PZL P.11c

The machine was supposed to be a “new P.7 with a different engine”, but in fact it became a completely new design. The strength calculations were carried out by engineer Piotr Bielkowicz. He designed a strong airframe, withstanding practically any load occurring in flight – the destructive coefficient for the wing was 19.1, and for the fuselage and tail 15, with a maximum of 13.5 in flight. It is worth remembering that these values were applied to the prototype, the PZL P.11c version had an even stronger airframe (especially the fuselage). Even the bombing attempts from a diving flight carried out before the war, with an additional (unforeseen in the design) load, caused only damage to the ailerons. It was great news for the Polish aviation after fresh experiences with SPAD 61C1s fighters, which had airfoils below the design strength and much below that in flight, and a significant improvement over the PZL P.1, whose poor airfoil sheeting almost eliminated the design from development (the accident of Major B. Orliński in August, 1928). The first prototype, with a smooth sheet metal wing, tail and fuselage borrowed from the PZL P.6, flew in August, 1931, with a weaker than expected Gnome et Rhône Jupiter VII 9Asb engine (French version of the Bristol engine). It had no cylinder covers, only individual fairings, later replaced by a narrow Townend ring. This copy appeared at various fairs and shows in Europe, arousing interest (and even willingness to buy, expressed by Romania, Yugoslavia, Portugal and Turkey). The second prototype – the P.11/II – received the expected Bristol Mercury IVA engine, as well as a wide engine cover, initially with exhaust pipes protruding halfway (from each cylinder). The ground tests were initially unsuccessful, the machine was vibrating strongly. Therefore, flight tests began late in December, 1931. Various propellers (Chaviere, Ratier, Letov) and fairings behind the cabin were tested, only the replacement of the engine with a new one and the propellers for the Bristol product (the Szomański propeller turned out to be better) eliminated most of the vibrations and the plane began to fly correctly. The third prototype powered by the Bristol Mercury IVS2 engine (475/555 hp) was intended for the US tour and participation in the National Air Races in Cleveland. It was the first (apart from the one for static tests) P.11 with a fine-grained surface and tail cover. The performance was unsuccessful, though, and the prototype was damaged.

pzl zd4