Trim follow-up systems
Some systems (Westland Sea King and Lynx) use only a combination of stick cancelling and pseudo-rate (differentiated attitude) feedback to provide a satisfactory control response and gust rejection. Whereas others are more complex, employing both attitude and rate gyros with force disconnects to alert the AFCS to an intended deviation from the current attitude datum. Ultimately, possibly even without the requirement to provide outer-loop modes such as airspeed hold or auto-ILS, the limited authority of series actuation will lead to some degradation of the long-term attitude-keeping performance. Whilst simply extending the travel of the series actuator would overcome this deficiency it is not a favoured solution since it would impact the safety case for the system (actuator runaways would have more severe consequences) and may degrade the control response characteristics due to excess damping. Usually a better alternative is to provide a mechanism that attempts to centralize the series actuators as they approach their end-stops thus ensuring that they operate with approximately full authority at all times. The key to a trim follow-up system is a device that can move the cockpit control thereby shifting the cyclic, collective or tail rotor pitch datum about which the series actuator operates. Usually the trim motor, which
Fig. 6.27 Effect of trim follow-up on gust response.
doubles as the parallel actuator in the cyclic channels, provides this trim follow-up capability. Activating a trim follow-up system also requires a logic circuit that can determine when to send a drive signal to this actuator. Since the series actuator position is often measured in order to provide a feedback signal to the actuation circuit, and possibly drive some form of cockpit indication, it is a simple matter to use this data to trigger movement of the parallel actuator/trim motor. As expected the application of trim follow-up improves gust rejection by simply increasing the effectiveness of the series actuator (compare Fig. 6.27 with Fig. 6.25).
Figure 6.28 confirms that trim follow-up extends the authority of the system by showing that the activity at the main servo now exceeds +1°. Figure 6.28 also highlights one of the potential disadvantages of trim follow-up systems for some pilots: unnatural and unexpected control activity in the cockpit. Although the pilot has selected a trim position for the desired airspeed the follow-up system causes the cyclic stick to migrate fore and aft depending on the degree of extra authority required by the series actuators to counter the atmospheric turbulence. Since the pilot has not selected a true autopilot mode he may be operating ‘hands-on’ and could find the uncommanded stick movements disconcerting.
Inappropriate stick forces during rapid control movements are perhaps of greater concern than stick activity during trimmed flight in turbulence. Suppose the pilot makes a small aft step input. Initially the series actuator will move in the same direction. This may cause the trim motor to activate driving the stick further aft. A short time later the aircraft starts to pitch-up and the rate feedback signal, augmented eventually by the attitude feedback signal if it is still retained, causes the series actuator to oppose the initial input. If this reversal is sufficiently large the trim motor may again activate but this time the stick will be driven in a direction opposite to the
Fig. 6.28 Effect of trim follow-up on main servo travel.
original input. Eventually the series actuator will approach null and the trim motor will be signalled to drive the stick back to the position selected by the pilot at the start of the input. If the pilot grips the control firmly during this process he may notice a disconcerting change in the feel of the control as the trim system is moved relative to the control. Problems with control response can be alleviated by reducing the trim rate. A slower acting trim motor is less likely to give perceptible stick migration during rapid controlling and will reduce the risk of catastrophic damage following a hard – over of the trim system. Rapid removal of unwanted stick forces following a gross change in aircraft attitude, a possible failing of a slow trim system, may be achieved by a trim release switch. Figure 6.29 shows the step response of an AFCS with a slow- acting trim follow-up system and Fig. 6.30 confirms that the gust rejection is still satisfactory.
Note that the ‘unrestrictedstick migration’ portrayed shows the hypothetical situation that would arise if the pilot made a step input and then allowed the stick to move in his hand as a function of the trim follow-up requirements. In reality the pilot would maintain a firm grip on the stick commensurate with the type of input made, in which case he would feel a change in the stick forces instead.