Evaluation of a Precision Hover Task with Added Rotorcraft Dynamics Using Time-Varying Cutoff Frequency

Crossover frequency has long been used to characterize the behavior of pilot-vehicle systems with known input forcing functions. For those cases when the input frequency is not known, the Army Aeroflightdynamics Directorate introduced the cutoff frequency concept as a surrogate measure of crossover frequency. Simply stated, cutoff frequency is a quantitative measure of pilot control activity determined from controller input power versus frequency. A limitation of the approach is that it is based on an average over the selected window size. To better understand how the pilot adapts over a run, a time-varying approach is needed. In this paper, wavelet-based scalograms are used to define a time-varying cutoff frequency. Scalograms provide a presentation of pilot input and rotorcraft output power signatures as they change over the selected time segment, akin to a time-varying power spectral density. From this measure a new metric, the power frequency, is introduced. To explore the utility of this metric, piloted simulation data from a recent study that investigated the impact of rotorcraft added dynamics on precision hover task performance are used. Five evaluation test pilots participated in the study conducted on the NASA Ames Vertical Motion Simulator. In the evaluation process, the pilots were first presented a simplified, uncoupled baseline rotorcraft model. Added rotorcraft dynamics in the form of a lead/lag second order dipole pair were then introduced. The impact of the added dynamics on task performance was explored as dipole damping was varied from run to run and, for selected cases, during a single run. Analysis results show the power frequency to be an effective means of differentiating run-to-run and pilot-to-pilot differences in pilot-vehicle system behavior.