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Aircraft Design Characteristics
Each aircraft handles somewhat differently because each resists or responds to control pressures in its own way. For example, a training aircraft is quick to respond to control applications, while a transport aircraft feels heavy on the controls and responds to control pressures more slowly. These features can be designed into an aircraft to facilitate the particular purpose of the aircraft by considering certain stability and maneuvering requirements. The following discussion summarizes the more important aspects of an aircraft’s stability, maneuverability and controllability qualities; how they are analyzed; and their relationship to various flight conditions.
Stability
Stability is the inherent quality of an aircraft to correct for conditions that may disturb its equilibrium, and to return to or to continue on the original flightpath. It is primarily an aircraft design characteristic. The flightpaths and attitudes an aircraft flies are limited by the aerodynamic characteristics of
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Positive static stability
Neutral static stabilityNegative static stabilityCGCGCGCGAppliedForceAppliedForceAppliedForce
Figure 4-18. Types of static stability.
the aircraft, its propulsion system, and its structural strength. These limitations indicate the maximum performance and maneuverability of the aircraft. If the aircraft is to provide maximum utility, it must be safely controllable to the full extent of these limits without exceeding the pilot’s strength or requiring exceptional flying ability. If an aircraft is to fly straight and steady along any arbitrary flightpath, the forces acting on it must be in static equilibrium. The reaction of any body when its equilibrium is disturbed is referred to as stability. The two types of stability are static and dynamic.
Static Stability
Static stability refers to the initial tendency, or direction of movement, back to equilibrium. In aviation, it refers to the aircraft’s initial response when disturbed from a given AOA, slip, or bank.
• Positive static stability—the initial tendency of the aircraft to return to the original state of equilibrium after being disturbed [Figure 4-18]
• Neutral static stability—the initial tendency of the aircraft to remain in a new condition after its equilibrium has been disturbed [Figure 4-18]
• Negative static stability—the initial tendency of the aircraft to continue away from the original state of equilibrium after being disturbed [Figure 4-18]
Dynamic Stability
Static stability has been defined as the initial tendency to return to equilibrium that the aircraft displays after being disturbed from its trimmed condition. Occasionally, the initial tendency is different or opposite from the overall tendency, so a distinction must be made between the two. Dynamic stability refers to the aircraft response over time when disturbed from a given AOA, slip, or bank. This type of stability also has three subtypes: [Figure 4-19]
• Positive dynamic stability—over time, the motion of the displaced object decreases in amplitude and, because it is positive, the object displaced returns toward the equilibrium state.
• Neutral dynamic stability—once displaced, the displaced object neither decreases nor increases in amplitude. A worn automobile shock absorber exhibits this tendency.
• Negative dynamic stability—over time, the motion of the displaced object increases and becomes more divergent.
Stability in an aircraft affects two areas significantly:
• Maneuverability—the quality of an aircraft that permits it to be maneuvered easily and to withstand the stresses imposed by maneuvers. It is governed by the aircraft’s weight, inertia, size and location of flight controls, structural strength, and powerplant. It too is an aircraft design characteristic.
• Controllability—the capability of an aircraft to respond to the pilot’s control, especially with regard to flightpath and attitude. It is the quality of the aircraft’s response to the pilot’s control application when maneuvering the aircraft, regardless of its stability characteristics.
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Reduced power allows pitch down
CGCLCGCL
Figure 4-20. Longitudinal stability.
Displacement
Positive static(neutral dynamic)Positive static(positive dynamic)Positive Static(negative dynamic)TimeDamped oscillationUndamped oscillationDivergent oscillation
Figure 4-19. Damped versus undamped stability.Longitudinal Stability (Pitching)
In designing an aircraft, a great deal of effort is spent in developing the desired degree of stability around all three axes. But longitudinal stability about the lateral axis is considered to be the most affected by certain variables in various flight conditions.
Longitudinal stability is the quality that makes an aircraft stable about its lateral axis. It involves the pitching motion as the aircraft’s nose moves up and down in flight. A longitudinally unstable aircraft has a tendency to dive or climb progressively into a very steep dive or climb, or even a stall. Thus, an aircraft with longitudinal instability becomes difficult and sometimes dangerous to fly.
　
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