What does the length of the blade refer to in helicopter terminology?

The length of the blade in helicopter terminology refers to the blade spar. The blade spar is the principal structural component of the rotor blade that runs along its length, providing strength and rigidity. It extends from the root to the tip of the blade, supporting the aerodynamic surfaces and tolerating the various loads encountered during flight.

Understanding the blade spar's role is crucial because it directly impacts the overall performance and structural integrity of the rotor system, notably in load distribution and resonance control during operation. The spar is responsible for maintaining the shape of the rotor blade and ensuring it can withstand the stresses generated during flight maneuvers, making it essential knowledge for any pilot.

In contrast, the chord line refers to the straight line connecting the leading and trailing edges of the rotor blade, the mean camber line is the locus of points halfway between the upper and lower surfaces of the airfoil, and the leading edge is the front edge of the airfoil section. While these terms are fundamental in understanding rotor blade design and aerodynamics, they do not specifically denote the length of the blade itself.