Propeller-Induced Moments (Torque, P-Factor, Slipstream, Gyroscope)

Propeller-induced moments

A running propeller produces not only thrust but also four side effects affecting the aircraft about its three axes. PPL pilots must understand these to safely handle take-off at full power.

The four effects are known in English as the "Four Left-Turning Tendencies" — all cause a leftward tendency for a right-turning propeller (US standard).

1. Torque

Mechanism:

• Newton's third law: when the engine turns the propeller to the right (viewed from behind), an equal reaction moment to the left acts on the aircraft (roll tendency).

Strength:

• Depends on engine power — strongest at full throttle.
• Low forward speed + high power = strongest effect.
• At low power: negligible.

When most felt:

• Take-off: full throttle + low speed.
• Go-around: sudden full throttle at low speed.
• Climb at high power.

Effect: roll tendency to the left (right-turning prop) → pilot must hold right aileron.

2. P-Factor (asymmetric thrust)

Mechanism:

• At high angle of attack (climb, take-off roll) the propeller blades move differently on right and left relative to the inflow.
• The descending blade on the right (right of cockpit, right-turning prop) has a higher AoA than the ascending blade on the left.
• → More thrust on the right than left → yaw moment to the left.

Visualisation:

• Imagine one blade pointing down and the other up.
• At high AoA the lower blade "sees" more inflow from ahead than the upper → unequal thrust.
• Since the propeller seen from front rotates clockwise, the right (descending) blade is stronger.

When strongest:

• High AoA: climb, take-off before liftoff (especially tailwheel!).
• At low speed (effect scales with AoA).

Effect: yaw tendency to the left → pilot must hold right rudder.

3. Spiraling slipstream

Mechanism:

• The propeller throws a slipstream rearward that rotates spirally (with propeller rotation direction).
• This spiral flow strikes the fin on the left side (right-turning prop).
• → Sideforce to the left on the fin → yaw moment to the left.

Visualisation:

• The propeller flow traces a corkscrew.
• Hits fuselage, wing root and fin.
• The fin is side-loaded → small sideforce.

When strongest:

• At low forward speed + high propeller rpm (spiral is "tighter" → hits more of the tail area).
• Take-off roll and climb.

Effect: yaw tendency to the left → pilot must hold right rudder.

Designer trick: some aircraft have a slightly offset fin or lift asymmetry to compensate.

4. Gyroscopic precession

Mechanism:

• A rotating propeller acts as a gyroscope.
• A force on the rotation axis takes effect 90° around in the direction of rotation (precession).

Example — tailwheel aircraft on take-off:

• On raising the tail (propeller moves forward-down):
• The "input" force is upward on the lower prop area.
• 90° in rotation direction (right-turning prop) = left in the aircraft.
• → Propeller pushes the aircraft to the left (yaw tendency).

Other manoeuvres:

• Pitch up (entering climb) → yaw moment right (right-turning prop).
• Pitch down → yaw moment left.
• Roll right → pitch moment down.
• Roll left → pitch moment up.

When relevant:

• Tailwheel aircraft during tail-raise on the take-off roll.
• Aerobatics during transitions.
• In normal nose-wheel PPL: weaker effect, noticeable on hard flap extension.

Summary of the four effects

Right-turning propeller (standard):

Consequence for PPL pilots: hold right rudder during take-off and climb — up to 20° pedal may be needed.

Left-turning propeller (rare)

Some twin engines have counter-rotating propellers — opposed rotation that mutually cancels the effects.

Example: Diamond DA42 has two counter-rotating propellers → no asymmetric effects.

For single-engine with left-turning propeller (e.g. Diamond DA40, Rotax engines): all effects go to the right → left rudder on take-off.

Practical application

Take-off roll (most common PPL application):

1. Apply full throttle: expect left roll and yaw tendency.
2. Right rudder applied progressively — on C172/PA-28 about 5–10° pedal.
3. Wings level with aileron against roll tendency.
4. Track centreline with rudder.
5. As speed increases the effect drops — reduce pedal input.

On go-around:

• Full throttle is applied suddenly → strong left tendency.
• Right rudder early and firmly.

On engine failure (multi-engine):

• Critical engine = the engine whose failure causes the largest yaw tendency or hardest manoeuvres.
• For right-turning engines: left engine is critical (more P-factor loss on right side).