Airflow and Reactions in the Rotor Disk Relative Wind Blade Twist  Rotor Blade and Hub Definitions Rotational Relative Wind (Tip-Path Plane) Section near root A Section in center B Section near tip C A B C Tip Trim tab Root Note: “More nose-down” tilt to blade section closer to tip Figure 2-14.  Blade twist. has advantages and disadvantages. The advantages are more  lift production at a given AOA than a symmetrical design,  an improved lift-to-drag ratio, and better stall characteristics.  The disadvantages are center of pressure travel of up to 20  percent of the chord line (creating undesirable torque on the  airfoil structure) and greater production costs. Knowledge of relative wind is essential for an understanding  of aerodynamics and its practical flight application for the  pilot. Relative wind is airflow relative to an airfoil. Movement  of an airfoil through the air creates relative wind. Relative  wind moves in a direction parallel to but opposite of the  movement of the airfoil.  [Figure 2-15] There are two parts to wind passing a rotor blade: Because of lift differential due to differing rotational relative  wind values along the blade, the blade should be designed  with a twist to alleviate internal blade stress and distribute  the lifting force more evenly along the blade. Blade twist  provides higher pitch angles at the root where velocity is  low and lower pitch angles nearer the tip where velocity  is higher. This increases the induced air velocity and blade  loading near the inboard section of the blade.  [Figure 2-14] The rotation of rotor blades as they turn about the mast  produces rotational relative wind (tip-path plane). The  term rotational refers to the method of producing relative  wind. Rotational relative wind flows opposite the physical  flightpath of the airfoil, striking the blade at 90° to the  leading edge and parallel to the plane of rotation; and it is  constantly changing in direction during rotation. Rotational  relative wind velocity is highest at blade tips, decreasing  uniformly to zero at the axis of rotation (center of the  mast).  [Figure 2-19] The angular position of the main rotor blades (as viewed from  above, as they rotate about the vertical axis of the mast) is  measured from the helicopter’s longitudinal axis, and usually  from its nose. The radial position of a segment of the blade is  the distance from the hub as a fraction of the total distance.  2-9 •  Horizontal part—caused by the blades turning  plus movement of the helicopter through the air  [Figure 2-16]  •  Vertical part—caused by the air being forced down  through the rotor blades plus any movement of the air  relative to the blades caused by the helicopter climbing  or descending  [Figures 2-17  and  2-18] •  Hub—on the mast, the attaching point for the root of  the blade, and the axis about which the blades rotate. [See Figure 1-7] •  Tip—the farthest outboard section of the rotor blade •  Root—the inner end of the blade and is the point that  attaches to the hub •  Twist—the change in blade incidence from the root  to the outer blade 