To see how free gyros are used in guided missiles to detect changes in attitude, refer to figure 9-10. Let us assume the missile shown in view A has a horizontal design attitude. A gyro within the missile has its spin axis in the vertical plane and is also gimbal-mounted. Any deviation in the horizontal attitude of the missile does not physically affect the gyro. In other words, the missile could roll and the gyro will maintain its position in space.

View B illustrates the last point. The missile has rolled about 30 but the gyro remains stable. If we could measure this angle, we would know exactly how far the missile deviated from the horizontal plane. This information could then be used to change the position of the control surfaces and correct or stabilize the

Figure 9-9.-Precessing a gyro.

Figure 9-10.-View A.-missile horizontal; View B.-missile rolls; View C.-missile pitches; View D.-missile yaws. In all cases, gyro remains fixed in space. 

missile. View C shows that a free gyro with a vertical spin axis can also be used to monitor or detect pitch.

Actually, a minimum of two free gyros is required to compensate for roll, pitch, and (the third factor) yaw. However, for yaw, a second gyro must be mounted so its spin axis is in the horizontal plane, as shown in view D. (You are looking down onto the missile.)