Pilot-induced Oscillations Research Articles

Smart-Cue and Smart-Gain Concepts to Alleviate Loss of Control

Unfavorable pilot-vehicle-system interactions including pilot-induced oscillations have long been an aviation safety problem. This was true 100 years ago when the Wright brothers first demonstrated powered flight, and it is still true today even for advanced fly-by-wire flight control systems. Although the effective aircraft dynamic properties involved in these events have been extensively studied and understood, similar scrutiny has not been paid to the many aspects ofthe primary manual control system that converts the pilot control inputs to motions of the control surfaces. It has often been tacitly assumed that the adoption of fly-by-wire systems has eliminated the primary manual control link as an important player in loss of control situations. Consequently, the impact of static and dynamic control system effects that distort ideal pilot-to-surface relationships, the near absence of manipulator tactile cues for some fly-by-wire systems, as well as the total elimination in fly-by-wire systems of some favorable cues present in traditional hydromechanical systems have not received detailed attention. The concept of dynamic has significantly evolved in work conducted by Systems Technology, Inc. during a two-phase program sponsored by NASA Dryden Flight Research Center. In this work the of interest results from control surface rate limiting and is quantified by the surface position error, whereas the distortion metric is the position lag. A force feedback cue (the constraining function) and/or a command path gain reduction are created when the position error exceeds the position lag (the alerting function). The smart-cue and smart-gain concepts, as described in this paper, are remarkable in their simplicity. The overall implementation does, however, require hardware in the form of a programmable, back-driven force feel system.

Stabilator Failure Adaptation from Flight Tests of NF-15B Intelligent Flight Control System

Adaptive flight control systems have the potential to be more resilient to extreme changes in airplane behavior. Extreme changes could be a result of a system failure or of damage to the airplane. A direct adaptive neural-network-based flight control system was developed for the National Aeronautics and Space Administration NF-15B Intelligent Flight Control System airplane and subjected to an inflight simulation of a failed (frozen) (unmovable) stabilator. Formation flight handling qualities evaluations were performed with and without neural network adaptation. The results of these flight tests are presented. Comparison with simulation predictions and analysis of the performance of the adaptation system are discussed. The performance of the adaptation system is assessed in terms of its ability to decouple the roll and pitch response and reestablish good onboard model tracking. Flight evaluation with the simulated stabilator failure and adaptation engaged showed that there was generally improvement in the pitch response; however, a tendency for roll pilot-induced oscillation was experienced. A detailed discussion of the cause of the mixed results is presented.

Metrics for the Evaluation of Pedal Force/Feel Systems in Transport Aircraft

A piloted, fixed-base, simulation study was conducted at NASA Langley Research Center to determine optimum rudder pedal force/feel characteristics for transport aircraft. After completion of this simulation study, an evaluation of four metrics for assessing rudder pedal force/feel system characteristics previously presented in the literature was conducted. This evaluation was based upon the numerical handling-qualities ratings assigned to a variety of pedal force/feel systems used in the simulation study. It is shown that the majority of rudder pedal force/feel system configurations that received poor normalized pilot ratings also failed one or more of the metrics. In addition, the majority of configurations that received satisfactory normalized pilot ratings also passed all of the metrics. Based upon the simulation results, a fifth metric was identified that improved the ability of the complete set of metrics to identify poor force/feel system designs. It is suggested that these metrics could form the basis of a certification requirement for transport aircraft. One of the pilot-induced oscillations that occurred in the simulation was analyzed and a possible triggering event was identified.

Identifying a Pilot-Induced Oscillation Signature: New Techniques Applied to Old Problems

Elimination of pilot-induced oscillations by design remains an elusive goal. Throughout the history of powered flight, as manual flight control systems have morphed and evolved, so too have the pilot-induced oscillations that often plague them. In the mid-1990s, the United States Air Force declared as part of its Unified Pilot-Induced Oscillation Theory program that the cumulative reduction of pilot-induced oscillations shall be 80% via criteria, 99% via evaluation methods, and 99.95% via detection and compensation. This too remains an elusive goal. One reason lies in the lack of consensus regarding the definition of exactly what is a pilot-induced oscillation. The intent of this paper is to coalesce the many definitions of pilot-induced oscillation into a single entity. Then, newly developed analysis techniques are applied to a flight-test database to expose the unique characteristics or signature of pilot-induced oscillations, at least as they apply to a representative mission operation: the probe-and-drogue aerial refueling task.

