Relaxed Static Stability Research Articles

Effects of Static Stability Margin on Aerodynamic Design Optimization of Truss-Braced Wing Aircraft

Currently, the aviation industry is facing an oil and energy crisis and is contributing much more greenhouse gas emissions to the environment. Aircraft design approaches, such as aerodynamic shape optimization, new configuration concepts, and active control technology, have been the primary and effective means of achieving goals concerning fuel burn, noise, and emissions. For now, the design problems of relaxed static stability (RSS, an active control technique) and truss-braced wing (TBW) configurations with high-fidelity aerodynamic shape optimization methods have been investigated widely to promote aerodynamic performance. Nevertheless, they are studied almost always separately, and the combination of exploration and refined design is rarely presented. Therefore, the purposes of this work are to evaluate the benefits of RSS on a full TBW wing–body–tail configuration under various flight conditions and the effects on multi-components and to further explore the potential and analyze the aerodynamic features with the combination of shape optimization and RSS. To address these issues, on the one hand, a range of seven static stability margins are adopted to evaluate its effects with a high-fidelity Reynolds-averaged Navier–Stokes solver. On the other hand, seven cases of drag minimization multipoint aerodynamic design optimization are performed, which are with 600 shape variables and 13 twist variables, subject to lift coefficient, trim, and thickness constraints. The results indicate that with RSS only, the initial configuration has a 2.39% drag reduction under cruise conditions and a 3.01% and a 5.24% drag reduction under two off-design conditions. Additionally, the effects on the multi-components are observed and analyzed. Moreover, all of the optimized configurations with RSS have 2.13%, 2.42%, and 2.12% drag reductions under cruise conditions, drag divergence conditions, and near-buffet-onset conditions, respectively. The most promising optimized configuration has a lift-to-drag ratio of 24.48 with an aerodynamic efficiency of 17.14. The evaluations with a series of off-design points also present high-level aerodynamic efficiency.

Angle-of-Attack Command Longitudinal Control for Supersonic Advanced Trainer Aircraft

This paper details the design procedure of developing an angle-of attack command longitudinal control system for the relaxed static stability aircraft in power approach mode. An advanced method of relaxed static stability is utilized for improving aerodynamic performance of a supersonic advanced trainer aircraft. The trainer aircraft adopts the relaxed static stability concept to improve the aerodynamic performance and to guarantee the aircraft stability. The longitudinal control law of the aircraft uses the dynamic inversion and proportional-plus-integral control methods. This paper addresses the analysis of the aircraft characteristics such as damping ratio, natural frequency, and gain and phase margins about the state variables for the longitudinal inner loop feedback design. In numerical simulations, the flight control performance of the developed angle-of-attack command longitudinal control system is compared with that of the existing pitch rate command longitudinal control system.

Relaxed static stability for four-wheel independently actuated ground vehicle

Abstract In order to maximize the advantage of four-wheel independently actuated (FWIA) technology, the paper proposes a new configuration and dynamics control method. It is called relaxed static stability (RSS). First, RSS allows lateral dynamics system of the vehicle to be inherently unstable to enhance overall structure design flexibility. Three FWIA electric vehicles (a racecar, a passenger car, and a military vehicle) are shown to demonstrate the benefit of RSS. By applying the RSS, the configuration flexibilities of the battery pack location of these vehicles are improved by 95.4%, 167% and 20%, respectively. Second, the closed-looped handling stability could be improved by yaw moment control through the pole assignment technique. The target pole’s positions of closed-looped lateral dynamics system are selected according to various performance requirements. Four cases of target pole locations are given as examples to show the principle of selecting the target pole locations for different type vehicles. A pole assignment controller is designed with the consideration of the parametric uncertainties caused by tire nonlinear behavior. Finally, an FWIA electric vehicle, which is built by the authors, is used as testbed to show the benefit of the RSS. The experiments in step steer and sine steer conditions prove the efficiency and robustness of the controller, as well as the convenience of adjusting target pole locations according to various performance requirements.

