Stability Augmentation System Research Articles

Analysis of the Stability Augmentation System of an Aircraft (SJ30) in Flight Turbulence

Analysis of the Stability Augmentation System of an Aircraft (SJ30) in Flight Turbulence

Flying qualities based time-varying stability augmentation system design for tiltrotor conversion control

Tiltrotors have three flight modes that pose control problems and quality defects during the conversion process. To address this, a novel flying qualities-based time-varying stability augmentation system is designed to achieve multi-mode, nonlinear, and time-varying stability. The system integrates a nonlinear time-varying control law with the flying qualities requirements for all three flight modes. It consists of an inner and outer loop control framework, where the control law in the inner loop is designed based on the Lyapunov theorem of stability. The reference models in the outer loop are derived from the flying qualities criteria to meet level one flying qualities requirements. To evaluate the conversion process, a time-varying flying qualities evaluation method is developed, which includes the conversion path, pilot model, and time-varying flying qualities index. The proposed time-varying stability augmentation control system is then tested through simulation during the conversion process. A pilot-aircraft closed-loop system is established for conducting experiments. Comparison between simulation results and pilot-in-loop experiment results demonstrates the effectiveness of the proposed control system. Furthermore, it proves that the evaluation method is suitable for analyzing time-varying systems. This research can be valuable in designing and evaluating stability augmentation controls for strongly time-varying systems.

Stability augmentation of pitch angle control for maximum power extraction of PMSG-based WTS with pitch actuator uncertainty via [formula omitted] adaptive scheme

This paper presents the stability augmentation of pitch angle control for a permanent magnet synchronous generator (PMSG)-based wind turbine system (WTS) with pitch actuator uncertainty via L1 adaptive scheme. To do this, a novel L1 adaptive scheme is firstly proposed to regulate the pitch angle at above-rated wind speed conditions and maintain the rated rotor speed in the presence of pitch actuator uncertainties in the obtained pitch system. Next, a non-affine PMSG system with uncertainties is utilized to represent the pitch regulated variable-speed WTS. Then, the L1 adaptive scheme is proposed as a stability augmentation system. At this time, the bound conditions are presented to prove the stability and performance evaluation of the proposed L1 adaptive scheme. Finally, through the simulation of the 20−MW PMSG-based WTS, the proposed algorithm proves its efficiency and applicability compared to the existing control methods.

Feasibility Investigation of Attitude Control with Shape Memory Alloy Actuator on a Tethered Wing

This study is aimed at assessing the feasibility of employing an innovative, smart-material-based control effector for an inflatable wing. A shape memory alloy (SMA) actuator is primarily investigated as a control effector in this work for its advantages of a simple actuation mechanism and a high force-to-weight ratio. This paper presents the design, control strategy and simulation results of the SMA actuator used as a stability augmentation system for a small-scale prototype kite. Stable flight of the kite is achieved during open wind tunnel tests using the SMA actuator. Based on experimental and simulation analyses, it is evident that the current SMA actuator is better for low-frequency actuations rather than stability augmentation purposes, as its performance is sensitive to practical conditions. The study also discusses potential improvements and applications of the SMA actuator.

The Study of Selected Aspects of the Suborbital Vehicle Return Flight Trajectory

The article presents the results of preliminary studies of the parameters of the return flight trajectory of a rocket plane for suborbital tourist flights into space. The rocket plane is designed as a tailless vehicle and has an unconventional arrangement of control surfaces: elevons and side plates that can rotate. The main aim of the research presented in this paper is to investigate the dynamic stability of the rocket plane and the response to control in the return suborbital flight. The secondary objective is to study the behavior of the rocket plane with respect to the initial state of the return flight. The key parameters taken into account in this study are the Mach number and G-load. Moreover, a study of the trim condition, dynamic stability and response to control of a rocket plane in the low part of the stratosphere is presented. The tests were carried out using a numerical simulation of the flight of a rocket plane. Dynamic stability was determined on the basis of time history analysis, and the results were compared with the results obtained by solving the eigenvalues problem. The results revealed that the rocket plane should be equipped with a Stability Augmentation System to improve short period damping at supersonic speeds at moderate altitudes. It can also be concluded that the maximum load G and Ma do not occur at the same height of flight. In terms of the effectiveness of the control surfaces, they start working at an altitude of 55 km. Due to the speed regime, the obtained results can be useful in the design of such objects as rocket planes, highly maneuverable and supersonic aircraft.

