This is a Reschke
wind-driver generator propeller or 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
Below you can see
the metal hub removed.