Helicopter blades need a main spar for strength. A finished spar can be seen together with the mould on the right. The mould for the skin is open and you can see the mating surfaces created by the domed mirror screws as well some red PVA release agent which I used to use (top right). The finish on these is almost mirror and the blades needed no finishing other than trimming edges and balancing. The black blade is part of an experimental lightweight carbon-fibre version, and the section of white blade has been cut through to show the spar in position on the leading edge. Naturally the spar has to be made smaller than the inside skin dimension (a lot of work!).
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