Wing structures often contain nonlinearities which affect their aeroelastic behavior and performance characteristics. Aerodynamic flows at transonic Mach numbers generate nonlinear aerodynamic forces on the wing affecting the aeroelastic response of the wing. Analysis techniques accounting for these structural and aerodynamic nonlinearities, and an understanding of their potential influence on the flutter mechanism of two-dimensional and three-dimensional wing-structures model are the main objective of this study. Two different categories of structural nonlinearities, i.e. (i) distributed nonlinearity and (ii) concentrated nonlinearity , are considered. The concentrated nonlinearities are mathematically modeled using Asymptotic Expansion method which based on on the Krylov-Bogoliubov-Mitropolski technique. The effective stiffness coefficient of a nonlinear element is defined as the ratio of the amplitude of the Fourier series expansion of the load and the amplitude of the displacement of that element. The effects of distributed nonlinearities on the aeroelastic characteristic of three-dimensional wing model are also investigated. The influences of this type of nonlinearity is treated in a quasi-nonlinear approach, which allows the variation of the the natural frequencies and damping factor of the structure model with respect to the amplitude of the motion. The transonic aerodynamic pressure distributions have been obtained by solving the unsteady Transonic Small Disturbance ( TSD ) flow equation using finite-difference techniques. An Alternating Direction Implicit ( ADI ) algorithm was used for two-dimensional flow model, and an Approximate Factorization ( AF ) algorithm was used for three-dimensional flow model. The finite-state generalized aerodynamic forces used in the aeroelastic analysis have been calculated by employing the Method of Harmonic Oscillation and the Pulse Transfer Function analysis. The solution of the aeroelastic equation in frequency domain is obtained by representing the equation in a finite-state form through the modal approach using Lagrange’s equation. The flutter boundary is obtained by solving this equation using the classical U-g method and root locus analysis. Flutter analysis of a two degree-of-freedom , two-dimensional typical wing sections with nonlinear torsional springs are studied. The aeroelastic responses of the system are obtained by integrating the nonlinear structural terms and aerodynamic terms simultaneously using Newmark-β and Wilson-θ methods. Flutter results obtained from both time integration and eigenvalue solutions are compared. These two results, in general, are in agreement. Flutter behavior of a simple three-dimensional swept wing model is also investigated. Comparison of the flutter boundary obtained by using the eigenvalue solution with flutter data from wind-tunnel experiments are made. / Ph. D.

The unsteady vortex-lattice method is used to calculate the pressure coefficients on thick and thin airfoils in steady and unsteady flowfields. The parameters which affect the results, such as time step and aspect ratio, are studied. The effects of Reynolds number and thickness of a wing in steady state and in oscillation are investigated. The present computed results for thick and thin wings are in close agreement with the experimental data. The numerical results obtained from a lifting-surface approximation are also in close agreement with the experimental data for a wing as thick as 18%. The lift and moment coefficients are affected by the thickness of a wing in oscillation and this effect is more noticeable for the moment coefficient. But to illustrate this it is necessary to go as high as 27% thickness. A wing in steady flight near a wavy surface, such as in the case of a large transoceanic wingship, is simulated by a wing oscillating in heave near a flat surface. In accord with the wingship, small aspect ratios and slight camber are considered. The numerical simulation predicts that the mean aerodynamic loads on a wing executing a simple-harmonic heaving motion are higher than the corresponding loads on the same wing in steady flight at the mean height and the same angle of attack. The increases are about the same for all heights. Hence, these preliminary results suggest that it would be beneficial to fly near the waves; that doing so would improve the aerodynamic efficiency. Also included in the present results are numerical simulation of the wakes that show the strong influences of the ground and the oscillations on their behavior. The unsteady vortex-lattice method is further used to investigate the effect of trailing vortices from a large leading wing on a trailing aircraft. The aerodynamic response of the trailing aircraft is examined by calculating the lift and drag forces and the pitch and roll moments. Furthermore, the aerodynamic response and the behavior of the wakes of the crossing wings are investigated. / Ph. D.

An investigation was conducted in the Langley 8-foot transonic tunnels to determine the aerodynamic loading characteristics of a thin conical cambered low-aspect-ratio delta wing in combination with a basic body and a body indented symmetrically for a Mach number of 1.2 in accordance with the supersonic area rule. The tests were conducted at Mach numbers from 0.60 to 1.12 and at 1.43 and at angles of attack generally from -4° to 20°. The wing vas conically cambered over the outboard 15 percent of each semispan. The wing had an aspect ratio of 2.31, 60° sweepback of the leading edge, and had NACA 65A003 airfoil sections parallel to the model plane of symmetry over the uncambered portion. The results of this investigation indicate that a leading-edge separation vortex forms at moderate angles of attack and causes the shape of the span load distribution to change markedly. Significant center of pressure movements are noted at transonic speeds. Indenting the body in accordance with the supersonic area rule had little effect on the aerodynamic loading characteristics. Comparisons with expert mental data for a similar plane wing indicates that the cambered wing is considerably more effective than the plane wing in utilizing the leading edge suction forces to produce thrust. A comparison between experimental and theoretical results indicates fair agreement around sonic speeds. / Master of Science

