Multi-Engine

Spiraling Slipstream

Each propeller leaves a corkscrew of air behind it. On a conventional twin only the left engine's spiral wraps inboard onto the fuselage and strikes the vertical fin — the right engine's spirals away outboard and misses the tail entirely. That fin force pushes the nose left, which happens to relieve the yaw when the right engine quits, and does nothing at all when the left one does.

Top-down · airflow from top

spiraling slipstream fin force & net yaw—

Power & airspeedHighCruise (fast, low power) → slow flight at climb power. High power and low airspeed wind the spiral tighter, so it hits the fin harder.

Right engine rotationCounter-rotating: the right engine's spiral now wraps inboard too and strikes the other side of the fin — a mirror image of the left engine.

Yaw budget · left engine failed

Asymmetric thrust—

Spiral on the fin—

Net yaw to hold—

Net yaw for each failure

Lose left engine
(right engine working)

Lose right engine
(left engine working)

Spiraling slipstream

PAST

PP-factor
AAccelerated slipstream
SSpiraling slipstream
TTorque