Inherent Aircraft Motions as Characteristics of Design

Once an aircraft is built, its flying qualities are the result of the effects of its mass (i. e., inertia), CG location, static margin, wing geometry, empennage areas, and control areas. Flying qualities are based on a pilot’s assessment of how an aircraft behaves under applied forces and moments. The level of ease or difficulty in control­ling an aircraft is a subjective assessment by a pilot. In a marginal situation, recorded test data may satisfy airworthiness regulations yet may not prove satisfactory to the pilot. Typically, several pilots evaluate aircraft flying qualities to resolve any debat­able points.

It is important that the design maintain flying qualities within preferred levels by shaping the aircraft appropriately. Whereas theoretical analyses help to minimize discrepancies, flying qualities can be determined only by actual flight tests. Like any other system analysis, control characteristics are rarely amenable to the precise the­ory due to a lack of exact information about the system. Therefore, accurate design information is required to make predictions with minimal error. It is cost-intensive to generate accurate design information, such as the related design coefficients and derivatives required to make theoretical analyses, which are conducted more inten­sively during Phase 2 of a project. Practically all modern aircraft incorporate active control technology (ACT) to improve flying qualities. This is a routine design exer­cise and provides considerable advantage in overcoming any undesirable behavior, which is automatically and continuously corrected.

Described herein are six important flight dynamics of particular design inter­est. They are based on fixed responses associated with small disturbances, making

Figure 12.12. Short-period oscillations and phugoid motion

the rigid-body aircraft motion linearized. Military aircraft have additional consider­ations as a result of nonlinear, hard maneuvers, which are discussed in Section 12.9. The six flight dynamics are as follows:

• short-period oscillation

• phugoid motion (long-period oscillation)

• Dutch roll

• slow spiral

• roll subsidence

• spin

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