Showing papers on "Aerodynamic force published in 1994"

State-Space Representation of Aerodynamic Characteristics of an Aircraft at High Angles of Attack

Abstract: Mathematical modeling of unsteady aerodynamic forces and moments plays an important role in aircraft dynamics investigation and stability analysis at high angles of attack. In this article the state-space representation of aerodynamic forces and moments for unsteady aircraft motion is proposed. Consideration of separated flow about an airfoil and flow with vortex breakdown about a slender delta wing gives the base for mathematical modeling using internal variables describing the flow state. Coordinates of separation points or vortex breakdown can be taken, e.g., as internal state-space variables. These variables are governed by some differential equations. Within the framework of the proposed mathematical model it is possible to achieve good agreement with different experimental data obtained in water and wind tunnels. These high angle-of-attack experimental results demonstrate considerable dependence of aerodynamic loads on motion time history.

The effects of wing rotation on unsteady aerodynamic performance at low reynolds numbers

Abstract: The downstroke-to-upstroke transition of many insects is characterized by rapid wing rotation. The aerodynamic consequences of these rapid changes in angle of attack have been investigated using a mechanical model dynamically scaled to the Reynolds number appropriate for the flight of small insects such as Drosophila. Several kinematic parameters of the wing flip were examined, including the speed and axis of rotation, as well as the duration and angle of attack during the wing stroke preceding rotation. Alteration of these kinematic parameters altered force generation during the subsequent stroke in a variety of ways. 1. When the rotational axis was close to the trailing edge, the model wing could capture vorticity generated during rotation and greatly increase aerodynamic performance. This vortex capture was most clearly manifested by the generation of lift at an angle of attack of 0°;. Lift at a 0°; angle of attack was also generated following rotation about the leading edge, but only if the downstroke angle was large enough to generate a von Karman street. The lift may be due to an alteration in the effective angle of attack caused by the inter-vortex stream in the downstroke wake. 2. The maximum lift attained (over all angles of attack) was substantially elevated if the wing translated backwards through a wake generated by the previous stroke. Transient lift coefficient values of nearly 4 were obtained when the wing translated back through a von Karman street generated at a 76.5°; angle of attack. This effect might also be explained by the influence of the inter-vortex stream, which contributes a small component to fluid velocity in the direction of translation. 3. The growth of lift with angle of attack was significantly elevated following a 7.5 chord stroke with a 76.5°; angle of attack, although it was relatively constant under all other kinematic conditions. 4. The results also indicate the discrepancies between transient and time-averaged measures of performance that arise when unsteady mechanisms are responsible for force generation. Although the influence of wing rotation was strong during the first few chords of translation, averaging the performance over as little as 6.5 chords of motion greatly attenuated the effects of rotation. 5. Together, these modeling results suggest that the unsteady mechanisms generated by simple wing flips could provide an important source for the production of aerodynamic forces in insect flight. Furthermore, the extreme sensitivity to small variations in almost all kinematic parameters could provide a foundation for understanding the aerodynamic mechanisms underlying active flight control.

Postinstability Behavior of a Two-Dimensional Airfoil with a Structural Nonlinearity

Abstract: A two-dimensiona l airfoil with a free-play nonlinearity in pitch subject to incompressibl e flow has been analyzed. The aerodynamic forces on the airfoil were evaluated using Wagner's function and the resulting equations integrated numerically to give time histories of the airfoil motion. Regions of limit cycle oscillation are detected for velocities well below the linear flutter boundary, and the existence of these regions is strongly dependent on the initial conditions and properties of the airfoil. Furthermore, for small structural preloads, narrow regions of chaotic motion are obtained, as suggested by power spectral densities, phase-plane plots, and Poincare sections of the airfoil time histories. The existence of this chaotic motion is strongly dependent on a number of airfoil parameters, including, mass, frequency ratio, structural damping, and preload.

