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Weisshaar, T. A.

Publications and source records attributed to Weisshaar, T. A..

35 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

On the track of practical forward-swept wings

Structural laminates which comprise wing-cover skins for forward swept winged aircraft are examined. The laminates are themselves composed of lamina arranged in a symmetrical and unbalanced fashion. The fibers are oriented so that no fiber has a counterpart in the same ply which is at an exact anti-angle to itself. The laminate orientation creates a wash-out in a forward swept wing and alleviates aeroelastic loading. Further discussion is devoted to center-of-pressure movement, flutter behavior, aeroelasticity and aeroelastic divergence, and wind tunnel testing of aerodynamically tailored wings. It is found that rotating the laminate to increase the divergence dynamic pressure decreases strain under aerodynamic loading. Flight tests with three models are reported, and it is concluded that divergence can be avoided by the use of an efficient composite structure.

Hertz, T. J.↗
NASA NTRS1982

Quadratic synthesis of integrated active controls for an aeroelastic forward-swept-wing-aircraft

State variable representations of flexible aircraft are obtained using a modeling approach which first defines a mean-reference axis coordinate system. The active system design approach for forward-swept-wing aircraft also expresses structural deformations of the wing in terms of free normal vibration modes relative to this mean reference axis. Calculations which assume quasi-steady incompressible aerodynamics are performed to obtain generalized force expressions for flight conditions. The use of this quadratic synthesis technique shows that increased performance and redundancy over decentralized approaches can be achieved using integrated active longitudinal stability augmentation and aeroelastic stabilization.

Gilbert, M. G.↗
NASA NTRS1981

Interactive aircraft flight control and aeroelastic stabilization

Several examples are presented in which flutter involving interaction between flight mechanics modes and elastic wind bending occurs for a forward swept wing flight vehicle. These results show the basic mechanism by which the instability occurs and form the basis for attempts to actively control such a vehicle.

Weisshaar, T. A.↗
NASA NTRS1979

An experimental and analytical investigation of the rail shear-test method as applied to composite materials

This report presents the results from an experimental and analytical investigation of the stress distributions occurring in a rail shear test. The effects of non-uniform stresses induced by differential thermal expansion, rail flexibility and specimen aspect ratio on measured shear modulus and ultimate strength of composite laminates are shown. A two-dimensional linearly elastic finite element model was used to analytically determine how various geometric parameters influenced the magnitude and distribution of inplane normal and shear stresses in a tensile rail shear specimen. Rail shear tests were conducted at room temperature and 589 K (600 F) on selected graphite-polyimide composite laminates using two titanium rail configurations. The analysis and test methods are discussed, and the results of the effects of the various parameters on shear modulus and ultimate strength are presented.

Garcia, R.↗
NASA NTRS1979

Analysis of graphite/polyimide rail shear specimens subjected to mechanical and thermal loading

A two-dimensional, linear-elastic, finite element analysis of selected graphite/polyimide rail shear test specimens was made to determine the mechanical loading and the effect of heating the specimen to a uniform temperature. The presence of specimen free edges and their influence on the accuracy of the rail shear test is discussed. Parameters include the length-to-width ratio of the specimen and the ply layup for symmetric, balanced laminates. Results presented include shear and normal stress distributions and the deflection behavior of various specimens caused by the mechanical loading and elevated temperature.

Weisshaar, T. A.↗
NASA NTRS1978

A simple method for designing structural models with closely spaced modes of vibration

A simple method for designing a mathematical model with closely spaced vibration modes is described. The design process begins with a reference model having specified geometry, continuous inertia and stiffness distributions, and degrees of freedom, all of which remain unchanged. Two natural frequencies of this model are then forced together by means of systematic perturbation of the model's discrete inertia and stiffness parameters. There is only one eigenvalue solution per design cycle, and the gradient vector is calculated directly from the resulting modal quantities. The minimization procedure employed is unconstrained. As applications, a cantilevered plane grid model with five degrees of freedom and a bending-torsion-oscillator with eleven degrees of freedom are treated.

Hallauer, W. L., Jr.↗
NASA NTRS1977

Lateral equilibrium of asymmetrical swept wings - Aileron control vs geometric twist

A static aeroelastic phenomenon unique to an aircraft with asymmetrically swept wings is discussed. A simple formula is derived from the analysis of a highly idealized model. The validity of this formula is examined through the use of a more sophisticated numerical analysis. Among the results of this analysis are the following: aileron settings of a few degrees are sufficient to trim such aircraft in roll-for-g-flight; the use of built-in twist in the form of initial negative dihedral provides an efficient alternative to aileron trim; if the wing is elastically tailored in a proper fashion, it may be possible to design a wing whose elastic deformation under airloads provides a form of self-trim in roll at the cruise q of the aircraft.

Weisshaar, T. A.↗
NASA NTRS1977

Aeroelastic stability characteristics of an oblique wing aircraft

Two different studies are presented involving a transport category aircraft with an oblique or asymmetrically swept wing. The first study concerns itself with the effect on flutter of changes in certain of the aircraft structural and inertial parameters. The second portion examines the subcritical damping characteristics of a flexible oblique wing aircraft. For this purpose, a recent version of the NASTRAN computer code is used. A comparison is made between the flutter speeds found using the traditional k analysis and the results found from computation of subcritical response using the p-k or 'British' flutter method.

