Roberto Gil Annes da Silva

Investigation on Transonic Correction Methods for Unsteady Aerodynamics and Aeroelastic Analyses

This paper presents an expedient transonic correction technique to compute unsteady pressure distributions and aeroelastic stability in the transonic flow regime. The transonic correction procedure here is an improvement of the downwash weighting method proposed previously by several authors. The previous downwash weighting methods could provide pressure and/or force corrections to some extent by applying different weighting methods on the lifting-surface self-induced downwash resulting from aeroelastic structural displacements or prescribed motions. However,the resulting pressure/force solutionswere often foundto be inconsistent, becausethey all failed toinclude the proper transonic unsteady and out-of-phase effects. Our improved downwash correction method is a rational formulation to include proper transonic effects, as this formulation is based on a successive kernel expansion procedure established in accord with the formal pressure-downwash relation. Accordingly, the developed transonic correction procedureisaproperandrational onethatis expectedtoyield moreconsistent aeroelasticsolutions.This procedure is now a fully developed program, known as the transonic weighting aerodynamic influence coefficient procedure in the ZAERO software system, or ZTAW. Computed results by ZTAW for the unsteady pressures and aeroelastic stability boundaries for four selected wing planforms (AGARD 445.6, F-5, LANN, Lessing wings) are found to be in good agreement with measured data. In contrast to the computational-fluid-dynamics-based methods of computational aeroelasticity, the present procedure is proven to be far more computationally efficient and industrially viable while yielding comparable aeroelastic solutions.

Sensitivity Study of Downwash Weighting Methods for Transonic Aeroelastic Stability Analysis

The paper is concerned with downwash correction methods for aeroelastic stability analyses in the transonic regime. A finite-difference Navier-Stokes code is used to calculate the unsteady aerodynamic loading due to dynamic angle-of-attack variations in three-dimensional transonic flow. The computed unsteady pressure coefficients are used as a reference state for flutter analyses using the downwash weighting method. The effects of the amplitudes of motion used in the calculation of nonlinear, unsteady reference pressures are addressed. The test case considered is the well-known AGARD wing 445.6 standard aeroelastic configuration. The configuration is subjected to rigid-body pitching oscillation about the midchord point at the root section. Flutter boundaries are computed using unsteady pressures, in the downwash correction methodology, as reference conditions to compute weighting operators. The results are compared with available experimental data and they indicate that the aerodynamic interference and viscous and thickness effects play an important role on the flutter prediction capability.

Navier-Stokes-based study into linearity in transonic flow for flutter analysis

A finite -difference Navier -Stokes code is used in order to study the linearity of aerodynamic loads with respect to the dynamic angle of attack in three dimensional transonic flow. Steady and unsteady pressure coefficients for prescribed rigid angle of attack motion are computed for a F -5 wing for which the method has been previously validated. The study is aimed at identifying the conditions under which approximate flutter analyses based on corrections to aerodynamic influence coefficients may be used. Results indicate that higher harmonics of the unsteady loads are present in the aerodynamic response and that the boundaries for linear behavior depend on the spanwise location along the wing.

Aircraft Control Based on Flexible Aircraft Dynamics

In this paper, the control law design for flexible aircraft is discussed. First, the traditional procedure of decoupling rigid-body and aeroelastic dynamics with low-pass and notch filters is addressed, with focus on controller performance as well as the resulting stability margin issues. A procedure based on a unified formulation of the flexible aircraft dynamics for flight control law design is proposed. In this procedure, the aeroservoelastic dynamics is assessed in the loop, and the offline filtering process is avoided. The formulation is applied to the virtual aircraft generic narrow-body airliner, with improvements in closed-loop performance and stability margins.

An Investigation on Modal Displacement Aerodynamic Effects on Downwash Weighting Methods for Transonic Flutter

The paper addresses further investigations on downwash correction methods for aeroelastic stability analyses in the transonic regime. The main concern is the investigation of the influence of the nature of the lifting surface motion, considering a general elastic body modal displacement to compute unsteady pressures. A finite-difference Navier-Stokes code is used to calculate the unsteady aerodynamic loads due to a three dimensional transonic flow. The unsteady pressure coefficients computed using this code are used as a reference state for flutter analyses based on a linearized aerodynamic theory using the downwash weighting method. The test case considered is the well-known AGARD wing 445.6 standard aeroelastic configuration. The results are compared with previous theoretical investigations.

Effects of the Aerodynamic Data in a MIMO System Identification Framework for Aeroelastic Analyses

The current paper is concerned with studying the effects of using different unsteady computational fluid dynamics data in order to generate the root locus for aeroelastic stability analysis. The dynamic system being considered in the present work is a NACA 0012 airfoil-based typical section in the transonic regime. The CFD calculations are based on the Euler equations and the code uses a finite volume formulation for general unstructured grids. A centered spatial discretization with added artificial dissipation is used, and an explicit Runge-Kutta time marching method is employed. Unsteady calculations are performed for several types of excitation on the plunge and pitch degrees of freedom of the dynamic system. These inputs are based on step and orthogonal Walsh functions. The use of system identification techniques is employed to allow the splitting of the aerodynamic coefficient time histories into the contribution of each individual mode to the corresponding aerodynamic transfer function. Such transfer functions are, then, interpolated and used in an aeroelastic stability analysis in the frequency domain. The present work compares the results provided for each case and attempts to contribute with guidelines for such analyses.