Helicopter Model Research Articles

A Differential-Flatness-Based Approach for Autonomous Helicopter Shipboard Landing

This paper presents a differential flatness-based approach to trajectory planning and tracking for helicopters and demonstrates this strategy for time-optimal autonomous helicopter shipboard landing. For planning and outer-loop tracking control, helicopter dynamics are captured by a simplified nonlinear model that is shown to be differentially flat, which enables the transformation of the original nonlinear system dynamics into an equivalent linear form with endogenous nonlinear constraints. For time-optimal trajectory planning, this equivalent model is used to construct a computationally-efficient optimization problem for free end-time end-state problems, where the decision-space is that of the trajectories of the flat outputs and their derivatives. While the attitude dynamics are stabilized by an inner loop controller, for tracking the planned trajectory a flatness-based outer loop control law is designed to stabilize the unstable zero dynamics. Further, analytical guarantees that relate inner loop tracking error to outer loop robust stability and tracking performance are derived. Simulations are performed on a high-fidelity numerical UH-60 helicopter model under ground effect and ship airwake disturbances, together with ship motion profiles collected from a naval environment to validate the proposed approach for realistic landing scenarios.

A Cascaded and Adaptive Visual Predictive Control Approach for Real-Time Dynamic Visual Servoing

In the past two decades, Unmanned Aerial Vehicles (UAVs) have gained attention in applications such as industrial inspection, search and rescue, mapping, and environment monitoring. However, the autonomous navigation capability of UAVs is aggravated in GPS-deprived areas such as indoors. As a result, vision-based control and guidance methods are sought. In this paper, a vision-based target-tracking problem is formulated in the form of a cascaded adaptive nonlinear Model Predictive Control (MPC) strategy. The proposed algorithm takes the kinematics/dynamics of the system, as well as physical and image constraints into consideration. An Extended Kalman Filter (EKF) is designed to estimate uncertain and/or time-varying parameters of the model. The control space is first divided into low and high levels, and then, they are parameterised via orthonormal basis network functions, which makes the optimisation- based control scheme computationally less expensive, therefore suitable for real-time implementation. A 2-DoF model helicopter, with a coupled nonlinear pitch/yaw dynamics, equipped with a front-looking monocular camera, was utilised for hypothesis testing and evaluation via experiments. Simulated and experimental results show that the proposed method allows the model helicopter to servo toward the target efficiently in real-time while taking kinematic and dynamic constraints into account. The simulation and experimental results are in good agreement and promising.

A vision-aided fuzzy adaptive sliding mode controller for autonomous landing of a nonlinear model helicopter on a moving marine platform

PurposeA vision-assisted fuzzy adaptive sliding mode controller is presented in this research and implemented on a nonlinear helicopter model, which is about to land on a moving ship. Stabilization of the dynamics and tracking the landing path are required, at the same time. This study aims to take one step closer to fully autonomous landing, which is a growing trend.Design/methodology/approachAn integrated guidance and control is considered for the model helicopter. A fuzzy logic is designed to adaptively choose the best control parameters for the sliding mode controller and solve the challenge of parameter tuning. A self-organizing matrix consisting of fuzzy sliding mode parameters is formed instead of a single parameter with the goal of enhancing controller tracking capability. A simple, precise and fast image recognition system based on OpenCV is used to detect the proper point for descending without getting any special data from the ship and by only using a general “H” sign.FindingsThe problem is simulated under intense disturbances, while the approach and landing performances are acceptable. Controller performance is compared and validated. Simulation results show the robustness, agility, stability and outperformance of the proposed controller.Originality/valueThe novelty of this paper is the designed procedure for using a simple image recognition system in the process of autonomous ship-landing, which does not use any special data sent from the ship. Besides, an improved nonlinear controller is designed for integrated guidance and control in this specific application.

Semi-Markov jump linear systems with bi-boundary sojourn time: Anti-modal-asynchrony control

This paper investigates the problem of control synthesis for a class of discrete-time semi-Markov jump linear systems, in which the sojourn time of each mode is bi-boundary (with upper and lower bounds). The system is subject to modal asynchrony, which means that the switchings of the mode-dependent controller to be designed lag behind the ones of the controlled plant, and the lag is mode-dependent. In contrast with the traditional mode-independent lag commonly assumed in the existing studies, not only is the modal lag more practical and general, but also it yields less conservatism of the controller design. By virtue of the semi-Markov kernel approach, the conditions on the existence of the anticipated stabilizing controllers capable of overcoming the modal asynchrony are derived. Illustrative examples including a class of vertical take-off and landing (VTOL) helicopter models are presented to demonstrate the necessity and the validity of the designed anti-modal-asynchrony controllers.

