Flight Envelope Research Articles

Attitude control for hypersonic reentry vehicles: An efficient deep reinforcement learning method

Aiming at the attitude control problem of hypersonic reentry vehicles (HRVs), a deep reinforcement learning (DRL) based anti-disturbance control method is proposed. First, a compound control framework consisting of a DRL-based auxiliary controller and a fixed-time anti-disturbance controller is proposed to improve the control performance under the premise of ensuring stability. Then, a novel value function approximation mechanism, named experience-based value expansion (EVE), is proposed to modify the value function update equation based on a two-dimensional replay buffer, which solves the DRL convergence problem brought by the HRV’s strong nonlinearities, tight coupling, and big flight envelope. Furthermore, a result-oriented encoder (ROE) is proposed to solve the DRL generalization problem brought by the HRV’s high uncertainties and unavailable real training environment. A bottleneck shape neural network structure is used for the DRL’s network structure to extract high-dimensional features and prevent overfitting to the training environment. Finally, abundant numerical comparative simulations demonstrate the effectiveness of the proposed efficient DRL algorithms and the DRL-based attitude controller.

Aerodynamic Analysis of Camber Morphing Airfoils in Transition via Computational Fluid Dynamics.

In this paper, the authors analyze an important but overlooked area, the aerodynamics of the variable camber morphing wing in transition, where 6% camber changes from 2% to 8% using the two airfoil configurations: NACA2410 and NACA8410. Many morphing works focus on analyzing the aerodynamics of a particular airfoil geometry or already morphed case. The authors mainly address "transitional" or "in-between" aerodynamics to understand the semantics of morphing in-flight and explore the linearity in the relationship when the camber rate is gradually changed. In general, morphing technologies are considered a new paradigm for next-generation aircraft designs with highly agile flight and control and a multidisciplinary optimal design process that enables aircraft to perform substantially better than current ones. Morphing aircraft adjust wing shapes conformally, promoting an enlarged flight envelope, enhanced performance, and higher energy sustainability. Whereas the recent advancement in manufacturing and material processing, composite and Smart materials has enabled the implementation of morphing wings, designing a morphing wing aircraft is more challenging than modern aircraft in terms of reliable numerical modeling and aerodynamic analysis. Hence, it is interesting to investigate modeling the transitional aerodynamics of morphing airfoils using a numerical analysis such as computational fluid dynamics. The result shows that the SST k-ω model with transition/curvature correction computes a reasonably accurate value than an analytical solution. Additionally, the is less sensitive to transition near the leading edge in airfoils. Therefore, as the camber rate changes or gradually increases, the aerodynamic behavior correspondingly changes linearly.

Superiority analysis of the cooled cooling air technology for low bypass ratio aero-engine under typical flight mission

The cooled cooling air technology (CCA technology) is beneficial to increase the aero-engine operating pressure ratio and turbine inlet temperature, while its influence mechanism during the full flight envelope is still ambiguous. A comparative analysis has been completed in this study to assess the flight performance of an aircraft engine with or without CCA technology. Based on the aircraft and aero-engine integrated technique, a novel analytical model is developed by combining the propulsion requirement analysis model with the engine design/off-design point analysis model. The precise submodules of the heat exchanger and the turbine blade in engine model improve the -practicability of analysis, which considers the influence of the flight mission on the heat transfer performance. This literature suggests that CCA technology could enhance the fuel weight at take-off phase, the weight-specific surplus power, the load factors, and the supersonic cruising Mach number by approximately 3–4.85% with re-optimizing the cycle parameters. The inlet temperature of turbine Tt4 rises by around 3% without the turbine blade temperature exceeding the limitation. This work further analyzes the most influential structural factor of the total engine performance and the transverse tube pitch is discovered as a noteworthy feature. Additionally, it is well known that the heat transfer and pressure drop of the CCA heat exchanger (CCA-HEX) change dramatically during the whole flight mission. Our findings provide valuable insights into a better understanding of the mechanisms of CCA-HEX characteristic change, in which poor heat transfer occurs at low altitudes and high Mach numbers, whereas the largest flow resistance appears in the high-altitude and low-Mach-number regionsin theopposite manner. In conclusion, CCA technology is a cost-effective method to improve the propulsion performance of the aircraft engine for flight mission.