Relaxed static stability based on tyre cornering stiffness estimation for all-wheel-drive electric vehicle

A novel dynamics control approach for all-wheel-drive electric vehicle (EV), relaxed static stability (RSS) approach is proposed with two advantages. Firstly, it allows vehicle lateral dynamics system to be inherent unstable to improve configuration flexibility. Secondly, handling performance could be improved based on closed-looped pole assignment with additional yaw moment. In this paper, basic control framework of RSS is proposed, including ‘Desired Pole Location’, ‘Pole Assignment’ and ‘Tyre Cornering Stiffness Estimation’ modules. The tyre cornering stiffness is estimated online to improve the robustness of the controller. The experiments based on an EV testbed show the performance and efficiency of RSS.

항공기 기수 숙임 현상 개선

The flight control system utilize RSS(Relaxed Static Stability) criteria in both longitudinal axes to achieve performance enhancements and improve stability. The aircraft using digital flight-by-wire flight control system receives aircraft flight conditions such as pitch, roll and yaw rate, normal acceleration from RSA(Rate Sensor Assembly) and ASA(Acceleration Sensor Assembly). These sensors has permissible measurement error related to system safety of an aircraft but, unexpected flight motions are happened by sensing errors such as offset, noise and etc. The unexpected pitch down tendency occurred by ASA sensor bias in 1g level flight with pilot hands-off. This paper addresses the design and verification of flight control law to improve of pitch down or up tendency caused by ASA sensor bias. The result of analysis and flight test reveals that pitch down tendency can be improved by pitch attitude feedback system.

항공기 비정상 자세, 고도 및 속도 회복을 위한 자동회복장치 설계 및 검증에 관한 연구

Relaxed Static Stability(RSS) has been applied to improve flight performance of modern version supersonic jet fighters. Flight control systems are necessary to stabilize an unstable aircraft and to provide adequate handling qualities. Also, flight control systems of modern aircraft employ many safety measure to cope with emergency situations such as a pilot unknown attitude flight conditions of an aircraft in night flight-testing. This situation is dangerous because the aircraft can lose if the pilot not take recognizance of situation. The system called the Pilot Activated Recovery System or PARS, provided a pilot initiated automatic maneuver capable of an aircraft recoveries in situations of unusual attitudes, speed and altitude. This paper addresses the concept of PARS with AARS(Automatic Attitude Recovery System), ATCS(Automatic Thrust Control System) and MARES(Minimum Altitude Recovery Estimation System), and this control law is designed by nonlinear control law design process based on model of supersonic jet trainer. And, this control law is verified by real-time pilot evaluation using an HQS(Handling Quality Simulator). The result of evaluation reveals that the these systems support recovery of an aircraft unusual attitude and speed, and improve a safety of an aircraft.

Adaptive Control of Advanced Fighter Aircraft in Nonlinear Flight Regimes

When advanced fighter aircraft fly at high angles of attack, unsteady aerodynamic effects, wing rock, and saturation of aerodynamic effectors can lead to difficulty in control and maneuverability. A novel adaptive output feedback control design based on dynamic inversion is investigated for aircraft which are operated in highly nonlinear flight regimes, where uncertainties in the form of both unmodeled parameter variations and unmodeled dynamics are common. The stability of the design is analyzed and validated with simulations using a modified NASA F-15 simulation. The simulation includes thrust vectoring and a differential stabiliator to provide increased control authority at high angles of attack and relaxed static stability to increase pitch maneuverability. The control designs include the use of pseudocontrol hedging techniques to exclude adaptation to control saturation.

