Flight dynamics nonlinearity assessment across a new aerodynamic attitude flight envelope.

摘要

A new asymmetric level aerodynamic attitude flight envelope is introduced in this dissertation. The aerodynamic attitude envelope is an angle of attack vs. sideslip angle region which describes the extent of where an aircraft can sustain a steady slipping horizontal flight condition. The new envelope is thus an extension of the more common speed-altitude symmetric level flight envelope. This new envelope can be used for design requirements, dynamic analysis, control synthesis, or performance comparison. Moreover, this envelope provides enhanced insight into trimability-controllability limitations within the aircraft design model. The aerodynamic attitude flight envelope is constructed for a six degree of freedom nonlinear simulation model of a high-performance aircraft. The constructed envelope is found to be asymmetric with respect to positive vs. negative sideslip values, due to the inherent asymmetry in the aerodynamic model database. Asymmetry and offset in the force and moment coefficient data could originate from experimental error, from model fabrication imperfections, from vortex-dominated flow, from data reduction flaws, or from other sources. The literature shows that vortex-dominated flow can cause significant side force, rolling moment, and yawing moment coefficient asymmetries. Details concerning the removal of asymmetry and offset in the aerodynamic data are given. The purpose behind removing the asymmetry and offset is to facilitate analysis of the new aerodynamic attitude flight envelope with an ideal aircraft model so that fundamental relationships can be more easily observed, and to provide a comparison with the non-ideal case previously computed. Based on the adapted and symmetrized aerodynamic data, a new aerodynamic attitude asymmetric level flight envelope is constructed and introduced.;Further, the six degree of freedom aircraft simulation model is analyzed with nonlinear index theory across this nontraditional flight envelope. Aircraft dynamic properties often change in a nonlinear way across operating conditions. Nonlinear index theory provides a new concept for measuring the strength of these changes for a given set of coordinates and is applied to the asymmetric aerodynamic attitude envelope with the original and the adapted and symmetrized aerodynamic data. This analysis provides new methodology and new insights into aircraft dynamics and control. The index analysis exposes certain flight condition regions in which nonlinearity strength is high. These regions are further investigated with both linear and nonlinear simulations. Because the nonlinearity index is based on the matrix two-norm, the index can sometimes overestimate the nonlinear strength. To circumvent this behavior, indices based on system matrix partitions and normalized state formulations are explored. Nonlinearity is a function of the coordinates used to express the dynamic system. Therefore, the nonlinearity index is also applied to the aircraft model, expressed with three different frame of reference sets for kinetics and kinematics, in order to determine the best, or most linear, coordinates among the three investigated sets.