4.stability-and-control. Aircraft Stability and Control
Stability is the inherent quality of an airplane to correct for conditions that may disturb its equilibrium and to return to or to continue on the original flightpath. It is primarily an airplane design characteristic. The flightpaths and attitudes an airplane flies are limited by the aerodynamic characteristics of the airplane, its propulsion system, and its structural strength. These limitations indicate the maximum performance and maneuverability of the airplane. If the airplane 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.
Stability is classified into two general types: static and dynamic.
Static Stability
Static stability refers to the initial tendency, or direction of movement, back to equilibrium after a disturbance. There are three categories:
- Positive static stability — the initial tendency of the aircraft to return to the original state of equilibrium after being disturbed.
- Neutral static stability — the initial tendency of the aircraft to remain in a new condition after its equilibrium has been disturbed.
- Negative static stability — the initial tendency of the aircraft to continue away from the original state of equilibrium after being disturbed.
Dynamic Stability
Dynamic stability refers to the aircraft response over time when disturbed from a given pitch, yaw, or bank.
- 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.
An airplane should have desirable stability characteristics in each of three axes — longitudinal (pitch), lateral (roll), and vertical (yaw).
Longitudinal Stability (Pitching)
Longitudinal stability is the quality that makes an airplane stable about its lateral axis. It involves the pitching motion as the nose moves up and down in flight. A longitudinally unstable airplane has a tendency to dive or climb progressively into a very steep dive or climb, or even a stall.
Longitudinal stability is largely determined by the relationship between three forces:
- The location of the center of gravity (CG) with respect to the center of lift (CL).
- The downward tail load produced by the horizontal stabilizer.
- The thrust line and pitching moments from power changes.
In most airplanes, the CG is located forward of the CL. This produces a nose-down moment that is balanced in cruise by a download on the horizontal tail (a negative lift force). The result is an airplane that, if disturbed nose-up, will tend to pitch back down, and if disturbed nose-down, will tend to pitch back up. A forward CG increases longitudinal stability but increases stall speed and elevator force required to flare. An aft CG decreases stability and may make recovery from stalls and spins difficult or impossible if loaded beyond the aft limit.
Lateral Stability (Rolling)
Stability about the airplane's longitudinal axis, which extends from nose to tail, is called lateral stability. This helps to stabilize the lateral or rolling effect when one wing gets lower than the wing on the opposite side. Four main design factors contribute to lateral stability:
- Dihedral — the upward angle of the wings from horizontal. When a wing drops, the lower wing presents a greater angle of attack to the relative wind, generating more lift to roll the aircraft back to level.
- Sweepback — swept wings produce a similar effect; the lower wing exposes more span perpendicular to the relative wind.
- Keel effect — a high-wing design with most of the fuselage below the wing acts like a pendulum, tending to right the aircraft.
- Weight distribution.
Directional Stability (Yawing)
Stability about the airplane's vertical axis (the sideways moment) is called yawing or directional stability. It is most easily achieved by a vertical fin (vertical stabilizer). When the airplane is yawed, the fin presents a greater surface to the relative wind, creating a force that yaws the airplane back into alignment with the relative airflow — much like a weathervane.
Dutch Roll and Spiral Instability
When lateral stability is greater than directional stability, the aircraft exhibits Dutch roll — a coupled oscillation of yaw and roll that is uncomfortable but not dangerous. When directional stability is much greater than lateral stability, the airplane exhibits spiral instability — slowly increasing bank and descending turn that, if uncorrected, develops into a steep spiral dive. Designers generally accept a slight Dutch roll tendency rather than spiral instability because Dutch roll can be damped with a yaw damper and is more easily noticed and corrected by the pilot.
Controllability
Controllability is the capability of an airplane to respond to the pilot's control inputs, especially with regard to flightpath and attitude. It is the quality of the airplane's response to the pilot's control application when maneuvering, regardless of its stability characteristics. Stability and controllability work in opposition: a highly stable airplane resists changes (low maneuverability), while a highly maneuverable airplane is less stable. Designers balance these qualities for the airplane's intended mission — trainers and transport aircraft favor stability, while fighters and aerobatic aircraft favor controllability.