Gradient-Based Optimization of Solar-Regenerative High-Altitude Long-Endurance Aircraft

In this paper we use gradient-based optimization to minimize the mass of a solar-regenerative high-altitude long-endurance (SR-HALE) flying-wing aircraft while accounting for nonlinear aeroelastic effects. We design the aircraft to fly year round at 35° latitude at 18km above sea level and subject the aircraft to energy capture, energy storage, material failure, local buckling, stall, longitudinal stability, and coupled flight and aeroelastic stability constraints. The optimized aircraft has an aspect ratio of 27:8, a surface area of 99:1m2, and a mass of 508:8 kg. Our results suggest that thick airfoils provide greater structural efficiency than increased carbon fiber reinforced polymer (CFRP) ply thicknesses. We also perform several parameter sweeps to determine sensitivity to altitude, latitude, battery specific energy, solar efficiency, avionics and payload power requirements, and minimum design velocity

Autonomous landing control of highly flexible aircraft based on Lidar preview in the presence of wind turbulence

This paper investigates preview-based autonomous landing control of a highly flexible flying wing model using short range Lidar wind measurements in the presence of wind turbulence. The preview control system is developed based on a reduced-order linear aeroelastic model and employs a two-loop control scheme. The outer loop employs the LADRC (linear active disturbance rejection control) and PI algorithms to track the reference landing trajectory and vertical speed, respectively, and to generate the attitude angle command. This is then used by the inner-loop using H∞ preview control to compute the control inputs to the actuators (control flaps and thrust). A landing trajectory navigation system is designed to generate real-time reference commands for the landing control system. A Lidar (light detection and ranging) simulator is developed to measure the wind disturbances at a distance in front of the aircraft, which are provided to the inner-loop H∞ preview controller as prior knowledge to improve control performance. Simulation results based on the full-order nonlinear flexible aircraft dynamic model show that the preview-based landing control system is able to land the flying wing effectively and safely, showing better control performance than the baseline landing control system (without preview) with respect to landing effectiveness and disturbance rejection. The control system’s robustness to measurement error in the Lidar system is also demonstrated

Technical Findings, Lessons Learned, and Recommendations Resulting from the Helios Prototype Vehicle Mishap

The Helios Prototype was originally planned to be two separate vehicles, but because of resource limitations only one vehicle was developed to demonstrate two missions. The vehicle consisted of two configurations, one for each mission. One configuration, designated HP01, was designed to operate at extremely high altitudes using batteries and high-efficiency solar cells spread across the upper surface of its 247-foot wingspan. On August 13, 2001, the HP01 configuration reached an altitude of 96,863 feet, a world record for sustained horizontal flight by a winged aircraft. The other configuration, designated HP03, was designed for long-duration flight. The plan was to use the solar cells to power the vehicle's electric motors and subsystems during the day and to use a modified commercial hydrogen-air fuel cell system for use during the night. The aircraft design used wing dihedral, engine power, elevator control surfaces, and a stability augmentation and control system to provide aerodynamic stability and control. At about 30 minutes into the second flight of HP03, the aircraft encountered a disturbance in the way of turbulence and morphed into an unexpected, persistent, high dihedral configuration. As a result of the persistent high dihedral, the aircraft became unstable in a very divergent pitch mode in which the airspeed excursions from the nominal flight speed about doubled every cycle of the oscillation. The aircraft s design airspeed was subsequently exceeded and the resulting high dynamic pressures caused the wing leading edge secondary structure on the outer wing panels to fail and the solar cells and skin on the upper surface of the wing to rip away. As a result, the vehicle lost its ability to maintain lift, fell into the Pacific Ocean within the confines of the U.S. Navy's Pacific Missile Range Facility, and was destroyed. This paper describes the mishap and its causes, and presents the technical recommendations and lessons learned for improving the design, analysis, and testing methods and techniques required for this class of vehicle

Nonlinear Aeroelastic Simulation of X-HALE: a Very Flexible UAV

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90708/1/AIAA-2011-1226-372.pd

Trajectory Control for Very Flexible Aircraft

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76453/1/AIAA-29335-906.pd

Simulation and optimization of takeoff maneuvers of very flexible aircraft

A generic framework for the simulation of transient dynamics in nonlinear aeroelasticity is presented that is suitable for flexible aircraft maneuver optimization. Aircraft are modeled using a flexible multibody dynamics approach built on geometrically nonlinear composite beam elements, and the unsteady aerodynamics on their lifting surfaces is modeled using vortex lattices with free or prescribed wakes. The open loop response to commanded inputs and external constraints is then fed into a Bayesian optimization framework, which adaptively samples the configuration space to identify optimal maneuvers. As a representative example, the proposed approach is demonstrated on a catapult-assisted takeoff. The specific modeling challenges associated to that problem are first discussed, including the effect of aircraft flexibility. An optimality measure based on ground clearance and wing root loads is then defined. It is finally shown that the link that ramp-length constraints introduce between acceleration, release speed, and wing root loads is the main driver in the optimal solution

X-HALE: A Very Flexible UAV for Nonlinear Aeroelastic Tests

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/83578/1/AIAA-2010-2715-581.pd

Dynamic Response of Highly Flexible Flying Wings

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77056/1/AIAA-2006-1636-998.pd

Technical Challenges Associated with In-Air Wingtip Docking of Aircraft in Forward Flight

Autonomous in-air wingtip docking of aircraft offers significant opportunity for system level performance gains for numerous aircraft applications. Several of the technical challenges facing wingtip docking of fixed-wing aircraft are addressed in this paper, including: close proximity aerodynamic coupling; mechanisms and operations for robust docking; and relative state estimation methods. A simulation framework considering the aerodynamics, rigid-body dynamics, and vehicle controls is developed and used to perform docking sensitivity studies for a system of two 5.5% scale NASA Generic Transport Model aircraft. Additionally, proof of- concept testing of a candidate docking mechanism designed to move the primary wingtip vortex inboard suggests the viability of such an approach for achieving robust docking

An Examination into the Defining Characteristics of Flexible Solar Aircraft Configurations Through Optimization

This paper examines the defining characteristics of various solar aircraft configurations through gradient-based multidisciplinary design optimization. We first present a general gradient-based solar aircraft optimization framework which accounts for nonlinear aeroelastic effects resulting from structural flexibility. We then apply this framework to several discrete SR-HALE aircraft geometric, structural, and propulsion system configuration choices to determine the defining characteristics of each configuration choice