Determination of the Blade Drag Coefficient

The average value of the drag coefficient, cd, for use in the anajy^, obtained by using an average value of angle of attack and the corresp^ ls may ^ coefficient as determined by wind tunnel tests of two-dimensional ai^f ^ing Figure 1.10 shows the characteristics of the NACA 0012 airfoil sectl^ Actions typical of the airfoils actually used on blades. These characteristics hav^ ?> Miich is from reference 1.1 and were synthesized from whirl tower tests of taken rotor over a wide range of collective pitch and tip speed by choosing * c°mplete drag coefficients that brought the test results into the best agreeing lift arJ theory. The results of two-dimensional wind tunnel tests of the same ^ Mth the used as a guide. Figure 1.10 shows the lift and drag coefficients as

^ions of

both the angle of attack and the Mach number. Also shown are other characteristics of importance to the helicopter aerodynamicist, which will be discussed later. The average lift coefficient for the rotor, is:

Сі — ай

But it was shown earlier that

cі ——

therefore,

6CT/o

a =———–

a

If it is assumed that the slope of the lift curve is equal to 6 per radian,

then:

a = Су/a radians

or

a = 57.3 C/ /a degrees

For the example helicopter at sea level, the value of Ct/g is 0.086, and thus the average angle of attack is 4.9°. The Mach number at the 75% radius station— which may be considered typical of the entire blade—is 0.43. From Figure 1.10, the corresponding value of the drag coefficient is about 0.010.