Chapter 1
Introduction
Standard aircraft design processes have traditionally been divided in the following stages []: * Feasibility studies: design needs to satisfy prescribed performance requirements along with other geographical and cost constraints, * Conceptual design: design on paper or on computer, involves analysis tools covering performance evaluation in different maneuvers, * Preliminary design: laying of proper moulds for building airframes, * Prototype building: finally arriving at a scaled down prototype, * Wind tunnel testing: on the developed prototype for aerodynamics database generation, * Flight testing: * Stability and control augmentation * Certification: requires an airplane to satisfy certain handling and flying qualities requirements.
This design process thus have nearly always left the stability and dynamics aspects to the end, where six degrees of freedom of motion are used to evaluate flying and handling qualities of aircraft. Finally, if these criteria do not match with the required satisfaction level of pilot or the industry standards, design of control systems are needed. To our knowledge (due to proprietary nature of the trade) and based on text book information, no analysis tools has been developed in the past for aircraft design which uses six degree of freedom equations of rigid aircraft motion right from the beginning. This is primarily due to the multi-objective iterative nature of the problems one needs to handle in order to arrive at an optimal configuration and also due to unavailability of several design parameters. With the development of aerodynamic database (DATCOM) and availability of analytical/empirical relations in terms of airplane airframe parameters, it becomes now feasible to use aircraft equations of six dof motion right from beginning of conceptual design phase of aircraft. There are numerous advantages of such methods, such as: * A common tool for all sizes of vehicle prototying, * Inclusion of flight and handling qualities criteria in this phase along with performance criteria, * Use of same model for stability augmentation, if needed, * Finally same model can be used for building autonomous capabilities for the vehicle and flight testing, and * The last but not the least, all peripherals of the aircraft conceptual design procedure covered using a single code.
Bifurcation analysis and continuation theory methodology (BACTM) has been used in the past for prediction of aircraft nonlinear motion [], for controlled maneuvering of aircraft into high-angle-of-attack flight regimes to investigate nonlinear flight phenomena [], for recovery of aircraft from nonlinear flight conditions, and for control protoptying to develop different maneuvering strategies of aircraft. All the above tasks make use of availability of extensive aerodynamic database for a particular airplane and a continuation technique. A continuation algorithm such as AUTO2000 solves all possible steady states of a given aircraft model as functions of a varying control parameter and also simulataneously computes stability of these steady states. Multiple steady states emerging as a result of bifurcations are also computed by AUTO2000. This results in a global map for aircraft dynamics, i.e. chart of all possible motions that an aircraft may exhibit in flight within its control deflection limits. Recently, development of a constrained bifurcation analysis (CBA) procedure has made it possible to carry out stability and performance evaluation of aircraft in constrained flight conditions, thus moving one step closer to the aircraft design process. The methodology proposed in this paper makes use of the CBA based numerical continuation procedure and availability of off-the-shelf aerodynamic models (DATCOM) written as analytical expressions in terms of aircraft design (geometric and location) parameters. Constrained bifurcation analysis of aircraft model (six dof equations) is carried out in flight conditions, such as, straight level flight with respect to some design parameters to arrive at an optimal configuration.
The paper is organized as follows. In section II, the aircraft model is described. An introduction to bifurcation analysis and continuation technique methodology (BACTM) is given in section III. In section IV, test cases are considered with few example results. A discussion on results is presented in section V with important highlights. Conclusions follow in section VI.
II The Aircraft Model
III Bifurcation Analysis and Continuation Theory Methodology
IV Design Example Problems
V Results and Discussions
VI Conclusions
VII References
Appendix