Stability Augmentation in Lateral Directional Dynamics: A Multivariable Approach

This paper delves into the concept of stability augmentation in lateral-directional dynamics, focusing on a multivariable approach. In aircraft control systems, ensuring lateral-directional stability is crucial for safe and efficient flight. The primary objective is to examine the feedback control system's role in enhancing stability and how it interacts with the aircraft's inherent dynamics. We begin by exploring the basic augmentation system for lateraldirectional dynamics, which includes the feedback of body-axis roll rate to ailerons and yaw rate to the rudder, commonly referred to as the yaw damper. One of the key challenges addressed in this paper is the need to overcome the interference caused by the yaw damper when coordinating turns. This interference results from the constant nonzero yaw rate present during coordinated turns. To address this issue, the concept of "transient rate feedback" is introduced, which uses a washout filter to differentiate the feedback signal and minimize its effect during steady-state conditions. The paper presents mathematical models and transfer functions that describe the behavior of the lateral-directional augmentation system. It discusses the impact of different parameters, such as the time constants of actuators and washout filters, on the system's performance. The design of the feedback loops for roll and yaw control is elaborated upon, emphasizing the trade-offs between roll response, dutch roll damping, and roll-yaw coupling. Simulation results are used to evaluate the effectiveness of the stability augmentation system in improving the aircraft's roll response and dutch roll damping under varying flight conditions. The paper concludes with insights into the challenges and benefits of multivariable control system design for lateral-directional stability augmentation in aircraf

A Linear-Active-Disturbance-Rejection-Based Vertical Takeoff and Acceleration Strategy with Simplified Vehicle Operations for Electric Vertical Takeoff and Landing Vehicles

A practical vertical takeoff and acceleration strategy is developed for manned electric vertical takeoff and landing vehicles, with a simple vehicle operation principle defined. Firstly, a 6-DOF model is established for 120 kg reduced-scale protype electric vertical takeoff and landing vehicles, with its physical control principles illustrated. Then, a simple vehicle operation method is defined for the vehicle, where the conventional operation method for fixed-wings and helicopters is considered for a friendly stick response definition for pilots with different backgrounds. The defined simple vehicle operation principles are realized by a control architecture with a linear-active-disturbance-rejection-control-based inner loop stability augmentation system and an airspeed-based mode selection outer loop. This system is then used to perform a four-stage vertical takeoff and acceleration strategy, which targets at a smooth and safe transition. The Monte Carlo simulation results and the strategy simulations prove that the proposed strategy, which achieves the design target perfectly, can be easily performed with the developed simple vehicle operation system, and that it has sufficient robustness performance to reject at least 20% of the model’s uncertainties.

Enhancing Ride Comfort and Stability of a Large Van Using an Improved Semi-active Stability Augmentation System

Enhancing Ride Comfort and Stability of a Large Van Using an Improved Semi-active Stability Augmentation System

Layout Analysis and Optimization of Airships with Thrust-Based Stability Augmentation

Despite offering often significant advantages with respect to other flying machines, especially in terms of flight endurance, airships are typically harder to control. Technological solutions borrowed from the realm of shipbuilding, such as bow thrusters, have been largely experimented with to the extent of increasing maneuverability. More recently, also thrust vectoring has appeared as an effective solution to ameliorate maneuverability. However, with an increasing interest for high-altitude airships (HAAs) and autonomous flight and the ensuing need to reduce weight and lifting performance, design simplicity is a desirable goal. Besides saving weight, it would reduce complexity and increase time between overhauls, in turn enabling longer missions. In this perspective, an airship layout based on a set of non-tilting thrusters, optimally placed to be employed for both propulsion and attitude control, appears particularly interesting. If sufficiently effective, such configurations would reduce the need for control surfaces on aerodynamic empennages and the corresponding actuators. Clearly, from an airship design perspective, the adoption of many smaller thrusters instead of a few larger ones allows a potentially significant departure from more classical airship layouts. Where on one side attractive, this solution unlocks a number of design variables—for instance, the number of thrusters, as well as their positioning in the general layout, mutual tilt angles, etc.—to be set according simultaneously to propulsion and attitude control goals. In this paper, we explore the effect of a set of configuration parameters defining three-thrusters and four-thrusters layout, trying to capture their impact on an aggregated measure of control performance. To this aim, at first a stability augmentation system (SAS) is designed so as to stabilize the airship making use of thrusters instead of aerodynamic surfaces. Then a non-linear model of the airship is employed to test the airship in a set of virtual simulation scenarios. The analysis is carried out in a parameterized fashion, changing the values of configuration parameters pertaining to the thrusters layout so as to understand their respective effects. In a later stage, the choice of the optimal design values (i.e., the optimal layout) related to the thrusters is demanded to an optimizer. The paper is concluded by showing the results on a complete numerical test case, drawing conclusions on the relevance of certain design parameters on the considered performance, and commenting the features of an optimal configuration.