비행제어시스템 설계 및 검증 절차

Relaxed static stability(RSS) concept has been applied to improve aerodynamic performance of modern version supersonic jet fighter aircraft. Therefore, flight control systems are necessary to stabilize an unstable aircraft, and provides adequate handling qualities and achieve performance enhancements. Standard FCSDVP (Flight Control System Design and Verification Process) is provided to reduce development period of the flight control system. In addition, if this process is employed in developing flight control system, it reduces the trial and error for development and verification of flight control system. This paper addresses the flight control system design and verification process for the RSS aircraft utilizing design goal based on military specifications, linear and nonlinear system design and verification based on universal software, handling quality test based on HILS(Hardware In-the-Loop Simulator) environment, and ground and flight test results to verify aircraft dynamic flight responses.

항공기 자세회복을 위한 자동회복장치 설계 및 검증에 관한 연구

Relaxed static stability(RSS) concept has been applied to improve aerodynamic performance of modem version supersonic jet fighter aircraft. Therefore, flight control system are necessary to stabilize an unstable aircraft and provides adequate handling qualities. Also, flight control systems of modem version aircraft employ a safety system to support emergency situations such as a pilot unknown attitude flight conditions of an aircraft in night flight-testing. This situation is dangerous because the aircraft can lose if the pilot not take recognizance of situation. Therefore, automatic recovery system is necessary. The system called the "Pilot Activated Recovery System" or PARS, provided a pilot initiated automatic maneuver capable of an aircraft recoveries in situations of unusual attitudes. This paper addresses the concept of PARS and designed using nonlinear control law design process based on model of supersonic jet trainer. And, this control law is verified by nonlinear analysis and real-time pilot evaluation using in-house software. The result of evaluation reveals that the PARS support recovery of an aircraft unusual attitude and improve a safety of an aircraft.

A Study on Longitudinal Phugoid Mode Affected by Application of Nonlinear Control Laws

Relaxed Static Stability (RSS) concept has been applied to improve aerodynamic performance of modern version supersonic jet fighter aircraft. The T-50 advanced supersonic trainer employs the RSS concept in order to improve the aerodynamic performance. And the flight control system stabilizes the unstable aircraft and provides adequate handling qualities. The T-50 longitudinal control laws employ a proportional-plus-integral type controller based on a dynamic inversion method. The longitudinal dynamic modes consist of short period with high frequency and phugoid mode with low frequency. The design goal of longitudinal control law is optimization of short period damping ratio and frequency using Lower Order Equivalent System (LOES) complying the requirement of MIL-F-8785C. This paper addresses phugoid mode characteristics such as damping ratio and natural frequency that is affected by the nonlinear control laws such as angle of attack limiter, auto pitch attitude command system and autopilot of pitch attitude hold.

Development of Flight Control Laws for the T-50 Advanced Supersonic Jet Trainer

The T-50 advanced supersonic jet trainer employs the Relaxed Static Stability (RSS) concept to improve the aerodynamic performance while the flight control system stabilizes the unstable aircraft and provides adequate handling qualities. The T-50 flight control laws employ a proportional-plus-integral type controller based on a dynamic inversion method in longitudinal axis and a proportional type controller based on a blended roll system with simple roll rate feedback and beta-betadot feedback system. These control laws are verified by flight tests with various maneuver set flight envelopes and the control laws are updated to resolve flight test issues. This paper describes several concepts of flight control laws used in T-50 to resolve those flight test issues. Control laws for solving the roll-off problem during pitch maneuver in asymmetric loading configurations, improving the departure resistance in negative angle of attack conditions and enhancing the fine tracking performance in air-to-air tracking maneuvers are described with flight test data.

제어법칙 개발을 위한 소프트웨어 전환장치 설계에 관한 연구

Relaxed Static Stability(RSS) concept has been applied to improve aerodynamic performance of modern version supersonic jet fighter aircraft. Therefore, the flight control systems are necessary to stabilizes the unstable aircraft and provides adequate handling qualities. The initial production flight control system are verified by flight test and it's always an elements of danger because of flight-critical nature of control law function and design error due to model base design method. These critical issues impact to flight safety, and it could be lead to a loss of aircraft and pilot's life. Therefore, development of an easily modifiable RFCS(Research Flight Control System) capable of reverting to a PFCS(Primary Flight Control System) of reliable control law must be developed to guarantee the flight safety. This paper addresses the concept of SSWM(Software Switching Mechanism) using the fader logic such as TFS(Transient Free Switch) based on T-50 flight control law. The result of the analysis based on non-real time simulation in-house software using SSWM reveals that the flight control system are switching between two computers without any problem.

