When I have cut curved braces or scoops on the bottom of rafters and floor joists, I used an axe first to blast away the bulk of the wood. Then an adze to shape it more and to remove the axe marks. Then a draw knife to shape it more and remove the adze marks. Then a curved bottom/sole spoke shave to shape if more and remove the draw knife marks.
When it is done it is usually very smooth and of a nice gentle arc.
Jim Rogers
