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