What was the Ju 52's "Doppelflügel"?

The Ju 52 had a low cantilever wing, with the center section built into the fuselage, and a structure of four pairs of circular-section duralumin spars under the corrugated skin characteristic of Junkers, which gave it torsional stiffness. Behind it, running along the entire trailing edge of each wing half and set well apart from it, ran a narrow control surface. Seen in profile it looked like a small auxiliary wing placed just behind the main one, hence its German name: Doppelflügel, "double wing."

That surface did two jobs. Its inboard section acted as a flap: lowering it reduced the stalling speed. The outboard section worked differentially as an aileron for roll control; it projected slightly beyond the wingtips and carried horn balances.

The idea of heavily balanced, separated surfaces was repeated at the rear. The braced horizontal stabilizer carried elevators with horn balance that also projected outward and left a visible gap relative to the stabilizer. All of this is part of the unmistakable silhouette of the Ju 52, nicknamed Tante Ju ("Auntie Ju") and Iron Annie, which can still be seen on the historic examples still flying in the 21st century, among them the Ju 52/3mg2e of the Lufthansa foundation, in whose in-flight photographs the Doppelflügel can be clearly seen. The Ju 52/3m was used in the 1930s mainly as a 17-seat airliner or as a utility transport for various civilian operators. From 1933, the Nazi regime demanded military versions from Junkers, and the Luftwaffe ended up acquiring thousands of examples as a basic transport; the main production version was the Ju 52/3mg7e.