Post-stall Maneuver Research Articles

A Weighted Feature Fusion Model for Unsteady Aerodynamic Modeling at High Angles of Attack

Unsteady aerodynamic prediction at high angles of attack is of great importance to the design and development of advanced fighters. In this paper, a weighted feature fusion model (WFFM) that combines the state-space model and neural networks is proposed to build an unsteady aerodynamic model for the precise simulation and control of post-stall maneuvers. In the proposed model, the influences of the physical model on neural networks are considered and adjusted by introducing a standardization layer and a new weighting method. A long short-term memory (LSTM) network is used to fuse two mappings: one from flight states to aerodynamic loads, and the other from low-fidelity data to high-fidelity data. Data from wind tunnel oscillation experiments at high angles of attack using a new kind of wire-driven parallel robot and the traditional tail support are used for verifying the proposed aerodynamic model. The output of the WFFM is also compared with predictions from other models, such as the state-space model, single LSTM model, and feature fusion model not including a feature weighting layer. Results demonstrate improved accuracy of the proposed model in the interpolation and extrapolation tests. Furthermore, the WFFM is applied to the flight simulation of F-16 with different control inputs. Compared with conventional models, the WFFM shows improved accuracy and better generalization capability.

Agile Fixed-Wing UAVs for Urban Swarm Operations

Fixed-wing unmanned aerial vehicles (UAVs) offer significant performance advantages over rotary-wing UAVs in terms of speed, endurance, and efficiency. Such attributes make these vehicles ideally suited for long-range or high-speed reconnaissance operations and position them as valuable complementary members of a heterogeneous multi-robot team. However, these vehicles have traditionally been severely limited with regards to both vertical take-off and landing (VTOL) as well as maneuverability, which greatly restricts their utility in environments characterized by complex obstacle fields (e.g., forests or urban centers). This paper describes a set of algorithms and hardware advancements that enable agile fixed-wing UAVs to operate as members of a swarm in complex urban environments. At the core of our approach is a direct nonlinear model predictive control (NMPC) algorithm that is capable of controlling fixed-wing UAVs through aggressive post-stall maneuvers. We demonstrate in hardware how our online planning and control technique can enable navigation through tight corridors and in close proximity to obstacles.We also demonstrate how our approach can be combined with onboard stereo vision to enable high-speed flight in unknown environments. Finally, we describe our method for achieving swarm system integration; this includes a gimballed propeller design to facilitate automatic take-off, a precision deep-stall landing capability, multi-vehicle collision avoidance, and software integration with an existing swarm architecture.

Numerical Virtual Flight Simulation of Quasi-Cobra Maneuver of a Fighter Aircraft

By using a computational-fluid-dynamics-based numerical virtual flight (NVF) simulator, a simplified quasi-Cobra maneuver of a typical third-generation fighter is simulated and the unsteady aerodynamics, such as the dynamic forces and moments and the flow separation, are analyzed preliminarily. The NVF simulator integrates an unsteady Reynolds-averaged Navier–Stokes (RANS) solver on dynamic unstructured/hybrid grids, a rigid-body dynamics solver, and user-defined flight control system. To analyze the complex multiphysics coupling problem, four cases with increasing complexity are studied, including 1) static state simulation; 2) one-degree-of-freedom (1-DOF) forced pitchup; 3) 1-DOF pitchup by open-loop control of the elevators; and 4) three-degree-of-freedom (3-DOF) quasi-Cobra maneuvering. By applying a cosine function open-loop flight control law to the elevators and introducing a simplified vector thrust model, the fighter can pitch up to a high angle of attack from the cruising state, and then it can pitch down quickly, the whole process is similar to a Cobra maneuver. Even though the RANS simulations are not necessarily highly accurate for the large angle of attack, the multidisciplinary application in this work demonstrates the potential of the NVF simulator for the simulations of realistic poststall maneuvers.

