Those were spinners to drive the sirens in the spats that were turned on when the Stuka went into a dive; the siren wail was an additional psychological aspect of a Stuka attack -- increase the schrecklichkeit ('frightfulness'). The sirens were added after it was noted that the dive brakes of the Stuka would emit a howling sound near the end of a high-speed dive. The siren, called the 'Jericho Trumpet', fell out of use during the Battle of Britain because the drag from the propellor decreased the speed and maneuverability of the already vulnerable aircraft.
