All modern propeller-driven aeroplanes have spinners over their propeller hubs. These spinners have the dual aerodynamic function of streamlining the engine installation and directing cool air into the openings in the cowling. The diagram below shows a typical spinner installation over a constant-speed propeller. The spinner bulkhead is installed on the propeller shaft flange and held in place by six spinner attaching bolts. The propeller is then installed so that the dowel pins in the propeller hub align with the holes in the flange. The propeller attaching nuts are installed and tightened to the torque value specified in the airframe maintenance manual.