Its rollout comes seven years after the launch of LM's previous longest 61.5-metre blade, the LM 61.5P, which was the world's longest series-manufactured rotor blade.
What is immediately striking about the blade is its enormous length compared with the small root diameter. Prominently visible at the round blade section measured from the root is a surprisingly simple-looking spoiler attachment. LM CTO Frank Nielsen explains that the spoiler geometry fits well with the inner blade section, enhancing aerodynamic-lift performance without substantially increasing resistance or drag.
Another interesting surface feature is the polyurethane tape. This has the feel of rubber and is used as an erosion-protection measure around the rounded leading edge.
The blade technology itself builds on LM's slender blade GloBlade programme, which first resulted in a 42.1-metre blade followed by larger 3MW sister products designed for rotor diameters up to 125 metres.
GloBlade combines advanced aerodynamics with in-house developed proprietary airfoils and proven materials technology. Nielsen said: "Materials technology is a decisive parameter to design and manufacture this relatively lightweight fibreglass blade for its length. We had to push glass/polyester blade technology to the next level to create a 73.5-metre blade for offshore application without using carbon."
LM's new airfoils are developed and tested in their own low-speed wind tunnel, with a capability to test blade sections facing real environmental conditions.
The combination of the blade monitoring system and turbine individual pitch control enabled a 10% reduction in load and blade deflection. "Thanks to the work done on the design together with Alstom experts, we were also able to retain a 3.2-metre root diameter similar to the LM 61.5P, while limiting blade mass to only 26 tonnes," says Nielsen.
A specialised team manufactures the upper and lower shells and separate shear-web reinforcements inside a hall dedicated to prototypes. After the glass fabric layers and balsa wood fillings have been put precisely into a mould, it is sealed by plastic foil. As a next step polyester resin is pulled into the mould by negative pressure and impregnates the fibres already laid out in the mould followed by a curing process. Nielsen says: "A key advantage of applying polyester against epoxy resin is it infuses very well and cures at room temperature, which makes it safe and secure."
Each new developed rotor blade is finally exposed to a static extreme loads bending test followed by a dynamic two-directional fatigue full 20-25 year life test. The programme is completed by a post-fatigue static test lasting in total about one year.