Verification of fighter aircraft command augmentation control laws by nonlinear simulation

The availability of digital MSI and LSI has made digital flight control systems more cost effective than analog systems for all but the simplest systems. The organization of a digital flight control system is concerned not only with implementation of the control laws and associated logic but also with the management of the data used in these computations and the organization of the computation. The configuration taken by a digital flight control system is dominated by the integrity and data handling requirements. This paper considers the impact of these factors on the design and operation of digital flight control systems with particular emphasis on the use of fiber optics for transmission of data between lanes.

Recently, the Plant Input Mapping (PIM) method was proposed as a digital-redesign (or an emulation) approach in digital control system design. This method guarantees stability of a redesigned digital control system for a wide range of sampling periods. In this paper the PIM method is applied to the design of a digital model-matching flight control system. In the model-matching control system design a plant must have no unstable zeros. However, unstable zeros appear for small sampling periods in a step invariance model of a continuous-time (CT) plant with the relative degree greater than two, even if the CT plant has no unstable zeros. Hence, in such a case, the direct-digital approach cannot be applied to the design of a digital model-matching control system. By contrast, the digital-redesign approach such as the Tustin's method or the PIM method is free from the problem. Simulation results for an unstable VTOL aircraft show that the proposed method gives better performance than the conventional design methods.

It is demonstrated that reconfigurable digital multiple model-following flight control systems incorporating fast-sampling error-actuated digital PID (proportional plus integral plus derivative) controllers can be easily designed using directly measurable input/output data. Thus, for example, it is shown in the case of the F-16 aircraft that desired flying qualities with respect to both pitch-rate and angle-of-attack response characteristics can be simultaneously achieved in the absence of sensor failures. In addition, in the case of failure of the angle-of-attack sensor, it is shown that desired flying qualities with respect to pitch-rate response characteristics and satisfactory flying qualities with respect to angle-of-attack response characteristics can still be achieved by reconfiguring the digital controller. >

Over the past forty years, the number of fighter aircraft controlled flight into terrain (CFIT) accidents per flight hour has not improved despite the addition of multiple manual warning systems. The Air Force Research Laboratory (AFRL) has undertaken a program to develop an automatic system that will take over when the pilot can no longer recover the aircraft, perform an automatic recovery and quickly return control to the pilot. It is called the Automatic Ground Collision Avoidance System or Auto GCAS. This system has been successfully demonstrated on a Block 50 F-16 with a digital flight control system as shown in Figure 1 and is transitioning to operational Block 40/50 F-16s. Until now the technology could not be applied to early model F-16s with analog flight control systems because of the lack of a high authority autopilot needed for the automatic recovery and the absence of a way for the flight control system to ascertain system integrity to insure safe operation. AFRL is now focused on finding a technical solution that will allow the implementation of Auto GCAS on aircraft with analog flight control systems. The effort is called Analog Auto GCAS. The Analog Auto GCAS project has developed an innovative method of adding redundant digital processor modules to the existing analog flight control computers of pre-block 40 F-16's. This will not only allow Auto GCAS to be added to F-16s with analog flight control computer but demonstrates a technical path forward for incorporating Auto GCAS onto other older aircraft with analog flight control computers. This paper provides a description of Auto GCAS, discusses the changes needed to implement Auto GCAS on an analog aircraft, and covers the results to date as well as the future path forward for implementing Auto GCAS on the pre-block 40 F-16 fleet.

From the early 1970s conventional digital and analogue electronic systems have gradually been superseded by microprocessor-based designs. Subsequent experience shows that their reliability rarely exceeded that of earlier designs, due mainly to software errors. The root cause was the lack of design formality and rigour in producing software. Yet in spite of this the micro has been tremendously successful in real-time systems. Clearly, the effects of software problems have been overstated. However, the one area which couldn’t tolerate such error rates is that of safety-critical software. Consequently work was put in hand to investigate this, especially in the aerospace field. One early study was that started in 1976 by Elliott Brothers, London, ‘the study of the design, development, analysis and testing of high integrity software for digital flight control systems’. This concluded that the most promising way forward was based on the semi-formal methods described in chapter 4. These proved to be very effective; yet error rates can still be unacceptably high. Better techniques are needed.

In the face of shrinking defense budgets, survival of the United States rotorcraft industry is becoming increasingly dependent on increased sales in a highly competitive civil helicopter market. As a result, only the most competitive rotorcraft manufacturers are likely to survive. A key ingredient in improving our competitive position is the ability to produce more versatile, high performance, high quality, and low cost of ownership helicopters. Fiber optic technology offers a path of achieving these objectives. Also, adopting common components and architectures for different helicopter models (while maintaining each models' uniqueness) will further decrease design and production costs. Funds saved (or generated) by exploiting this commonality can be applied to R&D used to further improve the product. In this paper, we define a fiber optics based avionics architecture which provides the pilot a fly-by-light / digital flight control system which can be implemented in both civilian and military helicopters. We then discuss the advantages of such an architecture.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