Minimum Parameters Learning-Based Dynamic Surface Control for Advanced Aircraft at High Angle of Attack

Aiming at the difficulty of post-stall maneuvering control modeling and control of advanced aircraft under unsteady aerodynamics, a control method with high control accuracy and fast computation speed is proposed based on Radial Basis Function (RBF) network with minimum parameters learning (MPL) and dynamic surface control (DSC) method. Firstly, the aerodynamic characteristics of post-stall maneuvers are analyzed based on the experimental data of large-scale oscillation wind tunnels, and the key factors affecting the unsteady aerodynamic forces are obtained. Then, an accurate unsteady aerodynamic model is established based on the improved extreme learning machine (ELM) method. Secondly, the influence of unsteady aerodynamic forces on the control of post-stall maneuvers is considered. For the uncertainty of advanced aircraft model, high angle of attack flight control laws based on RBF-DSC are designed. In order to improve the calculation speed of the above control law and optimize the parameters, a post-stall maneuver control law method based on MPL-RBF-DSC is designed, and the stability of the method is proved. The coordinated allocation of the conventional aerodynamic surfaces and thrust vectors is realized based on the daisy chain method. Finally, the typical maneuver simulation of “Cobra” is carried out, which highlights the advantages of the design method in this paper, such as high control accuracy, short calculation time and strong robustness.

LPV Modeling and Identification of Unsteady Aerodynamics for Fast Maneuvering Aircrafts

Fast maneuvers at large angles of attack are necessary for modern fighter aircrafts. The aerodynamic coefficients in this situation are unsteady, with the characteristics of hysteresis and fast time-varying. We propose an identification method based on the linear parameter varying (LPV) model. The model takes into account the hysteresis with respect to the angle of attack, and fast time varying with respect to the motion of post-stall maneuvers. By introducing the angular velocity as an exogenous variable, the model can guarantee the tracking ability of fast time-varying variations. We use the instrumental variable least squares (IVLS) algorithm to deal with the estimation problem. Finally, the experimental data obtained from maneuvers at large angles of attack from wind tunnel tests are used to validate the applicability and effectiveness of the proposed method.

Air Vehicle Management with Integrated Thrust-Vector Control

Vectored thrust is a powerful flow control technology that has long been used in rockets, launch vehicles, and other space systems. The technology has been more recently applied to atmospheric flight in conjunction with airbreathing propulsion systems. When fully integrated with other aerodynamic controls, vectored thrust has been shown to enable air vehicle management under unconventional flight conditions ranging from vertical takeoff or landing to agile maneuvering flight well beyond the aerodynamic stall boundary, the latter enabling close-in air-combat advantage. Capabilities demonstrated in several previously developed high-performance thrust-vectored experimental aircraft are reviewed and explored to illustrate the value and versatility of integrated thrust-vector (ITV) control technology. These capabilities include enhanced control authority at low airspeeds; deep, poststall maneuverability; tailless flight in both subsonic and supersonic flight regimes; extremely short takeoff and landing capability; as well as other benefits that can accrue from ITV implementation. Applications of this technology to contemporary aircraft are also discussed, and ITV nozzle concepts intended for use in conjunction with gas-turbine engines, including both mechanical and fluidic approaches, are briefly reviewed. A vision and rationale for incorporating ITV control on future civil and commercial aircraft designs are discussed, as are the likely areas of research needed for realization of the full potential of the ITV concept.

Dynamic surface control design of post-stall maneuver under unsteady aerodynamics

This paper presents an efficient method that overcomes the problem of the control design of the post-stall maneuver under unsteady aerodynamics. On the basis of adequate data of the large amplitude oscillation experiment device in wind tunnel test, the unsteady aerodynamics model with nonlinearity, coupling and hysteresis is established by the improved Extreme Learning Machine (ELM) method. Considering the nonlinearity of the longitudinal model of the advanced fighter and the aerodynamics characteristics of the post-stall maneuver, the control law under large attack angle is designed combining the backstepping method and the daisy chain allocation method. The first order filter is adopted to prevent the “differential explosion” problem. The designed control allocation law guarantees that the conventional surfaces and the vector nozzle deflect coordinately within the position limits and the rate limits. The Radial Basis Function (RBF) network is applied to model the uncertainty, and the stability of the proposed control law which considering the uncertainty is also proved. Digital simulations of the typical “Cobra” maneuver under the unsteady aerodynamics are completed with comparisons under different conditions. Simulations results verify the validity of the proposed control law under unsteady aerodynamics and the aerodynamics uncertainty.

