Showing papers in "Journal of Aircraft in 1984"

Dynamic load measurements with delta wings undergoing self-induced roll-oscillations

Abstract: The aerodynamic forces acting on a delta wing, mounted on a free-to-roll sting-balance apparatus, were measured. Two wing planforms having leading edge sweeps of 76 and 80 deg were tested, but only the wing with the 80 deg sweep would undergo periodic self-induced roll oscillation. The time dependent forces and roll angles for this wing were then recorded for various test conditions. In these tests a considerable drop in the average normal force of the free-to-roll wing was measured, relative to the normal force obtained in the static tests. Also, the helium-bubble flow visualization technique was used to gain some insight into the periodic motion of the separated leading edge vortices.

Aeroelastic flutter and divergence of stiffness coupled, graphite/epoxy cantilevered plates

Abstract: An analytical and experimental investigation was conducted to determine the aeroelastic flutter and divergence behavior of unswept, rectangular wings simulated by graphite/epox y, cantilevered plates with various amounts of bending-torsi on stiffness coupling. The analytical approach incorporated a Rayleigh-Ritz energy formulation and unsteady, incompressible two-dimensional aerodynamic theory. Flutter and divergence velocities were obtained using the \ -g method and compared to results of low-speed wind tunnel tests. Stall flutter behavior was also examined experimentally. There was good agreement between analytical and experimental results. Wings with negative stiffness coupling exhibited divergence, while positive coupling delayed the onset of stall flutter.

Leeside flows over delta wings at supersonic speeds

Abstract: An experimental investigation of the lee-side flow on sharp leading-edge delta wings at supersonic speeds has been conducted. Pressure data were obtained at Mach numbers from 1.5 to 2.8, and three types of flow-visualization data (oil-flow, tuft, and vapor-screen) were obtained at Mach numbers from 1.7 to 2.8 for wing leading-edge sweep angles from 52.5 deg to 75 deg. From the flow-visualization data, the lee-side flows were classified into seven distinct types and a chart was developed that defines the flow mechanism as a function of the conditions normal to the wing leading edge, specifically, angle of attack and Mach number. Pressure data obtained experimentally and by a semiempirical prediction method were employed to investigate the effects of angle of attack, leading-edge sweep, and Mach number on vortex strength and vortex position. In general, the predicted and measured values of vortex-induced normal force and vortex position obtained from experimental data have the same trends with angle of attack, Mach number, and leading-edge sweep; however, the vortex-induced normal force is underpredicted by 15 to 30 percent, and the vortex spanwise location is overpredicted by approximately 15 percent.

Fuel effects on gas turbine combustion-liner temperature, pattern factor, and pollutant emissions

Abstract: An anaytical study is made of the substantial body of experimental data acquired during recent Wright Patterson Aero Propulsion Laboratory sponsored programs on the effects of fuel properties on the performance and reliability of several gas turbine combustors, including J79-17A, J79 17C (Smokeless), F101, TF41, TF39, J85, TF33, and F100 Quantitative relationships are derived between certain key aspects of combustion, notably liner wall temperature, pattern factor, and exhaust emissions, and the relevant fuel properties, combustor design features and combustor operating conditions It is concluded that fuel chemistry, as indicated by hydrogen content and/or aromatics content, has a significant effect on flame radiation and liner wall temperature, but only a slight effect on the emissions of carbon monoxide (CO) and oxides of nitrogen (NOX) The physical properties that govern atomization quality and evaporation rates affect CO emissions, but other important performance parameters, such as NO X emissions and liner wall temperature, are sensibly independent of physical properties over the range of fuels studied

Wind shear terms in the equations of aircraft motion

Abstract: Conventional analyses of aircraft motion in the atmosphere have neglected wind speed variability on the scales associated with many atmospheric phenomena such as thunderstorms, low level jets, etc These phenomena produce wind shears that have been determined as the probable cause in many recent commercial airline ac cidents This paper derives the 6 deg equations of motion for an aircraft incorporating the variable wind terms The equations are presented in several coordinate systems (i e , body coordinates, inertia! coordinates, etc ) The wind shear terms, including the temporal and spatial gradients of the wind, appear differently in the various coordinate systems; these terms are discussed. Also, the influence of wind shear on inputs to computing the aerodynamic coefficients, such as the effects of wind velocity vector rotation on relative angular rates of rotation and on the time rate of change of angles of attack and sideslip, are addressed