Postinstability behavior of a two-dimensional airfoil with a structural nonlinearity

Abstract: A two-dimensiona l airfoil with a free-play nonlinearity in pitch subject to incompressibl e flow has been analyzed. The aerodynamic forces on the airfoil were evaluated using Wagner's function and the resulting equations integrated numerically to give time histories of the airfoil motion. Regions of limit cycle oscillation are detected for velocities well below the linear flutter boundary, and the existence of these regions is strongly dependent on the initial conditions and properties of the airfoil. Furthermore, for small structural preloads, narrow regions of chaotic motion are obtained, as suggested by power spectral densities, phase-plane plots, and Poincare sections of the airfoil time histories. The existence of this chaotic motion is strongly dependent on a number of airfoil parameters, including, mass, frequency ratio, structural damping, and preload.

Adaptive model architecture and extended Kalman-Bucy filters

Abstract: In radar systems, extended Kalman-Bucy filters (EKBFs) are used to estimate state vectors of objects in track. Filter models accounting for fundamental aerodynamic forces on reentry vehicles are well known. A general model structure accommodating the dynamics of reentry vehicles in both exoatmospheric and endoatmospheric flight is presented. The associated EKBFs for these various models are described and the resulting associated parameter estimation and identification problems are discussed. The effects of position, velocity, drag, and aerodynamic lift are described within a nested set of EKBF models. >

Modeling of aircraft unsteady aerodynamic characteristics. Part 1: Postulated models

Abstract: A short theoretical study of aircraft aerodynamic model equations with unsteady effects is presented The aerodynamic forces and moments are expressed in terms of indicial functions or internal state variables The first representation leads to aircraft integro-differential equations of motion; the second preserves the state-space form of the model equations The formulations of unsteady aerodynamics is applied in two examples The first example deals with a one-degree-of-freedom harmonic motion about one of the aircraft body axes In the second example, the equations for longitudinal short-period motion are developed In these examples, only linear aerodynamic terms are considered The indicial functions are postulated as simple exponentials and the internal state variables are governed by linear, time-invariant, first-order differential equations It is shown that both approaches to the modeling of unsteady aerodynamics lead to identical models

Unsteady aerodynamics of vortical flows: Early and recent developments

Abstract: The development of aerodynamic theories of streaming motions around bodies with unsteady vortical and entropic disturbances is reviewed. The basic concepts associated with such motions, their interaction with solid boundaries and their noise generating mechanisms are described. The theory was first developed in the approximation wherein the unsteady flow is linearized about a uniform mean lfow. This approach has been extensively developed and used in aeroelastic and aeroacoustic calculations. The theory was recently extended to account for the effect of distortion of the incident disturbances by the nonuniform mean flow around the body. This effect is found to have a significant influence on the unsteady aerodynamic force along the body surface and the sound radiated in the far field. Finally, the nonlinear characteristics of unsteady transonic flows are reviewed and recent results of linear and nonlinear computations are presented.

Modal coordinates for aeroelastic analysis with large local structural variations

Abstract: Time domain aeroelastic equations of motion are formulated in a way that allows large local structural variations with a state-space model that is based on a relatively small number of generalized coordinates. Freefree or restrained vibration modes are first calculated for a nominal finite element model loaded with relatively large fictitious masses located at the area of structural variations. These modes and the associated oscillatory aerodynamic force coefficient matrices are used to construct a time-domain model for a basic aeroelastic case where the fictitious mass contribution to the generalized mass matrix is removed. High-accuracy aeroelastic investigations of the effects of structural variations can then be performed by simply introducing mass, stiffness, and damping coupling terms. It is shown that the number of modes required for the investigation of large stiffness variations is substantially lower than that required when fictitious masses are not used, and only slightly larger than the number of modes required for direct aeroelastic analysis of a single structural case.

Aerodynamic Characteristics of a Hypersonic Viscous Optimized Waverider at High Altitudes

Abstract: The present paper addresses the applicability of the basic concept of waveriding at high altitudes, and the extent to which the large viscous forces degrade the aerodynamic performance of waveriders. The waverider under consideration was designed using a continuum flow methodology. It is shown that the lift-to-drag ratio of high-altitude/high-Knudsen-number waveriders can be expected to be significantly lower than their low altitude/low Knudsen number counterparts. The aerodynamic performance of a representative waverider which was optimized for a 90-km, Mach-25 application is studied for altitudes ranging from 97 km to 145 km and incidence angles of 0 to 30 deg. Typical values of the lift-to-drag ratio were computed to be in the range of 0 to 0.3. Friction forces are mostly responsible for this poor performance. Friction forces account for more than 93 percent of the drag and significantly reduce lift.

