Longitudinal stability & centre of pressure
Longitudinal stability: pitch-axis stability, about the lateral axis. PHAK: of the three axes, most affected by flight-condition variables. Unstable aircraft dives or climbs progressively into a steep dive, climb, or stall.
Most important of the three axes (TP 1102):
- CG position affects it most; speed, power, attitude also matter.
- Turbulent air and flap operation can disturb the balance.
- Tail resists unintended inputs, not coordinated control and throttle inputs.
Centre of pressure (CP): point where wing's total lift force acts. Moves with angle of attack (AoA) on most asymmetrical airfoils (TP 1102 ch.1).
| AoA change | CP moves |
|---|---|
| Increases (normal flight range) | Gradually forward |
| Decreases | Aft |
| Increases well beyond normal range | Starts moving back again |
- CP far enough aft: nose pitches down suddenly as wing stalls.
Pitching moment:
- CG forward of CP: nose-down tendency.
- CP forward of CG: nose-up moment.
- Designers set aft CG limit forward of CP for design speed, to keep equilibrium.
Three factors in static longitudinal stability:
- Wing location relative to CG.
- Horizontal tail location relative to CG.
- Tail surface area.
Sudden nose-up must change wing and tail moments to a restoring sum: nose returns down (nose-down returns up).
Wing alone: forward CL shift lifts leading edge further, so wing alone is inherently unstable.
Tail balance:
- Horizontal stabilizer well behind wing: CG leverage turns a small tail force into a large correcting moment (TP 1102).
- Most aircraft: wing CL aft of CG (nose-heavy).
- Horizontal stabilizer set at slight negative AOA: downward tail force balances it.
- Source: wing downwash striking top of stabilizer.
- Faster flight: more downwash, more tail-down force (except T-tails).
- Tail-down force equals added tail weight, up to several hundred pounds by type, like another passenger with baggage (TP 1102).
Trimmed-speed seeking:
- Speed drops: downwash and tail-down force drop.
- Nose pitches down: AOA and drag fall.
- Airspeed builds back up until tail force is restored; nose rises again.
- Aircraft is dynamically stable: each oscillation smaller than the last.
- Settles at speed where tail-down force exactly counters diving tendency.
Trap: expecting nose to pitch up at the stall.
References
- PHAK ch.5, Longitudinal Stability (Pitching)
- TP 1102 (Flight Training Manual)
- TP 1102 ch.1 Lift