Expert: Paul Soderman - 10/29/2000

Question
I am doing a final year project of B.E Mechanical Engineering on " Design and fabrication of a variable pitched, computer controlled aerogenerator( wind turbine)". My question is related to design of its blades. We are using NACA 2215 aerofoil, and have calculated aerofoil sections and their setting angle or pitch angle with other parameters,along the length of blade (i.e at different radial distance from hub). Now, the problem is that we do not know how one section must be placed relative to other sections. We have seen wind turbines having straight leading edge and thus, all the alignments to be on the trailing edge, and viceversa. Also, in some cases alignment was on both edges; but we do not know why one arangment is preferred on other.
  Please help us as no book we have access to answer this question.

Answer
If I understand, your question is - along what line should a wind turbine blade be twisted ?

I haven't worked much with with wind turbines, but our wind tunnel fans and all propellers are twisted with the tip down.  Usually, this is to enable a constant spanwise loading as much as possible.  The twist is chosen based on the spanwise velocity vectors (rotational and induced).  Another goal might be to have constant angle of attack along the span, but I believe constant loading is preferred.

Now, how to twist.  There may be mechanical or fabrication advantages to a twist about the leading or trailing edge.  But from an aerodynamic standpoint, the twist should be about the airfoil quarter chord.  At the quarter chord, the aerodynamic moments are constant (independent of angle of attack).  Thus, the torque that the blade would have to resist would be much more uniform along the span than it would if the blade were twisted about the leading edge for example.  Hence the forces and moments would be easier to handle by the structural elements.  (If the airfoil section were symmetrical, the moments about the quarter chord are zero.  Your blade will have a nose-down moment due to camber.)  With a leading edge twist, you could still carry the load to the root along the quarter chord, but the path is curved and more complicated structurally.