Effects of icing on performance of a research airplane

Abstract: The difference between the normal and actual rates of climb of a research airplane is used to measure the effect of icing on performance The icing conditions were encountered in the course of an extensive series of meteorological research flights in various locations and seasons Coefficients of lift and drag were determined for the airplane before and after icing encounters, and those coefficients were used to predict airplane per formance for various flight conditions The effect of icing was to increase the drag significantly, while there was little effect on the coefficient of lift In the course of these flights, characteristics of the icing clouds (such as hydrometeor size spectrum and phase, liquid water content, temperature, etc ) were also measured, and those characteristics are compared to the summaries in the Federal Aviation Regulations and to the data sources on which those summaries were based Most meaurements lie within the envelopes suggested by earlier studies For volume median diameters larger than 30/im, the liquid water contents were substantially lower than indicated by those summaries, but for other diameters measured liquid water contents extended up to and, in rare cases, exceeded the limiting envelopes of the Regulations Two exceptional cases of icing associated with droplets of 40 300 j*m diameter are also discussed The reduction in performance during these cases was anomalously large, although the liquid water content and volume median diameter did not indicate that these cases should have been potentially hazardous

Flutter Analysis Using Nonlinear Aerodynamic Forces

Abstract: The nonlinear effects of transonic aerodynamic forces on the flutter boundary of a typical section airfoil are studied. The flutter speed dependence on amplitude is obtained by utilizing a novel variation of the describing function method which takes into account the first fundamental harmonic of the nonlinear oscillatory motion. By using an aerodynamic describing function, traditional flutter analysis methods may still be used while including the effects of aerodynamic nonlinearities. Results from such a flutter analysis are compared with those of brute force time-marching solutions. The aerodynamic forces are computed by the LTRAN2 aerodynamic code for an NACA 64A006 airfoil at Mx = 0.86.

Wing design for minimum drag with practical constraints

Abstract: General design characteristics of wings that offer minimum drag while satisfying a variety of practical constraints are sought, including primarily a constraint upon structural weight The essential elements of aerodynamics and structures, when combined with analytical optimization, lead to a series of solutions for lift, chord, and thickness distributions across the span Although the optimized wings do not differ radically from current practice in these distributions, they can offer significantly lower drag For high subsonic wings of given structural weight, elasticity, and maximum load factor, forward rather than aft sweep promises a reduction in cruise induced drag of about 10% Wings whose weight is a large fraction of the total lift, as in sailplanes, may realize larger reductions

Applying slender wing benefits to military aircraft

Abstract: 1Therefore, in keeping with the spirit of the Wright brothers research and design accomplishments, as well as the scope of this meeting, I have elected to review some of the aerodynamic research performed at the Langley Research Center related to the application of slender wing benefits in the design of high-speed military aircraft In the context of this paper, slender wing benefits refer primarily to the supersonic performance and leading edge vortex flow associated with very highly sweptback wings Following a review of some early slender wing research, the paper presents several case histories of Langley contributions to the development of aircraft incorporating slender wing benefits and then summarizes some vortex flow technology that may contribute to future aircraft

Minimum induced drag of wings with curved planform

Abstract: Conclusions This study has shown that a small airfoil probe, consisting of a small canard wing mounted appropriately on an airframe and properly tapped, can serve as a viable alternative as a probe for angle-of-attack sensing on aircraft. An NACA 0012 airfoil section was used in wind tunnel tests in this study, and differential pressure coefficients greater than 3.0 at high angles of attack were achieved. These coefficients are an improvement by a factor of 2.0-3.0 over comparable coef- ficients obtained from hemispherical probes. References