항공기 세로축 무게중심의 변화에 따른 민감도 해석에 관한 연구

현대의 고성능 전투기는 공력성능 및 조종성능의 향상을 위하여 대부분 세로축 방향으로 항공기를 불안정하게 설계하는 정안정성 완화 개념을 채택하고 있다. 비행제어법칙의 설계 작업은 불안정하게 설계된 항공기에 안정성을 부여하고, 주어진 비행임무에 대하여 만족스런 조종성능을 발휘할 수 있도록 비행성능을 조작하는 일련의 과정이다. 세로축 무게중심은 무장형상, 연료상태 및 착륙장치의 위치에 영향을 받으며 항공기 안정성에 많은 영향을 미친다. 따라서 무게중심의 이동은 세로축 안정도 여유에 영향을 미친다. 본 논문에서는 운용 시에 발생 가능한 최대 후방 무게중심에 대해 항공기 안정성을 해석하였고, 비행시험을 통해 최종적으로 검증하였다. 선형해석 항목은 세로축 단주기 모드 특성 및 안정도 여유에 관하여 행하였으며, 비선형 해석 항목은 단주기 모드를 해석하기 위해 세로축 가진 입력에 대한 항공기 응답특성을 분석하였다. 또한, 최대 후방 무게중심에서 수행된 고받음각 비행시험 자료를 제시함으로써 T-50 고등훈련기의 비행 안정성을 제시하였다. An advanced method of Relaxed Static Stability (RSS) is utilized for improving the aerodynamic performance of modern version supersonic jet fighter aircraft. The flight control system utilizes RSS criteria in longitudinal axis to achieve performance enhancements and improve stability. The flight control law of T-50 advanced trainer employs RSS concept in order to improve the aerodynamic performance and guarantee aircraft stability. The longitudinal center of gravity(X-c.g) varies as a function of external stores, fuel state and gear position. Shifts in X-c.g relate directly to longitudinal static margin in aircraft stability. This paper deals the maximum aft X-c.g for critical aircraft loadings and checks static margin limits using sensitivity such as damping, natural frequency, gain and phase margin. And nonlinear analysis was conducted for such as short period input. And also, this paper shows the T-50 aircraft stability based on the result of high angle of attack flight such as upright and inverted departure.

T-50 세로축 장주기 모드 운동 특성에 관한 연구

An advanced method of Relaxed Static Stability (RSS) is utilized for improving the aerodynamic performance of modern version supersonic jet fighter aircraft. The flight control system utilize RSS criteria in both longitudinal and lateral-directional axes to achieve performance enhancements and improve stability. The T-50 advanced trainer employs the RSS concept in order to improve the aerodynamic performance and the flight control law in order to guarantee aircraft stability. The longitudinal two modes are the short period with high frequency and the phugoid mode with low frequency. The design goals of longitudinal control laws is concerned with the short period damping and frequency optimization using lower order equivalent system and utilizing the requirement of MIL-F-8785C. Analysis of short period mode has been and continues to be performed This paper addresses the analysis of aircraft phugoid node characteristics such as damping, natural frequency, and analysis of aircraft pitch motion that impacted by angle of attack limiter and auto pitch attitude control law.

Relaxed Static Stability Aircraft Design via Longitudinal Control-Configured Multi-disciplinary Design Optimization Methodology

This paper describes a multi-disciplinary design optimization approach to the conceptual design of a commercial aircraft with relaxed static stability. Longitudinal flight dynamics analysis and control design are performed concurrently with other disciplinary analysis to augment and improve handling qualities. The developed methodology enables control-configured designs providing higher freedom of change at the conceptual design stage. A design example demonstrates the effectiveness of the proposed integrated approach to improve performance goals.

