Validation Studies for Nonlinear Aeroelastic Trim and Stability Analysis of Highly Flexible Aircraft Zahra Sotoudeh ∗ , Dewey H. Hodges † Georgia Institute of Technology, Atlanta, Georgia 30332-0150 and Chong-Seok Chang ‡ Advanced Rotorcraft Technology, Inc., Mountain View, California, 94043 High Altitude, Long Endurance (HALE) aircraft are highly flexible, requiring nonlin- ear aeroelastic analysis. NATASHA (Nonlinear Aeroelastic Trim and Stability of HALE Aircraft) is a computer program developed to analyze the aeroelastic behavior of HALE aircraft. The underlying formulation is based on the published geometrically-exact, fully intrinsic beam equations. This paper presents a wide range of results from NATASHA, comparing them with well known solutions of beam stability and vibration problems, ex- perimental data or results from well-established computer codes. Despite the simplicity of the underlying formulation, results obtained confirm the accuracy of the analysis and its suitability for conceptual and preliminary design of HALE aircraft. Nomenclature a Deformed beam aerodynamic frame of reference b Undeformed beam cross-sectional frame of reference ∗ Graduate Research Assistant, Daniel Guggenheim School of Aerospace Engineering. Student member, AIAA. Email: Zahra@Gatech.edu † Professor, Daniel Guggenheim School of Aerospace Engineering. Fellow, AIAA. Email: Dhodges@Gatech.edu ‡ Aerospace Engineer; member, AIAA. Email: chong@flightlab.com Presented at the International Forum of Aeroelasticity and Structural Dynamics, Seattle, Washington, June 22 – 24, 2009. Copyright c  2010 by Z. Sotoudeh, D. H. Hodges and C. Chang. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.