Each half-wing was built around a pair of wooden spars and was covered with plywood, then joined centrally and supported just above the fuselage on cabane struts.

The cowling blended smoothly into the rest of the fuselage which was a 2 mm (0.08 in) thick ply-skinned semi-monocoque, tapering to the rear.

The pilot's open cockpit was just behind the trailing edge of the wing, where there was a small cut-out to improve his field of view.

The Sp.I's tailplane and fin were integral parts of the fuselage and also ply covered, as were the control surfaces; the elevators were divided.

The latter were soon corrected by a reduction in fuselage length and some rudder modifications and the aircraft was re-engined with a 89 kW (120 hp) Salmson 9Ac, a nine-cylinder modern radial.