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NASA NTRS1995

Wind gust models derived from field data

Wind data measured during a field experiment were used to verify the analytical model of wind gusts. Good coincidence was observed; the only discrepancy occurred for the azimuth error in the front and back winds, where the simulated errors were smaller than the measured ones. This happened because of the assumption of the spatial coherence of the wind gust model, which generated a symmetric antenna load and, in consequence, a low azimuth servo error. This result indicates a need for upgrading the wind gust model to a spatially incoherent one that will reflect the real gusts in a more accurate manner. In order to design a controller with wind disturbance rejection properties, the wind disturbance should be known at the input to the antenna rate loop model. The second task, therefore, consists of developing a digital filter that simulates the wind gusts at the antenna rate input. This filter matches the spectrum of the measured servo errors. In this scenario, the wind gusts are generated by introducing white noise to the filter input.

Gawronski, W.↗

NASA NTRS1959

Analysis of Acceleration, Airspeed, and Gust-Velocity Data From a Four-Engine Transport Airplane Operating Over a Northwestern United States Alaska Route

Acceleration, airspeed, and altitude data obtained with an NACA VGH recorder from a four-engine commercial transport airplane operating over a northwestern United States-Alaska route were evaluated to determine the magnitude and frequency of occurrence of gust and maneuver accelerations., operating airspeeds, and gust velocities. The results obtained were then compared with the results previously reported in NACA Technical Note 3475 for two similar airplanes operating over transcontinental routes in the United States. No large variations in the gust experience for the three operations were noted. The results indicate that the gust-load experience of the present operation closely approximated that of the central transcontinental route in the United States with which it is compared and showed differences of about 4 to 1 when compared with that of the southern transcontinental route in the United States. In general, accelerations due to gusts occurred much more frequently than those due to operational maneuvers. At a measured normal-acceleration increment of 0.5g, accelerations due to gusts occurred roughly 35 times more frequently than those due to operational maneuvers.

Engel, Jerome N.↗

DOE OSTI2022

Wind Turbine Gust Load Alleviation with Active Flow Control

Large wind turbine rotors are becoming more common in utility-scale wind power, especially for offshore wind plants. However, the trend toward large rotors can be limited by their ability to manage dynamic and extreme loads. To provide a safety margin for the rotor design and avoid catastrophic events such as tower strikes, extreme loads need to be controlled. The objective of this study is to develop and evaluate a feedback control system to alleviate extreme loads and reduce blade deflections under gust events using active flow control devices. We also propose a modification in the turbine controller to achieve further reduction in extreme loads. The extreme load reductions are evaluated under gust wind conditions with direction changes according to the IEC standard. The effects of the gust alleviation controller on turbine performance and fatigue loads are investigated as well. With the deployment of the gust alleviation controller and modified turbine baseline controller, the extreme loads and deflections reduce by up to 23%. The energy captured by the turbine is not affected by the proposed gust alleviation controller. The fatigue loads of various wind turbine components are either reduced or remain unchanged.

17 WIND ENERGY↗

NASA NTRSDate not supplied

The Effects of Freestream Turbulence on Wing-Gust Interactions

There is a growing interest in the experimental study of discrete vertical gusts motivated by the proliferation of small Uncrewed Air Systems. However, gusts are not the only unsteady flow disturbances that these vehicles are expected to encounter. Freestream turbulence is also a serious consideration. Previous work has indicated that freestream turbulence has significant effects on the flow around airfoils, so this work investigates the interaction of these two unsteady flow disturbances. Airfoils are pitched to mimic the flow angle change caused by a gust at different levels of incoming freestream turbulence. The lift generated during the gust encounters with elevated incoming freestream turbulence show that the effects on the performance of the airfoil are dependent on the turbulence intensity. Low turbulence intensity has beneficial effects, delaying stall and increasing the maximum lift, whereas higher freestream turbulence intensity delays stall without a corresponding increase in the maximum lift. A preliminary investigation of 15-trial ensemble-averaged flow fields suggests that freestream turbulence dominates the flow, but the flow features associated with the dynamic lift seen in the "clean" case are still somewhat present. However, lift curves and flow fields from individual trials indicates that the turbulence increases the unsteadiness in the interaction, but the general trends are dominated by the gust interaction. Further work is still needed to better determine the flow physics of these interactions and the interplay between repeatable and stochastic flow phenomena.

