Refer to figure.
To prevent the propeller from ‘wind milling’ the propeller blade is rotated from the fine-pitch stop through the coarse-pitch stop to reach the angle of attack to the relative airflow that produces zero lift; and referred to as ‘feathering’ the propeller. It’s usually approximately 85°/90° because it is the best compromise for the whole blade length and accounts for the blade twist.
This blade position stops the propeller from rotating because there is no lift force to produce a torque reaction; it reduces the drag to a minimum level and prevents damage being caused to the engine.
For a single-engine airplane feathering the propeller blades improves the glide performance because it does not require such a steep glide angle to maintain the required airspeed. As such, feathering the propeller increases the gliding range and endurance. For a multi-engine airplane, the decreased drag reduces the amount of thrust required to counteract the asymmetric force of the propeller of the failed engine and there is sufficient rudder authority remaining to enable the pilot to safely control the airplane.
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