THE DETECTION OF PILOT‐INDUCED OSCILLATIONS

Fly‐by‐wire systems enable superior control of chosen flight parameters. A pilot can modify stabilized parameters by adequate movement of control inceptors such as a side‐stick or trust lever. Fly‐by‐wire control reduces the load on a pilot and allows a pilot to focus on the main tasks. Unfortunately, the use of more complicated interfaces between human and machine can cause incorrect pilot behavior and in many cases lead to erroneous interactions between the operator and effective aircraft dynamics. The structure of control laws and dynamics of electromechanical actuators are especially important factors. They can influence unfavorable aircraft‐pilot coupling and can lead to pilot‐induced oscillations (PIO) in certain cases. The automatic detection of PIOs is presented in this paper. The practical realization of a PIO‐detector and examples of diagnostics of human‐machine systems are reported on in this article.

Model Predictive Controller Design to Suppress Rate-Limiter-Based Pilot-Induced Oscillations

Most severe pilot-induced oscillations (PIOs) are characterized as the presence of actuator rate limiting. Rate limiting causes phase lag and magnitude reduction between the limiter input and output when the input signal rate exceeds the rate limitation. This is the main cause of PIOs. Herein, two controllers, in which actuator rate limiting is redefined as input constraints, are designed based on the model predictive control (MPC) strategy and applied to the pilot-aircraft loop in both parallel and serial manners. In the parallel implementation, the MPC controller can suppress rate-limiter-based PIOs. In the serial implementation, the MPC controller can compensate the phase lag due to rate limiting, and hence, help the pilot prevent PIOs from occurring in advance.

Pilot-Induced Oscillation Analysis with Rate Limiter

This paper proposes a describing function method for limit cycle analysis of a system with a rate limiter. First, the rate limiter is defined and its describing function is derived in a strict manner.Then, the stability analysis using the describing function of the rate limiter is executed. In many cases of limit cycles, the output of the rate limiter becomes triangular, and in order to deal with such cases by the describing function, a new method to modify an open-loop transfer function is introduced. This method can predict the limit cycle occurrence more exactly, which is applicable to a prediction of a rate-limiter based PIO (Pilot-Induced Oscillation) of an aircraft.

ROBUST CONTROL APPLICATIONS

This plenary presentation will demonstrate the growing importance of robust control theory by describing its application to some non-trivial practical control problems. The talk will begin with a description of the design and flight test of a new batch of H∞ controllers for the Bell 205 helicopter. At the heart of each controller is an H∞ loop-shaping controller, augmented with a hand-tuned reference filter to improve tracking performance and to reduce a perceived phase lag which pilots had complained of previously. Flight testing revealed that, with such an architecture, it was relatively easy to get Level 1 handling qualities ratings in low aggression manoeuvres. Further fine tuning resulted in Level 1 qualities for high aggression manoeuvres and one controller performed to Level 1 standard in all manoeuvres tested.The second half of the talk will consider how robust control techniques can be used to design anti-windup compensators to counter problems associated with saturating actuators, such as pilot-induced oscillations in aircraft and stability/robust performance problems in state-of-the-art hard-disk drive servo systems.The paper is written in two parts: Part 1 considers helicopter control and Part 2 addresses saturation problems in high-performance head-positioning servo systems in high-density hard-disk drives.

Rudder Control Strategies and Force/Feel System Designs in Transport Aircraft

An analytical investigation of rudder control strategies in transport aircraft is undertaken. The study was motivated by an airline crash in which rudder usage by the pilot might have been a contributing factor. A detailed mathematical model of the human pilot is employed in a computer simulation to examine the closed-loop nature of rudder control strategies. Emphasis is placed on pilot-induced oscillation tendencies. A task suitable for pilot-in-the-loop flight-simulator evaluation of rudder control systems is proposed. A comparison of the pedal force/feel characteristics of three transport aircraft and a military helicopter is provided. The adverse effect that highly nonlinear pedal force/feel system characteristics can have on pilot/vehicle response predictability is demonstrated. A linearity index is defined to quantify the linear force/displacement characteristics of any force/feel system. Nomenclature A(−) =g ain factors in vehicle transfer functions ayps = lateral acceleration at pilot’s station, g

Internal Model and Saturating Actuation in Human Operation From View of Human-Adaptive Mechatronics

Human-adaptive mechatronics (HAM) is the area of mechatronics which adapts to the operator skill and assists its improvement. The analysis of human control action is one of the fundamental problems in the study of HAM. A special feature of human control action is the action being saturated with respect to the amplitude and velocity. At the same time, the human does not pay attention continuously to the response but intermittently scans and gets the information. In this paper, the continuous control action based on the scanned information is studied, and the desired trajectory of the human control action is considered to be generated by the closed-loop system including the internal model in the feedback path. Since the visual information is scanned intermittently, the closed-loop reference generator is considered as a sampled-hold system. The feedforward function of the cerebellum can be interpreted as the reference generator with a long scanning interval for the skilled operation. The saturating control action causing the pilot-induced oscillation is studied by taking the swing-up control of a single pendulum from the pendant to the upright position as an example. The two swing-up control laws are studied for reachability of the unstable nonlinear pendulum. One is the linear combination of sine function of the position and angular velocity, and the other is the variable-structure control for the sliding-mode function similar to the linear combination control law. The reachability is analyzed successfully by the color map.

