List the two basic properties of gyroscopes and explain them.
  • Describe the components of a universally mounted gyro.
  • Describe the factors that determine rigidity in a gyro.
  • List the factors that determine the direction of precession in a gyro.
  • Explain the right-hand rule for gyro precession.
  • Describe the term "Degree of Freedom" as it applies to gyros.
  • Explain the effect of apparent precession (apparent rotation).
  • Explain the purposes of erection systems.
  • Describe the use of gyros with only one degree of freedom.
  • Explain the purpose of an accelerometer.
  • Explain the principle on which accelerometers operate.
  • Explain the need for a pulse-counting accelerometer.


The word gyroscope was first coined by a French scientist, Leon Foucault, in 1852. It is derived from the Greek words "gyro," meaning revolution, and "skopien," meaning to view.

The gyroscope, commonly called a GYRO, has existed since the first electron was sent spinning on its axis. Electrons spin and show all the characteristics of a gyro; so does the Earth, which spins about its polar axis at over 1000 miles per hour at the Equator. The Earth's rotation about its axis provides the stabilizing effect that keeps the North Pole pointed within one degree of Polaris (the North Star).

Any rapidly spinning object - a top, a wheel, an airplane propeller, or a spinning projectile - is fundamentally a gyroscope. Strictly speaking, however, a gyroscope is a mechanical device containing a spinning mass that is universally mounted; that is, mounted so it can assume any position in space. Figure 3-1 shows a model of a gyro. As you can see, a heavy wheel (rotor) is mounted so that its spin axis is free to turn in any direction. The wheel spins about axis X; it can turn about axis Y, and it can turn about axis Z. With this mechanical arrangement, the spinning wheel can assume any position in space.

Figure 3-1. - Gyro model, universally mounted.

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Gyroscopes have two basic properties: rigidity and precession. Those properties are defined as follows:

RIGIDITY - The axis of rotation (spin axis) of the gyro wheel tends to remain in a fixed direction in space if no force is applied to it.

PRECESSION - The axis of rotation has a tendency to turn at a right angle to the direction of an applied force.

The idea of maintaining a fixed direction in space is simple to illustrate. When any object is spinning rapidly, it tends to keep its axis pointed always in the same direction. A toy top is a good example. As long as the top is spinning fast, it stays balanced on its point. Because of this gyro action, the spinning top resists the tendency of gravity to change the direction of its axis. You can think of many more examples. A bicycle is easier to balance at high speed than when it is barely moving. At high speed, the bicycle wheels act as gyros, and tend to keep their axes (axles) parallel to the ground.

Note that it is easy to move the gyro as long as you keep the axis POINTING in the SAME DIRECTION. The gyro resists only those forces that tend to change the direction of its axis. In a bicycle, since the axis of rotation (the wheel's axles) is horizontal, the wheels win resist any force that tends to tilt or turn them to the right or left.

If you can obtain a gyroscope top, you can do some instructive experiments with it. Hold the gyro top with its axis vertical as shown in figure 3-2 and start it spinning. As long as it is spinning fast, it will stay balanced. You can balance it on a string or on the point of your finger; the axis will stay vertical as long as the top is spinning fast. As we mentioned before, this ability of a gyro to keep its axis fixed in space is called RIGIDITY.

Figure 3-2. - A gyroscope top.


Now, if you stop the gyro top and turn its axis horizontal, and then start it spinning again, balancing one end on a pivot, (fig. 3-3), it won't fall. The top's axis will stay horizontal, resisting the tendency of gravity to change its direction Although the gyro will RESIST the force that gravity applies to it, the gyro will still RESPOND to that force. The gyro responds by moving its axis at a RIGHT ANGLE to the APPLIED FORCE.

Figure 3-3. - Gyro top with axis horizontal.

The axis will tilt in a direction 90 away from the applied force. This is called PRECESSION.

Figure 3-4 is another view of the same gyroscope. Its far end is still balanced on the pivot. Gravity is pulling down on the gyro. If the gyro rotor is turning in the direction shown by the arrow, the near end of the frame (axis) will move to the left. If the rotor were turning in the opposite direction, the frame would move to the right. Note that in each of these examples the direction of movement was displaced from the applied force (gravity) by 90. The axis stays horizontal, but the gyroscope responds to the force of gravity by rotating around the pivot.

Figure 3-4. - Gyro precession.

Gyro action may be summarized as follows: A spinning gyro tends to keep its axis pointing in the same direction. This is called RIGIDITY. If you apply a force that tends to change the direction of the spin axis, the axis will move at a right angle to the direction of the applied force. The direction of precession will be 90 clockwise from the applied force if the rotor is spinning clockwise (when viewed from the "free" end of the rotor's axis); if the rotor is spinning counterclockwise, the precession will be 90 counterclockwise. If the axis is horizontal, and you try to tilt it, the axis will turn. If the axis is horizontal, and you try to turn it, the axis will tilt. This second characteristic of a gyro is called PRECESSION.

Because of precession, we can control the direction that the spin axis points. This enables us to aim the spin axis where we want it to point. Without precession, the rigidity of the gyro would be useless.

Q.1 Can any rapidly spinning object be considered a gyroscope? answer.gif (214 bytes)
Q.2 In the drawing in figure 3-1, which axis is the gyro spin axis? answer.gif (214 bytes)
Q.3 What gyro property causes the gyro to remain in a fixed position? answer.gif (214 bytes)
Q.4 What type(s) of force does a gyro resist? answer.gif (214 bytes)
Q.5 In what direction will a gyro precess in response to an outside force? answer.gif (214 bytes)

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