What is blade angle?

• A. The angle between the blade and the wind direction
• B. The angle between the hub and blade root
• C. An angle formed by a line perpendicular to the crankshaft and the blade chord
• D. The angle the blade makes with the aircraft fuselage

Answer: (C)

In propeller terminology, blade angle is defined as the angle formed by the blade’s chord line and a line perpendicular to the crankshaft. The blade chord is the straight line from the blade’s leading edge to its trailing edge, and using a line perpendicular to the crankshaft provides a fixed reference tied to the engine’s rotation. This angle effectively describes the blade’s pitch.

The other ideas don’t fit this standard definition: the angle to wind direction would relate to relative wind or angle of attack, not the blade’s fixed pitch; the angle between the hub and blade root isn’t a recognized measure of blade pitch; and the angle with the aircraft fuselage isn’t a conventional or meaningful metric for blade orientation.