Recent research trends have indicated an interest in High-Altitude, Long-Endurance (HALE) aircraft as a low-cost alternative to certain space missions, such as telecommunication relay, environmental sensing and military reconnaissance. HALE missions require a light vehicle flying at low speed in the stratosphere at altitudes of 60,000-80,000 ft, with a continuous loiter time of up to several days. To provide high lift and low drag at these high altitudes, where the air density is low, the wing area should be increased, i.e., high-aspect-ratio wings are necessary. Due to its large span and lightweight, the wing structure is very flexible. To reduce the structural deformation, and increase the total lift in a long-spanned wing, a sensorcraft model with a joined-wing configuration, proposed by AFRL, is employed. The joined-wing encompasses a forward wing, which is swept back with a positive dihedral angle, and connected with an aft wing, which is swept forward. The joined-wing design combines structural strength, high aerodynamic performance and efficiency.
The results of the simulation of the complex, three-dimensional flow past the joined-wing of a HALE aircraft are used as an input for the structural analysis. The Reynolds-Averaged Navier-Stokes (RANS)-based flow solver, COBALT, provided detailed flow results for altitudes 30,000 ft and 60,000 ft for the cases with M=0.4 and α = 0°, M = 0.6 and α = 0°, and M = 0.6 and α = 12°. The surface static pressure from the flow analyses comprises the load transferred to the structural models developed in this study.
To date in the existing studies, only simplified structural models have been examined. In the present work, a semi-monocoque structural model is developed. All stringers, skin panels, ribs and spars are represented by appropriate elements in a finite-element model. Also, the model accounts for the fuel weight and sensorcraft antennae housed within the wings. Linear and nonlinear static analyses under the aerodynamic load are performed. Design optimization is performed to achieve a fully stressed design. The shell elements thickness and stringers cross-sectional area are properly resized to obtain a structure that meets the allowable stress in each element and is minimum weight. In addition to the stress constraints, deflection constraints are also imposed in the design optimization. As the joined-wing structure is prone to buckling, after the design optimization is complete linear and nonlinear bucking analyses are performed to study the global joined-wing structural instability, the load magnitude at which it is expected to occur, and the buckling mode. As this design and analysis study is aimed towards developing a realistic structural representation of the innovative joined-wing configuration, in addition to the global, or upper-level optimization, a local level design optimization is performed as well. At the lower (local) level detailed models of wing structural panels are used to compute more complex failure modes and to design details not included in the upper (global) level model. Proper coordination between local skin-stringer panel models and the global joined-wing model prevents inconsistency between the global and local level models.