Unsteady Aerodynamics↗

NASA NTRSDate not supplied

Flutter and Stochastic Gust Constraint Studies of the Undeflected CRM using MPhys

Aircraft design must ultimately address gust and flutter characteristics to achieve airworthiness. As modern aircraft continue to become more lightweight and flexible, these aeroelastic phenomena are more likely to drive the design process and should thus be considered early. Designing for ride quality due to typical gust spectra is also of interest to ensure a smooth passenger experience. The present work demonstrates the new implementation of stochastic gust constraints in MPhys/OpenMDAO, in addition to flutter and static stress constraints, among others. Design studies are shown for the NASA Common Research Model, including both fuel burn minimization as well as multiobjective optimization of fuel burn and landing gross weight. Two sets of optimization problems are covered, differing mainly by the gust quantity of interest, which include tip displacement and root bending moment. Fuel burn penalties imposed by the flutter and gust constraints are also discussed.

aeroelasticity↗

NASA NTRS1974

A passive gust alleviation system for light aircraft

A passive aeromechanical gust alleviation system has been examined for application to a Cessna 172. The system employs small auxiliary wings to sense changes in angle of attack and to drive the wing flaps to compensate the resulting incremental lift. The flaps also can be spring loaded to neutralize the effects of variations in dynamic pressure. Conditions for gust alleviation are developed and shown to introduce marginal stability if both vertical and horizontal gusts are compensated. Satisfactory behavior is realized if only vertical gusts are absorbed; however, elevator control is effectively negated by the system. Techniques to couple the elevator and flaps are demonstrated to restore full controllability without sacrifice of gust alleviation.

Roesch, P.↗

NASA NTRS1975

A passive gust alleviation system for a light aircraft

A passive aeromechanical gust alleviation system was examined for application to a Cessna 172. The system employs small auxiliary wings to sense changes in angle of attack and to drive the wing flaps to compensate the resulting incremental lift. The flaps also can be spring loaded to neutralize the effects of variations in dynamic pressure. Conditions for gust alleviation are developed and shown to introduce marginal stability if both vertical and horizontal gusts are compensated. Satisfactory behavior is realized if only vertical gusts are absorbed; however, elevator control is effectively negated by the system. Techniques to couple the elevator and flaps are demonstrated to restore full controllability without sacrifice of gust alleviation.

Roesch, P.↗

NASA NTRS1976

An analytical study and wind tunnel tests of an aeromechanical gust-alleviation system for a light airplane

The results of an analytical study of a system using stability derivatives determined in static wind tunnel tests of a 1/6 scale model of a popular, high wing, light airplane equipped with the gust alleviation system are reported. The longitudinal short period mode dynamics of the system are analyzed, and include the following: (1) root loci, (2) airplane frequency responses to vertical gusts, (3) power spectra of the airplane responses in a gust spectrum, (4) time history responses to vertical gusts, and (5) handling characteristics. The system reduces the airplane's normal acceleration response to vertical gusts while simultaneously increasing the pitching response and reducing the damping of the longitudinal short period mode. The normal acceleration response can be minimized by using the proper amount of static alleviation and a fast response system with a moderate amount of damping. The addition of a flap elevator interconnect or a pitch damper system further increases the alleviation while moderating the simultaneous increase in pitching response. The system provides direct lift control and may reduce the stick fixed longitudinal static stability.

Stewart, E. C.↗

NASA NTRS1977

A numerical investigation of the severe thunderstorm gust front

The structure of the thunderstorm gust front is investigated by a nonhydrostatic, two-dimensional (x,z) numerical model. In the model, which is dry, the production of negatively buoyant air by evaporation is parameterized via an externally imposed, local-cooling function. This parameterization sustains a steady cold downdraft, which drives the surface outflow and associated gust front. It is shown that two dominant factors influencing gust front structure in the vertical plane are the solenoidal field coincident with the front and surface friction, modeled by means of a simple bulk aerodynamic drag formulation. The circulation theorem is invoked to illustrate how solenoidal accelerations oppose the deceleration by surface friction. After the onset of a downdraft in the model, these opposing tendencies soon reach a balance. Thus, following a brief transient stage, the model gust front exhibits a persistent configuration as it propagates rapidly forward. The essential features of this configuration are examined and compared with both tower observations of gust fronts and laboratory models of gravity currents.