Invited Paper: Rotorcraft Design: The Crucial Influence of Safety from Concept to Fleet Support The 41st Alexander A. Nikolsky Honorary Lecture

It is an immense honor to have been selected to hold the prestigious 41st Nikolsky Lecture and to have the opportunity to synthesize my experiences with regards to the most important principle that permeates aeronautical engineering—"the concept of safety." Having worked in the rotary-wing field for 39 years, with growing levels of involvement and responsibilities, I have been involved in the design, development, and certification of many helicopter models at the Leonardo Helicopters Division (LHD; formerly Agusta and then AgustaWestland), such as A109, A119, EH101, A129, NH90, AW609. More recently, I had the full responsibility of design, development, certification, and entry into service of three new helicopter types within the "AW Family concept", specifically the AW139, AW189, and AW169. I am profoundly grateful for the mentors encountered in my professional life—Bruno Lovera and Santino Pancotti, both of whom were also honored with the Nikolsky Lectureship. In working with them, not a single day passed where the word "safety" was not mentioned. They taught me that "safety" shall be the mantra of every aeronautical engineer because it is our principal duty and responsibility, towards those who travel in, work on, and work with our products and entrust their lives to our work and professionalism daily. I have tried hard never to forget this lesson, and to convey this to the young engineers that I have had the chance and pleasure to work with. If I have been able to pass on this lesson successfully, through my work with others through this lectureship, it would be the greatest achievement of my life. In this vein, this paper is organized in three parts: (i) definitions and principles, along with some "philosophical" concepts; (ii) the application of these principles at Leonardo in the design of the latest generation of helicopters, and finally (iii) a discussion of emerging "safety technologies" that promise to improve the safety of future helicopters and operations.

Dual-loop Hovering Control for Unmanned Helicopter

The recent development of unmanned aerial vehicle flight control creates a strong demand for higher control accuracy and control quality. In this paper, the precise control and stable hovering problem are studied for unmanned helicopters. To solve the problem, we propose a hovering controller for dual-loop of unmanned helicopters. Firstly, the nonlinear mathematical model of unmanned helicopters is established based on Newton-Euler’s laws. Then, we respectively design the position model and attitude model. In the position model, we use the cascade PD controller to realize the precise control; In the attitude model, we use the cascade PD controller to realize the stable hovering. Simulation results show the effectiveness of the conclusions made in this paper.

Methodology of automated study of the workload of a helicopter pilot

The subject of the study is the contradiction between the need to conduct a study of the workload of helicopter pilots and the lack of methods regulated by regulatory and technical documents to ensure that such studies are carried out during the testing of new aircraft. The purpose of the study was to ensure the possibility of objectification of the pilot's workload during testing of modernized and created helicopter models, as well as in the process of flight simulator training. The author examines in detail such aspects of the topic as the development of a pilot workload research program; the completion of a helicopter by installing a set of technical means for studying attention reserves; determining the pilot's workload; determining the integral indicator of the pilot's workload and drawing up a conclusion based on the results of the workload study. The main conclusion of the conducted theoretical and experimental research is that the developed methodology for studying the workload of a helicopter pilot, based on determining the reserves of his attention when performing professional activities, provides an adequate definition of the workload when performing helicopter tests on semi-natural modeling complexes and during flight simulator training. The results of verification and research of the effectiveness of the developed solutions have shown that the objectification of the pilot's workload during helicopter testing provides an opportunity to substantiate industry recommendations on the refinement and improvement of the layout of helicopter cabins, to rationalize the points of the flight part of the test program and to intensify the process of professional training of flight personnel.

Research on Micro-motion Modeling and Feature Extraction of Passive Bistatic Radar Based on CMMB Signal

Passive bistatic radar has unique advantages in studying Micro-Doppler effect because of its system characteristics. China Mobile Multimedia Broadcasting (CMMB) as a broad range of radio and television signal, its signal adopts multi-carrier Orthogonal Frequency Division Multiplexing (OFDM) technology, can be used as a good illuminator. Based on the CMMB signal, the scattering point model of helicopter’s echo is established, and the influencing factors of Micro-Doppler frequency are analyzed. On this basis, the Singular-spectrum Analysis combined with Time-frequency analysis and autocorrelation method is used to extract the target micro-motion characteristics. The simulation results verify the rationality of the micro-motion model and the effectiveness of the feature extraction method in this paper.

