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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. 

## Blade Twist 

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] 

## Rotor Blade and Hub Definitions 

• 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 

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. 

## Airflow and Reactions in the Rotor Disk 

## Relative Wind 

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: 

• 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] 

## Rotational Relative Wind (Tip-Path Plane) 

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] 

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