Mitchell, K. E.↗

NASA NTRS1978

Dynamic wind-tunnel tests of an aeromechanical gust-alleviation system using several different combinations of control surfaces

Some experimental results are presented from wind tunnel studies of a dynamic model equipped with an aeromechanical gust alleviation system for reducing the normal acceleration response of light airplanes. The gust alleviation system consists of two auxiliary aerodynamic surfaces that deflect the wing flaps through mechanical linkages when a gust is encountered to maintain nearly constant airplane lift. The gust alleviation system was implemented on a 1/6-scale, rod mounted, free flying model that is geometrically and dynamically representative of small, four place, high wing, single engine, light airplanes. The effects of flaps with different spans, two size of auxiliary aerodynamic surfaces, plain and double hinged flaps, and a flap elevator interconnection were studied. The model test results are presented in terms of predicted root mean square response of the full scale airplane to atmospheric turbulence. The results show that the gust alleviation system reduces the root mean square normal acceleration response by 30 percent in comparison with the response in the flaps locked condition. Small reductions in pitch-rate response were also obtained. It is believed that substantially larger reductions in normal acceleration can be achieved by reducing the rather high levels of mechanical friction which were extant in the alleviation system of the present model.

Stewart, E. C.↗

NASA NTRS1980

Analytical design and evaluation of an active control system for helicopter vibration reduction and gust response alleviation

An analytical study was conducted to define the basic configuration of an active control system for helicopter vibration and gust response alleviation. The study culminated in a control system design which has two separate systems: narrow band loop for vibration reduction and wider band loop for gust response alleviation. The narrow band vibration loop utilizes the standard swashplate control configuration to input controller for the vibration loop is based on adaptive optimal control theory and is designed to adapt to any flight condition including maneuvers and transients. The prime characteristics of the vibration control system is its real time capability. The gust alleviation control system studied consists of optimal sampled data feedback gains together with an optimal one-step-ahead prediction. The prediction permits the estimation of the gust disturbance which can then be used to minimize the gust effects on the helicopter.

Taylor, R. B.↗

NASA NTRS1982

Study of rotor gust response by means of the local momentum theory

The vertical gust response of the helicopter rotor in cruising flight is studied analytically by means of the local momentum theory (LMT) and experimentally in a wind tunnel utilizing a gust generator. By introducing the unsteady aerodynamic effects into the LMT and by considering the elastic deformation of the rotor blade, the vibratory characteristics of flapping blades and of the rotor forces can be obtained. Since the LMT makes it possible to calculate the instantaneous load distribution along the rotor blade in desired azimuthal or timewise intervals, the effects of gradual penetration of the rotor into the gust can be studied and the Fourier analysis or power spectrum analysis of the rotor response applied to any kind of gust input. Some results obtained analytically are verified by the experimental tests performed in the wind tunnel which generates step, sinusoidal and random vertical gusts by the motion of a series of cascaded vanes upstream of the rotor.

Azuma, A.↗

NASA NTRS1983

A study of helicopter gust response alleviation by automatic control

Two control schemes designed to alleviate gust-induced vibration are analytically investigated for a helicopter with four articulated blades. One is an individual blade pitch control scheme. The other is an adaptive blade pitch control algorithm based on linear optimal control theory. In both controllers, control inputs to alleviate gust response are superimposed on the conventional control inputs required to maintain the trim condition. A sinusoidal vertical gust model and a step gust model are used. The individual blade pitch control, in this research, is composed of sensors and a pitch control actuator for each blade. Each sensor can detect flapwise (or lead-lag or torsionwise) deflection of the respective blade. The acturator controls the blade pitch angle for gust alleviation. Theoretical calculations to predict the performance of this feedback system have been conducted by means of the harmonic method. The adaptive blade pitch control system is composed of a set of measurements (oscillatory hub forces and moments), an identification system using a Kalman filter, and a control system based on the minimization of the quadratic performance function.