Roto-Stabilizer for Superb Pitch-Related Post-Stall Maneuvers and STOL

Stabilizers and their control surfaces are vital components in maneuvering an airplane during flight. However, a shortcoming of stabilizers is that they require airstream or propeller wash for them to work properly. In this work, we propose the concept of roto-stabilizer as viable substitution for conventional horizontal stabilizer. A key benefit of the proposed technique is its ability to exert powerful moment in the absence of forward airspeed or propeller wash. Proof of principle is demonstrated via computer simulations. Results reveal that new aerobatic maneuvers are made possible. Furthermore, when implemented in canard configuration, it is possible to achieve ultra-STOL and VTOL.

Dynamic test experiment system of single degree of freedom of Φ3.2 m wind tunnel

Post-stall maneuverability is a key technical indicator for modern fighter. It requires a fighter to maintain excellent dynamic behaviors at a high angle of attack. The dynamic stability derivative is a design input off light control system and one of its important sources is the wind tunnel test. In order to meet stricter quality requirements of test data, China Aerodynamics Research & Development Center (CARDC) has set up a forced oscillation dynamic test system based on single degree of freedom in the open test section of Φ3.2 m wind tunnel. Based on the electronic cam, in this system, the servo motor-decelerator directly drives the supporting rod. The system simplifies the structure of device, reduces the number of moving parts as well as the mechanical clearance. Features of the system include: (1) a large variation range of frequencies and amplitudes, a higher control precision and step-less regulation; (2) independent variation of angle of attack and sideslip angle so that the combination of angle of attack and sideslip angle can be more convenient with higher test efficiency; (3) a small wind tunnel wall interference and support interference. Same set of devices can enable small amplitude dynamic derivatives test, large amplitude oscillatory test, and static force measurement, all of which contribute to a consistent measurement basis. With this test system, we developed a simplified fighter model dynamic test, obtaining dynamic derivatives and unsteady aerodynamic loads. Results of our tests verified the rationality and reliability of data as well as a high precision of repeatability.

Robust post-stall perching with a simple fixed-wing glider using LQR-Trees

Birds routinely execute post-stall maneuvers with a speed and precision far beyond the capabilities of our best aircraft control systems. One remarkable example is a bird exploiting post-stall pressure drag in order to rapidly decelerate to land on a perch. Stall is typically associated with a loss of control authority, and it is tempting to attribute this agility of birds to the intricate morphology of the wings and tail, to their precision sensing apparatus, or their ability to perform thrust vectoring. Here we ask whether an extremely simple fixed-wing glider (no propeller) with only a single actuator in the tail is capable of landing precisely on a perch from a large range of initial conditions. To answer this question, we focus on the design of the flight control system; building upon previous work which used linear feedback control design based on quadratic regulators (LQR), we develop nonlinear feedback control based on nonlinear model-predictive control and ‘LQR-Trees’. Through simulation using a flat-plate model of the glider, we find that both nonlinear methods are capable of achieving an accurate bird-like perching maneuver from a large range of initial conditions; the ‘LQR-Trees’ algorithm is particularly useful due to its low computational burden at runtime and its inherent performance guarantees. With this in mind, we then implement the ‘LQR-Trees’ algorithm on real hardware and demonstrate a 95 percent perching success rate over 147 flights for a wide range of initial speeds. These results suggest that, at least in the absence of significant disturbances like wind gusts, complex wing morphology and sensing are not strictly required to achieve accurate and robust perching even in the post-stall flow regime.

