savings and corrosion resistance that it provides may make it attractive for this application.

The wood/epoxy composite blades produced by Howden also use the pivoting tip arrangement. While operating hours and machine numbers are not nearly as large as for the Danish designs, application with the wood/epoxy material system has not shown any unusual problems to date.

The Enertech and ESI designs of U.S. origin both used pivoting tip plates for overspeed protection. These were retained in place using threaded rod epoxy bonded into a veneer buildup at the blade tip. The load takeoff principle is identical to the well-tested root stud method and has proven highly reliable in service, except for cases of massive overspeed due to failure of the latch mechanism to deploy. This is not a material knowledge or fatigue problem.

Future wind turbines may employ techniques such as outboard blade ailerons or boundary layer control for overspeed protection. Such methods are not only potentially preferable from an aerodynamic standpoint, but also require much less disruption of the primary load path within the blade and could thereby reduce the possibility of long-term fatigue failure as well.

BLADE ROOT RETENTION

Glass-Reinforced Plastic (Grp) Blade Roots

For both fiberglass and wood/epoxy composites, the blade-to-hub interface requires a high-performance fatigue design that can take loads from the composite blade structure into a metal hub assembly. Mating these dissimilar materials in high-cycle fatigue has been a serious design challenge in both material systems.

Two major Danish fiberglass blade builders used a flanged root design. One variation, called the Hutter root after its originator, used thick unidirectional roving bundles wrapped around tubular bushings within the flange (Figure 4-5). These thick bundles bent around the root radius and terminated well up inside the root tube, so that an extensive bonding area was provided to the skin and spar reinforcing. Since the bolts attaching the blade to the machine passed through the flange bushings, the blade was

Figure 4-5

Flanged GRP blade root design. Source: Stoddard (1989).



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