George de Bothezat and his contribution into the world aviation and astronautics

The article dedicated the life and scientific way of George de Bothezat, the first Doctor of Sciences in the field of aviation. Together with Nikolay Zhukovsky, Igor Sikorsky, Stephen Timoshenko, Alexander Fan-der-Flit, and Alexey Lebedev, he was one of the organizers of the Air Fleet of the Russian Empire. He is the author of various inventions: gyroscopic sight and other types of aviation equipment. We analyze works by G. Bothezat on the impulse theory of propellers. In particular, the scientist derived formulas for ensuring the flight stability of airplanes and helicopters. He developed training ballistic tables, which allowed making corrections for the speed of the flight and the direction of the wind. We briefly describe a biography of G. Bothezat, focusing on the student period of his life in Kharkiv, Ukraine, and the reasons for G. Bothezat’s departure to the United States in 1918. It is stated that it was there that his talent as a designer and creator of helicopters of the original system was disclosed in the best way. In 1922, George Bothezat obtained the financial support of the American government to build a workable helicopter model without prototypes and experiments, only based on the results of calculations. The reasons why G.Bothezat did not manage to achieve the launch of the serial production of helicopters are analyzed. We also mention the activities of the company founded by G.Bothezat, which was engaged in the production of fans of a new type for the US Navy. The Bothezat system fans were installed at the Rockefeller Center in New York as well as in American tanks. It is emphasized that I. Sikorsky also used the works by G.Bothezat in his research. It is stated that the flight trajectory calculated by G.Bothezat in air and airless space was used in the development of the American program of a manned landing on the Moon using the “Apollo” system.

Safe Motion Planning and Learning for Unmanned Aerial Systems

To control unmanned aerial systems, we rarely have a perfect system model. Safe and aggressive planning is also challenging for nonlinear and under-actuated systems. Expert pilots, however, demonstrate maneuvers that are deemed at the edge of plane envelope. Inspired by biological systems, in this paper, we introduce a framework that leverages methods in the field of control theory and reinforcement learning to generate feasible, possibly aggressive, trajectories. For the control policies, Dynamic Movement Primitives (DMPs) imitate pilot-induced primitives, and DMPs are combined in parallel to generate trajectories to reach original or different goal points. The stability properties of DMPs and their overall systems are analyzed using contraction theory. For reinforcement learning, Policy Improvement with Path Integrals (PI2) was used for the maneuvers. The results in this paper show that PI2 updated policies are a feasible and parallel combination of different updated primitives transfer the learning in the contraction regions. Our proposed methodology can be used to imitate, reshape, and improve feasible, possibly aggressive, maneuvers. In addition, we can exploit trajectories generated by optimization methods, such as Model Predictive Control (MPC), and a library of maneuvers can be instantly generated. For application, 3-DOF (degrees of freedom) Helicopter and 2D-UAV (unmanned aerial vehicle) models are utilized to demonstrate the main results.

Towards Fast Nonlinear Model Predictive Control for Embedded Applications

Real-time implementation of NMPC has its limitations for the embedded applications such as solving a non-convex optimization problem within a sample time, high computational complexity, and limited onboard resources (such as processor speed, memory, power, etc.). To mitigate these limitations, we have proposed application of Successive Online Linearization-based NMPC (SOL-NMPC) to cater the fast sample time requirements of realtime embedded applications. The idea of SOL-NMPC is to derive a convex optimization (Quadratic Programming (QP)) problem at each sample time through the linearization of the nonlinear model which eventually results in a linear MPC formulation. Subsequently, the resulting LMPC is executed online by using state-of-the-art QP solvers. The developed SOL-NMPC is implemented on a low-cost STM32 Nucleo development board. The 2-DOF- Helicopter model is used to demonstrate the effectiveness of the proposed SOL-NMPC. Hardware-In-the-Loop (HIL) co-simulation results demonstrate the implementation feasibility for real-time control applications. The performance is compared with the classical NMPC implemented using the GRAMPC toolbox in C/C++ environment. The comparison results show that: (i) the proposed SOL-NMPC is comparatively faster and takes less computational resources as compared to classical NMPC, (ii) successfully satisfies the constraints, and (iii) however, at the cost of reduced complexity, one needs to scarify with the accuracy as compared to NMPC. Nevertheless, the accuracy obtained by the presented SOL-NMPC is acceptable in embedded applications over the high computational burden of the classical NMPC.

Stability Analysis of helicopter dynamics with incomplete information using MPC

The paper is devoted to the problem of seeking a helicopter motion control using MPC provided that the state vector is not fully observable. It is supposed that the helicopter motion is described by a dynamic system linearized near regimes with different fixed values of the forward speed. To solve the problem for an arbitrary value of the forward speed the interpolation strategy when the system matrices are decomposed in the basis of the finite functions is used. The proposed technique is applied to analyze dynamic behavior of three models of helicopters: ZD559-Lynx, SA330-Puma, S123-Bo105. The MPC approach permits to gain the stability of helicopters with complete and incomplete information in varying a time interval on which the system is under control and an interval over which a desired output is needed to achieve, initial conditions of a motion and values of the forward speed.

