Conceptual Aircraft Design

10Aircraft Weight and Centre of Gravity Estimation

10.1 Overview

An aircraft must ascend to heights by defying gravity and sustaining the tiring task of cruise – naturally, this is weight‐sensitive. Anyone who has climbed a hill knows about this experience, especially if one has to carry baggage. An inanimate aircraft is no exception; its performance suffers by carrying unnecessary mass (i.e. weight). At the conceptual design stage, aircraft designers have a daunting task of creating a structure not only at a low weight but also at a low cost, without sacrificing safety. Engineers also must be accurate in weight estimation well ahead of manufacture. This chapter presents a formal method to predict an aircraft and its component mass (i.e. weight), which results in locating the centre of gravity (CG) during the conceptual design phase. The aircraft inertia estimation is not within the scope of this book, except finding the area moment of inertia in Chapter 19.

In the past, in the UK and the USA, aircraft mass was expressed in foot pound system (FPS) units in pounds (lb_m) and weight in the same pound terminology (lb_f), a product of acceleration due to gravity and mass. This may create some confusion to those who are starting to use them for the first time. With the use of kg as mass in the System Internationale (SI), the unit for weight is a newton, which is calculated as the mass multiplied by gravitational acceleration (9.81 m s⁻²). This book uses both the FPS and SI systems; sometimes ...