Showing papers on "Aerodynamic force published in 1979"

A Digital Program for Calculating the Interaction Between Flexible Structures, Unsteady Aerodynamics and Active Controls

Abstract: A computer program, ISAC, is described which calculates the stability and response of a flexible airplane equipped with active controls. The equations of motion relative to a fixed inertial coordinate system are formulated in terms of the airplane's rigid body motion and its unrestrained normal vibration modes. Unsteady aerodynamic forces are derived from a doublet lattice lifting surface theory. The theoretical basis for the program is briefly explained together with a description of input data and output results.

Time-Dependent Response of a Two-Dimensional Airfoil in Transonic Flow

Abstract: A procedure is developed for the aeroelastic analysis of a two-dimensional airfoil in transonic flow. The fluid is assumed to be described by the unsteady low-frequency small-disturbance transonic potential equation for which a fully time-implicit integration scheme exists. Structural equations of motion are integrated in time simultaneously with the potential equation in order to predict the unsteady airfoil motion. As a computational example, a three-degree-o f-freedom NACA 64A010 airfoil is considered using representative values of the structural parameters. The method is shown to be both stable and accurate, and the time response for several choices of initial conditions and reduced freest ream density is presented. Oscillations with either growing or decaying amplitudes are indicated depending upon the prescribed initial conditions. OR the case of flow over an airfoil in a freestream at Mach numbers near 1, small amplitude motions of the body surface can produce large variations in the aerodynamic forces and moments acting on the structure. In addition, phase differences between the flow variables and the resultant forces may be great. These characteristics tend to enhance the probability of encountering aeroelastic instabilities in the transonic flow regime, and thus evidence a need for tech- niques of analyzing the coupled unsteady flowfield and resultant structural response in such situations. In the subsonic and supersonic cases, the governing flow equations may be linearized such that the aerodynamic forces depend upon the body motion in a linear fashion. Moreover, the resultant forces acting on the airfoil may be obtained through superposition by summing the contributions due to each of the various types of body motion permitted. This allows the linear structural equations of motion to be solved independent of the governing aerodynamic equations which provide only the force coefficients. Uncoupling of the fluid and structural equations is not, in general, possible for the transonic regime due to its inherent nonlinear nature. Recent advances in computational methods have made several approaches available for computing unsteady tran- sonic flows. While a number of different techniques have evolved and various physical problems have been con- sidered, M6 the unsteady body motion was generally prescribed as a known function of time, thereby precluding the simulation of true aeroelastic behavior. Only more recently have these procedures been applied to actual aeroelastic problems.17'18 It is the intent here to describe a method for obtaining the time-dependent response of a two- dimensional airfoil in transonic flow and to provide a computational example by applying this technique to a physical situation of practical interest. The governing aerodynamic equation of motion is assumed to be the unsteady low-frequency small-disturbance transonic equation for the velocity potential function which is capable of simulating nonlinear flow phenomena including irregular shock wave motions. Solutions to this equation have corn-

Curved centerline air intake for a gas turbine engine

Abstract: An inlet for a gas turbine engine is disposed about a curved centerline for the purpose of accepting intake air that is flowing at an angle to engine centerline and progressively turning that intake airflow along a curved path into alignment with the engine. This curved inlet is intended for use in under-the-wing locations and similar regions where airflow direction is altered by aerodynamic characteristics of the airplane. By curving the inlet, aerodynamic loss and acoustic generation and emission are decreased.

Control Law Synthesis for Active Flutter Suppression Using Optimal Control Theory

Abstract: This paper describes a study investigating the use of optimal control theory for the synthesis of an active flutter-suppression control law. For an example design application, a high-aspect-ratio cantilever wind-tunnel wing model is considered. The structural dynamics are represented by analytically computed natural frequencies and mode shapes. The three-dimensional unsteady aerodynamic forces for oscillatory motion are computed employing the doublet-lattice technique. With the aid of finite-order approximating functions for representing the aerodynamic forces in the time domain, the "flutter equations" are written in the standard state vector form. Linear optimal control theory is then applied to find particular sets of gain values which minimize a quadratic cost function of the states and controls. These control laws are shown to increase the flutter dynamic pressure by at least 50% at Mach numbers 0.7 and 0.9. The closed-loop system's control surface activity in a gust environment is also examined.

