Pilot-induced Oscillation Tendencies Research Articles

Model Reference Control for Reducing Pilot-Induced Oscillation Tendencies

Lags between pilot inputs and aircraft responses may lead to a pilot–vehicle system entering into a pilot-induced oscillation. A model reference control element can compensate for these lags, avoiding pilot-induced oscillations and improving tracking performance. Neural network compensation drives the combined controller–plant system to exhibit a closed-loop response similar to that of an idealized system, immune to the factors that trigger pilot-induced oscillations. Actuator rate limiting has been a historically ubiquitous cause of pilot-induced oscillations; meanwhile, aeroelastic effects pose a threat to future lightweight, flexible aircraft designs. Model reference control can reduce pilot-induced oscillation tendencies caused by either of these factors. This paper presents the methods used to train the model reference controller, including conditions for system stability under model reference control. It then showcases results from a pair of simulation experiments applying the control design to compensate for rate limiting and aeroelasticity, respectively. Results demonstrate that the model reference control scheme reduces pilot-induced oscillation tendencies and improves closed-loop tracking performance.

In-Flight Test Campaign to Validate PIO Detection and Assessment Tools

This paper describes a joint research campaign conducted by the German Aerospace Center (DLR) and the National Research Council Canada (NRC) to explore methods and techniques to expose rotorcraft pilot-induced oscillations (PIOs) during flight testing. A flight test campaign was conducted at NRC using the Bell 205 experimental aircraft. Results show that, particularly for the lateral axis, ADS-33 tasks can be successfully applied to expose PIO tendencies. Novel subjective and objective criteria were used during the test campaign. PIO prediction boundaries of the objective phase-aggression criteria (PAC) detection algorithm were validated through results obtained. This was the first use of PAC with data recorded in-flight. To collect subjective feedback, the aircraft–pilot coupling (APC) scale was used. This was the first use of the novel scale in-flight and received favourable feedback from the evaluation pilot. Modifications to ADS-33 mission tasks were found to successfully improve the ability to consistently expose PIOs.

Advancements in Predictions of Flying Qualities, Pilot-Induced Oscillation Tendencies, and Flight Safety

This work discusses the various groups of criteria for the prediction of the flying qualities (FQs) and pilot-induced oscillations developed at the Pilot-Vehicle Laboratory of the Moscow Aviation Institute over the last 25 years. The first criteria group includes requirements for the parameters of highly augmented aircraft. Four of these criteria for FQ and pilot-induced oscillation predictions were considered, and the same modification principles were proposed for all the criteria. The second criteria group is based on the requirements for the pilot–aircraft system, and some of the criteria developed at the Pilot-Vehicle Laboratory are discussed. The third criteria group developed at the Moscow Aviation Institute is used to calculate the pilot’s ratings group. Some of these criteria for single-loop, multichannel, and multimodal piloting tasks were considered. The relationship between the requirements for flight safety and the FQs demonstrates the necessity to modify these requirements for several aircraft classes.

Prediction of nonlinear pilot-induced oscillation using an intelligent human pilot model

This paper proposes a method to predict nonlinear Pilot-Induced Oscillation (PIO) using an intelligent human pilot model. This method is based on a scalogram-based PIO metric, which uses wavelet transforms to analyze the nonlinear characteristics of a time-varying system. The intelligent human pilot model includes three modules: perception module, decision and adaptive module, and execution module. Intelligent and adaptive features, including a neural network receptor, fuzzy decision and adaptation, are also introduced into the human pilot model to describe the behavior of the human pilot accommodating the nonlinear events. Furthermore, an algorithm is proposed to describe the procedure of the PIO prediction method with nonlinear evaluation cases. The prediction results obtained by numerical simulation are compared with the assessments of flight test data to validate the utility of the method. The flight test data were generated in the evaluation of the Smart-Cue/Smart-Gain, which is capable of reducing the PIO tendencies considerably. The results show that the method can be applied to predict the nonlinear PIO events by human pilot model simulation.

Longitudinal Pilot-induced Oscillation Tendencies Prediction Method of Receiver Aircraft in Hose-drogue Aerial Refueling

This paper proposes a new method for predicting longitudinal Pilot-Induced Oscillation (PIO) tendencies of receiver aircraft in hose-drogue aerial refueling. Conventional criteria of PIO are unsuited to predict the tendency while docking, owing to attitude and path of receiver aircraft needed to be control precisely. Based on mission-oriented flying qualities approach, the ground-based refueling simulation experiments have been conducted for receiver aircraft of different handling characteristics. The results show lower longitudinal PIO tendency requires adequate bandwidth of flight path response, appropriate pitch attitude zero of the short period mode and control gain. The new assessment method will be significant for flight control law design of aerial refueling mode.

Impact of turbulence and degraded visual environment on pilot workload

Offshore helicopter operations are frequently conducted in both turbulent and degraded visual environments (DVE). This investigation assesses the combined influence of turbulence and DVEs on pilot workload to identify first limits for safe operations. Flight tests using a simulation model of the research helicopter ACT/FHS (active control technology/flying helicopter simulator) flight mechanics model were conducted in the air vehicle simulator (AVES) at DLR Braunschweig. Tests were completed using four pilots, and results show the effects on pilot workload, task performance and control input activity. It was found that DVE and turbulence increase the workload and reduce task performance, but each in a different manner. Furthermore, the impact on control activity and pilot-induced oscillation tendencies are shown to have dependency upon the environmental conditions.

