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At least 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
NASA NTRS1978

The importance of quadrupole sources in prediction of transonic tip speed propeller noise

A theoretical analysis is presented for the harmonic noise of high speed, open rotors. Far field acoustic radiation equations based on the Ffowcs-Williams/Hawkings theory are derived for a static rotor with thin blades and zero lift. Near the plane of rotation, the dominant sources are the volume displacement and the rho U(2) quadrupole, where u is the disturbance velocity component in the direction blade motion. These sources are compared in both the time domain and the frequency domain using two dimensional airfoil theories valid in the subsonic, transonic, and supersonic speed ranges. For nonlifting parabolic arc blades, the two sources are equally important at speeds between the section critical Mach number and a Mach number of one. However, for moderately subsonic or fully supersonic flow over thin blade sections, the quadrupole term is negligible. It is concluded for thin blades that significant quadrupole noise radiation is strictly a transonic phenomenon and that it can be suppressed with blade sweep. Noise calculations are presented for two rotors, one simulating a helicopter main rotor and the other a model propeller. For the latter, agreement with test data was substantially improved by including the quadrupole source term.

Hanson, D. B.↗
NASA NTRS1978

Ultra-thin silicon solar cells for high performance panel applications

Solar cells have been fabricated which achieved the highest power to mass ratios and radiation stability yet reported for silicon devices. The thinnest cells (.04 mm) had initial efficiencies in excess of 2 watts per gram (AMO) and 1.7 watts per gram after an irradiation of 1 x 10 to the 15th equivalent 1 MeV electrons per square centimeter. The cells have been successfully interconnected by welding and filtered using a FEP bonded, ceria-doped microsheet of six mil thickness. Handling losses during cell manufacture and panel assembly may be minimized through the use of an integral reinforcing perimeter or ribs which remove almost all restrictions on cell thickness and area. Such a cell is typically composed of a main section which can be as thin as 0.015 mm and is supported at the edge by a thicker border (0.20 mm) of silicon.

Gay, C. F.↗
NASA NTRS1947

Boundary-layer and stalling characteristics of two symmetrical NACA low-drag airfoil sections

Two symmetrical airfoils, an NACA 633-018 and an NACA 631-012, were investigated for the purpose of determining their stalling and boundary-layer characteristics with a view toward the eventual application of this information to the problem of boundary-layer control. Force measurements, pressure distributions, tuft studies, and boundary-layer-profile measurements were made at a value of 5,800,000 Reynolds number. It was found that the 18-percent-thick airfoil stalled progressively from the trailing edge because of separation of the turbulent boundary layer. In contrast, the12-percent-thick airfoil stalled abruptly from a separation of flow near the leading edge before the turbulent boundary layer became subject to separation. From this it was concluded that if high values of lift are to be obtained with thin, high-critical-speed sections by means of boundary-layer control, the work must be directed toward delaying the separation of flow near the leading edge. It was found that the presence of a nose flap on the 12-percent-thick section caused the airfoil to stall in a manner similar to that of the 18-percent-thick section.

Mccullough, George B↗
NASA NTRS1991

Extension-bend-twist coupling behavior of thin-walled advanced composite beams with initial twist

An analytical model is developed for assessing the extension-bend-twist coupling behavior of nonhomogeneous anisotropic beams with initial twist. The model is formulated as a coupled two-dimensional boundary value problem, where the displacement solutions are defined with pretwist-dependent functions that represent the extension, bending, and torsion, and unknown functions that represent local in-plane deformations and out-of-plane cross-section warping. The unknown deformation functions are determined by applying the principle of minimum potential energy to a discretized representation of the cross section. Numerical results are presented that fully verify this approach and illustrate the strong extension-twist coupling behavior present in pretwisted beams with thin-wall laminated composite cross sections as a function of ply angle, initial twist level, and initial twist axis location. Cross-sections analyzed include; thin laminated rectangles with either asymmetric or symmetric ply stacking sequences and a thin-wall single cell D-section composed of a graphite/epoxy woven cloth.

Kosmatka, J. B.↗
NASA NTRS1951

On the theory of thin and thin-walled rods

Through the work of V. Z. Vlasov a theory of thin-walled rods has been established that is widely applicable in practice. This theory was extended by A. A. Umanski to thin-walled rods of closed profile section. The authors based their work on the concepts of the modern theory of shells. An attempt is made herein to construct a theory of thin-walled rods including the classical theory of deformation of thin rods by making use of a kinematic assumption.