This thesis outlines the effort in developing an integrated simulation framework for assessing the stability and control characteristics of an Unmanned Aerial Vehicle (UAV). The objectives are to understand flight instabilities experienced by the UAV and to propose solutions for the flight stability and control problems via an integrated modeling framework. The basic core of the framework involves the incorporation and integration of the UAV aerodynamic characteristics, flight stability derivatives and digital flight control system, into a six degree-of-freedom flight simulation model, the outputs of which are visualized graphically. To support the development of the simulation model, a comprehensive aerodynamic database for the UAV is required. Empirical aerodynamic estimation software such as USAF Digital Datcom and computational flow simulation software suites such as FLUENT and Solidworks Flo Works have been used to estimate the aerodynamic force and moments as well as stability and control derivatives of the UAV. By combining these estimates with wind tunnel data, a comprehensive aerodynamic characteristics model is constructed for augmenting the flight dynamics model of the UAV. Initially, a de-coupled longitudinal and lateral stability analysis based on the small disturbance theory has been developed to understand the stability modes of the UAV. A coupled longitudinal and lateral stability analysis was then developed to study the effects of the aerodynamic coupling caused by the single propulsion system to the stability and control characteristics. The model is created within the programmable software environments of d at lab^, ~imulink' and Aerospace s lock set@ which are used to construct the six-degree-of-freedom model for the dynamical motion of the UAV. The inputs to the model include the aerodynamic information based on the aerodynamic characteristics, stability derivatives, engine characteristics, flight variables and atmospheric disturbances. The outputs from this model are the responses of the flight variables depicting lateral, longitudinal and directional stability characteristics of the flight vehicle. Virtual Reality based systems such as the Flight Gear software has been used as a graphical interface for displaying the UAV trajectory, the orientation of the vehicle and the stability and control characteristics of the UAV based on the outputs from the model.

In airline service the reliability of a system is more than the confirmed failures for the system; it is the total removal experience of the system. Thus, the reliability of a system lies not only in the piece part failure rates, but also in the capability to verify and isolate failures. The application of digital technology brings several significant reliability improvements to automatic flight control systems (AFCS) when compared to contemporary analog systems. These advantages are demonstrated by the experience of the digital air data computer (DADC) of the DC-10 and the digital AFCS of the JA-37 Viggen. Experience with this equipment is reviewed, and the results are interpreted in terms of projections for airline DAFCS reliability. The digital system built-in test implemented by a stored program and the central processor gives a system integrity and dispatch reliability unequaled by analog systems. This high-integrity self-test reduces removal rates by giving line personnel a trustworthy tool and more complete automatic test processes for verifying maintenance actions. Digital circuit technology is directly suitable to largescale integrated circuits (LSIC) which reduce piece part counts and improve LRU reliability. Digital circuits are less subject to drift and the attendant difficulty to detect failures. These factors, coupled with the inherent high-integrity self-test, provide the basis for a significant improvement in reliability by the use of a digital automatic flight control system.

Beginning with process controls in the mid fifties, digital control systems have seldom met all the claimed advantages and in many cases have been outright failures. Airborne digital computers have just begun to appear in the commercial jet transport market after several years of experimental and military development. The success of these systems in meeting user expectations has not been overwhelming, which leads to the question: Why change from known analog flight control technology to digital? This paper examines that question by reviewing the state of the art in analog flight control system technology and evaluating this against current airline problems. A realistic means of evaluating claimed advantages for new technology is thus established. The advertised solutions of digital technology to analog problems are examined and, in some cases, debunked. However, consideration of the real problems and the capabilities of digital systems leads to the conclusion that there is a real payoff. Evidence from background studies at Boeing is offered to support this conclusion. A description of the digital flight control system electronics (FCSE) development program for the new Boeing 7×7 airplane is also provided. This program is currently in progress, and its specific objective is to assure that the real payoff is achieved for the subsequent production system.

The solution for the non-determinism of real-time expert systems is found in the state-based approach. Timing verification can be performed off-line before the expert system is taken in use. This approach has been verified on a digital (fly-by-wire) flight control system of a jet aircraft.

Flight evaluation of a digital multimode flight control system in anA-7D aircraft

Digital Model-Reference Flight Control of Aircraft with Unstable Sampling Zeros

Two variations of each of three types of digital optical transducers have been developed to provide the technology base for a digital optical flight control system for US Army helicopters. The transducer types are rotary position, linear position, and differential hydraulic pressure and all are electrically passive at the transducer location. The development and test of these six transducers has led to the selection of a reflective, time delay multiplexed concept using a single interconnecting fiber for use on the Advanced Digital Optical Control System Flight Demonstrator Program (ADOCS FDP).

The feedback I inearization method is appl ied to odation of aircraft failures occuring at the control effectors or the airframe. The failures are identified as parameter changes in the six-degree-offreedom nonlinear equations of motion by the recursive least square algorithm. The control parameters are updated using the latest estimated system parameters. In order to allow one to use digital computer in implementation, a discrete time servo controller is designed for the surface actuators and the engine, where the reference inputs are given by the continuous time control law. The performance of the RFCS is demonstrated through computer simulation using the nonlinear model of an aircraft which has half o f the right wing broken off.

The insertion of communication networks in closed-loop of digital control systems makes the system synthesis and analysis complex. In this paper, we consider the combination of classical reliability, risk and safety evaluation with typical control system through digital networked control systems. The main purpose is to study the influence of transmission delays and packet dropouts on the reliability of digital networked control systems by using a modified statistical method-Monte Carlo simulation. The behaviors of two types of fault are modelled by different statistical distribution. The control strategies we apply for digital control systems are Proportion Integration Differentiation control strategies, generated by Genetic Algorithm. In terms of a realistic system model, we evaluate and compare the influences on the reliability of digital networked control systems with different control strategies from quantitative aspect. Numerical examples are presented to illustrate the effectiveness and correctness of the approach.