AGARD two-dimensional aeroelastic configurations

Abstract: The development of reliable, efficient methods for the calculation of unsteady aerodynamic forces in the frequency-critical transonic speed regime can be enhanced by the availability of a limited number of test cases for the comparison of competing methods. Seven test cases are presented for airfoils with thickness from 6.0% to 16.5%: a biconvex parabolic arc airfoil, three conventional airfoils, and three cambered supercritical airfoils. The aerodynamic conditions such as Mach number, mean angle of attack, and oscillation amplitude and frequency are also given. Recommendations are made for uniformity in definition and reporting to enhance desired comparison for the aeroelastician.

Classical aerodynamic theory

Abstract: A collection of papers on modern theoretical aerodynamics is presented. Included are theories of incompressible potential flow and research on the aerodynamic forces on wing and wing sections of aircraft and on airship hulls.

Method for Calculating Wing Loading During Maneuvering Flight along a Three-Dimensional Curved Path

Abstract: The aerodynamic forces due to the nonsteady motion of a lifting surface along a three-dimensional path are analyzed. For this purpose, available steady-state calculation methods, based on potential theory, were modified by retaining the nonsteady terms in the governing equations. The surface motion is limited to such cases where the linear analysis can be applied. As an example, the time-dependent forces and moments of a slender wing in various nonsteady motions were studied.

Transonic flutter analysis of a rectangular wing with conventional airfoil sections

Abstract: Flutter analysts have encountered considerable analytical difficulties in the prediction of the flutter stability of aircraft operating in the transonic Mach number regime. Because of the shocks and nonlinearities of transonic flow the aerodynamic unsteady forces have been difficult to determine and have prohibited accurate determination of the flutter speed. The finite-difference relaxation method is used to determine the oscillatory transonic aerodynamic forces on a uniformly stiff cantilever rectangular wing in a flow field with mixed subsonic and supersonic regions together with shock waves. The flutter speed is determined at two transonic Mach numbers and is compared to the flutter speed obtained using a linear aerodynamic theory.

Wind Loads on Flat Plate Photovoltaic Array Fields

Abstract: The results of an experimental analysis (boundary layer wind tunnel test) of the aerodynamic forces resulting from winds acting on flat plate photovoltaic arrays are presented Local pressure coefficient distributions and normal force coefficients on the arrays are shown and compared to theoretical results Parameters that were varied when determining the aerodynamic forces included tilt angle, array separation, ground clearance, protective wind barriers, and the effect of the wind velocity profile Recommended design wind forces and pressures are presented, which envelop the test results for winds perpendicular to the array's longitudinal axis This wind direction produces the maximum wind loads on the arrays except at the array edge where oblique winds produce larger edge pressure loads The arrays located at the outer boundary of an array field have a protective influence on the interior arrays of the field A significant decrease of the array wind loads were recorded in the wind tunnel test on array panels located behind a fence and/or interior to the array field compared to the arrays on the boundary and unprotected from the wind The magnitude of this decrease was the same whether caused by a fence or upwind arrays

Aeroelastic Response Analysis of Two Dimensional, Single and Two Degree of Freedom Airfoils in Low-Frequency, Small-Disturbance Unsteady Transonic Flow