Estimation of Transport-Category Jet Airplane Maximum Range and Airspeed in the Presence of Transonic Wave Drag

One of the most difficult steps in estimating the cruise performance characteristics of high-subsonic transport-category turbofan-powered airplanes is the estimation of the transonic wave drag. Modern jet airplanes cruise most efficiently in the vicinity of the drag-divergence or drag-rise Mach numbers. In the initial design phase and later when the preliminary wind-tunnel and/or CFD computations and drag polars are known with increased accuracy, a method of estimating cruise performance is needed. In this study, a new semi-empirical transonic wave drag model using modified Lock’s equation was developed. For maximum range cruise estimations, an optimization criterion based on maximizing specific air range was used. The resulting nonlinear equations are of 12th- and 13th-order. Numerical Newton–Raphson nonlinear solvers were used to find real positive roots of such polynomials. The NR method was first tested for accuracy and convergence using known analytical solutions. A methodology for an initial guess was developed starting with the maximum-range cruise Mach without the wave-drag included. This guess resulted in fast quadratic convergence in all computations. Other novel features of this article include a new semi-empirical fuel-flow law, which was also extensively tested. Additionally, a semi-empirical turbofan thrust model usable for a wide range of bypass ratios and the entire flight envelope was developed. Such physics-based semi-empirical model can be used for a wide range of turbofans. The algorithm can be utilized to identify most beneficial input parameter values and combinations for the cruise flight phase. The model represents a powerful tool to estimate important cruise performance airspeeds located in the transonic regime. An intended application is in the conceptual development stages for early design optimizations of future airplanes. It is possible with additional effort to extend existing model capabilities to deal with supersonic transports optimal cruise parameters.

A Nonlinear Correction Method on Elastic Lift Coefficient for Civil Aircraft

Aerodynamic lift coefficient is nonlinear for an elastic aircraft flying at a high transonic speed, varying with Mach number and dynamic pressure. The common industry practice is that the correction on slope and intercept for linear segment of data curve of wind tunnel test, which is incapable of meeting increasing design requirements whereas aerodynamic coefficients are characterized by nonlinear nature. The nonlinear correction method for elastic lift coefficient is introduced by using equivalent angle of attack. Based on wind tunnel test data of rigid designed aircraft shape, elastic lift coefficient used for load and control analysis is directly obtained, which is applicable to different flight deformation state of a civil aircraft within the flight envelope. The wind tunnel test model was selected as a validation illustration. The analysis shows that the results obtained through the proposed correction method and the elastic test data are in a good agreement. The developed method takes into consideration both the computational efficiency and the precision.

Aerobatic Tic-Toc Control of Planar Quadcopters via Reinforcement Learning

This letter studies aerobatic tic-toc control of quadcopters. Tic-toc control enables rotorcraft to fly almost in the vertical plane rather than the horizontal plane. It is one of the most challenging manoeuvrers to achieve autonomously. The problem has to our knowledge not yet been studied for quadcopters. Studying it could expand their flight envelope and improve their performance in extreme, aerobatic flight tasks. In this letter, we employ a deep deterministic gradient policy approach to train reinforcement learning (RL) controllers based on carefully designed rewards. The obtained RL controllers are shown to generate two flight modes, spin and tic-toc. We analyse the properties of these flight modes and screen out unfavourable RL controllers. The qualified RL controller is then enhanced by combining it with PID and LQR controllers which achieves better flight performance and enables the quadcopter to track a moving reference point and recover to hovering flight status. Physical simulations using Simscape are presented to verify the proposed approach.