Handling Qualities Analysis of the Wright Brothers' 1902 Glider

From a handling-qualities standpoint, it can be argued that the Wright brothers’ 1902 glider represents their most significant design breakthrough on the basis that their subsequent aircraft retained the same basic control concept. This paper is a study of that aircraft, and results from wind-tunnel tests, modeling, and flight simulation trials are presented. Unstable in both longitudinal and lateral axes, the 1902 glider’s flight dynamics provide maneuverability but also a tendency to pilot-induced oscillation in tight tracking tasks. The Wrights taught themselves to fly on this aircraft and paved the way for the development of their powered flyer. This paper stands as a celebration of the achievements made by the Wrights 101 years ago and demonstrates the technological leaps they made in the process of designing, building, and flight testing the 1902 glider. The results are placed in the context of modern handling-qualities engineering.

Investigating The Role of Rate Limiting in Pilot-Induced Oscillations

From the Wright Flyer to fly-by-wire, the phenomenon of pilot-induced oscillations (PIO) has been observed on prototype, experimental, and operational military and commercial aircraft. The introduction of irreversible control systems with surfaces driven by powered actuators brought many benefits along with increased system complexity and the introduction of additional nonlinearities. Chief among these nonlinearities are the hardware and software rate limits associated with the control surface actuators. Basic sizing tradeoffs conducted in the design process set the maximum rate of an actuator, whereas software rate limits are introduced to prevent overdriving the control surface when loads or structural limitations exist. It is demonstrated that, when operating as intended, there are usually no ill effects associated with rate limits; however, certain conditions can lead to a highly saturated condition. This results in the sudden introduction of significant added phase lags to the pilot-vehicle system. In many cases, the end result is often PIO or other related loss of control events. One of the earliest well-documented PIO events involving rate limiting occurred on the first flight of the X-15. This event is significant in that common linear systems analysis techniques do not reveal a susceptibility to PIO. The analysis of X-15 flight 1-1-5 and the results of a more recent flight research program are presented in detail.

Fly-by-wire T and E challenge

Systems developers and testers have always assumed that human compensation is measurable, or, at least, that a cognizant and trained tester is able to identify and detect compensation. More than one study conducted at the Wright-Patterson large amplitude multi-mode aerospace research simulator (LAMARS) facility indicates that this is not necessarily true. Test pilots were able to compensate sufficiently to fly and meet defined performance standards on intentionally crippled aircraft flight control designs. These flight control systems were designed to trigger pilot-induced oscillations, but, in most cases, test pilots could compensate sufficiently to prevent pilot-induced oscillations and to control the simulated aircraft. Anecdotally, this points to a colossal deficiency in the test of highly augmented aircraft systems that has been borne out by multiple aircraft accidents in actual aircraft designs: natural pilot compensation is sufficient to allow faulty designs to reach production and operational service while hiding critical handling qualities cliffs that can lead to loss of an aircraft. This observation, if applied across the gamut of human factors experimentation, has vast ramifications for test and evaluation and development of all human interface systems.

Gain-Phase Margin Analysis of Pilot-Induced Oscillations for Limit-Cycle Prediction

This investigation attempts to forecast the limit cycles of pilot-induced oscillations (PIOs) by combining the gain-phase margin tester, the M-locus, and the parameter plane methods. First, one position- or rate-limited nonlinear element is linearized by the conventional means of describing functions. The stability of an equivalent linearized system with adjustable parameters is then analyzed using stability equations and the parameter plane method. Additionally, the minimum gain-phase margin of the PIO system at which a limit cycle can occur is determined by inserting the gain-phase margin tester into the forward open-loop system. Moreover, a simple method is developed to identify the intersections of the M locus and the constant gain and phase boundaries in the parameter plane. In so doing the exact relationship between the gain-phase margin and the characteristics of the limit cycle can be clearly determined. The results of this study demonstrate that these procedures can enhance the analysis of PIO over analysis by other methods in the literature. This approach is extended to PIO analysis with multiple nonlinearities.