Abstract: : A procedure is developed to obtain the aeroelastic responses of single and two degree of freedom aeroelastic systems in transonic flow. The fluid is described by the two dimensional unsteady low-frequency transonic potential equation based on small disturbance theory. The aerodynamic computer code LTRAN2 which is based on a fully implicit time integration scheme is employed to obtain the aerodynamic forces. The structural equations are simultaneously integrated with the potential flow equations by a numerical method and the aeroelastic responses are obtained. Results for neutrally stable responses are compared with those obtained by the U-g method. Aeroelastic responses are obtained for flat plates (single and two degree of freedom) at M = 0.7 by the linear part of LTRAN2. Examples of an NACA 64A006 airfoil at Mach numbers of 0.88 and 0.85 are also analyzed. Response results obtained for a single pitching degree of freedom system at M = 0.88 are compared with an existing solution. The response results obtained for a two degree of freedom system at M = 0.85 for neutrally stable condition are correlated with those obtained by the flutter analysis. Results also include stable and unstable response curves and their variations with altitude.

The Analysis of Propellers Including Interaction Effects

Abstract: Analytical and experimental studies have been undertaken on propellers operating in the unsteady flow field produced by interaction effects due to the fuselage, wing, and nacelles Methods have been developed and verified experimentally for determining the velocity field in which a propeller operates as well as its aerodynamic and dynamic response to this unsteady environment Methods are presented for predicting the net thrust of a propeller-wing-body combination as well as the unsteady thrust and torque acting on the propeller Sample calculations as well as wind tunnel and flight test results are presented which illustrates the sensitivity of a propeller to flow field in which it is operating

A Comparison of the Aerodynamic Characteristics of Eight Sailwing Airfoil Sections

Abstract: The operational characteristics of sailwings are discussed with emphasis placed on the importance of the trailing edge cable tension. The three dimensional aerodynamic characteristics were obtained from wind tunnel tests, and the results compared to determine the magnitude of the aerodynamic penalties paid for various structural simplifications. For the sectional thickness ratios, it is concluded that, while the basic double-membraned sailwing has exceptional aerodynamic performance, even superior for some applications to the conventional hardwing, any notable deviation from this configuration results in an unacceptably large performance penalty.

The influence of sweep on the aerodynamic loading of an oscillating NACA 0012 airfoil. Volume 1: Technical report

Abstract: Aerodynamic experiments were performed on an oscillating NACA 0012 airfoil utilizing a tunnel-spanning wing in both unswept and 30 degree swept configurations. The airfoil was tested in steady state and in oscillatory pitch about the quarter chord. The unsteady aerodynamic loading was measured using pressure transducers along the chord. Numerical integrations of the unsteady pressure transducer responses were used to compute the normal force, chord force, and moment components of the induced loading. The effects of sweep on the induced aerodynamic load response was examined. For the range of parameters tested, it was found that sweeping the airfoil tends to delay the onset of dynamic stall. Sweeping was also found to reduce the magnitude of the unsteady load variation about the mean response. It was determined that at mean incidence angles greater than 9 degrees, sweep tends to reduce the stability margin of the NACA 0012 airfoil; however, for all cases tested, the airfoil was found to be stable in pure pitch. Turbulent eddies were found to convect downstream above the upper surface and generate forward-moving acoustic waves at the trailing edge which move upstream along the lower surface.

Simplified Unsteady Aerodynamic Concepts with Application to Parameter Estimation

Abstract: A simplified aerodynamic force model based on the physical principle of Prandtl's lifting line theory and trailing vortex concept has been developed to account for unsteadiness in the aircraft dynamics. The wake is assumed to be compressed to a single shed vortex element of appropriate strength moving downstream at a speed sufficient to approximate the Wagner function. Results are presented illustrating the ability of the simplified theory to duplicate exact solutions in unsteady aerodynamics. Further, consideration is given to the utility of the model in a parameter identification application.

