Date of Award

Spring 2013

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Mechanical & Aerospace Engineering

Program/Concentration

Aerospace Engineering

Committee Director

Brett Newman

Committee Member

Ashraf Omran

Committee Member

Colin Britcher

Call Number for Print

Special Collections; LD4331.E56 T365 2013

Abstract

This thesis explores the application of nonlinearity index theory to the nature of stalling and spinning aircraft motions. Nonlinear dynamic phenomena related to stall and spin have been a concern to flight engineers and pilots for generations. Considerable past work has been directed to detect and understand the cause and nature of these phenomena. Methods such as bifurcation analysis, Volterra expansion series, and nonlinear simulation are a few of the nonlinear approaches developed to study this topic. Traditionally, however, linear analysis is the most widely used approach for studying aircraft dynamical behavior under a given set of acceptable conditions. This approach fails, however, when the generated experimental or computational model does not behave in the assumed linear manner, and in some cases may be incapable of describing stall/spin motions. The nonlinearity index technique provides a systematic way by which the nonlinearity of the dynamic system under consideration can be qualitatively and quantitatively sought. The original theory as applied to the initial value problem is presented here in the context of input excitation which is in inherent to flight mechanics. This concept is applied to the simplified, but nonlinear, models of the T-2C Buckeye trainer aircraft and the F-18 High Alpha Research Vehicle experimental aircraft undergoing stall or spin. This strength index is computed over a range of equilibrium conditions to detect potential flight conditions where nonlinearities are likely significant. These conditions are then further investigated via nonlinear simulation to gain additional understanding into the nonlinear dynamics. Results here show that the nonlinearity strength index is an effective and reliable measure for detecting nonlinear stall and spin motions including limit cycles and chaotic responses. Cases are discussed where the generated nonlinear simulation can pass through the nonlinear region and not reveal the underlying phenomenon, while the index technique clearly identifies the given behavior. Finally, traditional linear analysis techniques are considered which reveal that, in some cases, misleading or incorrect conclusions can be made, indicating that the nonlinearity index could be used to assess when linear analysis is appropriate or when a more sophisticated nonlinear approach is required.

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DOI

10.25777/xtb7-4q72

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