Session: 03-01 Mission-Adaptive Morphing UAVs

Paper Number: 171475

171475 - A Quick Aeroelastic Module for a Multidisciplinary Morphing Aircraft Design Software Package 

The research presented here details the development of a quick aeroelastic solver as part of a larger multidisciplinary morphing aircraft design software (SPARRO) for small Uncrewed Air Vehicles (UAVs) to understand which morphing arrangements are useful in various mission profiles. The solver detailed here can quickly estimate the flutter and divergence speeds from standard wing parameters: chord, span, taper ratio, sweep angle (sweeping at the mid-chord), airfoil type, material properties, and a battery mass distribution. The battery mass distribution can be arbitrary and allows exploration of different design configurations based on mission parameters. The module has two components – a simple FEA code that estimates the bending and torsional stiffnesses of the wing and the aeroelastic solution code. The aeroelastic component relies on an iterative solution of a fourth-order polynomial (based on so-called “simplified unsteady aerodynamics”) until an unstable root is found. Validation of the code was conducted in Altair HyperMesh and OptiStruct for a variety of example wings using the standard p-k method with shell elements for the structure and the doublet-lattice method for the aerodynamic loading. The discrepancies between validation and module results are discussed with potential mitigation options. Effects of the various parameters, such as taper ratio, sweep, and battery mass distribution on the flutter and divergence speeds are presented. The runtime of the code, as well as the effects of various parameters on the runtime are also discussed.

The wing used for validation has a span, chord, and skin thickness equal to 5.921 m, 0.5 m, and 1.5 mm, respectively. First, the structural mesh was created using second-order shell quad elements. The aerodynamic mesh was defined with 15 spanwise and 6 chordwise panels. Both meshes were connected essentially using a nearest-neighbor mapping for transferring loads and displacements between the meshes. Next, a table of approximate values for Mach numbers and reduced frequencies were entered and used for calculation of the aerodynamic influence coefficient matrices. As this is for small UAVs the density of air was assumed to be constant at sea level. The velocity sweep was from 60 m/s to 150 m/s for the aeroelastic p-k method. The solver input file is then exported and passed to OptiStruct. Flutter was then determined using plots of damping and frequencies  versus the velocity for the first bending and first torsional modes. As expected, and calculated by the developed aeroelasticity module, no divergence is predicted since none of the modes have zero frequency when the damping becomes positive (signaling instability). From the plots of  damping, flutter occurs in the torsional mode at around 103 m/s, which is extremely close to the SPARRO module prediction of 102.57 m/s. In the future it is planned to extend this analysis to tapered and swept-back wings. The goal is to provide better insight into the validity of the developed SPARRO code. The process of adjusting the SPARRO aeroelastic code to better match the new validation data will be started. Once that is complete, the aeroelastic code will be incorporated into the SPARRO framework which includes a full up CFD, CAD and mission capabilities.

Presenting Author: Yan Borden University of Michigan

Presenting Author Biography: Yan is a PhD candidate in Aerospace Engineering at the University of Michigan. He has a BS degree in Engineering Mechanics and Aerospace Engineering from the University of Wisconsin, Madison and an MS degree and Aerospace Engineering from the University of Michigan. He is in his 3rd year of his doctoral studies. He has worked on several projects for the Army Research Labs including on actuation for hypersonic vehicles using smart materials and on stability analysis for small morphing aircraft, the topic of this presentation.