Ilhan Tuzcu

Unified Theory for the Dynamics and Control of Maneuvering Flexible Aircraft

This work represents a new paradigm for the dynamics and control of maneuvering flexible aircraft. Using the system concept, the theory integrates seamlessly all the necessary material from the areas of analytical dynamics, structural dynamics, aerodynamics, and controls. It includes automatically all six rigid-body degrees of freedom and elastic deformations, as well as the gravity, propulsion, aerodynamic, and control forces, in addition to forces of an external nature, such as gusts. The seamless integration is achieved by using the same reference frame and the same variables to describe the aircraft motions and the forces acting on it, including the aerodynamic forces. The formulation is modular in nature, in the the sense that the structural model, the aerodynamic theory, and the controls method can be replaced by any other ones to better suit different types of aircraft, provided certain criteria are satisfied. A perturbation approach permits the separation of the equations of motion into a flight dynamics problem for the maneuvering aircraft rigid-body translations and rotations and an extended perturbation problem for the elastic deformations and perturbations in the rigid-body variables, where the second problem is subject to inputs from the maneuvering aircraft

The Lure of the Mean Axes

A variety of aerospace structures, such as missiles, spacecraft, aircraft, and helicopters, can be modeled as unrestrained flexible bodies. The state equations of motion of such systems tend to be quite involved. Because some of these formulations were carried out decades ago when computers were inadequate, the emphasis was on analytical solutions. This, in turn, prompted some investigators to simplify the formulations beyond all reasons, a practice continuing to this date. In particular, the concept of mean axes has often been used without regard to the negative implications. The allure of the mean axes lies in the fact that in some cases they can help decouple the system inertially. Whereas in the case of some space structures this may mean complete decoupling, in the case of missiles, aircraft, and helicopters the systems remain coupled through the aerodynamic forces. In fact, in the latter case the use of mean axes only complicates matters. With the development of powerful computers and software capable of producing numerical solutions to very complex problems, such as MATLAB and MATHEMATICA , there is no compelling reason to insist on closed-form solutions, particularly when undue simplifications can lead to erroneous results.

Effects of Flexibility on the Stability of Flying Aircraft

Traditionally, flying aircraft have been treated within the confines of flight dynamics, which is concerned, for the most part, with rigid aircraft. On the other hand, flexible aircraft fall in the domain of aeroelasticity. In reality all aircraft possess some measure of flexibility and carry out rigid body maneuvers, so that the question arises as to whether rigid body motions and flexibility in combination can affect adversely the stability of flying aircraft. This paper addresses this question by solving the eigenvalue problem for the following three cases: (i) the flight dynamics of a flexible aircraft regarded as rigid and whose perturbations about the flight path are controlled by feedback control, (ii) the aeroelasticity of a corresponding flexible aircraft prevented from undergoing rigid body translations and rotations, and (iii) the control of the actual flexible aircraft using the control gains derived in the first case by regarding the aircraft as rigid. This investigation demonstrates that it is not always safe to treat separately rigid body and flexibility effects in a flying flexible aircraft.

Control of flying flexible aircraft using control surfaces and dispersed piezoelectric actuators

The flight of relatively stiff aircraft can be adequately controlled by standard means, i.e., by the engine throttles and control surfaces. When the flexibility becomes a factor, the standard controls may not be sufficient. In several earlier papers, the authors have addressed the problem of dynamics and control of maneuvering flexible aircraft. Using a perturbation approach they separated the problem into a quasi-rigid flight dynamics problem for the flight variables, which tend to be large, and an extended perturbation problem for the perturbations in the flight variables and the elastic vibration, which tend to be small, where the second problem receives input from the first. It was suggested in the earlier papers that, in addition to the standard controls, the controls for the extended perturbation problem include actuators whose task is to control the vibration. In this paper, the possibility of controlling the vibration by means of piezoelectric actuators dispersed over the flexible structural components, and in particular over the wing and empennage, is explored. It is concluded that piezoelectric actuators can be effective in damping out vibration if adequate power sources can be provided. This is the first time that the feasibility of piezoelectric actuators has been investigated in a flight environment.

Control of flexible aircraft executing time-dependent maneuvers

A new generation of aircraft, namely, unmanned aerial vehicles (UAVs), and, in particular, autonomous UAVs, are expected to carry out very critical maneuvers, well in excess of what pilots are able to tolerate. A newly developed theory for the dynamics and control of maneuvering flexible aircraft is ideally suited for the analysis and design of such aircraft. The state equations for maneuvering flexible aircraft are nonlinear and of high dimension. With due consideration to the way aircraft are flown, the problem can be separated into one for the generally large aircraft motions on a given flight path and another one for small perturbations from the flight path and elastic vibration, where the second problem receives inputs from the first. The case in which the flight path represents a time-dependent maneuver, so that the system of perturbation equations is time varying is studied. Several developments designed to facilitate the treatment of time-varying systems are provided. In particular, included are 1) an explicit derivation of the matrices defining the state equations, 2) a new approach to the control of the perturbations from the flight path and the elastic vibration, and 3) expressions for the implementation of the controls in discrete time. Computer solutions for the system response can be carried out conveniently by the use of MATLAB® and MATHEMATICA. A numerical example illustrates the approach for the case of a flexible aircraft executing a pitch maneuver.