Robust Adaptive Control Laws for a Winged Re-entry Vehicle

The flight control system for reusable launch vehicles has to work for an extensive flight envelope ranging from hypersonic to subsonic Mach numbers. Since the plant dynamics are highly nonlinear, time-varying, and coupled, classical controllers are designed for large stability margins about the linearised points. This limits the performance of the classical controller, and the design process is iterative and time-consuming. Hence adaptive controllers are designed to handle such complex systems using a standard quadratic Lyapunov function. The existing parameter update laws of the model reference adaptive control system are not robust to bounded disturbances and unmodelled dynamics. Hence this paper proposes modified controller parameter update laws using a rectangular projection operator and barrier Lyapunov function, ensuring robustness to structured and unstructured uncertainties. The rectangular projection-based control law provides that the controller parameters remain within the specified upper and lower limits, whereas the barrier Lyapunov-based controller maintains the trajectory also within the specified error limit. The effectiveness of the proposed update laws is demonstrated for the automatic landing phase of a reusable launch vehicle.

Transition Analysis and Practical Flight Control for Ducted Fan Fixed-Wing Aerial Robot: Level Path Flight Mode Transition

For agile fixed-wing aerial robots, the vertical/horizontal flight mode transition is a fundamental requirement for high maneuverability. This letter focuses on a challenging implementation in which the aircraft conducts the transitional flight at fixed altitude along a straight path. Implemented on a small ducted fan fixed-wing, we propose a transition-corridor-based control strategy to conduct the flight transition neatly. The transition corridor is described as a region comprising all the admissible flight velocities, accelerations and attitudes, bounded by the capacity of both the propulsion and control system of the vehicle. By planning the transition trajectory in obedience with the identified transition corridor, as well as adopting a unified full envelope flight control scheme, the ducted fan fixed-wing successfully accomplishes the practical transitional flight. Experimental results are satisfactory in both flight mode transition and altitude stabilization.

An application of interval type-2 fuzzy model based control system for generic aircraft

This paper presents a design application of Interval Type-2 (IT2) Takagi–Sugeno (T–S)​ Fuzzy Model Based (FMB) control system for generic aircraft. The IT2 T–S FMB flight control system consists of an IT2 T–S fuzzy model and a fuzzy controller connected in a closed-loop. The IT2 T–S​ fuzzy model is obtained by linearizing the nonlinear aircraft dynamics about various representative points (equilibrium points) of the flight envelope with some fuzzy rules. The aircraft flight envelope parameters, i.e., operating altitude and aircraft speed are characterized as premise parameters and elements of stability and control derivative matrix are identified as consequent parameters of fuzzy model. Longitudinal dynamics of X-29A research aircraft is selected for design of IT2 T–S FMB control system. To achieve the optimum design flexibility, an imperfectly matched premise and membership function dependent (MFD) stability analysis is considered. In MFD stability conditions, the information of flight envelope parameters is included to capture the nonlinearity pertaining to variation in flight conditions. The closed-loop response of IT2 T–S FMB controller is presented at trim equilibrium points by taking four initial angle of attack flight conditions. The performance analysis of designed controller is also presented and discussed in comparison with Fuzzy LQR and LQR based optimal controllers. The simulation results reveal that proposed IT2 T–S FMB controller not only stabilizes the aircraft dynamics but also provides improved transient performance as compared with Fuzzy LQR and LQR based optimal controllers. This demonstrates the utility of IT2 T–S FMB control system for aircraft/ UAV’s related application.

Pseudospectral Continuation for Aeroelastic Stability Analysis

AbstractThis technical note is concerned with aeroelastic flutter problems: the analysis of aeroelastic systems undergoing airspeed-dependent dynamic instability. Existing continuation methods for parametric stability analysis are based on marching along an airspeed parameter until the flutter point is found—an approach that may waste computational effort on low-airspeed system behavior, before a flutter point is located and characterized. Here, we describe a pseudospectral continuation approach that instead marches outward from the system’s flutter points, from points of instability to points of increasing damping, allowing efficient characterization of the subcritical and supercritical behavior of the system. This approach ties together aeroelastic stability analysis and abstract linear algebra and, by reducing the sample space in which the analysis needs to take place, provides efficient methods for computing practical aeroelastic stability properties—for instance, flight envelopes based on maximum modal damping and the location of borderline-stable zones.

