Gust Load Alleviation Research Articles

Active Flow Control on the Stingray Uninhabited Air Vehicle: Transient Behaviour

The application of leading-edge separation control on an uninhabited air vehicle (UAV) with 50-deg leading-edge sweep is investigated experimentally in a full-scale close-return wind tunnel using arrays of synthetic jet actuators. Active flow control is used to enhance vehicle control at moderate and high angles of attack for takeoff and landing activities or gust load alleviation. The surface-mounted synthetic jet actuators are operated in various waveforms where the carrier frequency is at least an order of magnitude higher than the characteristic shedding frequency of the UAV. Actuation yields a suction peak near the leading edge, however, while excitation with a sinusoidal waveform results in a sharp suction peak near the leading edge; pulse modulation yields a larger and wider suction peak. The flow transients associated with controlled reattachment and separation of the flow over the UAV are investigated using amplitude modulation of the actuation waveform by measuring the dynamic surface pressure at different locations on the upper surface of the UAV’s wing. Phase-locked measurements show that the transients, associated with the onset of reattachment and separation, at high angles of attack are accompanied by the shedding of large-scale vortical structures and oscillations of the surface pressure. However, no oscillations are observed when the baseline flow is attached.

조종면을 이용한 돌풍하중완화 제어시스템 설계

항공기 조종면을 이용한 돌풍하중 완화기술에 대한 연구를 수행하였다 조종면 제어기를 포함하는 서보 공탄성 모델을 구성하였으며, 이에 대한 연속돌풍 응답해석을 수행하여 MSC/NASTRAN을 이용한 해석결과와 비교하여 시보 공탄성 모델을 검증하였다. 조종면 제어기 설계에는 출력 피드백을 이용한 최적 제어기법을 사용하였으며, 설계된 제어기에 대한 수치 시뮬레이션을 수행하여 돌풍하중 완화효과를 검증하였다. A study on gust load alleviation using aircraft control surface was performed. Aeroservoelastic model including control surface controller was formulated and validated by comparing the results of continuous gust response analysis with those of MSC/NASTRAN. Optimal control with output feedback was adopted for designing the control surface controller, and the effects of gust load alleviation was validated by performing the numerical simulation for the controller designed.

Output Feedback Control of the Nonlinear Aeroelastic Response of a Slender Wing

This paper presents the design of optimal constant gain output feedback based controllers for a nonlinear aeroelastic system. Controllers are designed for flutter suppression as well as gust-load alleviation. This controller architecture is one of the simplest, using direct feedback of the sensor outputs, but its performance is highly dependent on sensor selection and placement. Also, optimal design of such controllers require an accurate knowledge of the expected disturbance mode and gust spectrum. This paper presents results pertaining to the performance of SOF controllers for aeroelastic control (linear and nonlinear) and compares it to that of LQR and LQG controllers. Controllers are designed for various sensor placement. The gain and phase margins of the various controllers are also presented to understand the robustness characteristics. For optimal sensor placement and with knowledge of the disturbance, the constant gain output feedback controller performance and robustness was found to be equivalent to that of linear quadratic regulator and linear quadratic Gaussian controllers for the example considered. These controllers were also shown to be very easy to alter and combine with other controllers/filters for better overall system response.

Aeroservoelastic Analysis of the B-2 Bomber

In the early stages of the development of the B-2 bomber, the technical challenges posed by the aeroelastic characteristics of the all-wing aircraft were recognized. The cone guration’ s near-neutral pitch stability and light wing loading made the aircraft highly responsive to atmospheric turbulence. This dictated the requirement for an active digital e ight control system to provide both stability augmentation and gust load alleviation. The gust load alleviation e ight control system was designed by a multidisciplinary team using a combination of optimal and classical control design techniques. The analytical models included linearized approximations of the digital control law mechanization. Flight-test data analysis included the extraction of the vehicle open-loop response, which compared well with the analytical predictions. The multidisciplinary design approach resulted in the successful development of a control augmentation system that provides the B-2 with superb handling characteristics, acceptable low-altitude ride quality, and substantial alleviation of gust loads on the airframe.