Rotationally mounted flexible band wing

Abstract: A vehicle such as a missile (20) includes an aerodynamically shaped missile body (22) having a longitudinal centerline, a set of control surfaces (26) joined to the missile body (22), and, preferably, a propulsion system (28) operable to drive the missile body (22) forwardly. A cylindrical rotational bearing (32) is mounted on the missile body (22) with its cylindrical axis parallel to the longitudinal centerline (24) of the missile body. A flexible band wing (38) is supported from the rotational bearing (32). The flexible band wing (38) may rotate about the centerline (24) of the missile body (22) responsive to aerodynamic forces exerted on the missile body (22) and the flexible band wing (38) to aid in making maneuvers without requiring the missile (20) to bank to align the flexible band wing (38) with the direction of the maneuver.

Influence of aerodynamic forces in ice shedding

Abstract: Stresses in accreted ice on a typical airfoil impact ice caused by aerodynamic forces have been studied using finite element analyses. The objective of this study is to determine the significance of these stresses relative to values needed to cause ice shedding. In the case studied, stresses are not significant (less than 10 percent) when compared to the fracture value for airspeeds below a Mach number of 0.45. Above this velocity, the influence of aerodynamic forces on impact ice stresses should be considered in analyses of ice shedding.

Simulated Rarefied Aerodynamics of the Magellan Spacecraft During Aerobraking

Abstract: Aerodynamic loads upon the Magellan spacecraft during aerobraking through the atmosphere of Venus are computed at off-design attitudes with a direct simulation Monte Carlo (DSMC) particle method. This method is not restricted to the assumption of collisionless flow normally employed to assess spacecraft aerodynamics. Simulated rarefied flows at nominal altitudes near 140 km and an entry speed of 8.6 km/s were compared with simulated and analytic free-molecular results. Aerodynamic moments, forces, and heating for rarefied entry at all attitudes were 7-10% below free-molecular results. All moments acted to restore the vehicle to its nominal zeropitch, zero-yaw attitude. Suggested canting of the solar panels is an innovative configuration to assess gas-surface interaction during aerobraking. The resulting roll torques about the central body axis, as predicted in rarefied-flow simulations, were nearly twice that predicted for free-molecular flow, although differences became less distinct for thermal accommodation coefficients well below unity. In general, roll torques increased dramatically with reduced accommodation coefficients employed in the simulation. In the DSMC code, periodic free-molecule boundary conditions and a coarse computational grid and body resolution served to minimize the simulation size and cost while retaining solution validity.

The aeroelastic response of a two-dimensional airfoil with bilinear and cubic structural nonlinearities

Abstract: A two-dimensional airfoil with either a bilinear or cubic structural nonlinearity in pitch, and subject to incompressible flow has been analysed; the aerodynamic forces on the airfoil are evaluated using Wagner's function. The resulting equations are either integrated numerically using a finite difference method to give time histories of the airfoil motion, or solved in a semi-analytical manner using a dual-input describing function technique. For both types of nonlinearity regions of limit cycle oscillation (LCO) are detected for velocities well below the divergent flutter boundary. Using the finite difference method it is shown that the existence of the LCOs is strongly dependent on the initial conditions of the airfoil. Although the describing function method cannot predict the effect of initial conditions, it does give reasonable predictions of the velocity at which LCOs commence, and good predictions of the magnitude of the LCOs—at least for those cases where the LCO motion is predominantly period-one. The existence of the LCOs is strongly dependent on the properties of the airfoil. In some cases, most notably those with small structural preloads, regions of chaotic motion are obtained, as suggested by power spectral densities, phase-plane plots and Poincare sections of the airfoil time histories; the existence of chaos was confirmed for the cubic nonlinearity via calculation of the Lyapunov exponents, one of which is positive. The fact that chaotic motion is obtained with both bilinear and cubic nonlinearities suggests that it is not the discontinuous nature of the stiffness, associated with the bilinear nonlinearity, which is responsible for producing this chaotic motion.