Unsteady aerodynamics modeling for flight dynamics application

In view of engineering application, it is practicable to decompose the aerodynamics into three components: the static aerodynamics, the aerodynamic increment due to steady rotations, and the aerodynamic increment due to unsteady separated and vortical flow. The first and the second components can be presented in conventional forms, while the third is described using a one-order differential equation and a radial-basis-function (RBF) network. For an aircraft configuration, the mathematical models of 6-component aerodynamic coefficients are set up from the wind tunnel test data of pitch, yaw, roll, and coupled yawroll large-amplitude oscillations. The flight dynamics of an aircraft is studied by the bifurcation analysis technique in the case of quasi-steady aerodynamics and unsteady aerodynamics, respectively. The results show that: (1) unsteady aerodynamics has no effect upon the existence of trim points, but affects their stability; (2) unsteady aerodynamics has great effects upon the existence, stability, and amplitudes of periodic solutions; and (3) unsteady aerodynamics changes the stable regions of trim points obviously. Furthermore, the dynamic responses of the aircraft to elevator deflections are inspected. It is shown that the unsteady aerodynamics is beneficial to dynamic stability for the present aircraft. Finally, the effects of unsteady aerodynamics on the post-stall maneuverability are analyzed by numerical simulation.

1A2-C18 テールシックVTOL UAVの最小移動ポストストール姿勢遷移機動

Miniature vertical take-off and landing unmanned aerial vehicles (VTOL UAVs) make various missions possible alone. Since simple tail-sitter UAVs have any extra equipments for tail-sitting maneuver, attitude control is difficult problem. This paper discusses agile turnaround flight utilizing post-stall maneuver for tail-sitter VTOL UAVs. The minimum longitudinal travel distance turnaround strategy is formulated and optimization problem is solved. In order to achieve agile turnaround stably, a novel attitude control strategy for a tail-sitter VTOL UAV is implemented. The results show that the post-stall maneuver with proposed attitude controller achieved minimum travel distance turnaround stability, while conventional attitude controller fails.

Adaptive Materials and Aerostructures: Revolutionizing Aerospace Systems

This paper describes some of the most unique and dynamic aerospace technologies of our time. A brief historical overview traces modern Adaptive Aerostructures to their roots in subsystems of the F-14, through the bending, twisting plates of the 1980’s and the first warping-wing patents of the early 1990’s. This paper includes an overall description of the latest adaptive actuation technologies using Post-Buckled Precompressed (PBP) actuators, which enabled the first morphing-wing UAVs and world’s first post-stall maneuvering, convertible UAV which hovers like a helicopter then dashes like a missile. The paper describes the fundamental structural mechanics of these actuators and shows that they can provide up to an order of magnitude better performance than conventional adaptive actuators approaches. The paper describes future systems using such actuators in aircraft as small as 5mm to the nacelles of widebody aircraft. Coming advances are described including the latest and most capable flutter test and flight control surfaces for general aviation aircraft, business jets and commercial transports. The document concludes with a summary of the most modern optically adaptive materials and describes their performance in the first unclassified visual stealth aircraft.