Piloted Evaluation of a Control Allocation Technique to Recover from Pilot-Induced Oscillations

This paper describes the maturation of a control allocation technique designed to assist pilots in recovery from pilot-induced oscillations. The control allocation technique to recover from pilot-induced oscillations is designed to enable next-generation high-efficiency aircraft designs. Energy-efficient next-generation aircraft require feedback control strategies that will enable lowering the actuator rate limit requirements for optimal airframe design. A common issue on aircraft with actuator rate limitations is they are susceptible to pilot-induced oscillations caused by the phase lag between the pilot inputs and control surface response. The control allocation technique to recover from pilot-induced oscillations uses real-time optimization for control allocation to eliminate phase lag in the system caused by control surface rate limiting. System impacts of the control allocator were assessed through a piloted simulation evaluation of a nonlinear aircraft model in the NASA Ames Research Center’s Vertical Motion Simulator. Results indicate that the control allocation technique to recover from pilot-induced oscillations helps reduce oscillatory behavior introduced by control surface rate limiting, including the pilot-induced oscillation tendencies reported by pilots.

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

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.

Robust flight control design with handling qualities constraints using scheduled linear dynamic inversion and loop-shaping

A technique for obtaining a full-envelope decoupled linear flight control design is presented. The methodology begins with a reduced-order linear dynamic-inversion technique that is scheduled over the flight envelope. The reduced order dynamic inverter can offer a significant reduction in the number of state variables to be sensed or estimated as compared to typical applications of inverse dynamic control. The technique can provide desired input-output characteristics including control decoupling. The required gain scheduling of the reduced order dynamic inversion is straightforward. Uncertainty is introduced by perturbing the stability derivatives in the vehicle model at each of the flight conditions considered. The effects of uncertainty are then reduced by additional feedback loops involving a diagonal compensation matrix obtained through application of a loop shaping procedure based upon a quantitative feedback theory predesign technique. The tendency of quantitative feedback theory to produce high-bandwidth conservative designs is mitigated by the scheduling and decoupling associated with the dynamic inversion. Finally, handling qualities and pilot-induced oscillation tendencies are evaluated using a structural model of the human pilot implemented in an interactive computer program that can include the effects of nuisance nonlinearities such as actuator saturation. The proposed methodology is applied to the design of a lateral-directional flight control system for a piloted supermaneuvarable fighter aircraft.

Aircraft Handling Qualities and Pilot-Induced Oscillation Tendencies with Actuator Saturation

A pilot/vehicle analysis technique is offered that can provide predictions of handling-qualities levels and pilotinduced oscillation rating levels in single-axis tracking tasks, with linear or nonlinear vehicle dynamics. The technique is applied to the three vehicle conŽ gurations evaluated in the series of  ight tests sponsored by the U.S. Air Force that are identiŽ ed as HAVE LIMITS. The ability of the analysis technique to provide excellent quantitative predictions of the effects of actuator rate saturation on the handling qualities and pilot-induced oscillation susceptibility of the aircraft conŽ gurations is demonstrated. An interactive computer code is discussed that automates much of the pilot/vehicle analysis.

Development of the Probe-and-Drogue Handling Qualities Demonstration Maneuver

The development of the probe-and-drogueaerial refueling task as a handlingqualities demonstrationmaneuver is described. The probe-and-drogue task is an excellent test of handling qualities for precision  ying and is also a superb check for pilot-induced oscillation tendencies. The greatest difŽ culty with this maneuver has been in the deŽ nitionof the task itself including the deŽ nitions of speciŽ c performance limits. The task described herein differs from that described in the Notice of Change to Flying Qualities of Piloted Aircraft (MIL-STD-1797A) in that it requires precise drogue tracking in addition to a percentage of successful hookups. A task representative of that deŽ ned in MIL-STD-1797A that required at least three hookups in six attempts for desired performance was used as the evaluationmethod for theHAVEGAS project conducted by the U.S.Air Force Test Pilot School. This version of the maneuver, however, was found to be too easy even for poorly rated airplanes. Revised maneuver descriptions and performance requirements were developed and evaluated in the HAVE GAS II project that was undertaken as part of the U.S. Air Force Demonstration Maneuvers program and again conducted by the U.S. Air Force Test Pilot School. Three candidate tasks were evaluated in this test program: 1) the probe-and-drogue hookup task, 2) a new drogue tracking task, and 3) a new drogue pointing task. Although the drogue tracking task was found to be best for evaluating closed-loop handling qualities, it was limited by the fact that it would not reveal potential problems such as bow-wave effects that may be encountered within 5 ft of the basket. Thus it was concluded that a combination of the drogue tracking task and the probe-and-drogue hookup task should be  own to thoroughly evaluate closed-loop handling qualities in the aerial refueling mission.

An Adaptive Stick-Gain to Reduce Pilot-Induced Oscillation Tendencies

An Adaptive Stick-Gain to Reduce Pilot-Induced Oscillation Tendencies

Higher-order control system dynamics and longitudinal handling qualities

An experimental investigation of the effects of higher-order control system dynamics on the longitudinal handling qualities of a fighter airplane are discussed. This research was undertaken using the USAF/CAL variable stability T-33 airplane as an in-flight simulator. Different higher-order responses were simulated by altering the elevator feel system, elevator actuator, and airplane short-period characteristics. Essentially the same configurations were evaluated by two pilots using a revised pilot rating scale. One pilot also rated the configurations for their pilot-induced oscillations (PIO) tendencies. Comments and ratings were related to a response delay parameter. Many of the higher-order control systems investigated produced pronounced PIO tendencies in flight, and some were considered unflyable with certain higher-order characteristics. A comparison of fixed-base and in-flight evaluations indicated that configurations with significant PIO tendencies were rated poorer in flight, and configurations with little or no PIO tendencies were rated better in flight.