Published online by Cambridge University Press: 04 July 2016
Automatic navigation systems for aircraft often use gyroscopes to measure the changes in the direction of motion of the aircraft they are controlling. These measurements are fed back to the mechanisms moving the control surfaces of the aircraft.
Such a system is usually designed, in the first instance, assuming the aircraft structure to be rigid and such that the system is then stable.
Now suppose the aircraft structure, considered elastic, is disturbed in flight by, say, a gust. Normal modes of vibration of the structure will be excited, the resulting oscillations will be detected by the gyroscope and fed as error signals to the control surface actuators. If the actuators are capable of following these error signals, the control surfaces will also oscillate, exciting the structure further and so completing the cycle. This cycle may be unstable.