What is defined as a straight line through the leading edge of an airfoil?

The chord line is a fundamental concept in aerodynamics and is defined as an imaginary straight line connecting the leading edge and the trailing edge of an airfoil. This line serves as a reference for measuring various aerodynamic angles, such as the angle of attack, which is the angle between the chord line and the relative wind. Understanding the chord line is crucial for grasping how airfoils generate lift and how various factors affect their performance in flight.

The other options, such as blade spar, angle of attack, and relative wind, pertain to different aspects of helicopter aerodynamics and wing design, but they do not define the straight line through the leading edge of an airfoil. The blade spar refers to the structural component inside the rotor blades, angle of attack is the measurement concerning the chord line but does not define it, and relative wind is the wind experienced by the airfoil in flight, which interacts with the chord line but is not itself a straight line through the airfoil.