Research On The Longitudinal Inner Loop Configuration In ACLS

Smoothly landing a flying vehicle on an aircraft carrier moving in rough seas proves anextremely demanding task. Automatic Carrier Landing System (ACLS) significantly reducesthe impact caused by environment on carrier landing safety and relieves the pilot much of themanipulation press, thereby a higher landing success rate is achieved. The ACLS with avertical rate (H-Dot) reference in the autopilot control law demonstrates high performance ofrestraining ship-wake air disturbance, which has successfully been applied to F/A-18A fighteraircraft. This paper focuses on the design of longitudinal inner loop of H-Dot ACLS for a jetcarrier-based aircraft approaching a carrier.Firstly, considering some rational simplifying assumptions, a six degree of freedommotion model about this plane is established and the approximate trim in the reference flightcondition is accomplished. The subsequent linear modeling can be finished, depending on therough trim data instead of the precise one unknown. In addition, the fact that the small trimdeviation leads to divergent response of this bare aircraft in short time illuminates low pitchstatic stability of the aircraft and some simulation ascertains its longitudinal static instabilitynear the trim condition.Secondly, an accurate linearization to the full motion equations of the aircraft at itsreference motion point produces longitudinal small-disturbance linear state equations undercalm air. Further more, the additional forces and moments caused by wind are brought to thesolved linear model, resulting in longitudinal small-disturbance model under air turbulence.Both sets of linear models are specific as the example plane’s aerodynamic parameter valuesand derivatives of the reference are calculated and substituted into these state equations.Simulated comparison between the linear and nonlinear models indicates this linearizationmethod is reasonable.Again, analysis is turned to one of the main carrier landing errors--ship burble. For thistype of air disturbance, military standard MIL-HDBK-1797provides model selection andapplicable conditions, both of which are detailed in this article and the whole time domainsimulation diagrams about longitudinal coupled with vertical carrier air turbulence arepresented, besides a simple and convenient algorithm is used to simulate random componentof carrier airwake.At last, based on the structure of the F/A-18A H-dot ACLS, CAS (Control AugmentationSystem) and auto-throttle for the example plane approach landing in turbulent air are designed, simulated and verified. The control system frame of the aircraft is clear after principleselaboration and modeling for the autopilot inner loop, outer loop and the APCS (ApproachPower Compensator System). The comprehensive evaluation of control effect aims atoptimizing all the structural parameters of the flight control system from inner to outside bymanual regulation, programming and Simulink Design Optimization tools until theperformance of the configuration designed is very close to the published F/A-18A simulationoutcomes, with desired air turbulence rejection capability, satisfactory pitch flight qualitiesand qualified frequency characteristics meeting ACLS design standard. The conclusion is:design scheme adopted in this paper is logic and reasonable and H-dot control idea is not onlyadequate for F/A-18A carrier landing.