Fluid Mechanics and Thermodynamics of Turbomachinery, 5e

C C
0.4
0.2
–0
–0.2
–0.4
–0.6
twist). Because of this direct relationship between chordwise force and rotor torque,
CC serves as a convenient parameter for evaluating an aileron’s braking effectiveness.
Thus, if CC is negative it corresponds to a negative torque producing a rotor
deceleration. Clearly, it is desirable to have a negative value of CC available for all
angles of attack. Figure 10.28 shows some experimental results, Snyder et al. (1984),
illustrating the variation of the chordwise force coefficient with the angle of attack, a,
for aileron percent chord of 20 and 30% for several aileron deflection angles. The
general conclusions to be drawn from these results is that increasing the aileron chord
length and the aileron deflection angle contribute to better aerodynamic braking
performance.
Blade tip shapes
–0.8
–10 0 10 20 30 40 50
Angle of attack, deg
a = 20% chord
–60 deg
30% chord
–60 deg
30% chord
–90 deg
Wind Turbines 369
FIG. 10.28. Effect of chord length on chordwise force coefficient, C C, for a range of
angles of attack. (Adapted from Snyder et al. 1984, unpublished).
The blade geometry determined with various aerodynamic models gives no guidance
of an efficient aerodynamic tip shape. From a basic view of fluid mechanics, a strong
shed vortex occurs at the blade tip as a result of the termination of lift and this together
with the highly three-dimensional nature of the flow at the blade tip causes a loss of
lift. The effect is exacerbated with a blunt blade end as this increases the intensity of
the vortex.
Many attempts have been made to improve the aerodynamic efficiency by the addition
of various shapes of “winglet” at the blade ends. Details of field tests on a number
of tip shapes intended to improve performance by controlling the shedding of the tip
vortex are given by Gyatt and Lissaman (1985). According to Tangler (2000), test experience
has shown that rounding the leading-edge corner, Figure 10.29, with a contoured,
streamwise edge (a swept tip) yields good performance. Tip shapes of other geometries
are widely used. The sword tip also shown is often chosen because of its low noise
generation, but this is at the expense of a reduction in performance.
a