Active-Materials Induced-Strain Actuation for Aeroelastic Vibration Control

Recent achievements in the application of active- materials induced-strain actuation to counteract aeroelastic and vibration effects in helicopters and fixed wing aircraft are reviewed. First, the induced-strain actuation principles and capabilities are briefly presented. Next, the attention is focused on the smart rotor blade applications. Induced twist, active blade tip, and active blade flap are presented, with emphasis on experimental results. Then, fixed wing aircraft applications are considered. Experiments of active flutter control, buffet suppression, gust load alleviation, and sonic fatigue reduction are discussed. Conclusions and directions for further work are presented.

Review of Smart-Materials Actuation Solutions for Aeroelastic and Vibration Control

The paper reviews recent achievements in the application of smart-materials actuation to counteract aeroelastic and vibration effects in helicopters and fixed wing aircraft. A brief review of the induced-strain actuation principles and capabilities is done first. Attention is then focused on the smart rotor blade applications. Induced twist, active blade tip, and active blade flap are presented, with emphasis on experimental results. The fixed wing aircraft applications are considered next. Experiments of active flutter control, buffet suppression, gust load alleviation, and sonic fatigue reduction are discussed. Conclusions and directions for further work are presented at the end of the paper.

Finite-state aerodynamic modelling for gust load alleviation of wing–tail configurations

AbstractA finite-state aerodynamics methodology is proposed for the analysis of the forces generated by a gust. To illustrate and assess the methodology, gust-response and gust-alleviation applications are included. Finite-state aerodynamics denotes a technique to approximate aerodynamic loads so as to yield an aircraft model of the type ẋ = Ax + Bu (state-space formulation). In this paper, a finite-state formulation is proposed to include the presence of a gust. The aerodynamic loads to be approximated are evaluated here by using a frequency-domain boundary-element formulation; the flow is assumed to be irrotational except for a zero-thickness vortex layer (wake). The gust-alleviation application consists of determining a control law for reducing the response to a vertical gust disturbance, as measured by the centre of mass acceleration. Two optimal-control approaches are considered for the synthesis of the control law: one uses the classical linear-quadratic regulator (LQR), whereas the second includes the additional feed-forward of the gust velocity ahead of the aircraft. Deflections of ailerons and elevators are assumed to be the control variables. Numerical results deal with responses to both a deterministic ‘1 – cosine’ gust distribution and a stochastic von Kármán spectrum. They indicate that the finite-state aerodynamic model proposed is capable of approximating, with a high level of accuracy, both the aerodynamic loads induced by the aircraft kinematics variables and those induced by the control variables, over a wide frequency range.

An aerodynamic moment-controlled surface for gust load alleviation on wind turbine rotors

A method is described for limiting transient gust loading on horizontal-axis wind turbine rotors. The technique, known as aerodynamic moment control, is implemented by enclosing a pitchable section of the blade in an active control loop, using the external aerodynamic load as feedback variable. The actuator operates within an outer control loop, typically based on electrical power output. The properties of the actuator have been investigated by linear analysis, based on a constant-speed 330-kW wind turbine with active power control, and pitchable blade tips. Two cases were compared, in which the tip actuator was first implemented using position feedback (position control), then subsequently using aerodynamic moment feedback (moment control). The disturbance rejection properties of the overall power controller were found to improve in the latter case. A prototype aerodynamic moment controller has been demonstrated in wind tunnel tests. The controller was configured for an inherently unstable wing section, representing the pitchable tip of a wind turbine blade, at approximately 1/3 full scale. The response to external disturbances was investigated by introducing harmonic perturbations into the upstream airflow. The system successfully demonstrated the principle of aerodynamic moment feedback, although the actuator exhibited somewhat modest gust response characteristics due to the use of velocity feedback to enhance damping. The results of the tests, and the design implications for a full-scale wind turbine, are discussed.

