Aeroelasticity affects control but, in general, during the conceptual phase of the study, the aircraft is seen as a rigid body. The next phase takes into account the aeroelastic effects using an integral approach to fine-tune the control-surfaces design.

This chapter is not a definitive study of aircraft stability and control (see [1] through [4] for more details on the subject), but it qualitatively examines and provides an understanding of the geometrical arrangement of aircraft components that affect aircraft stability. The reason for discussing stability here is to provide experience through the use of statistics in shaping aircraft as early as possible so that, if necessary, fewer changes are required in subsequent design phases. This chapter presents a rationale for a designer’s experience and provides an opportunity to examine whether the final aircraft configuration reflects all other considerations at this stage of the design process. There are no changes in the worked-out examples.

Only the equations governing static stability are given to explain design features. A classic example of how stability affects aircraft configuration is the departure of what the Wright brothers accomplished with the “tail” in the front (see Section 1.2) by later designers to put the tail where it should be, at the back. The Wright brothers used a warping wing for lateral control; later designers introduced ailerons. A tail-in-front canard later returned to aircraft design with far better application than what the Wright brothers had contemplated.

12.1.1 What Is to Be Learned?

This chapter covers the following topics:

Section 12.2: Introduction to stability considerations affecting aircraft design Section 12.3: Basic information on static and dynamic stability

Section 12.4: Elementary theory examining uncoupled pitch and coupled directional and lateral stability to determine empennage size

Section 12.5: Current statistical trends in empennage-sizing parameters Section 12.6: Inherent aircraft motions as characteristics of design Section 12.7: Aircraft spinning

Section 12.8: Design considerations for stability Section 12.9: Military aircraft stability: nonlinear effects Section 12.10: Active control technology

12.1.2 Coursework Content

Readers may examine the final configuration to review its merits. There is little coursework in this chapter. (The aircraft configuration is unlikely to change unless performance falls short of the requirements; see Chapter 13.)

12.2 Introduction

Inherent aircraft stability is a result of the CG location, the wing and empennage sizing and shaping, the fuselage and nacelle sizing and shaping, and their relative locations. Because the initial control-surface positioning and sizing are accomplished