Lateral (roll) stability

Lateral Stability (Roll)

Lateral stability describes the behaviour about the longitudinal axis (roll). It ensures the aircraft returns to wings level after a roll disturbance.

Mechanism — dihedral effect

A roll induces sideslip (lateral flow). Various design measures convert this into a rolling moment back to wings level.

Dihedral (V-shape)

Dihedral = upward tilt of the wings.

• Positive dihedral (typical GA): tips higher than root — V shape.
• Anhedral (negative): tips lower than root — inverted V (rare in GA).

Effect on sideslip

Pilot rolls left → left sideslip → relative wind from below-left.

With positive dihedral:

• Left wing (lower): effective α rises → more lift.
• Right wing (higher): effective α drops → less lift.
• Result: rolling moment to the right → back to wings level.

Typical dihedral values

High dihedral (e.g. PA-28 with 5°) makes the aircraft strongly roll-stable — hence valued as a trainer and tourer.

Other factors

Wing position (high vs low wing)

High wing (C172, C152):

• Wing above fuselage centreline.
• On sideslip the fuselage pushes the lower wing up → effectively high α → more lift → roll stability.
• High-wings need less dihedral (C172 only 1.5°).

Low wing (PA-28, DA-40):

• Wing below fuselage centreline.
• On sideslip the fuselage obstructs the lower wing → effectively lower α → less lift → destabilises.
• Low-wings need more dihedral (PA-28 5°).

Mid-wing (Beech King Air): neutral.

Sweep

Swept wings create dihedral-like stability:

• On sideslip, the windward wing flows over a less-swept angle → more lift → stabilising.
• Strong effect on airliners (sweep 25-35°).

Keel effect

Large vertical area below CG (fuselage, gear):

• On sideslip aerodynamic force acts at the keel → roll-stabilising.
• Effective in some gliders and seaplanes.

Vertical fin

Large vertical tail also contributes to roll stability:

• On sideslip side-force acts on tail → also acts on roll (geometrically).

Too much roll stability — Dutch roll

For very roll-stable aircraft with low yaw stability, Dutch roll develops (see lesson "Coupled instabilities"):

• Yaw + roll out of phase.
• Oscillation "wags" around the vertical axis.
• Pilot can amplify or damp.

So: lateral stability alone is not enough — directional stability must also match.

Roll-axis oscillation modes

Roll subsidence

• Pure roll damping without oscillation.
• Mechanism: roll rate produces anti-roll moment via aileron leading-edge effect.
• Typical time constant 0.5-2 s in GA.

Spiral stability

• Very slow oscillation between roll and yaw.
• For most GA aircraft: slightly unstable → pilot must correct lightly every 30-60 s.
• High dihedral → stable, but trade-off with other factors.

Pilotage consequences

• High wing = roll-stable with less dihedral.
• Low wing = needs more dihedral, prone to crosswind-landing drift.
• On roll disturbance: aircraft corrects itself — only light aileron input needed.

Mass-position effect

Mass centre offset laterally (e.g. one passenger left, none right) creates a rolling moment:

• Pilot compensates with aileron or trim (glider).
• In C172 with 4 occupants: careful seat selection.