Evolution of airplane gust loads design requirements

Nomenclature a = lift curve slope, per rad #„, bn — Fourier series coefficients for e.g. acceleration c = wing mean geometric chord, ft FK — flight profile alleviation factor FKm = flight profile factor due to airplane weight FKZ = flight profile factor due to altitude g = acceleration due to gravity, 32.2 ft/s g;j, hn = Fourier series coefficients for true gust velocity H = gust gradient distance, ft K, KK = gust load alleviation factors L = scale of turbulence, ft M = Mach number q = dynamic pressure, psf /?, = maximum landing weight/maximum takeoff weight R2 = maximum zero fuel weight/maximum takeoff weight S = wing area, ft s = distance along flight path, ft T = local atmospheric temperature, °R 7,,, /„ = real and imaginary parts of the e.g. acceleration transfer function T() = ambient atmospheric temperature, °R U = gust velocity, fps, true airspeed (7dc = derived equivalent gust velocity, fps, equivalent airspeed (7ds = design gust velocity, fps, equivalent airspeed £/dt = derived true gust velocity, fps, true airspeed £/rct = design reference gust velocity, fps, equivalent airspeed Utr = power spectral scale factor V = aircraft velocity, fps, true airspeed VB = rough air penetration speed, Kt, equivalent airspeed Vc = design cruise speed, Kt, equivalent airspeed VD = design dive speed, Kt, equivalent airspeed W = aircraft weight, Ib Zmo = maximum operating altitude, ft y = ratio of specific heats Aft = incremental load factor, g fjif, = airplane mass parameter p = air density, slugs/ft Introduction S UBSONIC aircraft respond to atmospheric turbulent air motions or eddies of, approximately, 30-2000 ft in extent. Smaller eddies will generally be averaged out over the surface of the aircraft, larger eddies typically will not cause sharp or excessive aircraft accelerations or structural loads on the airplane. Aircraft in supersonic flight respond to ever longer wavelengths as the flight speed is increased. From the aircraft design standpoint, atmospheric turbulence may be separated into two categories: 1) turbulence, which contributes to aircraft structural fatigue and passenger inconvenience and discomfort, is generally associated with the less intense, smaller scales (small eddy size or higher spatial frequencies as characterized by the turbulence power spectrum); and 2) turbulence that can cause aircraft upset, passenger injury, and possibly structural damage or failure is associated with the more intense larger scales of the turbulence spectrum. Whereas the first category may be in the inertial subrange of the turbulence power spectrum where local homogeneity and stationarity may be assumed, the second category is definitely associated with the larger energy-containing scales of turbulence that are definitely nonstationary and inhomogeneous. In fact, the second type of atmospheric turbulence may not be turbulence at all, but may be a part of, or derive directly from, the ordered convective or geostrophic motions of the atmosphere. Frictionally induced turbulence in the planetary boundary layer is dependent on wind speed near the ground and the surface roughness. Convective turbulence in the planetary boundary layer is dependent on the lapse rate (the rate of temperature change with altitude) and the depth of the mixing layer. Thus, the wind speed and surface roughness and the lapse rate and mixing layer thickness largely determine the intensity and scale of the boundary-layer turbulence. This low altitude boundary-layer turbulence will affect landing and takeoff operations for the larger commercial aircraft that normally cruise at high altitude, and it will be the primary cause of disturbance for small aircraft that operate at low altitudes. Usually, it can be characterized as random, locally stationary, and Gaussian, and the turbulence scale at the surface is of the order of 1000 ft and increases with altitude. It is only natural that pilots avoid flying into areas of obviously rough air such as thunderstorms or even cumulus clouds;

Multigoal Programming for Structure/Control Design Synthesis with Fuzzy Goals

This paper considers the simultaneous structure/control design synthesis of a wing with an active control system called the gust load alleviation (GLA) system. The introduction of fuzzy goals for both the gust-induced stress with the control system off and the structural mass is the primary focus in the present paper. A simple wind tunnel model excited by a random gust is studied. The thickness of a FEM beam element modeling the wing spar and the feedback control gain are optimized to maximize the fuzzy decision with the prioritized design requirements, e. g., the control system's stability characteristics, and the standard deviation of the gust-induced stress and control surface deflection angle with the control system on. The sequential linear approximation method is proposed to obtain the optimum solutions for nonlinear multigoal problems with fuzzy goals.

