Lateral stability
Lateral stability: roll-axis stability. Aircraft tends to roll back to wings-level after a wing drops.
Four design features:
| Feature | What it is | Effect |
|---|---|---|
| Dihedral | Wingtip-up angle, root to tip | Low wing sideslips toward the drop, meets relative wind at higher angle of attack, more lift, rises to level |
| Sweepback | Swept wing | Adds "effective dihedral": dropped wing's leading edge more square to airflow, more lift, rises |
| Wing location | High wing vs low wing | High wing adds extra effective dihedral over low wing |
| Keel effect / weight distribution (high wing) | Fuselage hangs below wing like ship's keel below waterline, most side (keel) area above CG | In a slip, airflow pressure on upper keel area plus fuselage weight swinging back like a pendulum rolls aircraft to wings-level |
- Result: many high-wing types have little or no built-in dihedral. Low-wing need more.
- Anhedral: wing slants down root to tip. Reverse of dihedral.
In turns, lateral stability pushes back toward wings-level (TP 1102):
- Gentle turn: slight aileron pressure may be needed to hold bank.
- Beyond a certain angle: lateral stability fully overcome. Faster outer wing keeps making more lift, rolling aircraft further unless aileron holds.
| Turn | Why | Bank tendency |
|---|---|---|
| Power-off descending | Inner wing flies steeper spiral, raising its angle of attack enough to offset outer wing's speed advantage | Tends to hold constant |
| Climbing | Outer wing gets extra lift from greater speed and greater angle of attack | Tends to increase, aileron needed to hold |
Trap: dihedral not the only source. High-wing aircraft with little or no dihedral can still be laterally stable from wing location and keel effect alone. Also, bank does not hold in a climbing turn: it increases; aileron needed to hold.
References
- TP 1102 Basic Principles of Flight, Lateral Stability
- PHAK ch.5, Lateral Stability (Dihedral; Sweepback and Wing Location; Keel Effect and Weight Distribution)
- TP 1102 (Flight Training Manual)