Lightning strikes to an airplane in a thunderstorm

Abstract: The analysis of radar echoes from lightning at the moments of strikes to the NASA Langley Research Center's F-106B instrumented airplane proves that the airplane itself triggers the lightning, rather than intercepting naturally occurring flashes. In 1982 the UHF band radar at the NASA Goddard Space Flight Center/Wallops Flight Facility was used to guide the F 106B through the upper regions of thunderstorms so that the airplane might be struck by lightning. The UHF band radar data was analyzed to determine the nature and characteristics of direct lightning strikes to the airplane, and the airborne data was used to document the environmental con ditions favorable for such strikes The echo characteristics of the strikes were similar to those of intracloud flashes, and indicated that most of the time the airplane was part of the lighting channel. The probability of a direct strike to the F-106B during storm penetrations (PDS) is defined here as the ratio of the number of direct strikes to the airplane to the total number of flashes occurring in the radar resolution volume containing the airplane Correlations between the PDS and the intensity of rain, the intensity of turbulence, the ambient temperature, and the lightning flash rate in the storms penetrated were obtained The correlations indicated that the highest risk for the F-106B to be struck by lightning during penetrations in the upper regions of thun derstorms occurred under the following conditions: 1) ambient temperatures of -40 C and colder; 2) negligible to light precipitation; 3) negligible to light turbulence; and 4) lightning flash rates of < 10 per minute

Slender wings with leading-edge vortex separation - A challenge for panel methods and Euler solvers

Abstract: The demands of performance and maneuverability on modern fighter aircraft and missiles require slender wings, whose leading edge vortex flows result in increased lift at high angles of attack This paper reviews the history of the utilization of these vortex flows, and the development of theoretical methods for prediction of the nonlinear characteristics of slender wings Possible improvements based on the integral physical effects of the vortex flow are also considered Recent advances in the numerical solution of the time dependent Euler equations make Euler methods an attractive alternative to the methods based on potential flow solution ap proaches The latter are usually limited to subsonic speeds and demand as much effort to predict the flow around more complex configurations as do the Euler methods The latter even are able to predict flow separation effects to some extent, providing significant support for the evaluation of more complex shapes, thus improving the design procedures Results obtained by a panel procedure and by an Euler method are presented for different wings and wing body configurations Preliminary results obtained by applying these methods to vortex breakdown are included

Unsteady Flow Concepts for Dynamic Stall Analysis

Abstract: It is well established that there is a strong coupling between airfoil motion and boundary layer separation with attendant vortex shedding Unsteady flow mechanisms that influence this dynamic stall event have been described previously Until now sufficient information has not been available to determine the relative im portance of various unsteady flow effects, such as the time varying inviscid pressure gradient and the unsteady viscous boundary condition at the wall, the moving wall effect Recent experimental results provide the needed information, revealing how the mode of oscillation for the airfoil determines which unsteady flow effect will dominate Nomenclature chord length frequency dynamic overshoot parameter, Eqs. (5 7) section lift, coefficient c, = l/(p^ Ui/2)c Mach number section pitching moment, coefficient cm = mp c / Ka / M m q -pitch rate Re = Reynolds number based on chord length, =

Airfoil shape and thickness effects on transonic airloads and flutter

Abstract: A transient pulse technique is used to obtain harmonic forces from a time-marching solution of the complete unsteady transonic small-perturbation potential equation. The unsteady pressures and forces acting on a model of the NACA 64A010 conventional airfoil and the MBB A-3 supercritical airfoil over a range of Mach numbers are examined in detail. Flutter calculations at constant angle of attack show a similar flutter behavior for both airfoils, except for a boundary shift in Mach number associated with a corresponding Mach number shift in the unsteady aerodynamic forces. Differences in the static aeroelastic twist behavior for the two airfoils are significant. Nomenclature a = pitch axis location, referenced to midchord, in semichords b — semichord length c — chord length c/ = lift coefficient c/h = lift coefficient due to plunge c/ = lift coefficient due to pitch c^ = moment coefficient about c/4 cm — moment coefficient due to plunge cm = moment coefficient due to pitch Cp — pressure coefficient g — structural damping coefficient h — plunge displacement in semichords hj = plunge amplitude in semichord k — reduced frequency, bul V M — freestream Mach number m = airfoil mass per unit span ra — radius of gyration, referenced to pitch axis, in semichords

Application of transonic codes to aeroelastic modeling of airfoils including active controls

Abstract: A study is performed using aeroelastic modeling to investigate the stability behavior of airfoils in small-disturbance transonic flow. Two conventional airfoils, NACA 64.A006 and NACA 64A010, and a supercritical airfoil, MBB A-3, are considered. Three sets of unsteady aerodynamic data are computed using three different transonic codes (LTRAN2-NLR, LTRAN2-HI, and USTS) for comparison purposes. Stability results obtained using a constant matrix, state-space, aeroelastic model are presented in a root-locus format. Use of the state-space model is demonstrated through application to flutter suppression using active controls. Aeroelastic effects due to simple, constant gain, partial feedback, control laws that utilize displacement, velocity, and acceleration sensing are studied using a variety of control gains. Calculations are also performed using linear subsonic aerodynamic theory to reveal the differences between including and not including transonic effects in the aeroelastic model. Aeroelastic stability behavior of these airfoils is physically interpreted and discussed in detail.

Noise transmission characteristics of advanced composite structural materials

Abstract: Theoretical and experimental results from a study of noise transmission properties of large unstiffened panels which simulated aircraft outer skins and interior trim are reported. The investigation was performed to define the effects of composite structures on fuselage noise transmission relative to the transmissivity of aluminum structures. One-third octave band measurements were obtained in a two-room facility for measuring transmission loss. Center frequencies of at least 100 Hz were used, and 14 different composite panels, including samples of Kevlar, fiberglass, and graphite, were examined. Details of the composites fabrication techniques are provided, and an infinite panel theory transmission loss model is defined. The flexural rigidities of tape and fabric panels are calculated, as are the transmission losses, the coincidence frequency, and the critical frequency. The theory was determined to be accurate to within 1 dB of the measured transmission loss for mass-controlled specimens.

The influence of blade wakes on the performance of combustor shortened prediffusers

Abstract: Results of an experimental investigation of the influence of blade wakes on the performance of several combustor prediffusers are presented. Two curved and two straight wall diffusers of different length, area ratio, and turning angle were tested with a single-stage compressor sited at a number of positions relative to diffuser inlet. Only a small increase in loss was incurred when the wakes from the outlet guide vanes were allowed to decay within the prediffuser. Distance along the mean streamline was found to be the most significant parameter influencing the decay of the wakes. In outwardly curved diffusers, a distance of about four blade chord lengths was required for these effects to be minimized.

Circulation controlled STOL wing optimization

Abstract: By replacing the sharp trailing edge of an airfoil with a rounded Coanda surface, one can generate high lift at a moderate blowing rate. In such a circulation controlled (CC) airfoil, a two-dimensional wall jet is used to force the rear stagnation point down and increase the circulation and lift. The CC airfoil performance and blowing power requirements depend on both the wall jet momentum and the wall jet to freestream velocity ratio because a significant portion of the wall jet momentum is lost to shear over the Coanda surface. When using highpressure jet engine bleed air, an internal wall jet ejector can be used to optimize the wall jet velocity. This will reduce the required bleed airflow rate and cool the wing structure, in addition to providing boundary-layer control by suction. The applicability of CC high lift generation for short takeoff and landing aircraft was first demonstrated at West Virginia University in April 1974 with the WVU CC Technology Demonstrator STOL Aircraft. A second such CC technology demonstrator, a modified Navy A-6A was test flown five years later. The importance of optimizing the wall jet velocity is analyzed in this paper.

Benefits of dual wings over single wings for high-performance business airplanes

Abstract: An investigation was performed to compare closely coupled dual-wing aircraft and swept-forward sweptrearward (SFSR), dual-wing aircraft to corresponding single-wing aircraft to judge the advantages offered by aircraft designed with multiple-wing systems. The optimum dual-wing geometry used on the dual-wing designs were determined in an analytical study which investigated the twoand three-dimensional aerodynamic behavior of a wide range of dual-wing configurations in order to find the wing geometry that created the minimum cruise drag. This analysis used a multielement inviscid vortex panel program coupled to a momentum integral boundary-layer analysis program to calculate the two-dimensional aerodynamic data, which was then used as input for a three-dimensional vortex-lattice program, which calculated the three-dimensional aerodynamic data. The low drag of the dual-wing configurations is due to a combination of twoand three-dimensional drag reductions, and the structural advantages of the two wings, which permitted higher aspect ratios for the two wing systems, because of the wing tip structural connections.

Free-flight and wind-tunnel data for a generic fighter configuration

Abstract: A comparison of free-flight spark range and wind-tunnel data for a generic fighter configuration (standard dynamics model) is presented. The aerodynamic tests were conducted from 0.3 to 1.3 with the primary comparisons at the transonic Mach numbers. This paper shows the comparison of the zero angle-of-attack coefficients and derivatives since only relatively small oscillation amplitudes were achieved during the free-flight tests. However, it is believed that this comparison represents good agreement between these two sets of data obtained in different facilities using different test mechanisms and techniques.

Surface pressures on a flat plate with dual jet configurations

Abstract: Etude de l'effet de l'angle du jet par rapport a l'ecoulement transversal et des performances de configuration a deux jets en ligne et cote a cote

Twenty-Five Years of Handling Qualities Research

Abstract: paper reflects on 25 years (or more) of handling quality research and shares with the reader some of the author's resulting experiences and thoughts. When reaching back so far and considering all that has been accomplished, there are many facets of handling or flying qualities which could be covered and considered. However, the author chooses to limit discussion to those aspects concerned with the theory of handling qualities, in turn relating to closed-loop, pilot-vehicle, frequency-domain analysis and its application to handling and flight control problems. This is not to deny other aspects of handling qualities research which are beyond the scope of this limited exposition, such as: and inflightv-l2 simulation; rating systern~;~~'~b~p tirnal control operator models;'"1v workload con~epts;~~~~ and data etion and codifi~atisn.~~~ ~ Rather, it is to emphasize those aspects that the author is personally most familiar with, and which stress the design guidance role of handling qualities theory and practice. This has always been important and it is especially important now because of increasing dependence on sopliisticated flight control systems which can completely alter the way an airplane responds to the pilot's inputs. In fact, handling quality research has recently come up for its share of criticism RS being inadequate to cope with some of today's design problems. For example, Berry,3' in a recent article in Asfronaufics and Aeronautics and Gibson," in a paper before the AGARD Conference in Fort Worth, both decried the fact that there have been a rash of generic handling problems associated with high-performance aircraft having sophisticated flight control systems, and that such systems have not always reached their full potential to provide handling qualities superior to much simpler aircraft of the past. Against this background, first to be discussed are the basic aspects of handling or flying qualities and some of the early design problems that were solved; then, the growth of handling qualities theory in response to design demands; and, finally, how that theory has been applied and expanded over the years to become a valuable tool, especially useful in ns such as those that seem to be ics then, let handling qualities be dynamic and static properties of a ilot to fully exploit its performance riety of missions and roles. In other words, the limitations on the airplane do not originate in any kind of a pilot-vehicle control pro other aspects to the zed facets to handling.

Application of computational aerodynamics to airplane design

Abstract: OMPUTATIONAL aerodynamics has been defined as the engineering discipline which deals with the simulation of flowfields about aircraft through the numerical solution of the equations of fluid motion using digital computers This simulation process is referred to as numerical flow simulation; another way of simulating flow about airplanes is the wind tunnel Wind tunnel technology is almost as old as aeronautics; wind tunnel testing became the principal tool of aerodynamic design and development very early in the history of airplane engineering Of course in addition to the ex perimental approach analytical methods based on aerodynamic theory always have been used to obtain valuable design information; but in order to reduce the computational effort to a humanly tolerable level, many simplifying assumptions had to be introduced These simplifications not only had to do with the mathematical modeling of the physical problem e g inviscid incompressible flow but they also involved the geometry of the configuration and flow condition e g the lifting line concept to represent a wing These simplications did not make possible the calculation of many aerodynamic characteristics and posed a high degree of uncertainty about the accuracy and validity of the numerical results Therefore these results could be used only as design guidelines and large emphasis had to be placed on wind tunnel testing The advent of the digital computer and the impressive growth of its capabilities and cost effectiveness—as illustrated in Fig 1—now have made it possible to eliminate many of the simplifications that had been introduced in analytical methods This has increased the amount of in formation that can be extracted from a calculation and at the same time provides more accuracy and a greater range of validity for the results of numerical flow simulation This has turned computational aerodynamics into a major tool for airplane design This paper surveys the use of computational aerodynamics as a design tool Much development work in computational aerodynamics is of a fundamentally scientific nature con cerned with what type of numerical flow simulation is feasible within the boundaries of our knowledge of flow physics and numerical algorithms This paper deals with the engineering aspects of numerical flow simulation i e. what elements are practical in an environment where schedule and budget constraints predominate and where multidisciplinary in teraction is the way of life and particularly with the factors that affect its effectiveness in such an environment Even though computer system characteristics do fundamentally affect the effectiveness of computational aerodynamics they are not the subject of this paper The survey presented here is of necessity, selective and consequently incomplete The main intent of this paper is to highlight those elements of computational aerodynamics that are important for airplane application what form they have taken and to indicate what future developments are considered necessary to increase the general utility of computational aerodynamics Three dimensionality geometrical complexity and computational efficiency are stressed as overdriving considerations for design application First some general characteristics and needs of airplane design projects are discussed; within this context particular attention is given to the meaning of effectiveness and the identification of the computer code characteristics that affect it Next the code elements that are part of a typical numerical flow simulation are identified; the nature of these elements how they impact code effectiveness and how they are affected by considerations of effectiveness are addressed subsequently The author has drawn freely from developments at the Lockheed California Company to illustrate some points; for this he is indebted to many of his colleagues at Lockheed