LOADS - CYLINDERS↗
NASA NTRSDate not supplied

Corrugated Rollable Tubular Booms

A promising candidate for a new class of deployable composite boom is the COrrugated ROllable TUbular Boom (COROTUB), which is to be employed on future large space structures by the National Aeronautics and Space Administration (NASA). This is due to its two corrugated thin-shells forming a closed-section,which yields high bending and torsion stiffness, allowing for high dimensional stability. The corrugation geometry that dictates the boom cross-section shape is completely defined by closed-form analytical equations given a set of key input parameters available to the designer. Parametric studies changing two geometric variables are used to evaluate which parameters most influence the cross-section’s area moment of inertia and torsional constant. The number of corrugations, flattened height of the boom, and concave and convex corrugation radii are varied to study the complete boom design space. Deeper corrugations increase the local buckling loads (crimpling) of the thin-shell boom but reduce the area moment of inertia and therefore the global Euler buckling load of the compressive member,and thus a fine balance is sought. Finite element analyses of pure bending and compression load cases for 1.22 m boom specimens were carried out to study this design trade-off in detail. Based on previous work, several thin-ply composite laminates have been selected for the different shells that form the cross-section. Feasible boom designs with a maximized stiffness and load-carrying capacity will be manufactured to study their rollability and for experimental characterization. Structural characterization tests and additional numerical analyses will provide guidelines towards future corrugated boom designs and adequate boundary conditions to maximize the boom bending strength.

Deployable Booms↗
NASA NTRS1932

Strength Tests on Thin-walled Duralumin Cylinders in Torsion

This report is the first of a series presenting the results of strength tests on thin-walled cylinders and truncated cones of circular and elliptical section; it comprises the results obtained to date from torsion (pure shear) tests on 65 thin-walled duralumin cylinders of circular section with ends clamped to rigid bulkheads. The effect of variations in the length/radius and radius/thickness ratios on the type of failure is indicated, and a semi-empirical equation for the shearing stress at maximum load is given.

TRUNCATED - CYLINDERS, CIRCULAR - STRENGTH↗
NASA NTRS1947

Supersonic Wave Drag of Sweptback Tapered Wings at Zero Lift

On the basis of a recently developed theory for sweptback wings at supersonic velocities, equations are derived for the wave drag of sweptback tapered wings with thin symmetrical double-wedge sections at zero lift. Calculations of section wave-drag distributions and wing wave drag are presented for families of tapered plan forms. Distributions of section wave drag along the span of tapered wings are, in general, very similar in shape to those of untapered plan forms. For a given taper ratio and aspect ratio, an appreciable reduction in wing wave-drag coefficient with increased sweepback is noted for the entire range of Mach number considered. For a given sweep and taper ratio, higher aspect ratios reduce the wing wave-drag coefficient at substantially subcritical supersonic Mach numbers. At Mach numbers approaching the critical value, that is, a value equal to the secant of the sweepback angle, the plan forms of low aspect ratio have lower drag coefficients. Calculations for wings of equal root bending stress (and hence different aspect ratio) indicate that tapering the wing reduces the wing wave-drag coefficient at Mach numbers considerably less than the critical value but increases the drag coefficient at Mach numbers near the critical values. Comparisons on the basis of constant aspect ratio, however, indicate an increase of the wing wave-drag coefficient with taper at Mach numbers considerably less than the critical value and a decrease of the drag coefficient with taper at Mach numbers near the critical value.

WINGS - SWEEPBACK↗
NASA NTRS1920

The aerodynamic properties of thick aerofoils suitable for internal bracing

The object of this investigation was to determine the characteristics of various types of wings having sufficient depth to entirely inclose the wing bracing, and also to provide data for the further design of such sections. This type of wing is of interest because it eliminates the resistance of the interplane bracing, a portion of the airplane that sometimes absorbs one-quarter of the total power required to fly, and because these wings may be made to give a very high maximum lift. Results of the investigation of the following subjects are given: (1) effect of changing the upper and lower camber of thick aerofoils of uniform section; (2) effect of thickening the center and thinning the tips of a thin aerofoil; (3) effect of adding a convex lower surface to a tapered section; (4) effect of changing the mean thickness with constant center and tip sections; and (5) effect of varying the chord along the span.

