Introduction to Aircraft Aeroelasticity and Loads

22

Aeroelasticity and Loads Models

In this chapter, the building blocks that make up the aeroelastic and manoeuvre/gust load models will be introduced. Where possible, the models will be related to those used earlier in Part II of the book. Various comments on the requirements for structural and aerodynamic models are made for gusts and flutter in CS/AMC 25.341 and 25.629. Note that what is presented here is one way of developing suitable mathematical models for aeroelastics and loads, but other approaches could be used.

22.1 STRUCTURAL MODEL

22.1.1 Introduction

The basic mathematical model of the aircraft must be able to represent its vibration behaviour over the frequency range of interest, typically 0–40 Hz for large commercial aircraft and 0–60 Hz for small commercial aircraft. Thus the model will need to yield natural frequencies, modal masses and normal mode shapes in these frequency ranges. The model should adequately represent the aircraft complexity, including control surface and engine behaviour, and generate sufficiently accurate mode shapes.

22.1.2 Stiffness Model – ‘Beam-Like’ Representation

The traditional approach to determining a mathematical model for aircraft with fairly slender high aspect ratio wings was to recognize that the structure is ‘beam-like’ and then to represent the major aircraft components (e.g. wing, front fuselage, rear fuselage, tailplane, fin) by beams lying along reference axes positioned at, for example, the locus of shear centres (or flexural ...