Trim system evaluation

The final part of the FCMC assessment is to measure the beeper trim system characteristics and then to determine their suitability for the intended role. For a ‘beeper’ trim system using a trim motor the two important aspects are firstly the trim rate and secondly any lag in the system. Measurement of the rate is made on the ground by operating the trim and obtaining the displacement against time from the data replay. If no instrumentation is available then displacement against time is measured using a stopwatch and tapes fitted to the control. To conduct the airborne evaluation flight tasks are selected which require accurate trimming and other tasks that require more rapid displacement of the control. Too rapid a trim rate is distracting and frustrating for the pilot conducting the precision task while too slow a rate requires the pilot to hold the trim force for too long after a larger displacement. When measuring trim lag the trim control is moved in one direction and then trimmed in the opposite direction to measure the time it takes for backlash in the system to be taken up and the flight control to be moved.

5.2 ASSESSING STATIC STABILITY

5.2.1 Longitudinal static stability

The static stability of a helicopter will manifest itself to the pilot as the amount of forward stick required to maintain an airspeed greater than trim. The pilot will also ‘expect’ to perceive some change in control position as he trims the helicopter through its speed range in level flight. There are, therefore, two different test techniques:

(1) Apparent static stability tests. The helicopter is trimmed, in level flight, at a series of airspeeds from minimum to maximum. The control position data obtained is often referred to as the ‘Trimmed Flight Control Positions’ (TFCPs). As suggested earlier TFCPs can also be assessed in steady climbing and descending flight.

(2) Collective fixed static stability tests. The helicopter is trimmed at an airspeed and the collective fixed at ‘Power For Level Flight (PFLF). The pilot then attempts to hold an off-trim speed, either greater or less, and accepts the ensuing climb or descent. The stick position data obtained in this test is directly related to the strength of the speed stability (Mu) provided the rate of climb or descent is not excessive and the control power is constant.

The difference in the results obtained from these tests will depend on the pitch response of the helicopter to changes in collective pitch. In addition to the control position data the variation of pitch attitude with airspeed is also noted. A large change in attitude with speed may be used by the pilot to compensate for poor cyclic stick position cues. If, on the other hand, the variation in attitude is very small the attitude hold function of an AFCS would not be very effective as an airspeed hold.

5.2.1.1 Collective fixed test results

The variation of longitudinal cyclic pitch (Bl) with airspeed from a trimmed condition is relatively easy to estimate theoretically. The linearized form of the longitudinal equations of motion for a helicopter, with the centre of gravity situated at the body axes origin, can be written as follows:

m[U – rVe + qWe] = Xu. и + Xw. w + Xq. q – mgQ cos 6e + XBi. Bl + X0c. 6c

m[w – qUe + qVe] = Zu. и + Zw. w + Zq. q – mgQ sin 6e + ZBi. Bl + Z0c. 6c

Iyyq – Ixz r – Ixzp = Mu. и + Mw. w + Mq. q + MBl. Bl + Mqc. Qc

When performing speed stability testing the pilot endeavours to achieve the desired off-trim speed with the aircraft wings level and with no pitch, roll or yaw rate evident. Therefore: