Abstrac~The design and selection procedure of airfoil sections for small wind turbine blades is discussed and implemented in this paper. It is found that for blades up to 5 m long, two different airfoils mixed at the outer third of the span will be sufficient and have demonstrated good strength and aerodynamic characteristics. The rotorblade design was executed using glassfiber reinforced plastic and then installed on a 15 kW grid-connected-pitch-controlled machine. A static proof-load test indicated that the blade could withstand loads 10 times the normal working thrust, and a field performance test showed that the rotorblade has a 41.2% measured average power coefficient.
Technical Note these airfoil sections have their camber farther back, which provides some improvement in reducing the airfoil's C~max sensitivity to roughness effects. In addition, NACA 63nnn provides a lower C~ .... which helps control peak power. However, this characteristic is desirable only at the tip region of the blade, and when used at the inboard region, a degradation in energy production is expected. The LS-1 series have the opposite problem. This airfoil provides a desirable high Clmax toward the blade root, but contributes to excessive peak power when used at the outboard portion of the blade. This excessive peak power must then be controlled with an undesirable reduction in blade solidity or a less efficient blade operating pitch angle. Therefore, a mixture of two or more sections will be used in order to mitigate this problem.
The FX-S airfoil family has in its series the characteristics required for the outboard region, and they seem to gather the characteristics of both profiles, as can be seen in Fig. 1.
This family has a stable lift coefficient (Ca) at high angles of attack (low wind speeds), and an optimum (C~) at low angles of attack (high wind speeds). In addition, their moment coefficient (Cm) is smooth and almost constant over the whole range of operating angles of attack. The inboard region must be thicker and should have more material to take the higher stresses, in addition to having a smooth transition to the circular flange at the root. NACA 63-621 will be selected for the inboard region, which has these characteristics and is very similar to the FX 66S-196 profile. This profile similarity will simplify the construction of the transition from inner to outer regions.
DETERMINATION OF BLADE DATA
The design and construction of sophisticated wind turbine blades requires enormous amounts of data. Most important are those describing the geometry and structural characteristics such as length, thickness distribution, chord length distribution, twist, root connection, etc.