Thrust vector control of a V/STOL airship

Abstract: Potential concepts for thrust vector control of a modern airship were investigated using a six degree of freedom flight dynamics simulation. The specific thrust vectors simulated included those from two ducted fans mounted one on either side of the airship car, each having the capability of tilting in pitch and roll to give vertical and lateral thrust for control An auxiliary thruster at the bow or stern of the airship, which augments its directional control, was also considered It has been found that the tillable ducted fans provide the airship with greater operational flexibility especially during takeoff and landing Thrust vectoring to provide roll control was found to be effective while ground handling The bow/stern thruster was found to give excellent directional control, which significantly improved the airship lateral maneuverability at low speeds Thrust reversibility, thrust application rate, and tilt rate of thrust vectors were found to be important design parameters having considerable effect on airship flying qualities

Effect of geometry on airfoil icing characteristics

Abstract: A droplet trajectory computer code is used to predict the water droplet impingement characteristics of several low- and medium-speed airfoils. The maximum impingement efficiency, total collection efficiency, and limits of impingement are analyzed as functions of the airfoil geometry and freestream conditions. The airfoil geometry is represented by leading edge radius, maximum thickness, maximum camber, and angle of attack. The analysis shows that the primary effects are an increase in maximum impingement efficiency with a decrease in leading edge radius, a reduction in total collection efficiency for thicker airfoils, and a change in the limits of impingement for airfoils of different maximum camber.

Comparison of broadband noise mechanisms, analyses, and experiments on rotors

Abstract: A study is made of the various mechanisms which generate broadband noise on a range of rotors The sources considered are load fluctuations due to inflow turbulence, due to turbulent boundary layers passing the blades' trailing edges, and due to tip vortex formation Vortex shedding noises due to laminar boundary layers and blunt trailing edges are not considered as they can be prevented in most cases Various prediction methods have been reviewed and extended in some cases An extensive search was made of existing experiments and calculations based on the various prediction methods were made This study shows that present analyses are adequate to predict the spectra from a wide variety of experiments on fans, full scale and model-scale helicopter rotors, wind turbines, and propellers to within about 5 to 10 dB Better knowledge of the inflow turbulence improves the accuracy of the predictions The results of this study indicate that inflow turbulence noise depends strongly on ambient conditions and dominates at low frequencies Trailing edge noise and tip vortex noise are important at higher frequencies if inflow turbulence is weak Boundary layer trailing edge noise is important especially in the presence of large rotors; it increases slowly with angle of attack but not as rapidly as tip vortex noise, which can be important at high angles of attack for wide chord, square edge tips

A spatial model of wind shear and turbulence

Abstract: The purpose of the spatial model considered in the present investigation is to generate the wind environment for use by others for flight simulation. Winds and gusts are provided over any finite area (e.g., aircraft body) from which aircraft loads and moments may be calculated. Three-dimensional autospectral information and correlation are contained in the data. It is pointed out that the three-dimensionality as contained in the spatial model affords much greater realism than widely used one-dimensional models. The resulting simulated wind is a nonlinear, non-Gaussian combination of real atmospheric winds and Gaussian, three-dimensional turbulence modulated by gust intensities which may vary freely as desired over space. The turbulence as represented by a product of a varying gust intensity and simulated turbulence is nonlinear and non-Gaussian.