Shuttle entry trajectory reconstruction using inflight accelerometer and gyro measurements

Abstract: An error analysis has been made of a Shuttle postflight entry trajectory reconstruction process to obtain trajectory state estimation errors and to assess the impact of these errors on Shuttle aerodynamic force coefficient extraction. In this analysis, the entry trajectory is assumed to be reconstructed via numerical integration of onboard accelerometer and gyro measurements and constrained to satisfy ground-based radio tracking. The trajectory state estimation errors are calculated using a Kalman-Schmidt sequential filter assuming various measurement error models and combinations of ground-based tracking. The resultant trajectory estimation errors are analyzed in a simplified perturbation process to establish the accuracy to which postflight aerodynamic force coefficients can be determined. Results are presented which show that the principal error sources affecting the trajectory reconstruction and thus the force coefficient extraction, assuming perfect atmospheric density knowledge, are the accelerometer and gyro resolution, acceleration-sensitive gyro drifts, and the alignment uncertainties associated with integration on the Shuttle.

Flow and Force Equations for a Body Revolving in a Fluid

Abstract: Part I gives a general method for finding the steady-flow velocity relative to a body in plane curvilinear motion, whence the pressure is found by Bernoulli's energy principle. Integration of the pressure supplies basic formulas for the zonal forces and moments on the revolving body. Part II, applying this steady-flow method, finds the velocity and pressure at all points of the flow inside and outside an ellipsoid and some of its limiting forms, and graphs those quantities for the latter forms. Part III finds the pressure, and thence the zonal force and moment, on hulls in plane curvilinear flight. Part IV derives general equations for the resultant fluid forces and moments on trisymmetrical bodies moving through a perfect fluid, and in some cases compares the moment values with those found for bodies moving in air. Part V furnishes ready formulas for potential coefficients and inertia coefficients for an ellipsoid and its limiting forms. Thence are derived tables giving numerical values of those coefficients for a comprehensive range of shapes.

Subsonic longitudinal and lateral aerodynamic characteristics for a systematic series of strake-wing configurations

Abstract: A systematic wind tunnel study was conducted in the Langley 7 by 10 foot high speed tunnel to help establish a parametric data base of the longitudinal and lateral aerodynamic characteristics for configurations incorporating strake-wing geometries indicative of current and proposed maneuvering aircraft. The configurations employed combinations of strakes with reflexed planforms having exposed spans of 10%, 20%, and 30% of the reference wing span and wings with trapezoidal planforms having leading edge sweep angles of approximately 30, 40, 44, 50, and 60 deg. Tests were conducted at Mach numbers ranging from 0.3 to 0.8 and at angles of attack from approximately -4 to 48 deg at zero sideslip.

A computer program to generate equations of motion matrices, L217 (EOM). Volume 1: Engineering and usage

Abstract: The equations of motion program L217 formulates the matrix coefficients for a set of second order linear differential equations that describe the motion of an airplane relative to its level equilibrium flight condition. Aerodynamic data from FLEXSTAB or Doublet Lattice (L216) programs can be used to derive the equations for quasi-steady or full unsteady aerodynamics. The data manipulation and the matrix coefficient formulation are described.

Limit cycle stability of aerodynamically induced yaw oscillations

Abstract: The aerodynamic yaw moment is written generally as a Taylors-Fourier series. Truncation of the series and assembly of the model leads to a second order non-linear differential equation which bears some similarity to the Van der Pol equation. The system is then examined both on the phase plane and by an approximate stability analysis. The stable and unstable limit cycles, nested around periodic foci are generated and discussed. Finally, loop diagrams are introduced; it is felt that these will have applications beyond the bounds of this paper.

Theoretical study of aerodynamic characteristics of wings having vortex flow

Abstract: The aerodynamic characteristics of slender wings having separation induced vortex flows are investigated by employing three different computer codes--free vortex sheet, quasi vortex lattice, and suction analogy methods. Their capabilities and limitations are examined, and modifications are discussed. Flat wings of different configurations: arrow, delta, and diamond shapes, as well as cambered delta wings, are studied. The effect of notch ratio on the load distributions and the longitudinal characteristics of a family of arrow and diamond wings is explored. The sectional lift coefficients and the accumulated span loadings are determined for an arrow wing and are seen to be unusual in comparison with the attached flow results. The theoretically predicted results are compared with the existing experimental values.