Crittenden, J. B.↗
NASA NTRS1976

An investigation of aeroelastic phenomena associated with an oblique winged aircraft

Oblique wing aeroelasticity studies are reviewed. The static aeroelastic stability characteristics of oblique wing aircraft, lateral trim requirements for 1-g flight, and the dynamic aeroelastic stability behavior of oblique winged aircraft, primarily flutter, are among the topics studied. The similarities and differences between oblique winged aircraft and conventional, bilaterally symmetric, swept wing aircraft are emphasized.

Weisshaar, T. A.↗
NASA NTRS1976

Flutter of asymmetrically swept wings

Two formulations of the oblique wing flutter problem are presented; one formulation allows only simple wing bending deformations and rigid body roll as degrees of freedom, while the second formulation includes a more complex bending-torsional deformation together with the roll freedom. Flutter is found to occur in two basic modes. The first mode is associated with wing bending-aircraft roll coupling and occurs at low values of reduced frequency. The second instability mode closely resembles a classical bending-torsion wing flutter event. This latter mode occurs at much higher reduced frequencies than the first. The occurrence of the bending-roll coupling mode is shown to lead to lower flutter speeds while the bending-torsion mode is associated with higher flutter speeds. The ratio of the wing mass moment of inertia in roll to the fuselage roll moment of inertia is found to be a major factor in the determination of which of the two instabilities is critical.

Weisshaar, T. A.↗
NASA NTRS1975

An investigation of supersonic aeroelastic characteristics of oblique winged aircraft

Two formulations of the oblique wing flutter problem are presented: one formulation allows wing bending deformations and the rigid body roll degree of freedom while the second formulation includes bending-torsional deformation and roll degrees of freedom. Flutter is found to occur in two basic modes. The first mode is associated with bending-roll coupling and occurs at low reduced frequency values. The other instability mode is primarily one of classical bending-torsion with negligible roll coupling; this mode occurs at much higher reduced frequencies. The occurrence of bending-roll coupling mode leads to lower flutter speeds while the bending-torsion mode is associated with higher flutter speeds. The ratio of the wing mass moment of inertial in roll to the fuselage moment of inertia evidently plays a major role in the determination of which of the two instabilities is critical.

Weisshaar, T. A.↗
NASA NTRS1974

Static aeroelasticity and the flying wing, revisited

The roll dynamic behavior of some wing configurations is examined. Analysis shows that the static divergence instability which occurs for symmetrical or clamped sweptforward wings is modified by roll freedom. The instability found in the present highly idealized analysis is still of an aperiodic nature, but at a speed above the conventional divergence speed. The magnitude of this difference is seen to be a function of the ratio of the wing roll mass moment of inertia to the mass moment of inertia of the wing about the roll axis, and this difference may be quite significant.

Weisshaar, T. A.↗
NASA NTRS1974

Influence of static aeroelasticity on oblique winged aircraft

Interest has recently been expressed about the impact of static aeroelasticity on the lateral control of oblique winged aircraft. An attempt is made to illustrate, by use of a simple example, together with results from published literature, an aeroelastic phenomenon which is believed to be unique to oblique winged aircraft. This phenomenon is the aeroelastic roll moment and occurs because upward bending deflection of the sweptforward wing generates additional lift while the converse is true of sweptback wings.

Weisshaar, T. A.↗
NASA NTRS1972

Aeroelastic optimization of a panel in high Mach number supersonic flow.

Solution for a least-weight skin thickness distribution for a panel with a flutter parameter constraint. This panel weighs less than any similar constant thickness panel, but has the same critical supersonic panel flutter parameter. The panel rests on simple supports and is of sandwich construction. The span to chord ratio is large enough that the inertial, elastic, and aerodynamic behavior is one-dimensional. The Mach number is great enough that the aerodynamic forces acting on the upper panel surface may be accurately described by quasi-steady, linearized, supersonic aerodynamic theory. The final optimum design is obtained from theoretical and numerical methods adapted from optimal control theory. The results of this investigation show that the optimal panel thickness distribution is symmetric about the panel chord midpoint. Compared to a reference panel with constant thickness, optimum panels are found to be nearly 12% lighter.

Weisshaar, T. A.↗
NASA NTRS1972

Optimization of simple structures with higher mode frequency constraints.

Results of a study of least weight optimization of simple structures with a single natural frequency constraint. Variational techniques were used to derive the necessary equations, and numerical methods were used, where required, to find solutions to the resulting nonlinear, two-point boundary value problems. The numerical results suggest a similarity between the fixed fundamental frequency solution and solutions in which a single frequency other than fundamental is held fixed. Such a similarity is shown to occur under special circumstances, and the knowledge of the fundamental solution in these cases makes it possible to calculate the solution to problems with constraints on frequencies other than the fundamental.

Weisshaar, T. A.↗