12Stability 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.

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