Norton, F H↗
NASA NTRSDate not supplied

Bistable Deployable Composite Booms With Parabolic Cross-Sections

This paper investigates how stable equilibrium states in the extended and coiled configurations can be predicted in thin-shelled composite booms with parabolic cross-sections. These conic shapes potentially offer greater stiffness properties when compared to circular cross-sections, which is critical for improving the load bearing performance of deployed booms. Inducing bistability through the choice of composite shell layups in parabolic booms would allow for controllable self-deployment due to a less energetic coiled state when compared to monostable booms. An inextensional analytical model is used to predict the stable coiled diameters of tape spring and Collapsible Tubular Mast (CTM) booms with parabolic cross-sections. The parabolic section is discretized into circular segments using biarc spline interpolation, which allows them to be integrated into the strain energy minimization procedure used to obtain the equilibrium states. When the parabolic booms are parametrically compared against circular booms with identical layups, flattened height, and mass, the former are found to generally have better stiffness performance while being less efficient in stowed volume as evidenced by larger coiled diameters. Analytical coiled diameters and their strain energy are verified with finite element simulations for optimal parabolic tape spring and CTM booms.

Andrew J. Lee↗
NASA NTRS1982

A variational approach to the study of capillary phenomena

The problem of determining the free surface of a liquid in a capillary tube, and of a liquid drop, sitting first on a horizontal plane and then on more general surfaces is considered. With some modifications, the method applies to the study of pendent drops and of rotating drops as well. The standard capillary problem, i.e. the determination of the free surface of a liquid in a thin tube of general cross section, which resuls from the simultaneous action of surface tension, boundary adhesion and gravity is discussed. It turns out that in this case the existence of the solution surface depends heavily on the validity of a simple geometric condition about the mean curvature of the boundary curve of the cross section of the capillary tube. Some particular examples of physical interest are also be discussed. Liquid drops sitting on or hanging from a fixed horizontal plane are discussed. The symmetry of the solutions (which can actually be proved, as consequence of a general symmetrization argument) now plays the chief role in deriving both the existence and the regularity of energy-minimizing configurations. When symmetry fails (this is the case, for example, when the contact angle between the drop and the plate is not constant, or when the supporting surface is not itself symmetric), then more sophisticated methods must be used. Extensions in this direction are outlined.

Emmer, M.↗
NASA NTRS1982

An approximation concepts method for space frame synthesis

A method is presented for the minimum mass design of three dimensional space frames constructed of thin walled rectangular cross-section members. Constraints on nodal displacements and rotations, material stress, local buckling, and cross sectional dimensions are included. A high quality separable approximate problem is formed in terms of the reciprocals of the four section properties of the frame element cross section, replacing all implicit functions with simplified explicit relations. The cross sectional dimensions are efficiently calculated without using multilevel techniques. Several test problems are solved, demonstrating that a series of approximate problem solutions converge rapidly to an optimal design.

Mills-Curran, W. C.↗
NASA NTRS1984

Acoustic pressures emanating from a turbomachine stage

A knowledge of the acoustic energy emission of each blade row of a turbomachine is useful for estimating the overall noise level of the machine and for determining its discrete frequency noise content. Because of the close spacing between the rotor and stator of a compressor stage, the strong aerodynamic interactions between them have to be included in obtaining the resultant flow field. A three dimensional theory for determining the discrete frequency noise content of an axial compressor consisting of a rotor and a stator each with a finite number of blades are outlined. The lifting surface theory and the linearized equation of an ideal, nonsteady compressible fluid motion are used for thin blades of arbitrary cross section. The combined pressure field at a point of the fluid is constructed by linear addition of the rotor and stator solutions together with an interference factor obtained by matching them for net zero vorticity behind the stage.

Ramachandra, S. M.↗
NASA NTRS1984

Acoustic pressures emanating from a turbomachine stage

A knowledge of the acoustic energy emission of each blade row of a turbomachine is useful for estimating the overall noise level of the machine and for determining its discrete frequency noise content. Because of the close spacing between the rotor and stator of a compressor stage, the strong aerodynamic interactions between them have to be included in obtaining the resultant flow field. This paper outlines a three-dimensional theory for determining the discrete frequency noise content of an axial compressor consisting of a rotor and a stator each with a finite number of blades. The lifting surface theory and the linearized equation of an ideal, nonsteady compressible fluid motion are used for thin blades of arbitrary cross section. The combined pressure field at a point of the fluid is constructed by linear addition of the rotor and stator solutions together with an interference factor obtained by matching them for net zero vorticity behind the stage.