T-50 세로축 비행제어법칙 설계에 관한 연구

An advanced method of Relaxed Static Stability (RSS) is utilized for improving the aerodynamic performance of modem version supersonic jet fighter aircraft. The flight control system utilizes RSS criteria in both longitudinal and lateral-directional axes to achieve performance enhancements and improve stability. The T-50 advanced trainer employs the RSS concept in order to improve the aerodynamic performance and the flight control law in order to guarantee aircraft stability, The T-50 longitudinal control laws employ the dynamic inversion and proportional-plus-integral control method. This paper details the design process of developing longitudinal control laws for the RSS aircraft, utilizing the requirement of MIL-F-8785C. In addition, This paper addresses the analysis of aircraft characteristics such as damping, natural frequency, gain and phase margin about state variables for longitudinal inner loop feedback design.

비행제어법칙 설계 및 해석 절차에 관한 연구

An advanced method of Relaxed Static Stability (RSS) is utilized for improving the aerodynamic performance of modem version supersonic jet fighter aircraft. The flight control system utilizes RSS criteria in both longitudinal and lateral-directional axes to achieve performance enhancements and improve stability. Standard CLDA (Control Law Design and Analysis) process is provided that reduce the development period of the flight control system. In addition, if this process is employed in developing flight control laws, it reduces the trial and error development and verification of control laws. This paper details the design process of developing a flight control law for the RSS aircraft, utilizing military specifications, linear and nonlinea, analysis using XMATH and ATLAS(Aircraft, Tim Linear and Simulation), handling quality tests using the HQS (Handling Quality Simulator), and real flight test results to verify aircraft dynamic flight responses.

T-50 정밀추적 성능 향상을 위한 세로축 제어법칙에 관한 연구

An advanced method of Relaxed Static Stability (RSS) is utilized for improving the aerodynamic performance of modern version supersonic jet fighter aircraft. The laws of flight control system utilize RSS criteria in both longitudinal and lateral-directional axes to achieve performance enhancements. Particularly, the design of longitudinal control laws for utilizing RSS methods greatly affects the performance of the aircraft in Air-to-Air Tracking and Air-to-Ground modes, which improves weapon delivery. In the area of Air-to-Air Tracking, the development of longitudinal control laws aids in the fine tracking and gross acquisition of other aircraft. This paper proposes that new concept of longitudinal control law introduce in order to improve fine tracking performance in air-to-air tracking maneuver. Result of HQS pilot simulation and flight test, fine tracking performance improve without degradation of gross acquisition when new concept of control law is applied.

Aircraft flight control systems

AbstractThis paper presents a short account of the flight control systems used in commercial transport, military combat and general aviation aircraft. The effects of aircraft safety, reliability and weather delays on satisfactory aircraft operations are shown to be significant reasons for the extensive use of flight control systems. The principles of flight control, the sensors and actuators required and the various modes which can be selected are treated, together with a short account of the primary flying controls and the use of manual reversion in emergency situations. The paper concludes with a consideration of the fly-by-wire (FBW) and fly-fby-light concepts, and covers relaxed static stability, carefree manoeuvring and the use of canards before discussing some FBW flight control systems which are used in passenger aircraft.

Inner loop control of supersonic aircraft in the presence of aeroelastic modes

Longitudinal control system design is considered for a linearized dynamic model of a supersonic transport aircraft concept characterized by relaxed static stability and significant aeroelastic interactions. Two LQG-type controllers are designed using the frequency-domain additive uncertainty formulation to ensure robustness to unmodeled flexible modes. The first controller is based on a fourth-order model containing only the rigid-body modes, while the second controller is based on an eighth-order model that additionally includes the two most prominent flexible modes. The performance obtainable from the fourth-order controller is not adequate, while the eighth-order controller is found to provide better performance. Frequency domain and time-domain (Lyapunov) methods are subsequently used to assess the robustness of the eighth-order controller to parametric uncertainties in the design model.