Aviation Safety and Pilot Control Understanding and Preventing Unfavorable Pilot-Vehicle Interactions (1997) / Chapter Skim
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From page 126... ...
6Criteria for Assessing Aircraft-Pilot Coupling Potential This chapter examines the status of existing and proposed criteria for assessing APC potential. For fixed-wing aircraft with modest stability augmentation and conventional, fully powered surface actuating systems, Category I PIO tendencies are arguably reduced to a negligible level for aircraft that meet the flying qualities requirements in MIL-F-8785C69 or that are included in the newer MIL-STD-1797A.70 Rotorcraft that meet the requirements of ADS-33D68 should also be resistant to APC events.
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From page 127... ...
dynamic properties of a modern aircraft are similar to those of earlier aircraft, PIO-specific criteria should complement existing requirements. Design flexibility and features intrinsic to FBW technology allow -- and may require -- additional criteria for aspects of the FCS that are markedly different from conventional systems.
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PREREQUISITES FOR CRITERIA A useful criterion must satisfy three prerequisites: validity, selectivity, and ready applicability. Validity "Validity" implies that a criterion embodies properties and characteristics that define the environment of interest and are associated with parameter spaces covering the vast majority of known cases.
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Selectivity "Selectivity" demands that the criterion differentiate sharply between "good" and "bad" systems. Criteria that downgrade the performance of aircraft that are actually adequate may be too restrictive.
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The Gain/Phase Templates, including ω180/Average Phase Rate boundaries, consider the effective aircraft attitude dynamics as an element of an open-loop system exhibited as coordinates on a gain/phase chart. (This and the other criteria forms are illustrated below in connection with their short reviews.)
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Figure 6-1 Definitions of aircraft pitch attitude bandwidth and phase delay. Source: Adapted from Mitchell et al.49 Aircraft-Bandwidth/Phase Delay Aircraft-Bandwidth and Phase Delay are popular and effective criteria measures that have been successfully used to evaluate flying qualities.48,49 As illustrated in Figure 6-1, these are frequency-domain metrics that focus on particular characteristics of the aircraft attitude transfer function.
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needed to offset the time lags in effective aircraft dynamics)
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CRITERIA FOR ASSESSING AIRCRAFT-PILOT COUPLING POTENTIAL 133 These time-response attributes are the open-loop, effective aircraft properties. They are favorable for closed-loop piloted control because the pilot can close the loop acting in a pure-gain fashion in which pilot output is proportional to perceived error.
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the effective PVS. When the pilot is operating in a pure gain (i.e., synchronous)
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Figure 6-3 Aircraft-Bandwidth/Phase Delay parameters as indicators of PIO susceptibility for sample operational and test aircraft. Source: Adapted from Klyde.39 In Figure 6-3, a variety of operational and test aircraft are plotted against these basic boundaries.
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"excessive dropback" region permits the inclusion of the T-38 PIO, during which the pilot behavior was almost surely synchronous. One problem with the application of Aircraft-Bandwidth/Phase Delay criteria is the definition of the input to the effective aircraft.
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imply that the suggested Aircraft-Bandwidth/Phase Delay boundaries for PIO shown in Figures 6-2 and 6-3 assume synchronous-pilot control behavior. For most extended-rigid-body effective aircraft dynamics, where the higher frequency flexible modes do not significantly affect the phase or amplitude ratio*
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Figure 6-4 Bode and gain phase diagram presentations for Kc e-sτ/s. Source: Klyde.39 CRITERIA FOR ASSESSING AIRCRAFT-PILOT COUPLING POTENTIAL 138
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CRITERIA FOR ASSESSING AIRCRAFT-PILOT COUPLING POTENTIAL 139 TABLE 6-1 Idealized Rate-Command Controlled Element Characteristics τe (sec)
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of the frequency at 180-degree phase lag of the pitch attitude frequency response, ω180, and the average phase rate, are similar in form to the Aircraft-Bandwidth/ Phase Delay plot of Figure 6-3, except for the details. If synchronous-pilot activity is assumed, this formulation directly indicates the PIO frequency region.
