High aspect-ratio flying wing configurations designed for high altitude, long endurance missions are characterized by high flexibility, leading to significant static aeroelastic deformation in flight, and coupling between aeroelasticity and flight dynamics. As a result of this coupling, an integrated model of the aeroelasticity and flight dynamics has to be used to accurately model the dynamics of the flexible flying wing. Such an integrated model of the flight dynamics and the aeroelasticity developed by Patil and Hodges is reviewed in this dissertation and is used for studying the unique flight dynamics of high aspect-ratio flexible flying wings. It was found that a rigid body configuration that accounted for the static aeroelastic deformation at trim captured the predominant flight dynamic characteristics shown by the flexible flying wing. Moreover, this rigid body configuration was found to predict the onset of dynamic instability in the flight dynamics seen in the integrated model. Using the concept of the mean axis, a six degree-of-freedom reduced order model of the flight dynamics is constructed that minimizes the coupling between rigid body modes and structural dynamics while accounting for the nonlinear static aeroelastic deformation of the flying wing. Multi-step nonlinear dynamic inversion applied to this reduced order model is coupled with a nonlinear guidance law to design a flight controller for path following. The controls computed by this flight controller are used as inputs to a time-marching simulation of the integrated model of aeroelasticity and flight dynamics. Simulation results presented in this dissertation show that the controller is able to successfully follow both straight line and curved ground paths while maintaining the desired altitude. The controller is also shown to be able to handle an abrupt change in payload mass while path-following. Finally, the equations of motion of the integrated model were non-dimensionalized to identify aeroelastic parameters for optimization and design of high aspect-ratio flying wings. / Ph. D.

Ph. D.

With a look towards the future, which involves a push towards utilizing renewable energy sources and cementing energy independence for future generations, the design of more efficient aircraft and novel energy systems is of utmost importance. This dissertation looks at leveraging some of the benefits offered by inflatable wings for use in tethered kite-like systems towards the goal of designing a High Altitude Aerial Platform (HAAP). Uses of such a system include Airborne Wind Energy Systems (AWES), among others. The key bene- fit offered by such wings is their lightweight construction and durability, but challenges to aerodynamic performance arise out of their flexible nature and non-standard airfoil profile. Studying the aerodynamic behavior of such wings forms the critical focus of this research. This effort primarily encompasses an experimental investigation of two swept, tethered, inflatable wings conducted in the Virginia Tech Stability Wind Tunnel, and numerical CFD computations of these wings. The experiment was conducted in the modular wall configuration of the anechoic test section at speeds ranging from 15 − 32.5 m/s for three different tether attachment configurations and wings constructed out of two different fabric materials. Along with static aeroelastic deformation data using a 3D photogrammetry system, aerodynamic measurements were taken in the form of Pitot and static pressure measurements in the wake of the wing, force and moment measurements at the base of the mount, and tension measurements at the tether attachment locations. This provides a data set for validating static aeroelastic modeling approaches for such a system and highlights the dramatic effect of the variability in test configuration on the wing's aerodynamics. In addition to the wind tunnel tests, 3D steady RANS CFD computations of the rigid 3D scanned inflatable wing geometry were conducted in the wind tunnel environment for these configurations to validate the CFD modeling approach and highlight the level of detail necessary to accurately characterize the wing aerodynamic performance. Static aeroelastic deformation data from the 3D photogrammetry system, at a speed of 27.5 m/s, were also used to deform the 3D scanned inflatable wing geometry, and RANS CFD computations of this deformed inflatable wing were conducted at a wind tunnel speed of 27.5 m/s. Several turbulence models were investigated and comparisons were made with the wind tunnel test data. Good agreement was found with experimental data for the forces and moments and wake Pitot pressure coefficient contours. Comparisons were also made with the rigid wing CFD computations at the same tunnel speed of 27.5 m/s to illustrate the effect of static aeroelastic deformations on the aerodynamic performance, wake Pitot pressure coefficient contours and wing-tip vortex structures, of these flexible inflated wings. In effect, this research utilizes the synergy be- tween wind tunnel experiments and numerical CFD computations to study the flow behavior over inflatable wings and provide a comprehensive verification and validation approach for modeling such complex systems. / Doctor of Philosophy / With a look towards the future, which involves a push towards utilizing renewable energy sources and cementing energy independence for future generations, the design of more efficient aircraft and novel energy systems is of utmost importance. This dissertation looks at leveraging some of the benefits offered by inflatable wings for use in tethered kite-like systems towards the goal of designing a High Altitude Aerial Platform (HAAP). Uses of such a system include Airborne Wind Energy Systems (AWES), among others. The key benefit offered by such wings is their lightweight construction and durability, but challenges to aerodynamic performance arise out of their flexible nature and non-standard airfoil profile. Studying the aerodynamic behavior of such wings forms the critical focus of this research. This effort primarily encompasses an experimental investigation of two swept, tethered, inflatable wings conducted in the Virginia Tech Stability Wind Tunnel, and computer simulations of the aerodynamic flow over these wings. The experiment was conducted in the modular wall configuration of the anechoic test section at speeds ranging from 15 − 32.5 m/s for three different tether attachment configurations and wings constructed out of two different fabric materials. Along with measurements of the wing deformations using a 3D photogrammetry system, aerodynamic measurements were taken in the form of pressure measurements in the wake of the wing, force and moment measurements at the base of the mount, and tension measurements at the tether attachment locations. This provides a data set for validating static aeroelastic modeling approaches for such a system and highlights the dramatic effect of the variability in test configuration on the wing's aerodynamics. In addition to the wind tunnel tests, detailed computer simulations of the scanned inflatable wing geometry were conducted in the wind tunnel environment for these configurations to validate the computational modeling approach and highlight the level of detail necessary to accurately characterize the wing aerodynamic performance. The wing deformation data from the 3D photogrammetry system, at a speed of 27.5 m/s, were also used to deform the scanned inflatable wing geometry, and computer simulations of this deformed inflatable wing geometry were conducted at a wind tunnel speed of 27.5 m/s. Good agreement was found between the experimental and computational forces and moments and wake Pitot pressure coefficient contours. Comparisons were also made with the undeformed wing computations at the same tunnel speed of 27.5 m/s to illustrate the effect of wing flexibility on the aerodynamic performance. In effect, this research utilizes the synergy between wind tunnel experiments and numerical CFD computations to study the flow behavior over inflatable wings and provide a comprehensive verification and validation approach for modeling such complex systems.