Design optimization and off-design performance analysis of axisymmetric scramjet intakes for ascent flight

Scramjet engines are one of the most economical and reliable high-speed airbreathing propulsion technologies that can be used for low-cost satellite launchers and hypersonic atmospheric transportation. For access to space, the flight envelope practically comprises varying freestream conditions and altitude during ascent flight, constituting a complex optimization problem subject to various constraints from design and operation perspectives. This paper presents the results and insights obtained from a multi-objective optimization study of three classes of axisymmetric scramjet intakes, i.e., three-ramp, Busemann-based, and generic intakes represented by smooth Bézier curves, with the same intake mass flow rate for all classes. Optimization is conducted by means of surrogate-assisted evolutionary algorithms coupled with computational fluid dynamics simulations using a Reynolds-averaged Navier–Stokes flow solver for steady-state flowfields. It aims to minimize the intake drag and maximize the compression efficiency at Mach 7.7 at an altitude of 30 km simultaneously. It has been found that generic intakes, which offer greater local shape control, can achieve the highest compression efficiency of 94.8%, the highest total pressure recovery, and the highest flow uniformity at the intake exit. The Busemann-based intakes, on the other hand, can produce the highest static pressure ratio while incurring the lowest drag force. The utility of principal component analysis has effectively reduced the dimensions of the solutions, and has identified clusters of non-dominated solutions according to their characteristics such as geometric attributes, exit flow profiles, and wall property distributions, thereby conducing to obtain further physical insights into the optimal configurations.

Design and aerodynamic analysis of a VTOL tilt-wing UAV

The aerodynamic design and analysis of an Unmanned Air Vehicle, capable of vertical take-off and landing by employing fixed four rotors on the tilt-wing and two rotors on the tilt-tail, will be presented in this study. Both main wing and the horizontal tail can be tilted 90°. During VTOL, transition and forward flight, aerodynamic and thrust forces have been employed. Different flight conditions, including the effects of angle of attack, side slip, wing tilt angle and control surfaces deflection angle changes, have been studied with CFD analysis. For a Tilt-Wing UAV, there are challenges like high non-linearity, vulnerability to disturbances during hover and transition, cross-coupling between three axes, very low time to double amplitude of the open-loop system. Therefore, the aerodynamic analysis has been done in detail considering especially the challenges in transition phase of the flight envelope.

A review of propeller stall flutter

AbstractResearch on propeller performance has been reinvigorated by the development of new classes of vehicles, ranging from electrically powered fixed-wing aircraft, to multi-rotor electrical Vertical Take-off/Landing (eVTOL) vehicles and tilt-rotor aircraft. These types of aircraft utilise a range of modern propellers, often with more advanced planforms and features such as anhedral, and operate in flight envelopes that are outwith the traditional bands of performance. The use of advanced materials (mostly composites), high geometrical sweeps and variable angular velocities are the source of unsteady aerodynamics, that is often coupled with the blade’s structural response. Data from experimental investigations is mostly historic, with the majority of studies conducted before 1960, when aviation shifted rapidly towards jet propulsion. These studies lack in flutter boundary assessment. Modern propellers are likely to be pushed toward their flutter boundaries, but the experimental database published to-date is insufficient to provide flutter boundary assessment. This review examines the value of the available experimental research and the status of the state-of-the-art numerical methods, in order to establish the requirements for modern research on propeller stall flutter.