Wind loads on flat plate photovoltaic array fields. Phase III, final report

Abstract: The results of an experimental analysis (boundary layer wind tunnel test) of the aerodynamic forces resulting from winds acting on flat plate photovoltaic arrays are presented. Local pressure coefficient distributions and normal force coefficients on the arrays are shown and compared to theoretical results. Parameters that were varied when determining the aerodynamic forces included tilt angle, array separation, ground clearance, protective wind barriers, and the effect of the wind velocity profile. Recommended design wind forces and pressures are presented, which envelop the test results for winds perpendicular to the array's longitudinal axis. This wind direction produces the maximum wind loads on the arrays except at the array edge where oblique winds produce larger edge pressure loads.

Wind-tunnel/flight correlation study of aerodynamic characteristics of a large flexible supersonic cruise airplane CXB-70-1). 1: Wind-tunnel tests of a 0.03-scale model at Mach numbers from 0.6 to 2.53

Abstract: The longitudinal and lateral forces and moments for a 0.03 scale deformed rigid, static force model of the XB-70-1 airplane were determined. Control effectiveness was determined for the elevon in pitch and roll, for the canard, and for the rudders. Component effects of the canard, deflected with tips, variable position canopy, bypass doors, and bleed dump fairing were measured. The effects of small variations in inlet mass flow ratio and small amounts of asymmetric deflection of the wing tips were assessed.

Aerodynamic effects of an attitude control vane on a tilt-nacelle V/STOL propulsion system

Abstract: One possible technique for obtaining longitudinal control on a tilt-nacelle V/STOL aircraft is the use of a variable attitude vane assembly mounted in the propulsion system exhaust. Deflecting the vane produces large forces and moments without depending on forward speed of the aircraft. Tests are carried out in the Ames 40 by 80 ft wind tunnel on a large-scale tilt-nacelle V/STOL propulsion system with and without a variable attitude control vane assembly. Aerodynamic characteristics are analyzed in terms of nacelle aerodynamics, vane aerodynamics, and vane-induced effects on the nacelle aerodynamics. It is shown that the aerodynamic forces due to the nacelle without the vane can be a significant part of the total forces produced by the propulsion system. The control vane effectively produces large changes in pitching moment which are accompanied by significant changes in total lift and drag. The vane has a substantial effect on the propulsion-system aerodynamics. Other pertinent results are also given.

Upper stage jet impingement on separated booster

Abstract: The knowledge about the position of the detached shock and the aerodynamic forces acting upon a booster front during upper stage (sustainer) jet impingement on a separated booster is needed in connection with the problem of stage separation in the atmosphere for a multistage launch vehicle. The jet flow is squeezed in by the free-stream and characterised by a non-uniform transverse variation of flow properties. The flow field analysis of a trailing blunt body in such a non-uniform flow has been limited to experimental studies only. Most of the prior analytical treatments were restricted to the study of the effect of free-stream non-uniformities such as source flow and wake-like free-stream velocities on blunt bodies. A somewhat related problem of supersonic, underexpanded jet impingement upon an infinite flat plate in the absence of external stream has been studied by Belov et al and Gummer and Hunt.

Alleviation of the Side Force and the Yawing Moment Acting on a Slender Cone-Cylinder Body at High Angles of Attack, Using Small Jet Injection at Subsonic and Transonic Speeds

Abstract: : The effects of small symmetrical jets on the side forces of slender bodies at high angles of attack are investigated. Control as well as alleviation of these forces are considered. The effects of Reynolds number and blowing rate are investigated. Side force alleviation is obtained for subsonic and transonic flows. It is found that variation of Mach number affects the magnitude and direction of side forces.