Simultaneous structure/control design optimization of a wing structure with a gust load alleviation system

Simultaneous design optimization is considered for structure and control parameters of a wing with a gust load alleviation (GLA) control system. The application of a goal programming (GP) formulation to the design synthesis of an aeroservoelastic system is carried out by the use of a simple mathematical model in conjunction with a wind-tunnel model having a GLA control system. Numerical applications are based on a cantilever wing having an aileron surface controlled by wing-tip accelerometer feedback signals. System equations are obtained in the form of state equations, thus enabling the statistical characteristics of both the gust-induced wing stress and the control surface deflection angle to be evaluated using their standard deviations. The wing spar height and the controller feedback gain are simultaneously optimized to obtain the minimum spar weight while satisfying the following structure and control design constraints: 1) the spar stress is limited with the control system either on or off; 2) the control surface deflection angle is restricted; and 3) system stability should be guaranteed by incorporating a controller stability margin. Numerical examples demonstrate the successful application of a GP formulation for the simultaneous structure/control design synthesis by specifying priorities to the conflicting design constraints.

Active Vibration Control for Aircraft Wing

In active vibration control, gust load alleviation and flutter suppression method are studied for aircraft. For this purpose, an aero-servoelastic analysis code was developed using the finite element code MSC/NASTRAN. Moreover, a control law synthesis method was developed and applied to the LQG (linear quadratic Gaussian) method for reduced-order control law design. To validate both aeroservoelastic analysis and the control law synthesis code, the wind tunnel test of gust load alleviation was conducted in a 2 m low-speed wind tunnel of Nagoya Aerospace Systems Works using an aeroelastic wind model equipped with an actively controlled trailing edge surface. Analytical results were compared with the results from tests, and good agreement was obtained for wing acceleration. To estimate the effects of gust load alleviation and flutter suppression, they were applied to a 150-passenger twin-engine commercial airplane. For gust load alleviation, a 46.9% wing bending moment reduction was caused, as well as a flutter suppression of 30.7% flutter speed augmentation.

Multigoral Programming for Structure/Control Design Synthesis with Fuzzy Goals.

This paper considers the simultaaneous structure/control design synthesis of a wing with an active control system called the gust load alleviation (GLA) system. The introduction of fuzzy goals for both the gust-induced stress with the control system off and the structural mass is the primary focus in the present paper. A simple wind tunnel model excited by a random gust is studied. The thickness of a FEM beam element modeling the wing spar and the feedback control gain are optimized to maximize the fuzzy decision with the prioritized design requirements, e. g., the control system's stability characteristics, and the standard deviation of the gust-induced stress and control surface deflection angle with the control system on. The sequential linear approximation method is proposed to obtain the optimum solutions for nonlinear multigoal problems with fuzzy goals.

ACT wind-tunnel experiments of a transport-type wing

This paper summarizes the wind-tunnel experiments on the active control technology that have been conducted in the large low-speed wind tunnel of the National Aerospace Laboratory. The experiments include a gust load alleviation (GLA) and two active flutter suppression (AFS) tests. A wind-tunnel model of the cantilevered elastic wing is designed to simulate an energy efficient transport

Active Vibration Control Technology for Aircraft Wing.