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The focus of this study is on PIO to be solved in the development of modern flight control systems. Especially, it is important to investigate the fully-developed PIO as a worst case for the safety of piloted airplanes and to analyze the limit cycle phenomenon including the effects of actuator rate limiting and feedback control loop. Lateral-directional flight control system was taken for instance, this paper proposes a technique to design the feedback control law to have a positive effect on PIO to decrease the amplitude of the oscillation.

The role of CFD in aerodynamics, off-design

AbstractWe discuss the status, trends and long-term ambitions of Computational Fluid Dynamics (CFD) when applied away from the design point or concept, and therefore in the historically weak areas of CFD. This includes both undesirable flight conditions, such as stall, and undesirable products of the flow, such as noise. All pose the great challenge of turbulence, and accuracy is as dependent on the ideas behind the turbulence treatment as it is on computing power. A measured shift from Reynolds-averaged representations to large-eddy simulations will take place. Empiricism, both turbulence and engineering related, will recede only step by step over many years. Yet, CFD will make full use of every increase in computer power of this century. Increasingly, CFD will compete with flight tests, not just with wind tunnels, and will be validated by flight tests. Integration with other disciplines will allow us to predict crucial phenomena such as flutter, sonic fatigue, and pilot-induced oscillations. A gap will remain at any time between the phenomena amenable to a ‘grand challenge’ calculation and those amenable to a fully CFD-based design process, because certification involves thousands of conditions. The highest demand currently is in community and cabin noise for which industrial-accuracy methods are non-existent. On the other hand, gratifying progress has occurred in the area of stall and spin.

Nonlinear Prefilter to Prevent Pilot-Induced Oscillations due to Actuator Rate Limiting

For most modern recorded pilot-induced oscillations (PIOs), for example, YF-22, the flight records show that actuator rate limiting occurred. The root causes of PIOs in which actuator rate limiting occurs are demonstratedthrough simulations, and then a newly developed nonlinear rate limiter prefilter (RLPF) and a software rate limit (SWRL) filter are shown to be useful in preventing the PIO. Motion base simulator and flight-test results show that the RLPF/SWRL filter is successful in preventing departure and/or PIO.

Pilot-Induced Oscillation Analysis with Actuator Rate Limiting and Feedback Control Loop

Fully-developed pilot-induced oscillation (PIO) is an important issue to be solved in the development of modern fly-by-wire flight control systems. In this paper, the fully-developed PIO is analyzed as a worst case for the safety of piloted airplanes, including actuator rate limiting, feedback control loop, and pilot delay by using describing function method. It is shown that the predictions obtained with this method closely match results of the simulation in the frequency and the amplitude of the PIO limit cycle. And it demonstrates that the feedback control loop has a positive effect on PIO and decreases amplitude of the oscillation.

Recent experience in flight testing for pilot induced oscillations (PIO) on transport aircraft

AbstractThough the subject of pilot induced oscillations (PIO) is not new, it has garnered significant attention in the past several years. Since the mid-1990s, Boeing Commercial Airplanes has been conducting specific flight tests of its products in order to evaluate PIO tendencies. Beginning with the 777-200, a generic suite of test manoeuvres has been used to evaluate PIO tendencies on each product. The testing has been conducted on a ‘window of opportunity’ basis with the intent to gather data and evaluate each model. To date, specific evaluations have been carried out on six different aircraft models spanning a wide range of aircraft sizes, inertial characteristics, and control system implementations. Each manoeuvre in the generic suite is discussed in detail. In addition, along the way, a large number of other manoeuvres have been used at various times to evaluate PIO tendencies. These are briefly described. No single test manoeuvre or technique has been identified which provides effective discrimination of PIO tendencies. Finally, the subject of the pilot in the loop is discussed with regard to achieving consistency in conducting PIO evaluations.

Simulator Platform Motion Effects on Pilot-Induced Oscillation Prediction

Simulator motion platform characteristics were examined to determine if the amount of motion affects pilotinduced oscillation prediction. Five test pilots evaluated how susceptible 18 different setsof pitch dynamics were to pilot-induced oscillations with three different levels of simulation motion platform displacement: large, small, and none. The pitch dynamics were those of a previous in-e ight experiment, some of which elicited oscillations. These in-e ight resultsserved as truth data for the simulation. As such, the in-e ight experiment wasreplicated asmuch as possible. Objective and subjective data were collected and analyzed. With large motion, pilot-induced oscillation and handling qualities ratings matched the e ight data more closely than with small motion or no motion. Also, regardlessof the aircraft dynamics, large motion increased pilot cone dencein assigning handling qualitiesratings, reduced safety pilot trips, and lowered touchdown velocities. Whereas both large and small motion provided a pitch rate cue of high e delity, only large motion presented the pilot with a high e delity vertical acceleration cue.