Combating adverse yaw

Adverse Yaw and How to Counter It

Adverse yaw is the unwanted yaw motion opposite to the intended roll caused by aileron deflection. A critical side-effect the pilot must counter.

Mechanism

When the pilot applies aileron left:

• Left aileron up → lift drops on left → left wing drops.
• Right aileron down → lift rises on right → right wing rises.

Drag difference:

• Right wing (aileron down): increased induced drag (higher lift → more tip vortex).
• Left wing (aileron up): reduced drag.

Result: more drag right → yaw right, i.e. opposite to intended roll direction (left).

How strong is adverse yaw?

Strength depends on:

• Aileron deflection size.
• Angle of attack (higher α → higher CL → stronger vortex → more drag difference).
• Airspeed (lower speed → more aileron deflection required → more adverse yaw).
• Aileron design (see below).

Practically noticeable: significant at low speed (approach), small in cruise.

Design countermeasures

1. Differential ailerons

Asymmetric travel:

• Aileron going up (on the descending wing): larger travel.
• Aileron going down (on the rising wing): smaller travel.

Effect:

• Up aileron disturbs more flow → more drag on descending wing.
• Down aileron disturbs less → less drag on rising wing.
• → Drag difference shifted to the aileron side → reduces or eliminates adverse yaw.

Use: widespread in modern GA (DA-40, many Cessnas).

2. Frise ailerons

Special geometric form:

• Aileron going up: leading edge pushes into free flow under the wing → produces its own drag on descending wing.
• Aileron going down: closes flush with trailing edge → minimal drag.

Effect: increases drag on descending wing → compensates adverse yaw.

Use: many PA-28 variants, some Cessnas.

3. Coupled aileron-rudder

Mechanical coupling between aileron and rudder:

• Aileron deflection automatically actuates rudder in the same direction.
• Examples: some trainers (e.g. Tomahawk), gliders.

4. Spoilers instead of ailerons

Spoilers for roll control:

• Extending spoiler reduces lift on one side → roll that way.
• No adverse yaw (no lift rise on the opposite side).
• Examples: many gliders, some airliners.

Pilot countermeasures (in every aircraft)

Coordinated rudder with aileron

Standard technique:

• On aileron deflection left: simultaneously left rudder pedal.
• Magnitude: proportional to aileron deflection and airspeed.

Practical practice:

• Coordinated turn exercise: dose aileron + rudder so ball stays centred.
• At low altitude (pattern): pay particular attention.

At stall recovery

• No sudden aileron inputs at stall — can amplify adverse yaw → spin risk.
• Wings level via rudder + pitch-down for stall recovery.

Example — comparison across aircraft

Stall-spin risk from adverse yaw

Classic scenario

1. Base-to-final turn at low speed.
2. Pilot rolls late → tighter bank required → more aileron input.
3. More adverse yaw → pilot compensates with too much rudder.
4. Skidding turn → one wing stalls earlier (uncoordinated).
5. Spin at low altitude → fatal.

Prevention: roll in time, modest speed reserve, no skidding turns.

Self-test

• Try with an instructor an aileron-only roll without rudder.
• Feel yaw opposite to roll direction.
• Then with coordinated rudder — difference clear.