Stability Considerations Affecting Aircraft Configuration

12.1 Overview

Chapter 11 completed the aircraft configuration in the conceptual study phase of an aircraft project by finalizing the external dimensions through the formal-sizing and engine-matching procedures. The design now awaits substantiation of aircraft per­formance to ensure that the requirements are met (see Chapter 13). Substantiation of aircraft performance alone is not sufficient if the aircraft-stability characteristics do not provide satisfactory handling qualities and safety, which are flying qualities that have been codified by NASA. Many good designs required considerable tailor­ing of the control surfaces, which sometimes affected changes to and/or reposition­ing of the wing and incorporated additional surfaces (e. g., dorsal fin and ventral fins).

Preliminary stability analyses, using semi-empirical methods (e. g., DATCOM and RAE data sheets [now ESDU]), are conducted during the conceptual study as soon as the three-view aircraft configuration is available. The analyses include the CG location (see Chapter 8) and preliminary stability results from geomet­ric parameters (e. g., surface areas, wing dihedral, sweep, and twist), which are determined from past experience and statistics. Aircraft dynamic-stability analy­sis requires accurate stability derivatives obtained from extensive wind-tunnel and flight testing. These are cost-intensive exercises and require more budget appropri­ation after the project go-ahead is obtained in the next phase (i. e., Project Defini­tion, Phase 2). Manufacturing philosophy is firmed up during Phase 2 after aircraft geometry is finalized, when the jig and tool designs can begin. Phase 2 activities are beyond the scope of this book.

New-generation aircraft incorporate artificial stability such as the use of FBW technology, which is control-configured vehicles (CCV). This is a good example of a systems approach (see Figure 2.1) to aircraft design. Phase 1 activities of commer­cial transport aircraft design with FBW can begin with available statistics of similar designs and then proceed to developing the aircraft-control laws. Advanced combat aircraft design requires the control laws to establish the initial FBW architecture at an early stage, which is not addressed in this book. For this reason, the author sug­gests that coursework on complex designs be postponed until the basics are learned. This book is limited to conventional aircraft design, a generalized procedure that also can be applied to CCV designs.

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:

Introduction to stability considerations affecting aircraft design

Basic information on static and dynamic stability

Elementary theory examining uncoupled pitch and coupled

directional and lateral stability to determine empennage size

Current statistical trends in empennage-sizing parameters

Inherent aircraft motions as characteristics of design

Aircraft spinning

Design considerations for stability

Military aircraft stability: nonlinear effects

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
empirically from statistical data, the important aspect of whether the aircraft has safe-handling characteristics is not examined. This chapter highlights some of the lessons learned on how to arrange aircraft components relative to one another.

The pitching motion of an aircraft is in the plane of aircraft symmetry (about the Y-axis, elevator-actuated) and is uncoupled with any other type of motion. Direc­tional (about the Z-axis, rudder-actuated) and lateral (about the X-axis, aileron- actuated) motions are not in the plane of symmetry. Activating any of the controls (e. g., rudder or ailerons) causes a coupled aircraft motion in both the directional and lateral planes.

Finally, at the end of a project, flight tests reveal whether the aircraft satisfies the flying qualities and safety considerations. Almost all projects require some type of minor tailoring and/or rigging of control surfaces to improve the flying qualities as a consequence of flight tests – in hindsight, possibly making it better than what was initially envisaged. For civil aircraft designs, this is a routine procedure and is neither expensive nor a major hurdle to program milestones. Military aircraft design projects are preceded by technology demonstrators, which results in obtaining vital information for the final design that may incorporate configuration changes from the lessons learned. The design still must undergo fine-tuning as a result of flight tests. This is a relatively more expensive and time-consuming process, but it saves funds by minimizing errors during the design of military aircraft, which often incorporates cutting-edge advanced technologies that are yet to be operationally proven.

Designers should be aware of the preferred flying qualities so that the aircraft is configured intelligently to minimize changes in the final stages; this is the main objective of this chapter.

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