This is a Reschke 50 cm. or 19 inch wind-driven Hilfspropeller.  It was typically mounted onto a generator which was attached to an undercarriage strut or leg.  The generator and propeller were positioned such that the wind stream generated by the aircraft's main propeller and the forward momentum of the aircraft in flight would drive the little Hilfspropeller to turn and by so turning the propeller would force the generator to create an electrical current which in turn would power equipment on the aeroplane, such as heated suits for flightcrew. Below you can see the metal hub removed.