Longitudinal Stability

081-04-03Pablo Asensio Martínez2026-04-011 min

Key Forces

  • Lift ($L$): Acts through the Aerodynamic Center (AC). Normally behind the CG.
  • Weight ($W$): Acts through the Center of Gravity (CG).
  • Tail Load: Usually a downward force to balance the Nose Down moment caused by Lift being behind Weight.

Static Stability Requirement

For positive static longitudinal stability, the Center of Gravity (CG) must be forward of the Aerodynamic Center (AC) of the whole aircraft (Neutral Point).

  • If CG is too far Aft: Stability is reduced. The aircraft becomes more maneuverable but harder to control (twitchy). If CG is behind the Neutral Point, the aircraft becomes unstable.
  • If CG is too far Forward: Stability is increased (very stable), but controllability is reduced (heavy elevator). Greater tail downforce is needed -> More Lift needed -> More Drag -> Higher Fuel Consumption.

Factors Affecting Stability

  1. CG Position: Forward = Stable. Aft = Unstable.
  2. Tailplane Area: Larger tailplane = More stability.
  3. Tail Moment Arm: Longer distance between CG and Tail = More stability.

Dynamic Longitudinal Stability

  • Phugoid: A long period oscillation (speed and altitude exchange, Angle of Attack constant). Lightly damped but easily controlled by pilot.
  • Short Period Oscillation: A rapid pitching oscillation (Angle of Attack changes). Heavily damped naturally. If undamped, results in PIO (Pilot Induced Oscillation).