Natural frequency and relative damping requirements for the longitudinal long-term mode vary with pilot attentiveness. As might be expected a more heavily damped mode is required if the pilot’s attention is divided between the flying task and other
Fig. 5.8 Dynamic stability requirements (ADS-33).
duties. Figure 5.8 compares longitudinal long-term mode data from a light singled – engined helicopter with the requirements of ADS-33E [5.2]. Notice that if the pilot is fully engaged in attitude and flight path control (definition of full-attention) the aircraft has Level 1 handling qualities (as defined in Section 5.7.1). However, as soon as his attention is divided the low level of relative damping confirms Level 2 handling qualities. Ministry of Defence Standards take a different approach by assuming that the pilot will either be actively monitoring the performance of an AFCS or flying under autopilot control whilst more fully involved in ancillary duties. In each case a minimum acceptable time is specified before the pitch attitude can vary by more than 1 degree.