High-precision maneuvers at high angles-of-attack are not properly addressed by even the most advanced aircraft control systems. Here we present our control design procedure and indoor experimental results with a small fixed-wing autonomous glider which is capable of executing an aggressive high angle-of-attack maneuver in order to land on a perch. We first acquire a surprisingly accurate aircraft model through unsteady flight regimes from real kinematic flight data taken in a motion-capture arena. This model is then used in a numerical nonlinear (approximate) optimal control procedure which designs a feedback control policy for the elevator deflection. Finally, we report our experimental data demonstrating that this simple glider can exploit pressure drag to achieve a high-speed perching maneuver.

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Experiments in Fixed-Wing UAV Perching

Birds routinely execute post-stall maneuvers with a speed and precision far beyond the capabilities of our best aircraft control systems One remarkable example is a bird exploiting post-stall pressure drag in order to rapidly decelerate to land on a perch Stall is typically associated with a loss of control authority, and it is tempting to attribute this agility of birds to the intricate morphology of the wings and tail, to their precision sensing apparatus, or their ability to perform thrust vectoring Here we ask whether an extremely simple fixed-wing glider (no propeller) with only a single actuator in the tail is capable of landing precisely on a perch from a large range of initial conditions To answer this question, we focus on the design of the flight control system; building upon previous work which used linear feedback control design based on quadratic regulators (LQR), we develop nonlinear feedback control based on nonlinear model-predictive control and 'LQR-Trees' Through simulation using a flat-plate model of the glider, we find that both nonlinear methods are capable of achieving an accurate bird-like perching maneuver from a large range of initial conditions; the 'LQR-Trees' algorithm is particularly useful due to its low computational burden at runtime and its inherent performance guarantees With this in mind, we then implement the 'LQR-Trees' algorithm on real hardware and demonstrate a 95 percent perching success rate over 147 flights for a wide range of initial speeds These results suggest that, at least in the absence of significant disturbances like wind gusts, complex wing morphology and sensing are not strictly required to achieve accurate and robust perching even in the post-stall flow regime

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Robust post-stall perching with a simple fixed-wing glider using LQR-Trees.

This paper describes a new recursive algorithm for the approximation of time varying nonlinear aerodynamic models by means of a joint adaptive selection of the model structure and parameter estimation. This procedure is called Adaptive Recursive Orthogonal Least Squares (AROLS), and is an extension and modification of the classical Recursive Orthogonal Least Squares (ROLS). This algorithm is considered to be particularly useful for indirect fault tolerant flight control, making use of model based adaptive control routines. After the failure, a completely new aerodynamic model can be elaborated recursively with respect to structure as well as parameter values. The performance of the identification algorithm is demonstrated on some simulation data sets.

Online Aerodynamic Model Structure Selection and Parameter Estimation for Fault Tolerant Control

This paper focuses on full six degree-of-freedom (6-DOF) aerodynamic modeling of small UAVs at high angles of attack and high sideslip in maneuvers performed using large control surfaces at large deflections for aircraft with high thrust-to-weight ratios. Configurations such as this include many of the currently available propellerdriven RC-model airplanes that have control surfaces as large as 50% chord, deflections as high as 50 deg, and thrust-to-weight ratios near 2:1. Airplanes with these capabilities are extremely maneuverable and aerobatic, and modeling their aerodynamic behavior requires new thinking because using traditional stability derivative methods is not practical with highly nonlinear aerodynamic behavior and coupling in the presence of high propwash effects. The method described in this paper outlines a component-based approach capable of modeling these extremely maneuverable small UAVs in a full 6-DOF realtime environment over the full envelope that is defined in this paper to be the full ±180 deg range in angle of attack and sideslip. This method is the foundation of the aerodynamics model used in the RC flight simulator FS One. Piloted flight simulation results for four small RC/UAV configurations having wingspans in the range 826 mm (32.5 in) to 2540 mm (100 in) are presented to highlight results of the high-angle aerodynamics modeling approach. Maneuvers simulated include tailslides, knife edge flight, high-angle upright and inverted flight (“harriers”), rolling maneuvers at high angle (“rolling harriers”) and an inverted spin of a biplane (“blender”). For each case, the flight trajectory is presented together with time histories of aircraft state data during the maneuvers, which are discussed.

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Modeling full-envelope aerodynamics of small UAVs in realtime

The ability of birds to perch robustly and effectively is a powerful demonstration of the capabilities of nature's control systems. Their apparent robustness to gust disturbances is particularly remarkable because when the airspeed approaches zero just before acquiring a perch, the influence of aerodynamic forces, and therefore potentially the control authority, is severely compromised. In this paper we present a simplified closed-form model for a fixed-wing aircraft which closely agrees with experimental indoor perching data. We then carefully examine the LTV controllability along an optimized perching trajectory for three different actuation scenarios - a glider (no powerplant), a fixed propeller, and a propeller with thrust vectoring. The results reveal that while all three vehicles are LTV controllable along the trajectory, the additional actuators allow the perch to be more easily acquired with less control surface deflections. However, in all three cases, disturbances experienced just before reaching the perch cannot be effectively rejected.

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On the controllability of fixed-wing perching

The usefulness of mini unmanned aerial vehicles has been proven by several applications in the domain of reconnaissance, surveillance and search-and-rescue applications in dangerous areas. This paper describes the development of a xed-wing mini-UAV platform, the sensing and guidance and control systems for use in laboratory, indoor and outdoor ight testing. The systems developed are demonstrated with autonomous hover experiments on the UAV. Furthermore, an ecient way of modeling the dynamics of the vehicle is presented and supported with experimental data obtained during autonomous hover and high angle of attack ight experiments.

Flight modeling and experimental autonomous hover control of a fixed wing mini-UAV at high angle of attack

This paper presents the preliminary work done of a flight validation process for several existing and under-development metrics-driven adaptive flight control methods. This is key element in the larger NASA Intelligent Resilient Aircraft (IRAC) program, which seeks to increase overall aircraft safety through dramatic improvements in stability, maneuverability, and probability of safe landing under adverse conditions - including aircraft damage. This project enables innovations in adaptive control approaches to be validated in flight tests that include simulated and actual damage as part of the fundamental research elements of the IRAC program. This paper briefly describes the available equipment which is a key element in the project. Furthermore, a model structure is proposed in order to capture qualitatively the impact of damage on the dynamics.

In flight validation of adaptive flight control methods

This paper presents the preliminary work done for a flight validation process for several existing and under-development metrics-driven adaptive flight control methods. This is key element in the larger NASA Intelligent Resilient Aircraft (IRAC) program, which seeks to increase overall aircraft safety through improvements in stability, maneuverability, and probability of safe landing under adverse conditions including aircraft damage. This paper focuses on development of the simulation environment, which is necessary to test and develop the control architectures before flight testing. It proposes a fully nonlinear six degree of freedom dynamic model of the vehicle that captures qualitatively severe damage. Example damage cases are partial loss of wing surface, loss of control effectivenes, engine failures etc. This affects the dynamic model on three apsects: shift of center of gravity, change of aerodynamic coefficients and reduced controllability of the vehicle. We propose a model which structure combines the different aerodynamic contributions of the surfaces to capture modifications of the dynamics due to damage.

Hugo De Blauwe

Papers

Flight modeling and experimental autonomous hover control of a fixed wing mini-UAV at high angle of attack

In flight validation of adaptive flight control methods

Nonlinear modeling and simulation of small Unmanned Aerial Vehicle with substantial damage