Numerical and Experimental Investigation into the Aerodynamic Benefits of Rotorcraft Formation Flight

The use of formation flight to achieve aerodynamic benefit applied to rotorcraft has, unlike its fixed-wing counterpart, received little attention in the literature. This document presents a proof-of-concept of rotorcraft formation flight from two independent investigations: a numerical study of a fully articulated helicopter influenced by an upstream helicopter wake and a wind-tunnel experiment featuring two small-scale helicopter models with fixed-pitch blades. Both cases feature a representation of two helicopters in a diagonal, staggered formation aligned on the advancing side of the main rotor, but do not simulate directly comparable flight conditions. The vertical and lateral alignment of the two helicopters is varied in order to observe the achievable reductions in main rotor power required during cruise flight. The wind-tunnel experiment data yield an estimated maximum total power reduction for the secondary aircraft of approximately 24%, while the numerical models yield reductions between 20% and 34% dependent on flight velocity. Both experiments predict a higher potential for aerodynamic benefit than generally observed for fixed-wing formations, which is attributed to the asymmetric velocity profile induced by the wake of the upstream rotor. Optimal lateral alignment of both experimental and numerical results is found to feature overlap of the rotor disk areas, rather than tip-to-tip alignment, as a result of the circular rotor disk area. Experimental data show an optimal vertical alignment of the secondary rotorcraft below the primary, due to the self-induced vertical displacement of the rotor wake, which is absent from the numerical results due to the application of a flat wake assumption. The results show a promising potential for rotorcraft formation flight, though due to the limited nature of the models used, conclusions cannot be generalized. The potential aerodynamic benefit indicated by the present study invites further research in the field of rotorcraft formation flight.

Static Analysis of Unsteady Aerodynamics Wake of Simplified Helicopter Model Via Simulation Work

Computational tools have led and helped researchers in providing advanced results, notably in rotorcraft research, as flow around the helicopter is dominated by complex aerodynamics and flow interaction phenomena. This research work aimed to evaluate the aerodynamic computational results on a simplified model helicopter when the model was subjected to the angles of attack 0°, -5°, -15°, and -20°, respectively. The study also examined the unsteady flow behaviour on the three-dimensional elliptical shape of a fuselage equipped with a rotor hub of the single rotor blade. The computational domain for the aerodynamic flow field was created within the size of 7 m (length) x 5 m (width) x 5 m (height). Results showed that an increase in the angle of attack in the rotor component caused additional drag of about 34% to 45% whilst the fuselage component contributed about 55% to 65% to drag increment. Also, a significant value of total pressure from -235 Pa to 250 Pa demonstrated along the simplified model helicopter distinctly showed that the complexity of geometry caused adverse pressure. The findings of this research work could potentially improve the understanding of complex flow surrounding the helicopter that has always baffled the aerodynamicists.

Improvement of Computational Efficiency for Fast ISAR Image Simulation Through Nonuniform Fast Fourier Transform

The image-domain ray-tube integration formula (IRIF) is an efficient way to build a large database for synthetic-aperture radar automatic target recognition (SAR-ATR) because the IRIF can rapidly generate an ISAR image of a computer-aided design (CAD) model. Additionally, calculation of the IRIF can be accelerated by a nonuniformly sampled impulse train and the nonuniform fast Fourier transform (NUFFT). In a numerical implementation, upsampled ISAR images are required and the IRIF can calculate the upsampled ISAR images directly in image-domain. However, image-domain upsampling increases the computation time because the NUFFT calculates a matrix whose size is increased by the upsampling factor. In this letter, we propose a method to minimize the increase in computation time caused by image-domain upsampling. We derive a formula to approximately predict the scattered far-field under the small-angle condition and accelerate the formula by utilizing the nonuniformly sampled impulse train and the NUFFT. The NUFFT is used to calculate a far-field matrix without upsampling. Then, zero-padding in the field-domain and the FFT are utilized to generate an upsampled ISAR image. To validate the proposed method, ISAR images of a tank and a helicopter CAD models were simulated and were compared with those of the previous methods.