no abstract provided by author / Master of Science

An investigation to determine the low-speed lateral-directional characteristics of a generic 74-degree delta wing-body configuration employing the latest generation, gothic planform vortex flaps has been conducted. In addition, the theoretical estimates from VORSTAB were compared against experimental data to aid in documenting this new method. VORSTAB is an extension of the Quasi-Vortex-Lattice Method of Lan which empirically accounts for vortex breakdown effects in the calculation of longitudinal and lateral-directional aerodynamic characteristics. The experimental results indicated that leading-edge deflections of 30 and 40 degrees significantly reduce the magnitude of the wing effective dihedral relative to the baseline for a specified angle of attack or lift coefficient. For angles of attack greater than 15 degrees, these flap deflections reduce the configuration directional stability despite improved vertical tail effectiveness. Asymmetric leading edge deflections are shown to be inferior to conventional ailerons in generating rolling moments. Asymmetric leading-edge deflections are effective in producing side force at moderate to high angles of attack. VORSTAB lateral-directional calculations provide ballpark estimates at low to moderate angles of attack. The theory does not account for vortex flow induced, vertical tail effects at high angles of attack and should not be used for this angle of attack region. The empirical formulae for predicting vortex burst effects are not reliable in their present form. Although the basic trends are correct, the magnitude of the predicted vortex burst effect is typically over-estimated. / Master of Science

A computational model is presented which predicts the force, stroke, and energy needed to overcome aerodynamic loads encountered by morphing wings during aircraft maneuvers. This low-cost model generates wing section shapes needed to follow a desired flight path, computes the resulting aerodynamic forces using a unique combination of conformal mapping and the vortex panel method, computes the longitudinal motion of the simulated aircraft, and closes the loop with a zero-error control law. The aerodynamic force prediction method has been verified against two more expensive codes. This overall model will be used to predict the performance of morphing wings and the requirements for the active material actuators in the wings. / Master of Science

A series of short stiffened panel designs which may be applied to a preliminary design assessment of an aircraft wing rib is presented. The computer program PASCO is used as the primary design and analysis tool to assess the structural efficiency and geometry of a tailored corrugated panel, a corrugated panel with a continuous laminate, a hat stiffened panel, a blade stiffened panel, and an unstiffened nat plate. To correct some of the shortcomings in the PASCO analysis when shear is present a two-step iterative process using the computer program VICON is used. The loadings considered include combinations of axial compression, shear, and lateral pressure. The loading ranges considered are broad enough such that the designs presented may be applied to other stiffened panel applications. An assessment is made of laminate variations, increased spacing. and non-optimum geometric variations, including a beaded panel. on the design of the panels. / Master of Science

The aerodynamic-structural-control design of a simplified wing and a forward-swept composite wing are studied. In the first example, the wing is modeled as a beam with a control surface near the wing tip. The torsional stiffness is the only physical property varying along the span. The aerodynamic model is based on strip theory, and the control model is based on output feed-back control. With the structural-control interaction being the main focus, two different approaches are taken for the simplified wing design: (1) a sequential approach, (2) an integrated approach. In each approach the wing is designed for minimum weight subject to divergence and control deflection constraints. The results of this study indicated that while the integrated approach produced a better design than the sequential approach, the difference was minimal. In the second example, a forward-swept composite wing is designed for a high subsonic transport aircraft. The structural analysis is based on finite-element method. The aerodynamic calculations are based on vortex-lattice method, and the control calculations are based on output feed-back control. The wing is designed for minimum weight subject to structural, aerodynamic/performance and control constraints. Efficient methods are used to calculate the control deflection and efficiency sensitivities which appear as second order derivatives in the control constraint equations. To suppress the aeroelastic divergence of the forward-swept wing, and to reduce the gross weight of the design aircraft, two separate cases are studied: (1) combined application of aeroelastic tailoring and active controls, (2) aeroelastic tailoring alone. The results of this study indicated that, for this particular example, aeroelastic tailoring is sufficient for suppressing the aeroelastic divergence, and the use of active controls was not necessary. / Ph. D.

System reliability based design of aircraft wings is studied. A wing of a light commuter aircraft designed according to the FAA regulations is compared with one designed by system reliability optimization. Both the level III, and the advanced first order, second moment (AFOSM) method are employed to evaluate the probability of failure of each failure element of the system representing the wing. In the level III method the statistical correlation between failure modes is neglected. The AFOSM method allows to evaluate the sensitivity derivatives of the system safety index analytically. Furthermore, it accounts for the statistical correlation between failure modes. The results demonstrate the potential of stochastic optimization, and the importance of accounting for the statistical correlation between failure modes. Finally, it is shown that the problem associated with discontinuity of sensitivity derivatives, encountered when using second order Ditlevsen upper bounds to estimate the system failure probability, is circumvented if a penalty function method is used for optimization. / Ph. D.

Steady, inviscid, supersonic flow past conical wings is studied within the context of irrotational, nonlinear theory. An efficient numerical method is developed to calculate cones of arbitrary section at incidence. The method is fully conservative and implements a body conforming mesh generator. The conical potential is assumed to have its best linear variation inside each cell; a secondary interlocking cell system is used to establish the flux balance required to conserve mass. In regions of supersonic cross flow, the discretization scheme is desymmetrized by adding the appropriate artificial viscosity in conservation form. The algorithm is nearly an order of magnitude faster than present Euler methods. It predicts known results as long as the flow Mach numbers normal to the shock waves are near 1; qualitative features, such as nodal point lift-off, are also predicted correctly. Results for circular and thin elliptic cones are shown to compare very well with calculations using Euler equations. This algorithm is then implemented in the design of conical wings to be free from shock waves terminating embedded supersonic zones adjacent to the body. This is accomplished by generating a smooth cross-flow sonic surface by using a fictitious gas law that makes the governing equation elliptic inside the cross-flow sonic surface. The shape of the wing required to provide this shock-free flow, if such a flow is consistent with the sonic surface data, is found by solving the Cauchy problem inside the sonic surface using the data on this surface and, of course, the correct gas law. This design procedure is then demonstrated using the simple case of a circular cone at angle of attack.