From Water Tunnel to Poststall Flight Simulation: The F/A-18 Investigation

Introduction T HE prediction of poststall maneuvers and departure characteristics of high-performance aircraft continues to be challenging,1;2 due, in particular, to the highly three-dimensional nature of the motions and aerodynamic responses. A case in point is the F/A-18 aircraft (Fig. 1), which has experienced departures from controlled  ight, typically involving large sideslip excursions with attendant high body-axes rate variations leading to high angles of attack ® (Ref. 4). The lateral–directional departure susceptibility of the F/A-18 provided part of the motivation within the NASA High-Alpha Technology Program for the improvement of high-alpha predictionsbased on ground testing. The gyrations and responsesexperiencedon departureare intrinsicallynonplanar, that is, involvingarbitraryrotationsnot alignedwith theprincipalaxesof the aircraft.3 When the  ight mechanical deŽ ciencies of an F/A-18 simulation were identiŽ ed, a Canadian programwas undertaken to enhance simulator Ž delity.6 The areas in which the airframe model could be improved included the poststall aerodynamicdatabase, the multivariabletable lookuparchitecture,and the aerodynamicmodel. The present paper focuses on the improvements to the database. The shortcomings of the poststall aerodynamic database were attributed largely to facility interference and scaling effects not accounted for and gaps in the parameter space covered. The original rotary database for F/A-18 was generated in the NASA Langley Research Center Spin Tunnel.11 The F/A-18 poststall maneuvering aerodynamics is dominated by forebody/leading edge extension (LEX) vortex interactions at moderately high angles of attack (30 55 deg). In the light of the sensitivity of the vortex interactions to sideslip angle (Ref. 12), it was deemed necessary to generate a more detailed rotary database to deŽ ne the associated relationshipsover a large sideslip range. When the advantages of the water-tunnel approachwere considered, it seemed appropriate to use this ground-testmethodology to generate supplementary data. The orbital platform rotary balance system (OPLEC) at the Institute for Aerospace Research (IAR) water tunnel, a diagnostic tool for studying support interference7;8 and unsteady wall interference, was subsequently used in highresolution tests as well as diagnostic experiments15i18 on the baseline F/A-18. A thorough understanding of scale effects on vortex

Optimization of Vertical Plane Cobralike Pitch Reversal Maneuvers

Four time-optimal, vertical plane, cobralike pitch maneuvers corresponding to different sets of boundary conditions have been studied for the F-18 high-angle-of-attack research vehicle aircraft. These maneuvers are poststall maneuvers that enable an aircraft that is initially followed by a pursuing aircraft to shift positions, that is, the initial evader turns into the pursuer. The initial pursuer is assumed to remain at constant speed and altitude. In addition an 80-deg pitch reversal maneuver has been considered that was optimized using a Chebyshev approach. A nonlinear programming and collocation method was successfully used to find the optimal vertical plane trajectories in open-loop form. All trajectories that were found have been put through a six-degree-of-freedom simulation utilizing a nonlinear inversion closed-loop control technique. The results showed that the optimal vertical plane trajectories can be flown quite accurately, using longitudinal and lateral thrust vectoring, along with the conventional aerodynamic controls.

Optimization for Fighter Aircraft Vertical-Plane Maneuvering Using Poststall Flight

A time-optimal, vertical-plane, evasive-offensive maneuver using poststall e ight is found for a e ghter aircraft similar to an F-16. An evader aircraft initially followed by a pursuer aircraft transfers position using the verticalplane maneuver, becoming the pursuer. The aircraft that was initially the pursuer is assumed to continue e ight at constant velocity and constant altitude. A direct numerical method, collocation with nonlinear programming, proveswellsuitedforsolvingthisproblem.Theevaderaircraft’ soptimaltrajectoryisfoundtobeacobramaneuver, one of the vertical poststall maneuvers. Allowing the evader to use thrust vectoring yields a small improvement in e nal time. Nomenclature CD = drag coefe cient CL = lift coefe cient D = drag force e = unit vector directed along the longitudinal axis of an evader g = gravitational acceleration h = altitude J = cost function L = lift force m = mass Nz = normal acceleration r = position vector of a pursuer relative to an evader S = wing area T = thrust t = time v = velocity x = downrange position a = angle of attack c = e ight-path angle q = air density s = thrust vectoring angle s LMT = absolute value of limitation of thrust vectoring angle Subscripts e = value for initial evader f = value at e nal time

