Swiss Junkers Ju 52 belly at the Hahnweide Oldtimers Meet 2011.
The Ju 52 had a low cantilever wing, the mid-section of which was built into the fuselage, forming its underside. It was formed around four pairs of circular cross section duralumin spars with a corrugated surface that provided torsional stiffening. A narrow control surface, with its outer section functioning as the aileron, and the inner section functioning as a flap, ran along the whole trailing edge of each wing panel, well separated from it. The inner flap section lowered the stalling speed and the arrangement became known as the Doppelflügel, or "double wing".
Hahnweide Oldtimers Meet 2011
Canadian Airline Blog