Balsa boards with a density of up to one hundred and thirty grams per cubic decimetre. Modellers favour this figure because it combines low weight with enough stiffness to be useful: the board does not give way under a thumb, it stays flat on the bench, and it still adds almost nothing to the all-up weight.
Typical uses are ribs, formers, the more heavily loaded areas of sheeting, and infill wherever the part will be handled during assembly. The figure turns straight into a weight budget: a sheet 100 mm wide, one metre long and 2 mm thick works out at roughly 26 grams at this density, so the sheeting of a small wing can be totted up before anything is cut. How the density grades relate to one another is explained in balsa sheet for models, with harder stock for formers and wing roots in Finnish aircraft plywood.
