So you throw away about 10% of the energy. Hmm. Actually EDY, a decent airfoil can provide substantial lift over a range of attack angles (the angle between the apparent wind and the blade; usually good to 12 degrees). Furthermore the lift coefficient increases with attack angle. Lift or power out also depends on the apparent wind speed which decreases toward the hub, and on the blade width. So there are a number of design parameters that can be fiddled with. Hence different blade angles for different designs. In my opinion it's not necessary, for instance, for the blade to get wider toward the root. One could use a constant width blade and increase the attack angle toward the root, but the twist in the blade might become severe (around 70 degrees?) requiring a thick piece of material.
