Artificial stability – Mach trimmers and yaw dampers

In principle the pilot can control an unstable motion, by operating the controls directly to provide suitable forces and moments to oppose the motion. For example, in the case of the Dutch roll, the rudder is extremely effective in suppressing the yaw and hence controlling the motion. If the motion is of high frequency and poorly damped, however, this makes the aircraft very tiring to fly, and at some frequencies the pilot’s reactions will be such that he will not be able to ‘follow’ the motion correctly. In this event his efforts may well make a bad situation worse.

One way of overcoming the problem is to relieve the pilot of this part of his task altogether by the use of an automatic control system. In the case of the Dutch roll, the yawing motion can be sensed, both in terms of the degree of yaw and the rate at which it is developing, by the use of gyroscopically based instruments. In this case a position gyro can be used to sense the degree of yaw and a rate gyro to sense its rate. Once the information concerning the aircraft motion is available the rudder can be moved automatically to provide the required correction. Such a device is present on all large modern jet trans­port aircraft and is known as a yaw damper.

Details concerning the design of either the gyros or the damper control system are outside the scope of this book, however it is perhaps interest­ing to mention a few features which must be considered before leaving the subject.

One obvious feature is that the control system employed in the yaw damper must be able to distinguish between a conscious control input on the part of the pilot, and the control movement generated as a result of the unwanted motion. Thus the total movement must be determined as a combination of both inputs. Another point which must be carefully considered is the integrity of the control system. Should failure occur, the safety of the aircraft must not be comprom­ised. This means that either suitable back-up must be provided, or the system must revert to full manual control on failure. In the latter event the character­istics of the aircraft must be such that manual flight is reasonably possible, even if not very pleasant.

Further damping can be provided by the use of a similar system to control the ailerons in such a way as to oppose the rolling component of the motion. This system is known as a ‘roll damper’.

As mentioned above, the longitudinal characteristics deteriorate due to the rapid centre of pressure movement which results from comparatively small changes in the aircraft operating condition in transonic flight. This again can be ‘fixed’ by the use of a suitable automatic control system. This system uses elevator movement to compensate for the change in centre of pressure and is known as a ‘Mach trimmer’.