A technique to obtain the sensitivity of the static aeroelastic response of a three dimensional wing model is designed and implemented. The formulation is quite general and accepts any aerodynamic and structural analysis capability. A program to combine the discipline level, or local, sensitivities into global sensitivity derivatives is developed. A variety of representations of the wing pressure field are developed and tested to determine the most accurate and efficient scheme for representing the field outside of the aerodynamic code. Chebyshev polynomials are used to globally fit the pressure field. This approach had some difficulties in representing local variations in the field, so a variety of local interpolation polynomial pressure representations are also implemented. These panel based representations use a constant pressure value~ a bilinearly interpolated value, or a biquadratic ally interpolated value. The interpolation polynomial approaches do an excellent job of reducing the numerical problems of the global approach for comparable computational effort. Regardless of the pressure representation used, sensitivity and response results with excellent accuracy have been produced for large integrated quantities such as wing tip deflection and trim angle of attack. The sensitivities of such things as individual generalized displacements have been found with fair accuracy. In general, accuracy is found to be proportional to the relative size of the derivatives to the quantity itself. / Ph. D.

An analysis was presented for predicting lift and drag characteristics of flat-top wing-body combinations at supersonic speeds. These combinations consist of a wing mounted above an expanding body with their apexes being coincident. The assumptions with which the analysis was made are the following: 1. The linear theory was applicable. 2. The leading edge or the wing was coincident or ahead of the body shock. 3. Condition of zero base drag (static pressure at the base equal to the stream static pressure). The analysis was carried out by considering the individual terms which appear in the lift-drag relations separately, and utilizing the most recent theoretical methods to determine them. The analysis was applied to two flat-top wing-body combinations; namely, a semiconical body with an arrow planform wing, and a 3/4 power semibody with a diamond planform wing. For these combinations a free-stream Mach number of 3.35 satisfied the condition for the wing leading edge and the body bow shock to be coincident. To obtain a check on the analysis, the results were compared with experimental data at a Mach number of 3.35. / Master of Science

