Longitudinal Stability (Pitch)

Longitudinal stability describes the aircraft's behaviour about the lateral axis (pitch) after a disturbance. It ensures the aircraft maintains pitch trim.

Mechanisms

1. Horizontal stabilizer

Main component for longitudinal stability:

Produces downward lift in normal flight (negative lift on the stab).
On pitch-up (nose up) → α at stab increases → more down-lift → nose-down pitching moment → corrects disturbance.

2. CG position

CG forward of aerodynamic centre (neutral point, NP) → stable.
CG aft of NP → unstable.
Static margin: NP − CG, in % of mean chord.
Typical GA: 5-15 % positive static margin.

3. Wing downwash

Downwash behind the wing affects α at the stab.
On pitch-up: less downwash → more α at stab → more correction.

Consequences of CG position

Forward CG

More stable flight:

Higher stall speed: more elevator travel required for high α → stab produces more down-lift → effectively more weight to be carried by the wing.
More damped pitch.
Higher yoke force for manoeuvres.
Better stall recovery.

Aft CG

Less stable:

Lower stall speed: less down-lift at stab needed.
More sensitive to pitch disturbances.
Easier to manoeuvre (less yoke force).
Reduced trim drag → more efficient.
Stall recovery harder (insufficient stab down-force).

Aft of NP (negative static margin)

Statically unstable.
Pilot must constantly correct.
In normal GA never certified.

Special case: CG limits in POH

Mass and balance (Subject 030) must keep CG within the forward and aft limits:

Forward limit: stability upper limit and yoke-force limit.
Aft limit: stability lower limit (static margin > 5%).
Pilot is responsible.

Pitch-axis oscillation modes

Phugoid

Long, damped oscillation between pitch and airspeed.
Mechanism: pitch-up → speed drops → less lift → nose drops → speed rises → lift returns → pitch-up.
Period: 30-90 s (typical GA).
Very damped in stable aircraft — oscillates 1-2 times and calms.
Pilot scarcely notices in normal flight.

Short-period pitch

Fast, short oscillation at nearly constant speed.
Mechanism: α change → stab responds → α corrected.
Period: 2-5 s.
Strongly damped in stable aircraft.

Trim system

Pilot trims the aircraft to desired pitch state.
Trim tab on elevator (variable) or anti-balance tab.
With correct trim: yoke can be released → aircraft holds pitch.

Tail designs

Conventional tail (Cessna 172): stabiliser + elevator.
T-tail (PA-28, DA-40): stabiliser on top of vertical fin.
All-moving stabiliser (stabilator): no separate stab+elevator. Example: Cherokee, Mooney.
Canard (forward instead of rear): Example: Velocity SE.

Efficiency aspects

Aft CG = lower trim drag → better range (airliners deliberately fly CG near aft limit).
Forward CG = more stable but higher drag.
Trade-off between stability and efficiency.

Stall with various CG positions

CG forward: stall at higher speed (no centre-of-mass correction needed), classic stall recovery.
CG aft: lower stall speed, but deep stall possible (stab in vortex field → no recovery effect). Very dangerous.

Construction measures

CG shift through loading or fuel burn (CG moves depending on tank location).
Trim system compensates.