Heat Transfer and Residence Time in Lean Direct Injection Fuel Galleries

Abstract In radially staged lean direct injection (LDI) systems, pilot fuel plays an important role in cooling the mains fuel gallery in regions of the flight envelope where the mains fuel is stagnant. Under these conditions, managing wetted wall temperatures is vital to avoid the formation of carbonaceous particles, which become deposited on the surfaces of the fuel gallery and can lead to a deterioration in system performance. The prediction of wetted wall temperatures therefore represents an important aspect of the injector design phase. Such predictions are often based on injector thermal models, which tend to rely on the application of convective boundary conditions from empirical heat transfer correlations. The use of these correlations leads to questions over the accuracy of predicted wetted wall temperatures and therefore uncertainty over the probability of deposition. This paper seeks to improve on the current situation by applying the inverse conduction technique to determine heat transfer coefficients (HTCs) specific to the pilot fuel gallery. These HTCs are crucial for determining wetted wall temperatures in the pilot and mains fuel galleries and principally govern the risk of deposition in the stagnant mains. The pilot heat transfer data are further examined alongside measurements of the pilot residence time distribution, which together control the risk of pilot deposition at low fuel flow rates. Both the heat transfer and residence time measurements demonstrate the opportunity for future fuel gallery design refinement and provide the supporting data to facilitate this.

An Online Data-Driven LPV Modeling Method for Turbo-Shaft Engines

The linear parameter-varying (LPV) model is widely used in aero engine control system design. The conventional local modeling method is inaccurate and inefficient in the full flying envelope. Hence, a novel online data-driven LPV modeling method based on the online sequential extreme learning machine (OS-ELM) with an additional multiplying layer (MLOS-ELM) was proposed. An extra multiplying layer was inserted between the hidden layer and the output layer, where the hidden layer outputs were multiplied by the input variables and state variables of the LPV model. Additionally, the input layer was set to the LPV model’s scheduling parameter. With the multiplying layer added, the state space equation matrices of the LPV model could be easily calculated using online gathered data. Simulation results showed that the outputs of the MLOS-ELM matched that of the component level model of a turbo-shaft engine precisely. The maximum approximation error was less than 0.18%. The predictive outputs of the proposed online data-driven LPV model after five samples also matched that of the component level model well, and the maximum predictive error within a large flight envelope was less than 1.1% with measurement noise considered. Thus, the efficiency and accuracy of the proposed method were validated.

A New Architecture of Morphing Wing Based on Hyperelastic Materials and Metastructures With Tunable Stiffness

Morphing wings with the ability of shape changing may enlarge the flight envelop of air vehicles. This is particularly important for small-scale aircrafts demanding maneuverability and adaptability in dynamic environments. However, to design a shape-changing mechanism suitable for small drones is very challenging due to extreme requirements raised by dimensional constraint, actuating limits, and weight restriction in Micro Air Vehicles (MAVs). A novel design method of morphing wings is proposed for small-scale aircrafts in this paper. The morphing wing is composed of a heterogeneous architecture, including three components: a longitudinal spar with pneumatic actuators, chordwise stiffening ribs with cable-driven metastructures, and a compliant skin. Through employing both pneumatic networks with hyperelastic behavior and modified pantographic metastructures with tunable stiffness, structural deformation along both spanwise and chordwise. Mechanical behaviors of this morphing wing were investigated thoroughly by theorical analysis, numerical verification, and experimental validation. Theorical predictions agree well with the results of 3D Finite Element Method (FEM). With optimal design determined through design of experiment (DOE) and FEM, prototyped morphing wings were fabricated accordingly. The aerodynamic performance was examined by a wind tunnel test with a Particle Image Velocimetry (PIV) instrument. Results demonstrate the efficiency and capacity of this morphing wing to adjust aerodynamic layout of aircraft in a low-speed airflow environment. This proposed design provides a promising solution applicable to small-scale aircrafts with morphing wings.