Frequency Domain Analysis of F-16 Aircraft in a Variety of Flight Conditions

Examining the flight quality of an aircraft to ensure the stability of the aircraft, increase maneuverability, and make the aircraft easier to control by the pilot necessitates an examination of the natural stability of the system. Within the scope of the paper, the frequency domain response of the F-16 aircraft dynamics is analyzed using Simulink models considering two different flight regimes because the frequency-domain methods have many distinct and important advantages over time-domain methods. Aerodynamic, propulsive, and atmospheric databases are used to create the nonlinear model. The trim analysis for cruise flights is carried out to obtain trim parameters. The aircraft is numerically linearized using the small perturbation theory. The linearized dynamics for each trim condition are used to create transfer functions for each input. The linear model is subsequently examined in the frequency domain to obtain information about the dynamic behavior of the aircraft, and flight quality analysis was examined by considering the lateral and longitudinal modes of the aircraft by international standards. It has been clearly understood the stability augmentation system design has critical importance for the modes with unstable or long steady-state duration.

Frequency Domain Analysis of F-16 Aircraft in a Variety of Flight Conditions

Examining the flight quality of an aircraft to ensure the stability of the aircraft, increase maneuverability, and make the aircraft easier to control by the pilot necessitates an examination of the natural stability of the system. Within the scope of the paper, the frequency domain response of the F-16 aircraft dynamics is analyzed using Simulink models considering two different flight regimes because the frequency-domain methods have many distinct and important advantages over time-domain methods. Aerodynamic, propulsive, and atmospheric databases are used to create the nonlinear model. The trim analysis for cruise flights is carried out to obtain trim parameters. The aircraft is numerically linearized using the small perturbation theory. The linearized dynamics for each trim condition are used to create transfer functions for each input. The linear model is subsequently examined in the frequency domain to obtain information about the dynamic behavior of the aircraft, and flight quality analysis was examined by considering the lateral and longitudinal modes of the aircraft by international standards. It has been clearly understood the stability augmentation system design has critical importance for the modes with unstable or long steady-state duration.

Moving Spatial Turbulence Model for High-Fidelity Rotorcraft Maneuvering Simulation

A moving spatial turbulence model is developed for rotorcraft maneuvering simulation under different flight conditions. The recursive algorithms are adopted to model its distributed longitudinal turbulence components, which are correlated with the lateral and vertical axes to form a local spatial turbulence field. The flow field is constrained around the rotorcraft by following its movement, and the corresponding turbulence components are updated at a constant spatial interval. The statistical properties along the longitudinal, lateral, and vertical directions have been validated against the von Kármán theory. A synthetic simulation environment consisting of a flight dynamics model and a pilot model is constructed to demonstrate the effectiveness of the turbulence model. Its performance is validated by comparing the power spectral densities of both rotorcraft responses and pilot controls in turbulence against flight test data. The piloted simulation results on an Approach-to-Hovering task show that the handling qualities ratings are susceptive to the level of turbulence and significantly increase when performing aggressive controls. The simulation also accurately predicts the expected effect of varied aircraft speed, wind speed, turbulence intensity, and stability augmentation system on piloted handling qualities rating for rotorcraft flight in turbulence.

Sensor and Process Noises Reduction using a Luenberger State Estimator with a Stability Augmentation System for a Hypersonic Transport Aircraft

This paper firstly, presents an autopilot strategy for a Hypersonic Transport Aircraft (HST) using a Stability Augmentation System (SAS) with a Luenberger estimator. The SAS is designed using Linear Quadratic Regulator (LQR) theory which, for HST, benefits the guaranteed robust dynamic stability provided three theoretical requirements are met. The Luenberger estimator is incorporated into the autopilot design to estimate the state variables of the aircraft for the SAS. In the dynamic response simulation, sensor and process noises are inserted into the mathematical model. However, to date, knowledge of the sensor and process noises at the speeds and heights where the aircraft will be flying is limited. The simulation shows that the Luenberger estimator significantly filters the noise. This is an advantage for the HST as prior knowledge of the noises is not necessary when designing the Luenberger estimator.