Three-dimensional flow calculations for a projectile with standard and dome bases

Abstract: Test firings of the 155-mm XM825 artillery projectile have shown that its flight performance was affected by configurational changes to the base cavity. This was an unexpected result, and a clear understanding of why these changes affected the flight behavior did not exist. A computational study has been made for the two different base-cavity configurations which were flight tested. Flowfield computations have been performed at 0.8

Approximate Heat Transfer Methods for Hypersonic Flow in Comparison with Results Provided by Numerical Navier-Stokes Solution

Abstract: In the design of an hypersonic configuration heat-transfer plays an essential role For computations within aerothermal predesign fast and sufficiently accurate heat prediction methods are required In this work approximate methods for computing skin friction and heat transfer for hypersonic flow have been studied The evaluation of heat transfer and skin friction by using these methods only requi- res the flow data at the boundary layer edge and the surface tem- perature Combining the approximate viscous methods with surface inclination methods leads to a very efficient overall method Total aerodynamic forces and heat transfer for a complete configuration are computed withon seconds on workstations assembly Approximate heat transfer and skin friction results are obtained here for a hypersonic drag stabilized missile, a waverider configuration, se- veral blunted cone cases, a swept fin with blunt leading edge and a complete missile (axisymmetric body with 6 fins in the aft)

Rarefied-flow shuttle aerodynamics flight model

Abstract: Robert C. Blanchard*NASA Langl_ Research Center, Hampton, Virginia 23681andKevin T. Larman * and Christina D. Moats _;Lockheed Engineering and Sciences Company, Hampton, Virginia 23666A model of the Shuttle Orbiter rarefied-flow aerodynamic force coefficients has been derived from the ratioof flight acceleration measurements. The in-situ, low-frequency (<1 lq[z), low-level (,-_1 × 10-6 g) accelerationmeasurements are made during atmospheric re-entry. The experiment equipment designed and used for this taskis the High Resolution Accelerometer Package (HiRAP), one of the sensor packages in the Orbiter ExperimentsProgram. To date, 12 HiRAP re-entry mission data sets spanning a period of about 10 years have been processed.The HiRAP-derived aerodynamics model is described in detail. The model includes normal and axial hypersoniccontinuum coefficient equations as functions of angle of attack, body-flap deflection, and devon deflection. Normaland axial free molecule flow coefficient equations as a function of angle of attack are also presented, along withflight-derived rarefied-flow transition bridging formulae. Comparisons are made between the aerodynamics model,data from the latest Orbiter Operational Aerodynamic Design Data Book, applicable computer simulations, andwind-tunnel data.aCgKnMMmicro-gSVX,Y,Z6olPSubscriptsA

Kinematics and aerodynamics of the velocity vector roll

Abstract: The velocity-vector roll is defined as an angular rotation of an airplane about its instantaneous velocity vector, constrained to be performed at constant angle of attack (AOA), no sideslip, and constant velocity. Consideration of the aerodynamic force equations leads to requirements for body-axis yawing and pitching rotations that must be present to satisfy these constraints. Here, the body-axis rotations and the constraints are used in the moment equations to determine the aerodynamic moments required to perform the velocity-vector roll. The total aerodynamic moments, represented in the reference body-axis coordinate system, are then analyzed to determine the conditions under which their maxima occur. It is shown, for representative tactical airplanes, that the conditions for maximum pitching moment are strongly a function of the orientation of the airplane, occurring at about 90 deg of bank in a level trajectory. Maximum required pitching moment occurs at peak roll rate and is achieved at an AOA in excess of 45 deg. The conditions for maximum rolling moment depend on the value of the roll mode time constant. For a small time constant (fast response) the maximum rolling moment occurs at maximum roll acceleration and zero AOA, largely independent of airplane orientation; for a large time constant, maximum required rolling moment occurs at maximum roll rate, at maximum AOA, and at 180 deg of bank in level flight. The maximum yawing moment occurs at maximum roll acceleration and maximum AOA and is largely independent of airplane orientation. Results are compared with those obtained using conventional assumptions of zero pitch and yaw rates and show significant improvement, especially in the prediction of maximum-pitching-moment requirements.

