From about 1950 to his death in February 1981, Iosif Silimon was Romania's most prominent glider designer, his aircraft distinguished by his initials.
[2] The cantilever high wing common to both models was built around a single spar with a plywood covered torsion box ahead of it around the leading edge.
Both had two seat, tandem cockpits with a moulded perspex canopy in two parts, stretching from a little behind the nose into the wing leading edge.
The underside of the IS-13's fuselage curved smoothly, gradually decreasing the diameter towards the tail but that of the IS-12 narrowed more abruptly under the wing.
Both models had a short landing skid under the forward fuselage and a semi-recessed, fixed and unsprung monowheel under the wing and fitted with a brake.
The horizontal tails, mounted on top of the fuselages, were similar straight tapered surfaces but set 130 mm (5.1 in) further forward on the fillet on the IS-13, with the result that the IS-12's cut-out for rudder movement in the elevators was not needed.