Robust maneuvering envelope estimation based on reachability analysis in an optimal control formulation
This paper discusses an algorithm for estimating the safe maneuvering envelope of damaged aircraft. The algorithm performs a robust reachability analysis through an optimal control formulation while making use of time scale separation and taking into account uncertainties in the aerodynamic derivatives. Starting with an optimal control formulation, the optimization problem can be rewritten as a Hamilton-Jacobi-Bellman equation. This equation can be solved by level set methods. This approach has been applied on an aircraft example involving structural airframe damage. Monte Carlo validation tests have confirmed that this approach is successful in estimating the safe maneuvering envelope for damaged aircraft.
Dynamic Flight Envelope Assessment and Prediction
1. Summary This paper will discuss a three year research program being conducted by the Boeing Phantom Works, St. Louis, MO. This program addresses the NASA Integrated Resilient Aircraft Control (IRAC) Topic 1.3, Dynamic Flight Envelope Assessment and Prediction. The Dynamic Flight Envelope Assessment and Prediction system will provide NASA with flight safety enhancements through a process to identify on-line fault anomalies for a damaged air vehicle, and measure changes in structure dynamics mode shapes due to the damage. The system also provides adaptive control to maintain the damaged aircraft within allowable structure limits and suppress any adverse, dynamic structural mode interaction with the flight control system. The primary focus of this paper is on ControlCentric Modeling, combining rigid body models with dynamic flexible structure and loads models to provide the basis for on-line adaptive control of the structure load properties of the aircraft. This system addresses structural anomalies caused by battle damage to military aircraft and structure failures to civil aircraft caused by incidents such as fatigue, composite structure delamination, corrosion, mid-air collisions, or improper assembly. Products will be algorithms and processes that can provide safe control of structures for damaged tactical and transport aircraft. The development approach builds on the NASA Intelligent Flight Control System technology, already demonstrated on a research F-15 for adaptive control of the aerodynamic capability of a damaged aircraft. This basis will be reviewed in the paper. The significance of this program is to add a control capability to prevent exceeding structure load limitations or dynamic interaction on an aircraft that experiences failures to structure elements, thereby increasing the safety and survivability of air vehicles. 2. Program Objectives The objectives of this program are to:
Damage initiation and propagation in composite shells subjected to impact
Abstract The paper investigates the impact-induced damage initiation and propagation for laminated composite shells under low-velocity impact. A three-dimensional eight-node non-conforming element is described to analyze interlaminar stress distribution, and progressive failure. The damage analysis is performed by using Tsai–Wu quadratic failure criterion, Tsai’s damage modes and an additional delamination formula at all Gaussian points. The damage modes considered are matrix cracking, fiber breakage and delamination. The progressive failure is expressed by reducing stiffness of the material at all failed Gaussian points. Special attention is centered on curved laminates. Impact energy threshold for the onset of damage and transient damage size are predicted for composite shells. The analyses of the flat and curved laminates are compared for discussing their different damage mechanism. In addition, the influence of the stacking sequence, the thickness and the radius of curvature on damage behavior of composite shells are studied.