Implications of Recent Rotary Rig Results for Flight Prediction

Analysis of rotary balance data on a generic aircraft model revealed the nature of the flow mechanisms present in subscale experiments, which included the existence of support interference. Recent results on the F/A-18 confirmed the existence of a threshold rotation rate in the rotary data and revealed that in the nonlinear domain at elevated angles of attack, alternatively deterministic or chaotic behavior will prevail, depending on whether or not the magnitude of the initial rotation rate exceeds the threshold value. This has important implications for flight mechanics predictions and control system design as the poststall maneuvering envelope often will include the hysteresis loop. Moreover, dynamic data generated to date in the nonlinear domain have often been restricted to this inner region, limited by the threshold rates, and do not provide any information about the basic hysteretic behavior. Only a comprehensive investigation of steady-state hysteresis and support interference, as in the case of the F/A-18, can provide the basis needed for flight predictions in this domain

Minimum-Time Maneuvers of Thrust-Vectored Aircraft

The new aircraft technology of thrust vectoring (TV) enables the execution of new types of poststall maneuvers. Aircraft agility and e ight safety are greatly enhanced. The objective and main contribution of the present work are the suboptimal solution of minimum-time pitch-reversal (there and back ) and yaw-reversal standard agility comparison maneuvers (SACOM). It is demonstrated that, due to asymmetric and coupling moments at high ® and high ¯, pure pitch and yaw maneuvers are impossible; a full six-degree-of-freedom model must be employed. A novel parametric two-stage suboptimization algorithm was developed to copewith the complexity ofthe optimal control problem. All four aerodynamic and three TV controls are simultaneously active in the execution of a low g load, nearly pure 80-deg pitch-reversal and a 30-deg yaw-reversal SACOM of an F-15B aircraft.

The X-31 aircraft: Advances in aircraft agility and performance

The X-31 enhanced fighter maneuverability (EFM) demonstrator has pioneered agile flight in the post-stall flight regime and explored integrated multi-axis thrust vectoring across a broad flight envelope. Its maneuvering achievements include sustained flight up to 70 degrees angle of attack, velocity vector rolls in deep post-stall conditions, and post-stall turns from high entry to exit speeds with ultra low turning/transitional conditions. The concept of post-stall maneuverability was extensively studied in simulations preceding initiation of the X-31 program. These simulations provided a baseline for tactical utility demonstrations and vehicle design requirements. Post-stall maneuverability was not achieved without encountering and mitigating the effects of highly unsteady, asymmetric, vortex-dominated flow-fields associated with post-stall flight. Anomalies in vehicle response to control inputs were observed at high angles of attack, as were differences between simulator and actual flight parameters due to a misrepresentation of the effects of these complex flowfields. Some preliminary force and moment data for the X-31 configuration during dynamic maneuvers are provided to highlight the complex nature of the flowfield. The X-31 aircraft's enabling capabilities, including multi-axis thrust vectoring and integrated flight/propulsion control also provided performance enhancements across the entire flight envelope. In what were known as ‘quasi-tailless’ experiments, conventional aerodynamic control surfaces were used to reduce or eliminate the stabilizing influence of the vertical stabilizer, while the vehicle's multi-axis thrust vectoring capability was used for restabilization. Properly exploited, these technologies can lead to the reduction or elimination of traditional aerodynamic control surfaces, which provides profound improvements in vehicle range, weight, payload, and low observability. This review focuses on some of the principal aerodynamic issues encountered during the course of the X-31 program, the resulting lessons learned, and the derived results concerning the complex aerodynamic effects associated with exploiting this aircraft's unique capabilities. Suggested research and technology development issues which came to light during the X-31 program are also discussed.

Primjena vektoriranog potiska s osvrtom na superupravljivost istraživackog zrakoplova X-31A

Vectored propulsion is a new concept of aircraft attitudecontrol by engines jet-deflection. Thrust vectoring researchhas become actualized during the last ten years, mainly formilitary application.The carried out experimental projects confirm the effectivenessof the thrust-vectoring systems in both conventionalaircraft attitude control and post-stall maneuverability enhancement.Apart from military aviation, the results of thrust-vectoringresearch are applicable in civil aviation, especially in STOLand VTOL aircraft design.