NOVELTY - The extension portion (30) includes segments (130a-130e) located adjacent one another in a span wise manner. Each segment has a first surface (140a) spaced from second surface (150a) at a proximal region (170), and connected to the second surface at distal region (160) to form an extended trailing portion of rotor blade. The segments are made from fiber reinforced plastics material, thermoplastic material, wood or laminate material.    USE - Extension portion for rotor blade of wind turbine (claimed).    ADVANTAGE - Provides an extension portion to enable blade portion of rotor blade to retain a more constant chord, thus optimizing aerodynamic performance of the rotor blade. Improves structural loading and design of rotor blade, while reducing chord-wise loading caused by directing loads around an extreme trailing edge of rotor blade, by straightening rearmost edge of blade portion via extension portion. Forms surfaces of segments of extension portion from a light weight material to minimize contribution of extension portion to overall weight of rotor blade and experienced edge wise loads. Inhibits transmission of loads between, and fluid leakage through the segments of extension portion through the provision of an interface and a sealing unit, respectively. Facilitates transportation of rotor blade as the extension and blade portions can be separately transported to an installation site. Lowers transportation cost of rotor blade by reducing maximum width of the rotor blade. Requires smaller ovens for curing and occupies less room for main shell mold since blade portion of rotor blade and main shell mold are narrowed. Enhances product flexibility by changing or replacing extension portions when damaged.    DETAILED DESCRIPTION - The transmission of loads between, and fluid leakage through the segments are inhibited by an interface (180) and a sealing unit, respectively. The sealing unit can be a brush or lip seal, or one of the group of elastomeric, inflatable and labyrinth seals. The sealing unit includes a sealing member made from elastomeric material. INDEPENDENT CLAIMS are included for the following:    (1) a wind turbine; and    (2) a wind turbine installation.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic perspective view of the rotor blade extension portion.    Extension portion (30)    Segments (130a-130e)    First surface (140a)    Second surface (150a)    Distal region (160)    Proximal region (170)    Interface (180)