Development of Modern Control Laws for the AH-64D in Hover/Low Speed Flight

Modern control laws are developed for the AH-64D Longbow Apache to provide improved handling qualities for hover and low speed flight in a degraded visual environment. The control laws use a model following approach to generate commands for the existing partial authority stability augmentation system (SAS) to provide both attitude command attitude hold and translational rate command response types based on the requirements in ADS-33E. Integrated analysis tools are used to support the design process including system identification of aircraft and actuator dynamics and optimization of design parameters based on military handling qualities and control system specifications. The purpose is to demonstrate the potential for improving the low speed handling qualities of existing Army helicopters with partial authority SAS actuators through flight control law modifications as an alternative to a full authority, fly-by-wire, control system upgrade. NOTATION ACAH attitude command attitude hold DH direction hold DVE degraded visual environment HH height hold HQ handling qualities MCLAWS modern control laws PH position hold RC rate command SAS stability augmentation system TRC translational rate command UCE usable cue environment __________________________ 1 Harding Consulting, Inc., jeffrey.w.harding@us.army.mil 2 Westar Aerospace and Defense Group, Inc. 3 University of California, Santa Cruz Presented at the American Helicopter Society 62 nd Annual Forum, Phoenix, AZ, May 9-11, 2006. Copyright © 2006 by the American Helicopter Society International, Inc. All rights reserved. INTRODUCTION The AH-64 Apache was designed in the late 70’s and went into service as the US Army’s most advanced day, night and adverse weather attack helicopter in 1986. The flight control system was designed to meet the relevant handling qualities requirements based on MIL-F-8501 (Ref. 1). Only slight improvements were made to the flight control system during the D model upgrade years later. Although the AH64 was designed to operate in all conditions, there were no dedicated handling qualities requirements to account for the increased pilot workload associated with operating in a degraded visual environment (DVE). As a result, the handling qualities are not optimum for all conditions. Operation in desert environments, where brown-outs are often encountered during takeoffs and landings, has resulted in increased accident rates. These accidents are associated with the pilot’s loss of situational awareness due to a lack of visual cues and represent both a safety and cost concern. The US Army Safety Center recognizes this trend throughout the helicopter fleet. In a recent Safety Center accident investigation study, the number one material fix toward improving army aviation safety and reducing accidents by as much as 50% was identified as improving the hover and low speed handling qualities (Ref. 2). Similar findings were reported by Key based on Army helicopter pilot error mishap data (Ref. 3). Since the design of the original AH-64 flight control laws, over 20 years of research in helicopter flight controls and handling qualities has shown that there is a degradation in handling qualities for near-earth tasks as the pilot’s visual environment degrades. These degraded handling qualities result in higher pilot work load and increased accident rates. The research has also shown that the degraded handling qualities can be overcome by changing the control response type to provide increased stability. The results of this research led to the development of a new handling qualities specification for military rotorcraft ADS-33E (Ref. 4). ADS-33E incorporates a usable cue environment (UCE) rating scale to account for the lack of visual cues while operating at night and poor weather conditions. As the UCE degrades, the helicopter control response type must be improved from a rate command, to an attitude command, to a translational rate command system in order to maintain satisfactory handling qualities. All current army helicopters were designed before the specification was developed; however, flight control system upgrade programs are now required to meet some portions of the new specification. The Army’s long term goal is to have all helicopter flight control systems for both new and legacy aircraft designed or upgraded to meet the more stringent handling qualities requirements of ADS-33E. In 2005, the Aviation Engineering Directorate initiated a program to develop modern control laws (MCLAWS) for the AH-64D. The term modern, in this paper, refers to updated control laws (compared to the legacy system) that are designed specifically to meet ADS-33E handling qualities requirements by implementing new response types such as attitude command attitude hold (ACAH) and translational rate command (TRC). The goal was to apply the latest technology and analysis tools to develop new control laws for improved AH-64D handling qualities in hover and low speed flight using the existing mechanical control system. The program leveraged previous research on achieving an ACAH response type with limited authority systems (Refs. 5, 6) and a demonstration program on the UH-60 Black Hawk (Refs. 7, 8). The UH-60 program involved the design of modernized control laws to provide an ACAH response in low speed flight. The program included both a simulation evaluation and a flight test demonstration. The results confirmed the improved handling qualities for hover-related mission tasks over the legacy control laws using the existing ±10% authority SAS. A key element in the success of the UH-60 MCLAWS program was the use of an integrated tool set for modeling, analysis and simulation. These same integrated tools provide the foundation for the work presented in this paper. Modeling was performed using Simulink ® for graphic programming. System identification of aircraft and actuator dynamics was accomplished in the frequency-domain using CIFER ® (Comprehensive Identification from Frequency Responses, Ref. 9). Control law analysis and optimization was performed using CONDUIT ® (Control Designers Unified Interface, Ref. 10) with desktop simulation provided by RIPTIDE (Real-time Interactive Prototyping Technology Integration Development Environment, Ref. 11). CIFER ® , CONDUIT ® and RIPTIDE were all developed by the US Army Aeroflightdynamics Directorate. This paper presents the development of modern control laws to improve the hover and low speed handling qualities of the AH-64D using the existing aircraft hardware including the force trim system and partial authority SAS actuators. Development was based on a linear flight dynamics model previously identified from frequency response flight test data using CIFER ® (Ref. 12). The identified model was linked to the control law model to form a closed loop simulation in Simulink ® . An overview of the model following control law architecture used to achieve the required ADS-33E response types is presented. The impact of actuator saturation on the design is discussed. The primary focus of the paper is on the use of CONDUIT ® to perform analysis and control law optimization against multiple handling qualities and control system specifications. Although piloted handling qualities ratings are not presented, the control law design was flown in RIPTIDE to evaluate the closed loop response characteristics in a piloted simulation environment. OBJECTIVE The objective of this study was to develop new flight control laws for the AH-64D to achieve Level 1 handling qualities in the day and in degraded visual environments (DVEs) in accordance with ADS-33E. The DVEs that were considered during the design and development of the system include a moonless, overcast night and brown-out conditions from dust kicked up during near earth operations. Both of these conditions result in a Usable Cue Environment of 3 (UCE=3). The mission task elements to be considered were those for attack rotorcraft. The constraints on the study were that these objectives be achieved through software upgrades to the flight control laws with no significant changes to the mechanical flight controls. The existing flight control system includes mechanical linkages from the pilot and copilot/gunner stations to the primary actuators. Partial authority SAS servos are built into the primary actuators and are capable of augmenting the actuator output by ±10% of the total pilot control authority in the lateral, directional and collective axes. The pitch SAS has 20% forward and 10% aft authority. The trim feel system consists of a magnetic brake which allows the pilot to reset the stick forces using a force trim release button on the cyclic stick. There are no trim actuators on the AH-64D.