An experimental and computational investigation of a low-canard high-wing aircraft design has been conducted. The aircraft studied has a canard and wing of similar chord and airfoil section. The canard is approximately half the span of the main wing and both surfaces are untwisted and unswept. Canard incidence with respect to the zero angles of attack line is 4° and the main wing has an incidence of 1° and a dihedral of 3°. Force and moment data were obtained in two separate wind tunnel test entries in the VPI Stability Tunnel. The first of these entries were concerned with longitudinal characteristics while the second dealt primarily with lateral/directional characteristics. Flow visualization was also done in both sets of tests. Lift characteristics showed an apparent onset of stall and then a second rise in the lift curve. The aircraft displayed stable characteristics in both the longitudinal and lateral/directional cases. However, the pitch break at the onset of stall was unstable. The “double peak” lift curve as well as the unstable pitch break have been attributed to the canard tip-vortex interaction with the main wing. Test Reynolds numbers were 260,000 for the first set and 300,000 for the second series of tests. Computational cases were run using both an uncambered vortex lattice method and a general three-dimensional constant doublet and source panel method. Lift curve slope and static margin were obtained from the vortex lattice code and agree well with the experiment. All aerodynamic forces and moments were predicted by the doublet panel method PMARC. Longitudinal data was obtained using a symmetric 3200 panel model while lateral/directional data was taken using a 1600 panel model. Both the lift curve slopes and the pitching moment slopes compare well between the computational cases and the experimental data. The actual values for a given angle of attack, however, differ and remain unexplained. This is possibly due to either canard wing interaction effects, wind-tunnel-model manufacturing flaws, model mount or tunnel installation interference or a data reduction error. / Master of Science

Past research at this university has proven the feasibility of the multi-disciplinary design of a complex system involving the complete interaction of aerodynamics and structural mechanics. Critical to this design process, is the ability to accurately and efficiently calculate the sensitivities of the involved quantities (such as drag and dynamic pressure) with respect to the design variables. These calculations had been addressed in past research, but it was felt that insufficient accuracy had been obtained. The focus of this research was to improve the accuracy of these sensitivity calculations with a thorough investigation of the computational aspects of the problem. These studies led to a more complete understanding of the source of the errors that plagued previous results and provided substantially improved sensitivity calculations. Additional research led to an improvement in the aerodynamic-structural interface which aided in the accuracy of the sensitivity computations. Furthermore, this new interface removed discontinuities in the calculation of the drag which the previous model tended to yield. These improvements were made possible with the application of shape functions in surface deflection analysis, instead of the previous ‘zonal’ approach. Other factors which led to accuracy improvements were changes to the aerodynamic model and the paneling scheme. Final studies with the optimization process demonstrated the ability of the improved sensitivities to accurately approximate the design problem and provided useful results. Additional studies on the optimization process itself provided information on move limit restrictions and various constraint problems. / Master of Science

M.S.

The dissertation is focused on the large computational costs of integrated multidisciplinary design. Efficient techniques are developed to reduce the computational costs associated with integrated structural-aerodynamic design. First efficient methods for the calculations of the derivatives of the flexibility matrix and the aerodynamic influence coefficient matrix are developed. An adjoint method is used for the flexibility sensitivity, and a perturbation method is used for the aerodynamic sensitivity. Second a sequential optimization algorithm that employs approximate analysis methods is implemented. Finally, a modular sensitivity analysis, corresponding to the abstraction of a system as an assembly of interacting black boxes, is applied. This method was developed for calculating system sensitivity without modifying disciplinary black-box software packages. The modular approach permits the calculation of aeroelastic sensitivities without the expensive calculation of the derivatives of the flexibility matrix and the aerodynamic influence coefficient matrix. / Ph. D.

Parawing vehicles may have unusual values of many of the mass and aerodynamic factors affecting dynamic lateral stability and control. These unusual characteristics are due in large part to the fact that the center of gravity of parawing vehicles is located far below the parawing, whereas conventional aircraft usually have the vertical center-of-gravity location near the plane of the wings. The present thesis is an analytical investigation of the dynamic lateral stability and control of a typical parawing vehicle. The analysis was made using three-degree-of-freedom, rigid body equations of motion. Stability derivatives used in the calculations were obtained from static and dynamic force tests of a parawing model with rigid leading-edge and keel members. The analysis is treated mainly in terms of the effects of vertical center-of-gravity position, since this was found to be the most significant factor affecting the lateral stability and control of the hypothetical vehicle. / Master of Science