Files in this item
Files  Description  Format 

application/pdf Merrett_Craig.pdf (4MB)  (no description provided) 
Description
Title:  Aeroservoviscoelasticity theory: lifting surfaces, plates, velocity transients, flutter, and instability 
Author(s):  Merrett, Craig G. 
Director of Research:  Hilton, Harry H. 
Doctoral Committee Chair(s):  Hilton, Harry H. 
Doctoral Committee Member(s):  OstojaStarzewski, Martin; Bodony, Daniel J.; Bragg, Michael B. 
Department / Program:  Aerospace Engineering 
Discipline:  Aerospace Engineering 
Degree Granting Institution:  University of Illinois at UrbanaChampaign 
Degree:  Ph.D. 
Genre:  Dissertation 
Subject(s):  Aeroelasticity
aeroservoelasticity viscoelasticity aeroviscoelasticity velocity transients flutter panel flutter dynamic stability 
Abstract:  Modern flight vehicles are fabricated from composite materials resulting in flexible structures that behave differently from the more traditional elastic metal structures. Composite materials offer a number of advantages compared to metals, such as improved strength to mass ratio, and intentional material property anisotropy. Flexible aircraft structures date from the Wright brothers' first aircraft with fabric covered wooden frames. The flexibility of the structure was used to warp the lifting surface for flight control, a concept that has reappeared as aircraft morphing. These early structures occasionally exhibited undesirable characteristics during flight such as interactions between the empennage and the aft fuselage, or control problems with the elevators. The research to discover the cause and correction of these undesirable characteristics formed the first foray into the field of aeroelasticity. Aeroelasticity is the intersection and interaction between aerodynamics, elasticity, and inertia or dynamics. Aeroelasticity is well suited for metal aircraft, but requires expansion to improve its applicability to composite vehicles. The first is a change from elasticity to viscoelasticity to more accurately capture the solid mechanics of the composite material. The second change is to include control systems. While the inclusion of control systems in aeroelasticity lead to aeroservoelasticity, more control possibilities exist for a viscoelastic composite material. As an example, during the layup of carbonepoxy plies, piezoelectric control patches are inserted between different plies to give a variety of control options. The expanded field is called aeroservoviscoelasticity. The phenomena of interest in aeroservoviscoelasticity are best classified according to the type of structure considered, either a lifting surface or a panel, and the type of dynamic stability present. For both types of structures, the governing equations are integralpartial differential equations. The spatial component of the governing equations is eliminated using a series expansion of basis functions and by applying Galerkin's method. The number of terms in the series expansion affects the convergence of the spatial component, and convergence is best determined by the von Koch rules that previously appeared for column buckling problems. After elimination of the spatial component, an ordinary integraldifferential equation in time remains. The dynamic stability of elastic and viscoelastic problems is assessed using the determinant of the governing system of equations and the time component of the solution in the form exp (lambda t). The determinant is in terms of lambda where the values of lambda are the latent roots of the aeroservoviscoelastic system. The real component of lambda dictates the stability of the system. If all the real components are negative, the system is stable. If at least one real component is zero and all others are negative, the system is neutrally stable. If one or more real components are positive, the system is unstable. In aeroservoviscoelasticity, the neutrally stable condition is termed flutter. For an aeroservoviscoelastic lifting surface, the unstable condition is historically termed torsional divergence. The more general aeroservoviscoelastic theory has produced a number of important results, enumerated in the following list: 1. Subsonic panel flutter can occur before panel instability. This result overturned a long held assumption in aeroelasticity, and was produced by the novel application of the von Koch rules for convergence. Further, experimental results from the 1950s by the Air Force were retrieved to provide additional proof. 2. An expanded definition for flutter of a lifting surface. The legacy definition is that flutter is the first occurrence of simple harmonic motion of a structure, and the flight velocity at which this motion occurs is taken as the flutter speed. The expanded definition indicates that the flutter condition should be taken when simple harmonic motion occurs and certain additional velocity derivatives are satisfied. 3. The viscoelastic material behavior imposes a flutter time indicating that the presence of flutter should be verified for the entire life time of a flight vehicle. 4. An expanded definition for instability of a lifting surface or panel. Traditionally, instability is treated as a static phenomenon. The static case is only a limiting case of dynamic instability for a viscoelastic structure. Instability occurs when a particular combination of flight velocity and time are reached leading to growing displacements of the structure. 5. The inclusion of flight velocity transients that occur during maneuvers. Two and threedimensional unsteady incompressible and compressible aerodynamics were reformulated for a time dependent velocity. The inclusion of flight velocity transients does affect the flutter and instability conditions for a lifting surface and a panel. The applications of aeroservoviscoelasticity are to aircraft design, wind turbine blades, submarine's stealth coatings and hulls, and land transportation to name a few examples. One caveat regarding this field of research is that general predictions for an application are not always possible as the stability of a structure depends on the phase relations between the various parameters such as mass, stiffness, damping, and the aerodynamic loads. The viscoelastic material parameters in particular alter the system parameters in directions that are difficult to predict. The inclusion of servo controls permits an additional design factor and can improve the performance of a structure beyond the native performance; however overcontrol is possible so a maximum limit to useful control does exist. Lastly, the number of material and control parameters present in aeroservoviscoelasticity are amenable to optimization protocols to produce the optimal structure for a given mission. 
Issue Date:  20110525 
URI:  http://hdl.handle.net/2142/24148 
Rights Information:  Copyright 2011 Craig G. Merrett 
Date Available in IDEALS:  20110525 
Date Deposited:  201105 
This item appears in the following Collection(s)

Dissertations and Theses  Aerospace Engineering

Graduate Dissertations and Theses at Illinois
Graduate Theses and Dissertations at Illinois