I use Inkscape for my aeromodelling plans which is just a simple 2-D SVG drawing tool. It gives you Bezier curves, straight lines, standard shapes (quadrilaterals, circles, ellipses) and most importantly for me scaling and rotation of shapes in 1/100th of one degree steps. It has quite a few output formats, including dxf which is handy if you want to drive any kind of cutting machine including laser cutters or CNC*. It's dead easy to get to grips with, an hour or two tops. The Bezier tool is especially handy for tracing from existing plans/images.
If you need "proper" CAD features have a look at LibreCAD. Understands most of the common CAD file formats including dxf.
I don't usually use it because the font support isn't good (it has a poor selection of baked in fonts - very 1980s ) and it can't import PDFs which is a pain as there are a lot of tatty old plans that I've been able to tidy up in Inkscape because it can.
Both are properly free and run on Windows, *nix. Inkscape also runs on OS-X.
Lurk.
*I have a set of aluminium French curves CN cut for me from my Inkscape source DXFs.