Design and Flight Test of a Stability Augmentation System for a Flexible Aircraft

Control law design for flexible aircraft with coupled flight and structural dynamics is currently a challenge. If not correctly addressed during control law design, the aeroservoelastic coupling can negatively affect the lateral-directional stability of the aircraft. In this context, this paper describes a methodology for the design of a stability augmentation system for the Instituto Tecnológico de Aeronáutica X-HALE flexible aircraft and proposes a performance index for numerical optimization of the closed-loop feedback gains. Comparisons between open-loop and closed-loop numerical simulation results show how the proposed control system attenuates the Dutch-roll response. An aeroservoelastic analysis is made in which it its demonstrated that the control system has small but beneficial effects on aeroelastic stability. Finally, experimental data obtained via flight tests with the real aircraft demonstrate the effectiveness of the control system in practice.

Incremental Dual Heuristic Dynamic Programming Based Hybrid Approach for Multi-Channel Control of Unstable Tailless Aircraft

Actor-critic based online reinforcement learning control has been proved to be promising method for control of aerial vehicles. However, it is difficult to guarantee high-level success rate of initial training and to tune the large amount of parameters for actors and critics considering unstable multi-input and multi-output (MIMO) aircraft. In order to facilitate and simplify the training of the actor and critic for unstable aircraft, classic stability augmentation system (SAS) is designed for the open-loop aircraft first. Then the online incremental model based dual heuristic dynamic programming (IDHP) method, which has been proposed recently, is extended in application to design a multi-channel robust adaptive controller, and MIMO form network structures are designed and determined for the actors and critics considering the three-channel coupling issues. Consequently, the classic SAS and the IDHP controller make up a novel hybrid control framework. In this control framework, the SAS takes charge of counteracting the unstable eigenvalues of the open-loop aircraft system, and the IDHP takes charge on guaranteeing robust and adaptive performance for high-performance tailless aircraft equipped with the SAS. Specifically, the introduction of the classic control method decreases the difficulty of the initial training for multi-channel IDHP controller. The tuning process for initial parameters of actor and critic neural networks in multiple channels is greatly facilitated. Without the help of SAS, the initial training for multi-channel IDHP controllers of unstable plants is almost impossible to succeed. Finally, the novel hybrid control architecture and method are validated using the Innovative Control Effectors (ICE) model, which has unstable modes in the longitudinal dynamics. Typical aerodynamic model uncertainties are numerically simulated to demonstrate the effectiveness of the proposed control method.

流れ場統合飛行制御の耐乱気流性能のシミュレーションによる評価

In the future UAM (Urban Air Mobility) society, a wide variety of small aircraft, including drones, are cooperatively operated in urban spaces at lower altitudes. Unlike in undisturbed high-altitudes for large airliners, the UAM airspace might be atmospherically unstable, and severe turbulence like sudden crosswinds and downdrafts between buildings should be assumed. Such microclimate phenomena inherent to urban space are difficult to predict, and are a major threat to the safe operation of small lightweight aircrafts flying at low speeds. Hence, there is a great need for powerful attitude stabilization system that can back up manual control and typical SAS (Stability Augmentation System). In this study, the Flow Field Integrated Flight Control is proposed as a solution to this problem. The surface pressure field is used to obtain wind gust characteristics and improve flight stability. The feedback gain is tuned based on real-time modeling of the relationship between the aircraft motion and the surface pressure field, including the effect of turbulence. The proposed design method does not require any models in advance, such as aerodynamic model, mass properties, and turbulence models. The optimal pressure sensor location is also investigated by simulations.

Improved Reinforcement Learning Using Stability Augmentation With Application to Quadrotor Attitude Control

Reinforcement learning (RL) has been successfully applied to motion control, without requiring accurate models and selection of control parameters. In this paper, we propose a novel RL algorithm based on proximal policy optimization algorithm with dimension-wise clipping (PPO-DWC) for attitude control of quadrotor. Firstly, dimension-wise clipping technique is introduced to solve the zero-gradient problem of the PPO algorithm, which can quickly converge while maintaining good sampling efficiency, thus improving the control performance. Moreover, following the idea of stability augmentation system (SAS), a feedback controller is designed and integrated into the environment before training the PPO controller to avoid ineffective exploration and improve the system’s convergence. The eventual controller consists of two parts: the first is the result of the actor neural network in the PPO algorithm, and the second is the output of the stability augmentation feedback controller. Both of them directly use an end-to-end style of control commands to map the system state. This control architecture is applied in the attitude control of the quadrotor. The simulation results show that the quadrotor can quickly and accurately track the command and has a small steady-state error after the training by the improved PPO algorithm. Meanwhile, compared with the traditional PID controller and basic PPO algorithm, the proposed PPO-DWC algorithm with stability augmentation framework has better performance in tracking accuracy and robustness.

