Wind Turbine Airfoils and Rotor Wakes - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 394

Comparative performance tests, with and without these tip devices, were conducted on
a two-bladed commercial HAWT, 10 m in diameter. Each of the tip devices replaced a
length of blade equal to approximately 5 percent of the rotor radius, so the swept area re-
mained constant during these tests. Modest performance improvements of the order of 3
percent were predicted for these tips, using the best available non-planar aerodynamic model.
However, a small but significant reduction in power of about 3 percent to 6 percent was mea-
sured. While the theoretical improvement may still be achievable by proper adjustment of the
non-planar elements, these tip devices do not appear promising for significantly increasing
the power of a HAWT rotor.
Figure 6-14 illustrates a tip shape that has been found to be more desirable for HAWT
blades [Tangler 1993]. Its characteristics are

-- rounded leading-edge corner , to eliminate the start of separation caused by

large, three-dimensional adverse pressure gradients at a sharp corner;

-- finite tip chord (approx. c /2), to maintain a good Reynolds number and lift-to-

drag ratio at the tip;

-- straight trailing edge , to provide a pitching moment that is dynamically stable

( i.e. nose-down).

This tip shape has been used successfully on SERI airfoil blades (Fig. 6-10(b)), both thin
and thick.

Figure 6-14. Aerodynamically desirable tip shape for a HAWT blade. [Tangler 1993]