M.Simons The World's Vintage Sailplanes 1908-45
In France after the military collapse in 1940, with permission from the Germans, gliding was encouraged by the Vichy Government. Engineers of the SNCASO group decided to work on a high performance sailplane. The result was the all-metal
SO-P1. It first flew in 1941.
The wing was notable for its very thin aerofoil, made possible only by the metal spar. This was a built-up box of sheet webs and angle extrusions, situated at 25% of the chord aft from the leading edge. The upper and lower flanges were heavy gauge plates which also formed part of the outer skin of the wing. To the flange plates were riveted long lengths of piano-type hinge. These were used to attach the separately built leading and trailing portions of the wing. For maintenance and repair these whole sub-assemblies could be separated again by withdrawing the hinge pins. The front part of the wing formed the torsion box, with built-up metal ribs and alloy skin. Aft of the spar, the wing was fabric covered over pressed metal sheet ribs. A light, secondary spar of 'I' section carried the ailerons, which were slotted and balanced, hinged on external brackets underneath the wing, and driven by internal pushrods.
Upper and lower surface airbrakes were fitted. The wing was mounted on the fuselage at shoulder level and had a pronounced 'gull' dihedral, the outer panels even drooping slightly, and the extreme tip canted up again at a small angle. The mainspar was connected to the fuselage main frame by a single tapered bolt, drag and torsion loads being carried by an internal diagonal strut, pin-jointed at both ends.
The fuselage was unusually wide, 760 mm instead of the usual 600. This was, according to the designers, not only to provide a comfortable cockpit but to minimise the fuselage drag in the presence of the wing. Internally there were two strong main frames and a diagonal frame forming the seat back, with a central keel member running forward, carrying the seat, controls, and landing skid attachments. The rest of the fuselage was a metal shell structure approaching the true monocoque, with no longerons and only light metal hoops to maintain the shape and stiffness. The skins were 1 mm thick in the forward regions, reduced to 0.8 mm for the tail cone, and 0.64 mm at the extreme nose. The cockpit canopy was a plastic moulding. Aft, the fuselage tapered to a point so the tail was somewhat vulnerable. With a wingtip down the elevator could touch the ground.
In 1942 when the Germans occupied the whole of France, the sailplane was put into storage. When the war ended, the SO-P1 needed its fabric covering renewed but was otherwise ready for flight. In 1947 it went with a government-sponsored French team to the US National Championships held at Wichita Falls in Texas. The SO-P1 made a flight of 354 km, which was at the time a national record for France.
SO-P1: Span, 16.00 m. Wing area. 16 sq m. Aspect ratio. 16. Empty weight. 250 km. Flying weight, 341 kg. Wing loading, 21.3 kg/sq m. Aerofoil, SNCASO special design. 10% thick. Best glide. 1 : 30. Minimum sinking speed, 0.7 m/sec at 70 km/h.