Saito, S.↗

NASA NTRS1989

NACA/NASA research related to evolution of U.S. gust design criteria

This paper traces the evolution of gust design criteria in the U.S. particularly from the standpoint of research that was used in the substantiation for the various versions in the evolution. The mathematical models of airplanes and of atmospheric turbulence and their rationale are described. Emphasis is given to the revisions and refinements made starting in the 1920's up to the present time. The major steps, beginning with the sharp edged gust formula, are traced through the modified formula specifying ramp-platform gusts and later to one-minus-cosine gusts and finally to criteria for continuous gust analyses. The influence of aircraft design developments on design criteria development needs is also addressed. A brief summary of military criteria is included. Significant discussion is devoted to measurements that have been made, including onboard recordings, to provide an extensive data base of (1) atmospheric turbulence experienced in routine flight operations, (2) specially-instrumented research aircraft measurements to provide atmospheric characterization for various flight and meteorological conditions, and (3) comparisons of measured and calculated aircraft responses in turbulence. Some features of the instrumentation used will be depicted.

Murrow, Harold N.↗

NASA NTRS1990

Influence of airfoil nose radius on sound generated by gust interactions

The sound radiated by interaction of a short wavelength gust (k much greater than 1) with a symmetric thin airfoil is analyzed. The theory is based on a linearization of the Euler equations about the subsonic mean flow past the airfoil. The sound generation mechanism is found to be concentrated in a local region surrounding the parabolic nose of the airfoil; the size of this local region scales on the gust wavelength. Airfoil thickness produces terms of relative order O(sq rt S) in the far field sound, where S is a Strouhal number based on the airfoil nose radius. At low Mach numbers, entropy gusts generate much less sound than vorticity gusts. However, for O(1) Mach numbers and moderate values of S, entropy and vorticity gusts produce similar sound levels.

Tsai, C. T.↗

NASA NTRS1992

Airfoil wake and linear theory gust response including sub and superresonant flow conditions

The unsteady aerodynamic gust response of a high solidity stator vane row is examined in terms of the fundamental gust modeling assumptions with particular attention given to the effects near an acoustic resonance. A series of experiments was performed with gusts generated by rotors comprised of perforated plates and airfoils. It is concluded that, for both the perforated plate and airfoil wake generated gusts, the unsteady pressure responses do not agree with the linear-theory gust predictions near an acoustic resonance. The effects of the acoustic resonance phenomena are clearly evident on the airfoil surface unsteady pressure responses. The transition of the measured lift coefficients across the acoustic resonance from the subresonant regime to the superresonant regime occurs in a simple linear fashion.

Henderson, Gregory H.↗

NASA NTRS1992

Forcing function effects on unsteady aerodynamic gust response. II - Low solidity airfoil row response

The paper investigates the fundamental gust modeling assumption on the basis of a series of experiments performed in the Purdue Annular Cascade Research Facility. The unsteady period flow field is generated by rotating flows of perforated plates and airfoil cascades, with the resulting unsteady periodic chordwise pressure response of a downstream low solidity stator row determined by miniature pressure transducers embedded within selected airfoils. When the forcing function exhibited the characteristics of a linear-theory gust, the resulting response on the downstream stator airfoils was in excellent agreement with the linear-theory models. When the forcing function did not exhibit linear-theory gust characteristics, the resulting unsteady aerodynamic response of the downstream stators was much more complex and correlated poorly with the linear-theory gust predictions. It is shown that the forcing function generator significantly affects the resulting gust response, with the complexity of the response characteristics increasing from the perforated-plate to the airfoil-cascade forcing functions.

Henderson, Gregory H.↗

NASA NTRS1925

Forces on Airships in Gusts

In this report it is shown that determining the instantaneous angle of pitch, the acceleration of the gust is as important as its maximum velocity or yaw. Hitherto it has been assumed that the conditions encountered in gusts could be approximately represented by considering the airship to be at an instantaneous angle of yaw or pitch (according to whether the gust is horizontal or vertical), the instantaneous angle being tan to the (-1) power (v/v), where v is the component of the velocity of the gust at right angles to the longitudinal axis of the ship, and v is the speed of the ship. An expression is derived for this instantaneous angle in terms of the speed and certain aerodynamic characteristics of the airship, and of the maximum velocity and the acceleration of the gust, and the application of the expression to the determination of the forces on the ship is illustrated by numerical examples.

Burgess, C P↗