The design of the rotor plays a very important role in the overall performance of a wind turbine; the rotor is also one of the most complex parts to properly design. While the hope with the bamboo designs is to create a blade flexible enough to be able to furl in high winds, I have also been investigating blades with much more complicated geometries (changing chord length, twist angle, and airfoil geometry along the radius of the blade) that would allow them to stall in high winds as a form of power control.
Airfoil geometry lies at the heart of blade design. Each and every airfoil exhibits its own unique traits as a function of angle of attack (alpha) and the local Reynolds number (Re). The characteristic of most interest to us is the ratio of lift forces to drag forces (Cl/Cd). Some airfoils are characterized by a large Cl/Cd for a wide range of alpha, while other airfoils have a much more defined “sweet spot” after which the performance of the airfoil degrades significantly. In addition, some airfoils respond better to different types of flow and blade soiling. It is very important to understand these characteristics when attempting to optimize the shape of the blade.
I am currently basing my design off of two airfoils (FX 63-137 and S834) for the root and the tip of the blade. NREL has developed several airfoils for small rotors (S822, S823, S833, S834, S835); if time allows, I’d like to compare the performance of the FX 63-137 airfoil to other thick airfoils designed for the root of a blade, such as S823 and S833.