Aviation Safety and Pilot Control Understanding and Preventing Unfavorable Pilot-Vehicle Interactions (1997) / Chapter Skim
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Pages 181-191
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From page 181... ...
Appendix C Details of Aircraft-Pilot Coupling Examples This appendix provides quantitative details of the APC examples discussed in summary form in the body of the report. Essentially Linear Oscillatory Aircraft-Pilot Coupling Events The simplest oscillatory APC events occur when both the effective aircraft and the pilot dynamics act as quasi-linear and time-stationary system elements.
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Figure C-1a Bode and Nichols diagrams for a synchronous PVS of an aircraft with low susceptibility to oscillatory APC events. Source: McRuer.42 APPENDIX C 182
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Figure C-1b Bode and Nichols diagrams for a synchronous PVS of an aircraft with high susceptibility to oscillatory APC events. Source: McRuer.42 APPENDIX C 183
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corresponding to the neutral stability frequency, ωu (6.17 rad/sec) , where the pilot-effective aircraft open-loop phase angle is -180 degrees.
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aircraft bandwidth (ωBW)
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Some Nonlinear Characteristics That Can Lead To Flying Qualities Cliffs Common Cliff Producers Conceptually, the cliff metaphor evokes a picture of sudden large changes in aircraft motions associated with relatively slight changes in pilot activity. When cliff-like changes are caused only by an increase in the pilot's output amplitude, the pilot-aircraft system is not behaving like a linear system.
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Command-Path Gain-Shaping Most modern FCSs incorporate gain-shaping in the pilot's command path. The shaping is usually shallow around neutral, with the gain (control gradients)
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support the view that rate limiting exacerbates time lag effects.3,14,15,28,31,39 But how and when the additional time lags enter the closed-loop system have not been well documented in detailed experiments. Although there is a consensus that rate limiting phenomena are important factors in fully developed, severe APC events encountered in operational situations, the transitional system behavior on the way to rate limiting and severe PVS oscillations has not been recorded in enough detail for a complete understanding.
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large enough to keep the system essentially always on the rate limit. Figure C-3b gives a describing function for the amplitude ratio and phase angle as functions of a frequency ratio in which the normalizing frequency is the "onset frequency" (ωON)
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Figure C-3 Time domain and transfer characteristics for fully developed rate limiting.
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attentive pilot, this baseline condition would exhibit a gain margin of about 3/2 and a phase margin of perhaps 25 degrees. In this essentially linear regime, the closed-loop system will perform well.
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