Thanks very much for the backup Glen on the 'hollow' term, which seems to be somewhat esoteric as to its history and application. Your access to these kinds of resources is as always extremely impressive!!!
I understand what Manuel is going for here, a bit more descriptive term for what seems to be a rather unusual geometric shape in cross section, and eludes the standard terms such as 'triangular' or three edged 'trefoil' .
The hollow term seems defeated in degree as an accurate term as it is more applicable toward the noted German process for fullering to lighten and strengthen blades, and may refer to various cross sections.
The term 'foliate' seems better applied to decorative style than to geometric shape, and Im not sure 'keeled' will work either as it is a structural term which brings to mind instantly the boat cross section in which the arcs are convex rather than concave. Excellent word there Fearn!!
Mellifluous!! and well placed suggestion.......Manolo, your sense of humor is as always, priceless! on the keeling term.
While we may still be at 'square' one trying to find a better geometrically based term for the section on these blades, I really admire the excellent interaction here in working toward ideas.
All the best,
Jim