The change in pitch at a propeller blade from the root to the tip ensures...
Refer to figure.
The propeller blade is an airfoil and as all airfoils have specific characteristics referring to Chord, Camber, Thickness etc.
Blade Angle or Pitch: The angle between the blade Chord and the plane of rotation. The propeller itself is twisted so the blade angle changes from root to tip. The greatest blade angle, or the highest pitch, is at the root while the smallest Angle of Incidence or smallest pitch is at the tip. For reference purposes, the blade angle is measured at a point 75% of the blade length from the root.
Angle of Attack: The path of the propeller through the air determines the direction of the relative airflow. The angle between the blade Chord and the relative airflow is the Angle of Attack (α). The Angle of Attack (α) is the result of Propeller Rotational Velocity (RPM) and Aircraft Forward Velocity (TAS).
Α propeller blade designed with the same blade angle throughout its entire length would be inefficient because as airspeed increases in flight, the portion near the root would have a negative Angle of Attack while the blade tip would be stalled. Also because the rotational velocity of the blade increases from root to tip, lift forces would be different at the entire blade and lift moments (loads) would cause structural damage.
Twisting the blade eliminates the different Loads acting on the entire blade and equalizes thrust production on the blade from root to tip.
The change in pitch at a propeller blade from the root to the tip ensures a nearly constant Load by a constant effective Angle of Attack over the entire length of the blade.
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