Which term corresponds to the plane traced by the blade tips during rotation?

• A. Chord Line
• B. Angle of Incidence
• C. Tip Path Plane
• D. Resultant relative wind

Answer: (C)

The planed geometry traced by the outer ends of the blades as they rotate is called the tip path plane. It represents the plane in which the blade tips move around the hub. In hover, this plane is essentially aligned with the rotor disk, but in forward flight the plane tilts as a result of flapping and cyclic input, which helps balance lift between the advancing and retreating blades. The other terms describe different blade characteristics: the chord line is simply the line along the blade’s cross-section, the angle of incidence is the blade’s angle relative to the local wind, and the resultant relative wind is the vector sum of the aircraft’s airspeed and the induced flow. Thus, the term that specifically corresponds to the plane traced by the blade tips is the tip path plane.