Balsa sheet is the lightest timber used in modelling. It cuts with one pass of a sharp blade, sands down in moments and, for its weight, holds a shape remarkably well, which is why it becomes sheeting, ribs, infill and whole airframes for flying models.
Balsa sheet is graded by density, and that figure decides how hard and how stiff the board is. Everything else about a board follows from it: how deeply it dents, how cleanly it cuts and how much load it will carry before it folds.
A lower density means a lighter, softer board for sheeting and infill; a higher one gives stiffer stock for structural parts. The grades are named in grams per cubic decimetre, which is the same figure as kilograms per cubic metre. Choose by grain direction as well: straight-grained board resists bending, cross-grained board curves more willingly around a former. Where balsa stops being stiff enough, for formers and wing roots, Finnish aircraft plywood takes over, and hardwood strip and dowel are in wooden profiles and strip.
