High-fidelity Aeroelastic Analysis of Very Flexible Aircraft

HIGH altitude, long endurance (HALE) aircraft offer capabilities found nowhere else in the spectrum of machine flight, but their design and analysis offers unique challenges. From their observation altitude above 50,000 ft, HALE systems can relay telecommunication signals, perform intelligence, surveillance, and reconnaissance (ISR) functions covering large swaths of land for extended periods of time, and collect valuable data for atmospheric research. ISR capabilities form the primary mission for SensorCraft platforms – designed for at least 30 hours of on-station endurance at a range of 2000 nautical miles [1]. NASA’s ERAST [2] program gave rise to a family of all–wing aircraft designed with science missions in mind, which set multiple records for endurance and sustained high-altitude flight. DARPA’s Vulture [3] program pushes the endurance envelope farther, aiming to demonstrate an aircraft that can loiter for 5 years, between 60,000 and 90,000 ft, with a 1000lb payload. Aircraft meeting these performance requirements have characteristic design traits: (1) slender, high aspect ratio wings with thick airfoils, and (2) very low structural weight. Long, lightweight wings tend be very flexible, and even in trimmed flight, HALE aircraft are often deformed beyond the range in which small displacements can be assumed. At high altitude, the speed of sound is low relative to sea level, thus the Mach regime is higher. Since thick airfoils have a lower critical Mach number, these higher Mach flows can generate highly nonlinear aerodynamics in the form of shocks along the wing upper surface. Numerous researchers have tackled the challenges of HALE aeroelasticity. Van Schoor and von Flotow [4] were among the first who studied this problem. Using linear finite element analysis (FEA) and 2D unsteady strip theory aerodynamics, they pointed out that including flexible deformation modes has a dramatic effect on predicted aircraft stability. Several other studies on HALE wings have used low–order aerodynamics [5–14]. First, Patil, Hodges, and Cesnik authored a series of works [5, 6, 8] about the coupling of an exact intrinsic beam model with Peters’ finite–state aerodynamics [15] and its application to ∗Graduate Research Assistant, Department of Aerospace Engineering, AIAA Member, [email protected] †Professor, Department of Aerospace Engineering, AIAA Associate Fellow, [email protected]