Dynamic Forces From Single Gland Labyrinth Seals: Part I—Ideal and Viscous Decomposition

Abstract: A theoretical and experimental investigation on the aerodynamic forces generated by a single gland labyrinth seal executing a spinning/whirling motion has been conducted. A lumped parameter model, which includes the kinetic energy carryover effect, is presented along with a linear perturbation solution technique. The resulting system is nondimensionalized and the physical significance of the reduced parameters is discussed. Closed-form algebraic formulas are given for some simple limiting cases. It is shown that the total cross force predicted by this model can be represented as the sum of an ideal component due to an inviscid flow with entry swirl and a viscous part due to the change in swirl created by friction inside the gland

Study on wings of flying microrobots

Abstract: The conventional method for calculating aerodynamic forces generated on objects is based on the aerodynamics in the high Reynolds number flow, like the airflow around the wings of airplanes, because little is known about the low Reynolds number flow. Using this method to calculate the aerodynamic force acting on a flying microrobot, the obtained results may differ from the actual values. For analysis of this difference. The large-scale models of wings for flying microrobots were made, whose wing lengths are 3 cm and 1.5 cm respectively. With these wings the experiments were done in the low Reynolds number flows (Re=385, 752). The drag forces measured in these experiments were compared with the ones from calculations. As a result, in both flows, the measured values were larger than the calculated one. Using semiconductor surface machining technology, we also fabricated a flapping mechanism with the wings that generate the difference of the drag forces during upstroke and downstroke movements. >

Effect of Wind Tunnel Acoustic Modes on Linear Oscillating Cascade Aerodynamics

Abstract: The aerodynamics of a biconvex airfoil cascade oscillating in torsion is investigated using the unsteady aerodynamic influence coefficient technique. For subsonic flow and reduced frequencies as large as 0.9, airfoil surface unsteady pressures resulting from oscillation of one of the airfoils are measured using flush-mounted high- frequency-response pressure transducers. The influence coefficient data are examined in detail and then used to predict the unsteady aerodynamics of a cascade oscillating at various interblade phase angles. These results are correlated with experimental data obtained in the traveling-wave mode of oscillation and linearized analysis predictions

Wind turbine blade aerodynamics: The analysis of field test data

Abstract: Data obtained from the National Renewable Energy Laboratory site test of a wind turbine (The Combined Experiment) was analyzed specifically to capture information regarding the aerodynamic loading experienced by the machine rotor blades. The inflow conditions were shown to be extremely variable. These inflows yielded three different operational regimes about the blades. Each regime produced very different aerodynamic loading conditions. Two of these regimes could not have been readily predicted from wind tunnel data. These conditions are being subjected to further analyses to provide new guidelines for both designers and operators. The roles of unsteady aerodynamics effects are highlighted since periods of dynamic stall were shown to be associated with brief episodes of high aerodynamic forces.

Measurement of unsteady airloads on an oscillating engine and a wing/engine combination

Abstract: Experimental investigations of unsteady aerodynamic forces were performed on an oscillating ejector engine model and a wing/engine combination in the subsonic and transonic flow regimes. The experimental results were compared with theoretical results. The aim was to determine how well, in reality, the mathematical aerodynamic models commonly used for flutter calculations correspond to the flow conditions on an engine. The investigations on the isolated ejector engine demonstrated that linear lifting surface theory provides quite accurate unsteady aerodynamic forces. The effects of the Mach number and reduced frequency are described correctly. For the wing/engine combination, the unsteady interference effect of the engine oscillation on the lower side of the wing is strongly influenced by flow separation at the wing/pylon connection. In general, the unsteady aerodynamic forces induced by the engine on the wing are small and are therefore of minor influence on the unsteady airloads of an oscillating wing.