NONLINEAR AEROELASTIC ANALYSIS OF JOINED-WING AIRCRAFT
Joined-wing aircraft are being designed for ‘sensorcraft’ configuration. Joined wings lead to multiple load paths and constraints. The focus of this work is to understand the nonlinear structural as well as aeroelastic behavior of joined wings. The paper presents a formulation for the nonlinear aeroelastic analysis of joined wings. Results are presented for two joined wings configurations. Overall, the joined wings are found to be stiffer than single wing configuration. The ‘non-planar’ joined wing is found to be stiffer than the ‘planar’ joined wing. The structural dynamic characteristics of the joined wings are also presented. The bending modes are similar to the cantilevered beam modes while the torsional vibrations are restricted to the unconstrained part of the main wing. The structural analysis showed negligible nonlinear effects for static deformation as well as for structural dynamics characteristics. KEY-WORDS: Jointed wing, wing defection.
In recent years there has been a push towards the design and development of uninhabited aerial vehicles (UAVs). UAVs are being designed for various missions including atmospheric sensing, border monitoring, military reconnaissance and combat (UCAV). It is expected that UAVs would not only take over multiple conventional roles in civilian as well as military service, but also undertake new unconventional missions in the future. One example of an unconventional a mission is the ‘sensorcraft.’ Sensorcraft is being designed for longrange, high-altitude, intelligence, surveillance and reconnaissance (ISR). Sensorcraft is a joined-wing design aimed at providing an unobstructed field of view around the vehicle.1 Weisshaar and Lee2 present the history of joined-wing aircraft. The paper also presents aeroelastic tailoring studies. Livne3 presents a literature survey of the present status of technical development. Aeroelastic stability and response are critical in the design of joined-wing aircraft. Joined-wing aircraft differ considerably from conventional aircraft. The wings are expected to be long and flexible leading to high deformation. Analysis of such wings may require geometrically nonlinear structural and aerodynamic analysis. The constraints imposed by the joint are likely to impose stress concentration. Additional buckling-like nonlinearity have also been reported.4 Analysis of joint mechanisms and its effect on the structure (local and global) is essential. Linear, cantilevered wing analysis tools, which are routinely used for conventional wings, cannot be used to analyze joined wings. The aeroelastic analysis and design of such joined-wing aircraft is the topic of this paper. PRESENT WORK
A nonlinear aeroelastic analysis methodology has been developed and is being implemented. The analysis is centered around a geometrically exact structural model for the dynamics of beam-like structures.5 This analysis accounts for large deformation of the wing and fuselage. The analysis has been coupled with various aerodynamic models to investigate the nonlinear aeroelastic behavior and flight dynamics of High Altitude, Long Endurance (HALE) aircraft.6, 7 Joint constraint equations are added to the nonlinear structural formulation. The structural analysis is then coupled with an unsteady, vortex-lattice aerodynamic model.8 THEORY
The structural formulation used in the present research is based on the mixed variational formulation for dynamics of moving beams.5 Equations of motion are generated by including the appropriate energies in a variational principle followed by application of calculus of variation. The final equations are presented here. By using simple shape functions, the mixed variational formulation leads to a set of coupled nonlinear differential equations in terms of the element nodal displacements (u) and rotations (ө), nodal internal forces (F)...
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 Hodges, D. H., “A Mixed Variational Formulation Based on Exact Intrinsic Equations for Dynamics of Moving Beams,” International Journal of Solids and Structures, Vol. 26, No. 11, 1990, pp. 1253 – 1273.
 Patil, M. J., Hodges, D. H., and Cesnik, C. E. S., “Nonlinear Aeroelasticity and Flight Dynamics of High-Altitude Long-Endurance Aircraft,” Journal of Aircraft, Vol. 38, No. 1, Jan. – Feb. 2001, pp. 88 – 94.
 Patil, M. J., Hodges, D. H., and Cesnik, C. E. S., “Limit Cycle Oscillations in High-Aspect-Ratio Wings,” Journal of Fluids and Structures, Vol. 15, No. 1, Jan. 2001, pp. 107 – 132.
 Hall, K. C., “Eigenanalysis of Unsteady Flows About Airfoils, Cascades, and Wings,” AIAA Journal, Vol. 32, No. 12, December 1994, pp. 2426–2432.
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