Figure 2-9

CHAPTER 2

HELICOPTER AERODYNAMICS WORKBOOK

Figure 2-9

On nonsymmetrical airfoils, the center of pressure of upper and lower surfaces do not act

directly opposite each other, and a pitching moment is produced. As the AOA changes, the

location of the distributed pressures on the airfoil also changes. The net center of pressure (sum

of upper and lower) moves forward as AOA increases and aft as AOA decreases, producing

pitching moments. This characteristic makes the center of pressure difficult to use in

aerodynamic analysis. Since the moment produced about the aerodynamic center remains

constant for pre-stall AOA, it is used to analyze airfoil performance with lift and drag

coefficients.

Pitching moments are an important consideration for airfoil selection. Torsional loads are

created on the blades of positively cambered airfoils due to the nose down pitching moment

produced during increased AOA. These torsional loads must be absorbed by the blades and

flight control components, and initially this resulted in structural blade failure and excessive

nose-down pitching at high speeds. Early helicopter engineers consequently chose symmetrical

airfoils for initial designs, but have since developed cambered blades and components with high

load-bearing capacity and fatigue life.

For the TH-57, rotor blade designers combined the most desirable characteristics of

symmetrical and nonsymmetrical blades, resulting in the "droop-snoot" design (figure 2-10).

This incorporates a symmetrical blade and a nonsymmetrical "nose" by simply lowering the nose

of the blade. The resulting blade performance characteristics include low pitching moments and

high stall AOA the retreating blade. The significance of this second characteristic will be

covered in chapter 3.

2-8 ROTOR BLADE AERODYNAMICS