A fitness sharing based ant clustering method for multimodal optimization of the aircraft longitudinal automatic carrier landing system

With the flight envelope becoming larger and larger, the Automatic Carrier Landing System (ACLS) is becoming a complex large-scale system, and the corresponding control parameter design has been the most important key link to ensure the safety and flight quality of aircraft carrier landing missions. In this paper, a Fitness Sharing based Ant Clustering (FSAC) method is presented for use in multimodal optimization of the longitudinal ACLS and to reveal the hidden properties of the solution space. In doing so, first, the lattice rule based space sampling strategy is used to create the individual ant sequences. Then a fading memory fitness sharing function is applied to modify the clustering strategy and regroup the ants for multimodal optimization of the search space. An adaptive learning strategy is developed to dynamically adjust the search scope of the ant colony. Moreover, an online health monitor is used to replace the weak ants with new ones in a timely manner in order to keep robustness of the FSAC. Finally, the multimodal feasible solutions which are good candidates for the ACLS design are presented and the characteristics of the solution space are also analyzed. The result shows that many large solution sets are located at the bottom of the solution space, whereas on the upper side of the solution space, the number of feasible solutions decreases sharply. An F/A-18 model is used as a test bed and the simulations are carried out in various wind conditions to demonstrate the effectiveness and feasibility of the proposed method.

Adaptive backstepping controller design of quadrotor biplane for payload delivery

Performance of the UAVs for a particular application can be enhanced by hybrid design, where take-off, hover, and landing happen like rotary-wing UAVs, and flies like fixed-wing UAVs. A backstepping controller and an adaptive backstepping controller are designed for trajectory tracking and payload delivery in a medical emergency or medical substance delivery like vaccine delivery in the presence of wind gust. Simulation results show that the backstepping controller effectively tracks the trajectory during the entire flight envelope, including take-off, hovering, the transition phase, level flight mode, and landing. A comparison between Backstepping, Integral Terminal Sliding Mode (ITSMC) and Adaptive Backstepping controllers for payload delivery show that the adaptive backstepping controller effectively tracks the altitude and attitude. ITSMC is capable of tracking the desired trajectory for a change in the mass but has sluggish response. The backstepping controller generates a steady-state error in altitude during the mass change in biplane quadrotor.

실시간 PQ-RRT* 알고리즘을 이용한 유도 시스템 개발과 궤적 추종 제어기의 통합 설계 연구

Herein, the integrated design process of guidance and control systems required for the design of autonomous flight systems for aircraft is discussed. The system can generate a trajectory using real-time path planning, re-plan the path, and accurately track the created trajectory. The real-time path planning is implemented using a rapidly-exploring random tree-based algorithm. Furthermore, the problems with the real-time rapidly-exploring random tree-based algorithm and possible solutions are discussed accordingly. The control system is implemented by incremental backstepping control, which ensures trajectory tracking performance across the entire flight envelope. To verify the series of processes, simulations are run using the high-fidelity Bo-105 model. The effectiveness of the real-time PQ-RRT* and path optimization algorithm proposed in this work is verified by the simulation results.

Flight control methodologies for Neptune aerocapture trajectories

In this work, a comparison of potential flight control methodologies for Neptune aerocapture trajectories is presented. Lifting trajectories pertaining to bank angle modulation and direct force control as well as ballistic trajectories pertaining to continuously-variable drag modulation are investigated. A parametric study of vehicle configurations is explored to quantitatively compare the flight envelope between lifting and ballistic trajectories. A closed-loop numerical predictor–corrector aerocapture guidance architecture is utilized to unify each flight control technique for trajectory comparisons. A series of Monte Carlo simulations of blunt body Neptune aerocapture trajectories are conducted to assess each flight control’s robustness to uncertainties in vehicle aerodynamics, atmospheric knowledge, and entry state. Direct force control can achieve 100% successful science orbit insertion within a 120 m/s total ΔV budget. Bank angle modulation can achieve 100% successful science orbit insertion within a 300 m/s total ΔV budget. Continuously-variable drag modulation can achieve 99.3% successful science orbit insertion within a 190 m/s total ΔV budget but at 3 times lower peak stagnation point convective heating rate.