Flight Dynamics Modeling, Trim, Stability, and Controllability of Coaxial Compound Helicopters

Coaxial compound helicopters feature high-speed characteristics that are not possessed by conventional helicopters. The aim of this paper is to develop a flight dynamics model of coaxial compound helicopters and investigate the trim, stability, and controllability. A flight dynamics model including a coaxial rotor model and a propeller model is introduced. Based on the flight dynamics model, the trim of hover and level flights of a coaxial compound helicopter is performed. Considering the influence of flapping motions, stability and control derivatives are assessed under the small disturbance hypothesis by using a numerical differentiation method. Then, stability and response of the coaxial compound helicopter are presented. The results of trim show good agreement with those of the reference, which verifies the flight dynamics model. The speed static stability derivative is stable in whole trim range, while the angle of attack static stability derivative is unstable. The yawing control with collective differential of the coaxial compound helicopter will induce serious roll effect in high-speed flights.

Adaptive neural network-based fault-tolerant control for a three degrees of freedom helicopter

In this paper, the tracking control problem of a three degrees of freedom (3-DOF) helicopter with unknown actuator faults, model uncertainties and external time-varying disturbances is solved by using the proposed adaptive neural network-based fault-tolerant control (FTC) strategy. Firstly, the mathematical model of the 3-DOF helicopter is established. Then, neural networks (NNs) are utilised to approximate and compensate the unknown functions in the presence of the controlled system. It should be stressed that hyperbolic tangent functions are introduced to reduce the negative effects caused by NN approximation errors and external time-varying disturbances. Moreover, auxiliary functions are designed to compensate filter errors, which further enhance the tracking performance of the closed-loop system. Finally, simulation results are provided to verify the effectiveness of the proposed FTC scheme.

Investigation on effects of time-frequency characteristics of ship airwake on helicopter flight characteristics

A conventional helicopter flight dynamics model, which can be coupled with ship airwake date, is developed in this study. In the method, the ship airwake data is obtained by the high-accuracy DES model, and a strategy which can transmit CFD data to the flight dynamics model is established based on the "one-way" coupling idea. Then, the SFS2 ship model and UH-60A helicopter are chosen as a combination to investigate the influences of the spatial and temporal characteristics of ship airwake from the aspects of control margins and unsteady level. The time-averaged simulation results show that for the counterclockwise-rotor helicopter, although pilot could have more collective pitch margin under crosswind condition compared to the headwind condition, he might possess much less pedal margin due to the sidewash in the airflow. The unsteady results indicate that the unsteady loading level of the helicopter would increase significantly under the crosswind condition compared to the headwind condition due to the increase of turbulent density in the airwake. Furthermore, for the conventional helicopter, the disturbances on the forces and moments which along the rotor hub (i.e., thrust and yaw moment) are the critical factors that increasing the pilot workload during the landing procedure.

Adaptive neural network-based active disturbance rejection flight control of an unmanned helicopter

To enable the unmanned helicopter to fly autonomously in precise paths and reduce the influence of internal and external unknown disturbances of the unmanned helicopter, this paper proposes the adaptive radial basis function (RBF) neural network-based active disturbance rejection controller (ADRC). This controller is abbreviated as RBF-ADRC. Firstly, this paper introduces the flight dynamics model of the unmanned helicopter and the control features of the traditional ADRC. Subsequently, this paper uses modern control theory to establish a state observer and uses adaptive RBF neural network to estimate the unknown total disturbance. Finally, this paper constructs the unmanned helicopter's trajectory tracking control system based on the RBF-ADRC controller. The simulation results of the spiral ascent and the “8”-figure climb maneuver flight prove that the anti-disturbance, robustness and tracking accuracy of the RBF-ADRC are better than the traditional ADRC and proportion-integration-differentiation (PID) control methods.

Simulation Experiment Analysis and Verification of Ship Resonance for Shipboard Helicopter

According to the dynamic characteristics of shipboard helicopter landing, the time-domain load of ship movement on the fuselage under complex sea conditions, the nonlinear dynamic characteristics of rotor damper, and the influence of rotor blade flapping movement and aerodynamic force are considered. The fuselage, rotor lagging damper, blade flapping and lagging movement and aerodynamic model are modeled separately as sub-models, and then the subsystems are integrated according to mechanical principles to finally establish the ship resonance analysis model of helicopter. The method for simulation experiment of ship resonance is put forward. The whole process of ship resonance experiment is dynamically demonstrated by digital simulation technology, which simulates the excitation load on the fuselage caused by the rise and fall and rolling movement of the ship. Actually simulates the lifting collective pitch and conducts periodic excitation by disturbing the control stick. Based on the transient response of the time-domain simulation dynamic system under disturbance, the stability of ship resonance is analyzed. The accuracy of the simulation method is verified through project cases, and the simulation experiment analysis techniques for accurately predicting the unstable boundary of ship resonance are mastered, which provides design and analysis means instead of the experiment that cannot be carried out in model development.