Directional Stability (Yaw)

Directional stability describes behaviour about the vertical axis (yaw). It ensures the nose returns to the relative wind after a yaw disturbance.

Main mechanism — vertical fin (vertical stabiliser)

The vertical fin acts like a weathercock:

On yaw to the left → relative wind from the right.
Fin sees flow from the right → produces sideforce to the left at the tail → produces yaw moment to the right → corrects disturbance.

Other factors

Fuselage

Fuselage shape influences yaw stability:

Long narrow fuselage with mass behind CG: destabilising (like an arrow with mass at front).
Bulbous nose: slightly destabilising.

Sweep

Swept wings contribute to yaw stability:

On yaw the wings effectively cross the air at different sweep angles.
Leading wing: less sweep → higher drag → yaw correction.

Single-engine propeller effect

Propeller slipstream spirals around the rear fuselage:

Hits the vertical fin at an angle → produces constant yaw (to one side away from slipstream).
Most single-engine aircraft (Cessna 172, PA-28): yaw to the left via P-factor + spiral flow.
Compensation: rudder trim tab preset or pilot with rudder pedals.

Relationship to pitch axis — straight & diagonal

When yaw is corrected:

Yaw = 0 → nose into relative wind.
However: sideslip can still occur if bank doesn't match.
Coordinated flight: yaw + bank matched → ball centred.

Yaw oscillations

Dutch roll (see next lesson "Coupled instabilities")

Yaw + roll in opposite phase.
In very roll-stable aircraft with low yaw stability.

Adverse yaw

With aileron deflection: rising wing has less drag, descending wing more → unwanted yaw opposite to roll direction.
See lesson "Combating Adverse Yaw".

Snake

Short, rapid yaw oscillation on some twin-engine aircraft.

Design layout

Fin size

Larger fin:

More yaw stability.
But: higher drag.
Spiral instability can increase (fin produces too strong yaw recovery → roll amplification).

Example

C172 has a moderate fin → well balanced.
Gliders often have smaller fins (less drag, less stability — pilot compensates with rudder).
Airliners with yaw damper can have smaller fin (yaw damper compensates).

Yaw damper

Modern aircraft (airliners, high-performance twins) have a yaw damper:

Automatic system that produces rudder inputs to damp yaw oscillations.
Particularly Dutch roll is damped.
PPL trainers have no yaw damper — pilot damps with pedals.

Design measures for yaw stability

Larger fin: direct effect.
Strake: small leading-edge extension on tail fuselage.
Ventral fin: additional lower tail.
Wing sweep: indirect effect.

Operational notes

In crosswind: fin tries to turn nose into wind → pilot must counter with rudder on approach.
On engine failure in twin: unequal thrust → strong yaw effect → pilot compensates with rudder.
In slip or skid: pilot deliberately violates coordination — typically in crosswind landing.