A METHOD FOR PREDICTING THE STABILITY CHARACTERISTICS OF AN ELASTIC AIRPLANE Volume I -FLEXSTAB Theoretical Description

摘要

A theoretical development of the FLEXSTAB system is presented. The development integrates the theoretical mechanics of a flexible body with a low frequency unsteady aero¬dynamic theory employing linear influence coefficients based on finite element approxima¬tions. The theoretical mechanics resolve the dynamics of a flexible aircraft into structural dynamics of free vibration modes superimposed on rigid body dynamics. This resolution is made using a mean reference frame for structural motions and leads to two important features of the FLEXSTAB system:one, a logical merger of quasi-steady and dynamic aeroelasticity through the residual flexibility approximation;and, two, a logical basis for incorporating into the analysis empirical, rigid aircraft aerodynamic data. The aerodynamic theory is applicable to subsonic and supersonic flow and multiple wing-body-tail-nacelle configurations. Aero¬dynamic influence coefficients are derived using a paneling scheme which lends itself to empir¬ical corrections. Finally, the theoretical aero- and structural dynamics are integrated, conserv¬ing energy of the system and thereby yielding equations of motion appropriate to stability evaluation. These equations are expressed for a steady, reference motion to determine trim and static stability. They are also expressed in terms of unsteady perturbations about the reference motion to determine dynamic stability by characteristic roots or by time histories following an initial perturbation or following penetration of a discrete gust flow field.