Circular arc airfoil wing: why does the wing nose down twist due to camber?
I think it may be related to the geometric construction of the blade : 

Usually taper is made by scaling down airfoil at wingtip, which doesn't alter angle of attack at all. 



In this case the airfoil itself is cut along a line going from root trailing edge to tip mid-chord, which alters angle of attack with desired negative twist.