Generalized Quantitative Stability Analysis of Time-Dependent Comprehensive Rotorcraft Systems

Rotorcraft stability is an inherently multidisciplinary area, including aerodynamics of rotor and fuselage, structural dynamics of flexible structures, actuator dynamics, control, and stability augmentation systems. The related engineering models can be formulated with increasing complexity due to the asymmetric nature of rotorcraft and the airflow on the rotors in forward flight conditions. As a result, linear time-invariant (LTI) models are drastic simplifications of the real problem, which can significantly affect the evaluation of the stability. This usually reveals itself in form of periodic governing equations and is solved using Floquet’s method. However, in more general cases, the resulting models could be non-periodic, as well, which requires a more versatile approach. Lyapunov Characteristic Exponents (LCEs), as a quantitative method, can represent a solution to this problem. LCEs generalize the stability solutions of the linear models, i.e., eigenvalues of LTI systems and Floquet multipliers of linear time-periodic (LTP) systems, to the case of non-linear, time-dependent systems. Motivated by the need for a generic tool for rotorcraft stability analysis, this work investigates the use of LCEs and their sensitivity in the stability analysis of time-dependent, comprehensive rotorcraft models. The stability of a rotorcraft modeled using mid-fidelity tools is considered to illustrate the equivalence of LCEs and Floquet’s characteristic coefficients for linear time-periodic problems.

Energy Efficiency Enhanced Landing Strategy for Manned eVTOLs Using L1 Adaptive Control

A new landing strategy is presented for manned electric vertical takeoff and landing (eVTOL) vehicles, using a roll maneuver to obtain a trajectory in the horizontal plane. This strategy rejects the altitude surging in the landing process, which is the fatal drawback of the conventional jumping strategy. The strategy leads to a smoother transition from the wing-borne mode to the thrust-borne mode, and has a higher energy efficiency, meaning a better flight experience and higher economic performance. To employ the strategy, a five-stage maneuver is designed, using the lateral maneuver instead of longitudinal climbing. Additionally, a control system based on L1 adaptive control theory is designed to assist manned driving or execute flight missions independently, consisting of the guidance logic, stability augmentation system and flight management unit. The strategy is verified with the ET120 platform, by Monte Carlo simulation for robustness and safety performance, and an experiment was performed to compare the benefits with conventional landing strategies. The results show that the performance of the control system is robust enough to reduce perturbation by at least 20% in all modeling parameters, and ensures consistent dynamic characteristics between different flight modes. Additionally, the strategy successfully avoids climbing during the landing process with a smooth trajectory, and reduces the energy consumed for landing by 64%.

Performance Comparison Between Piezoelectrically Induced Stresses and Active Control for Aeroelastic Stability Augmentation

Recent works have addressed piezoelectrically induced stresses as a potential technique for aeroelastic stability augmentation of biclamped structures. Because active-control-based techniques are conventionally used for aeroelastic stabilization, this paper presents a comparative study on the effectiveness of piezoelectrically induced stresses and active control for aeroelastic stability augmentation. A finite-element-method-based piezoaeroelastic model is proposed employing two-node, eight-degree-of-freedom smart beam elements and an unsteady, strip-theory-based aerodynamic approach involving vertical gust components. A stability augmentation system was designed for stiffness control and used for comparison in a particular wing configuration. Analyses involving the stability margins, input signal energy, and the response to vertical gust were performed and discussed. The results showed that piezoelectrically induced stresses and active control can provide equivalent flutter speed increase for the two cases of gain margin considered. However, the system can become unstable due to control signal saturation when submitted to high-amplitude gusts, which was not observed when piezoelectrically induced stresses is used. On the other hand, it was observed that the active control required a much smaller amount of energy for stabilization. In general, it was noticed that the piezoelectrically induced stresses technique was not as effective as active control to increase the flight envelope of biclamped structures, because active control can provide equivalent aeroelastic stability improvement with smaller amount of energy. However, it is found to be a promising strategy to be used on emergency devices, where the aeroelastic stability of biclamped structures needs to be guaranteed in critical aerodynamic disturbance conditions.