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Figure 6-5 Gain/Phase Template, ω180/Average Phase Rate Boundaries. Source: Gibson.24 CRITERIA FOR ASSESSING AIRCRAFT-PILOT COUPLING POTENTIAL 141
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The elementary system dynamics (Yc = Kc, Kc /s, and Kc /s2) used to develop the empirical data base had no higher-frequency net lags.
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TABLE 6-2 Prediction of PIO Susceptibility with Smith-Geddes Attitude-Dominant Type III Criterion for Operational and Test Aircraft Aircraft ωPIO (rad/ sec)
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Illustrative Example When the Smith-Geddes criterion is applied to the elementary example of an idealized rate-command effective aircraft (i.e., Yc = Kce-jωie /jω) , the criterion frequency (ωc)
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frequency of Category I PIOs using the existing formula for ωc is higher than has actually been observed. This implies that the effects of higher frequency lags are underestimated in the Smith-Geddes determination of the criterion frequency.
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points to changes that might be made to reduce the PIO tendency. It also indicates the most sensitive frequency regions of the closed-loop PVS, provides a basis for selecting test inputs, and is useful in several other ways.
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frequency using the Smith-Geddes estimate for wc can be improved by using Equation 6-7. This approach is suggested not only by its successful fit to the Have PIO data but also by the (possible)
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Thus, if this empirical equation can be generalized, the estimated PIO frequency is somewhat greater than 111 percent of the frequency predicted for a synchronous pilot interacting with the aircraft's attitude dynamics. This result has some interesting implications.
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boundaries were drawn for flying qualities levels and for PIOs. The PIO boundaries are shown in Figure 6-7.
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Dropback The Dropback parameter, ∆θpeak, which is illustrated in Figure 6-9, deals with metrics, such as qpeak/qss, derived from the attitude response to step inputs. Dropback may not appear to be an accurate measure of closed-loop PVS response because, strictly speaking, it is an open-loop aircraft response.
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Figure 6-9 Pitch rate overshoot and pitch attitude dropback. Source: Nelson and Landes.54 CRITERIA FOR ASSESSING AIRCRAFT-PILOT COUPLING POTENTIAL 151
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Because the pulse applied as the test input is really two pulses separated by 10 sec, Dropback is not sensitive to high-frequency effective delays. For instance, the Dropback for the rate-command controlled element with an effective time delay of Equation 6-2 is zero, just as it is for a pure Yc = Kc/s rate command transfer function.
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if the gain is too sensitive.42 In past experiments, this variable has been fairly well controlled to be near optimum levels for conventional cockpit longitudinalcontrol devices. This has not been true of lateral control devices.
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individual Level 1 and Level 2 flying qualities requirements is not known. Consequently, critical PIO-related requirements are "buried" among other, less important requirements.
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mission tasks: air-to-air tracking, power approach, air-to-ground tracking, boom tracking, and formation flying. Clearly, HQDT and similar tasks, along with capture tasks, expose some PIO tendencies and can readily be extended to mission tasks other than air-to-air or air-to-ground tracking.
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Non-Oscillatory APC Events The specter of novel, non-oscillatory APC events associated with new FCS features and functions made possible by FBW technology was raised in Chapter 2. These events tend to occur in situations near control limits that create discrepancies between what the aircraft is doing and the pilot's expectations.
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easily. This phenomenon may underlie some of the three-dimensional PIOs described in Chapter 1.
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region in the gain/phase domain. The boundaries for this region are based primarily on an examination of a large number of configurations that exhibited Category II PIOs when the onset frequency appeared within those boundaries (Figure 6-10)
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the context of the more complete data set available from the analysis phases. For example, dropback as a time-domain measure might be more convenient for assessing flight test tasks than the more elaborate frequency sweeps required for Aircraft-Bandwidth/Phase Delay or other frequency-domain measures.
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There are several areas where available criteria need to be improved. For example, the Smith-Geddes Type III frequency formulation should be fine tuned to take more current data into account.
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