In active vibration control technology, gust load alleviation and flutter suppression technologies are studied for aircraft. For this purpose, an aero-servoelastic analysis code was developed using the MSC/NASTRAN by MHI. Moreover, a control low-synthesis method was developed, being applied to LQG (linear quadratic Gaussian) method for reduced order control law design. To validate both the aeroservoelastic analysis and the control law synthesis code, the wind tunnel test of gust load alleviation was conducted in a 2m low-speed wind tunnel of Nagoya Aerospace Systems Works using an aeroelastic wind model equipped with an actively controlled trailing edge surface. Analysis results were compared with the results from tests and good agreement with test results was obtained for the wing acceleration. To estimate the effects of gust load alleviation and flutter suppression technologies, they were applied to a 150-passenger twin-engine commercial airplane. For gust load alleviation, a 46.9% wing bending moment reduction effect was presented and flutter suppression, 30.7% flutter speed augmentation effect was presented.

突風荷重軽減機構を含む片持矩形弾性翼の最適構造設計

突風荷重軽減機構を含む片持矩形弾性翼の最適構造設計

片持弾性矩形翼の突風荷重軽減に対する制御系設計と風洞実験

This paper describes designs of the gust load alleviation (GLA) system and its wind-tunnel tests. The objective is to verify the effectiveness of control system syntheses which we have examined theoretically in the active flutter suppression (AFS). Applying the syntheses to the GLA system of a cantilevered elastic rectangular wing, the 1st or the 2nd order controllers can be constructed. It is shown in the wind-tunnel tests that the bending moment at the wing root is reduced to 66.2% of the control-off value as the best by using a designed reduced-order controller. Furthermore, the robustness for the parameter perturbation of the plant is verified in the tests where the wind velocity is 50% higher than the designed wind velocity.

Controller Designs of a Gust Load Alleviation System for an Elastic Rectangular Wing

This paper proposes two design methods of reduced-order controllers for gust load alleviation (GLA) systems of an elastic wing, and examines the control performance using both simulation studies and wind-tunnel experiments. One of the methods is based on the use of the generalized Hessenberg representation (GHR) in the time domain, and the other method is the one in the frequency domain termed the Nyquist frequency approximation (NFA). The former yields quasi-optimal controllers in the sense of minimizing quadratic performance indices, whereas the latter can yield controllers with increased stability margin. Applying these methods to the design of GLA systems of a cantilevered elastic rectangular wing, low-order controllers, the 1st- or the 2nd-order, can be constructed, and they showed as good performance as the LQG compensator.

Gust load alleviation of a transport-type wing - Test and analysis

The present paper describes experimental and analytical results of gust response of a 1/9-scale transport-type wing with a gust load alleviation system. A reduced-order feedback control filter is formed with the aid of a modified optimal control theory. The system using the feedback filter has been confirmed to be effective against both sinusoidal gust and atmospheric turbulence. Detailed comparison is made by taking into account the effect of location of the leadingand the trailing-edge control surfaces. In general, agreement between analysis and experiment is good.

Reduction of the ultimate factor by applying a maximum load concept

The ultimate factor for fixed-wing aircraft has not been reduced from 1.5 for more than 50 years. Many arguments can be made clearly indicate that the prediction of loads is more accurate today. A case can also be made that stresses can be determined with more accuracy by applying sophisticated finite-element methods. On the other hand, opponents can claim that, owing to these sophisticated prediction methods, the structure is much more exploited and therefore the factor of safety against failure should remain unchanged. Because these arguments cannot be settled—espec ially when dealing with a certification agency—a maximum load concept is proposed. The flight control system for a naturally unstable aircraft will limit the design parameters, such as acceleration, acceleration rate, velocity, attitude, in such a way that the limit design loads are not exceeded. The reliability of this load limitation is as high as that of the flight control system (FCS) itself, assuming no condition can be tolerated that will make the aircraft unstable or uncontrollable. Load cases that cannot be limited by the FCS—particularly landing cases—are still considered with an ultimate factor of 1.5. Also, loads arising from gusts must be treated with 1.5 if there is no gust load alleviation system available. Fortunately, most of the design cases for fighter airplanes arise from maneuvers, which is the reason that considerable structural weight reductions can be achieved when the proposed concept is applied.