Ramachandra, S. M.↗
NASA NTRS1987

Composite box beam analysis - Theory and experiments

Beam theory is widely used as a first approximation in numerous structural applications. When applied to composite beams, the accuracy of beam theory becomes questionable because (1) the shearing and warping deformations become significant, as the shearing stiffness of composite laminates is often very low, and (2) several elastic couplings can occur that strongly influence the behavior of composite beams. The torsional behavior of thin-walled composite beams has important implications for aeronautical structures and is deeply modified by the above nonclassical effects. This paper presents two comprehensive analysis methodologies for composite beams and describes experimental results obtained from a thin-walled, rectangular cross-sectional beam. The theoretical predictions are found in good agreement with the observed twist and strain distributions. Out-of-plane torsional warping of the cross-section is found to be the key factor for an accurate modeling of the torsional behavior of such structures.

Bauchau, O. A.↗
NASA NTRS1991

Periodic blade loads of a high speed propeller at small angle of attack

A code has been developed to predict the periodic aerodynamic loads of an advanced turboprop propeller. The analytical formulation accounts for flow three-dimensionality and flow periodicity due to the propeller inclination. The flow past the blade sections is computed using a thin layer Navier-Stokes solver. An iterative procedure is used to account for the induced axial and rotational velocities. The viscous periodic results are obtained for an eight-bladed Hamilton Standard SR-7L advanced propeller at a cruise Mach number of 0.813 and 35,000 ft. altitude. The results are shown for flow field quantities and performance parameters during the blade passage in the plane of rotation illustrating the periodic nature of blade flow separation and shocks. The time averaged coefficients of thrust and power are computed and compared with available flight test data. The results obtained show excellent agreement at cruise conditions for small nacelle angles of attack.

Takallu, M. A.↗
NASA NTRS1928

Pressure Distribution over a Wing and Tail Rib of a VE-7 and of a TS Airplane in Flight

This investigation was made to determine the pressure distribution over a rib of the wing and over a rib of the horizontal tail surface of an airplane in flight and to obtain information as to the time correlation of the loads occurring on these ribs. Two airplanes, VE-7 and TS, were selected in order to obtain the information for a thin and a thick wing section. In each case the pressure distribution was recorded for the full range of angle of attack in level flight and throughout violent maneuvers. The results show: (a) that the present rib load specifications in use by the Army Air Corps and the Bureau of Aeronautics, Navy Department, are in fair agreement with the loads actually occurring in flight, but could be slightly improved; (b) that there appears to be no definite sequence in which wing and tail surface ribs reach their respective maximum loads in different maneuvers; (c) that in accelerated flight, at air speeds less than or equal to 60 per cent of the maximum speed, the accelerations measured agree very closely with the theoretically possible maximum accelerations. In maneuvers at higher air speeds the observed accelerations were smaller than those theoretically possible. (author)

Crowley, J W , Jr↗
NASA NTRS1999

Predicted Aerodynamic Characteristics of a NACA 0015 Airfoil Having a 25% Integral-Type Trailing Edge Flap

Using the two-dimensional ARC2D Navier-Stokes flow solver analyses were conducted to predict the sectional aerodynamic characteristics of the flapped NACA-0015 airfoil section. To facilitate the analyses and the generation of the computational grids, the airfoil with the deflected trailing edge flap was treated as a single element airfoil with no allowance for a gap between the flap's leading edge and the base of the forward portion of the airfoil. Generation of the O-type computational grids was accomplished using the HYGRID hyperbolic grid generation program. Results were obtained for a wide range of Mach numbers, angles of attack and flap deflections. The predicted sectional lift, drag and pitching moment values for the airfoil were then cast in tabular format (C81) to be used in lifting-line helicopter rotor aerodynamic performance calculations. Similar were also generated for the flap. Mathematical expressions providing the variation of the sectional lift and pitching moment coefficients for the airfoil and for the flap as a function of flap chord length and flap deflection angle were derived within the context of thin airfoil theory. The airfoil's sectional drag coefficient were derived using the ARC2D drag predictions for equivalent two dimensional flow conditions.

Hassan, Ahmed↗