Introduction
In straight and level, constant-speed flight, forces acting on an aircraft are in equilibrium. Drag is offset by thrust, and weight is offset by lift. The angle at which relative wind meets the wing, known as angle of attack, creates a pressure difference that produces lift. Aircraft velocity is projected along the flight path, while pitch angle is determined by the relationship between flight path and angle of attack.
Static Stability
Static stability refers to an aircraft's ability to return to a balanced condition after a disturbance. This is achieved when the aerodynamic center (AC) is located behind the center of gravity (CG). In such a configuration, the aircraft naturally regains equilibrium without control inputs. Excessive stability can make an aircraft overly sensitive to gusts, while insufficient stability can make control difficult without augmentation systems.
Long and Short Period Oscillations
Aircraft motion about its three principal axes—longitudinal, lateral, and vertical—produces roll, pitch, and yaw, respectively. Long period oscillations, such as Dutch roll and phugoid, are inherent static characteristics. Short period oscillations are dynamic and result from control inputs or disturbances like gusts. Desirable aircraft behavior includes inherent damping of both oscillation modes; an unstable aircraft requires significant pilot attention.
Effects of Vertical Currents
Vertical atmospheric currents affect aircraft by altering angle of attack and lift. When an aircraft encounters an upward current, angle of attack increases, leading to a load factor increase at the CG. The change in load factor can be estimated using the formula Δn = 0.5ρVwCLα / (W/S). A downward current decreases load factor. Additionally, vertical currents cause angular accelerations about the CG due to the aircraft's tendency to restore balanced flight, resulting in local load factor variations forward and aft of the CG.
Examples
Example 1: For a generic transport aircraft with wing area 130 m², mass 60,000 kg, true airspeed 225 m/s, and vertical current velocity 10 m/s, an upward current increases load factor at the CG by 0.62, to a total of 1.62. A downward current decreases it to 0.38.
Example 2: With a pitch-down angular acceleration of 0.3 rad/s² (17.2°/s²) from an upward current, local load factor 15 m aft of the CG increases by an additional 0.46, reaching 2.08. Conversely, a downward current with the same angular acceleration (pitch-up) results in a local load factor of -0.08 aft of the CG, sufficient to lift unrestrained occupants and items toward the ceiling.