Evolution of Modern Wind Turbines Part A: 1940 to 1994 - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 147

that the rotor speed passes rapidly through resonance with tower bending modes (and
others) during starting and stopping. As shown in Figure 3-28, low-frequency cylindrical
steel designs require much less material than stiff towers of the same height, and therefore
cost considerably less. Safe, soft-tower design technology also permits taller towers, to take
advantage of positive wind shears and increase the average wind speed at the rotor.
Figure 3-28. Trend of HAWT tower weight with height, stiffness, and type of
construction. [Frederick and Savino 1986]