In this paper, flexible aircraft with strut-braced wings are modeled by using a low-order strain-based nonlinear beam formulation with free rigid-body motions, which is coupled with a 2-D finite-state unsteady aerodynamics. Since strain variables are independent degrees of the beam formulation, the displacement constraints between the main wings and bracing members are considered using Lagrange multipliers. The complete equations of motion are derived by following Hamilton's Principle and discretized into constant-strain elements. The strain-based aeroelastic formulation with the inter-member displacement constraints is flexible and allows modeling and analysis of arbitrary aircraft platforms. Specific focus of the current study is placed on exploring the impact of the spanwise joint location between the main wings and struts on the aircraft's aeroelastic characteristics. The aeroelastic formulation and parametric studies presented in this paper provide a solid ground for further platform and structural design optimization, as well as the control synthesis of flexible aircraft with strut-braced wings, subject to aeroelastic and flight dynamic constraints.
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