Freeplay

Freeplay will inevitably exist in any mechanical control system due to wear and backlash between the various components. Freeplay that exists between the flight controls and the blade pitch change linkage is termed total system freeplay while wear or backlash between the flight controls and the spring feel unit is termed trim system freeplay.

Measuring total system freeplay (for an irreversible system) is performed with rotors stopped and an observer stationed by the rotor head. The pilot makes control inputs in the axis under investigation and the observer confirms that a blade pitch change has taken place. The process is repeated using gradually smaller inputs until there is no resultant blade pitch change. Results can be confirmed with blades turning by observing the tip path plane and observing when there is no response to the control.

Total system freeplay is always undesirable as it will delay the aircraft response to inputs and therefore adversely affect handling qualities during tasks such as precision hovering which involve small, high-frequency control inputs.

Determining trim system freeplay is rather more straightforward and involves measuring the distance through which the control can be moved without needing to overcome a force. Although generally undesirable, some pilots like the opportunity that trim system freeplay affords to make small control adjustments without having to overcome B + F. Although it is different to a TCDB, a band of trim system freeplay does share the characteristic that control displacements within the band do not require operation of the trim system as there will be no force to hold off.

5.1.1 Assessing mass balance and control dynamics

The mass balance characteristics of the cyclic and collective are assessed to determine the tendency of the control to move due to the influence of gravity or other accelerations. Clearly this will usually only be a problem where there is no force feel system fitted or the force gradient is very shallow. The assessment is made initially with no control friction set to establish a base-line condition, and then repeated with ‘normal’ amounts of friction. Any manoeuvres which produce forces on the controls such as pull-ups/push-overs and steep turns can be employed. Poor mass balance characteristics can increase the pilot’s workload as it prevents the control being released for other than brief periods or requires any adjustable friction devices to be set to possibly undesirably high levels.

Identifying the control dynamics is an essential part of any FCMC assessment. This consists of evaluating the effect of control ‘raps’ and releases from an off-trim condition as well as assessing the effect of any biomechanical feedback and the mass balancing of controls. Although some limited testing can take place on the ground the majority of tests need to be conducted in-flight. Release-to-trim (RTT) tests of the cyclic and control raps of the cyclic and collective are approached with caution in case the control dynamics lead to a divergent aircraft response. For obvious reasons the tests are not conducted with the aircraft at the edges of the cleared flight envelope. The procedure involves two crew members; the handling pilot gives a countdown before each RTT or rap while the other crew member positions both hands near the control, ready to suppress any divergent response. A series of incremental RTTs or raps in each axis are made and the control dynamics recorded. For a well-damped response it is usually sufficient to record the number of overshoots but for less damped responses time histories may be needed. An example of poor dynamics includes a collective control where if a rap is made and only a small amount of friction has been set the subsequent aircraft heave motion will cause a collective displacement in the opposite direction; this forces a divergent oscillation which may be difficult to suppress.

Part of this process of assessing control dynamics involves looking for biomechanical feedback. This is a problem related to FCMC that can degrade handling qualities and is the process whereby the motion of the aircraft causes the pilot’s arm to move resulting in an unintended control input. Biomechanical feedback is most likely to occur when there is little B+ F in the control system and the control has poor mass balance characteristics. The pilot’s seating position may also be a factor if it prevents the pilot achieving a position where the arm can be braced to prevent inadvertent movements caused by aircraft motions. This coupling between the pilot and the control may be a problem when flying in turbulence as gusts will cause an aircraft disturbance that will subsequently result in a control input. If this is combined with poor control dynamics the result may be very serious.