Figure 2-7

CHAPTER 2

HELICOPTER AERODYNAMICS WORKBOOK

Figure 2-7

AIRFOILS

Airfoils fall into two categories: symmetrical and nonsymmetrical. A symmetrical airfoil

has identical size and shape on both sides of the chord line, while a nonsymmetrical airfoil has a

different shape and size on opposite sides of the chord line. Cambered airfoils are in the

nonsymmetrical category (figure 2-2).

PITCHING MOMENTS

Now let us investigate the different aerodynamic characteristics of these airfoils regarding the

aerodynamic center and center of pressure of each type. The aerodynamic center is the point

along the chord where all changes in lift effectively take place and where the sum of the

moments is constant. The sum of the moments is constant for any AOA. On a symmetrical

blade, the moment is zero. The center of pressure is the point along the chord where the

distributed lift is effectively concentrated and the sum of the moment is zero. On symmetrical

airfoils, it is co-located with the aerodynamic center. On cambered airfoils, the center of

pressure moves forward as AOA increases. The center of pressure of the upper and lower

surfaces of a symmetrical airfoil act directly opposite each other. The aerodynamic center and

center of pressure are co-located; therefore, no moment is produced even though the total lift

force changes with change in AOA (figures 2-8 and 2-9).

2-6 ROTOR BLADE AERODYNAMICS