What causes relative wind?

• A. Horizontal wind due to rotor wash
• B. Vertical ascent of air
• C. Forward airspeed
• D. Updraft above rotor

Answer: (C)

Relative wind is the air flowing past the rotor or airfoil that moves opposite to the aircraft’s motion through the air. It’s produced by the aircraft’s actual movement through the surrounding air, so when the helicopter has forward airspeed, air approaches from the nose and the rotor blades cut into that oncoming flow. This oncoming air defines the blade’s relative wind and sets the effective angle of attack needed for lift.

Rotor downwash or vertical air movement from the rotor itself can create flow through the disk, but the fundamental cause of the relative wind the rotor blades encounter during forward flight is the forward speed of the aircraft. The other options describe air movements that aren’t the primary driver of the relative wind perceived by the blades.