Dutch Roll
The term Dutch roll refers to a tendency for an aircraft to roll whenever there is yaw. Swept wing aircraft are particularly susceptible, and many are equipped with yaw damper, which is an automatic device that senses yaw and counters it with corrective control inputs before the Dutch roll oscillations can develop. Gregory N. Brown and Mark J. Holt describe the phenomena as follows:
Dutch roll is caused by an aircraft’s tendency to sideslip slightly when the aircraft yaws. One wing yawing forward in this situation changes the effective span between left and right wings. The wing yawed forward momentarily creates more lift than the one of the other side. The result is that the forward wing rises and starts a rolling movement. The problem is aggravated by the fact that the forward wing, due to its increased lift, also has more drag, pulling that wing back once again and starting an oscillation in the other direction.5
Forces during Takeoff
There are many forces that produce left yaw tendencies in most aircraft and must be controlled by rudder,
Torque—caused by prop and engine: movement in one direction causes movement in another.
Precession—a gyroscopic force whereby pressure exerted on a spinning mass will cause a reaction 90° along the direction of rotation.
Asymmetric thrust or P-factor—during high-pitch attitudes (climbs), the downward moving side of the prop disk produces greater lift than the upward moving side. Viewed from the pilot seat, the right side of the prop disc is the down-going side and therefore produces greater thrust than the up-going left side (in most conventional engines).
Slipstream—propeller slipstream spirals around the fuselage and strikes the vertical stabilizer on the port side.
Critical Engine
The effect of asymmetric thrust in multi-engine aeroplanes is to create what is referred to the critical engine. The critical engine is defined as the engine that, should a failure occur, will most adversely affect aircraft performance and control.
On a twin-engine aircraft when both propellers turn clockwise (as viewed from the rear of the aircraft), the failure of the left engine will have the more adverse effect because the remaining thrust from the right engine, owing to asymmetric thrust, would be further from the longitudinal axis than would be the case if the right engine failed and only the left were producing thrust.
Note that some twin-engine aircraft do not have a critical engine as the right engine has a counter-rotating propeller.