Figure 12-2.  Feathered propeller. As a review, the constant-speed propellers on almost all  single-engine airplanes are of the non-feathering, oil- pressure-to-increase-pitch design. In this design, increased  oil pressure from the propeller governor drives the blade  angle towards high pitch, low rpm. In contrast, the constant-speed propellers installed on most  multiengine airplanes are full feathering, counterweighted,  oil-pressure-to-decrease-pitch designs. In this design,  increased oil pressure from the propeller governor drives  the blade angle towards low pitch, high rpm—away from  the feather blade angle. In effect, the only thing that keeps  these propellers from feathering is a constant supply of high- pressure engine oil. This is a necessity to enable propeller  feathering in the event of a loss of oil pressure or a propeller  governor failure. The aerodynamic forces alone acting upon a windmilling  propeller tend to drive the blades to low pitch, high rpm.  Counterweights attached to the shank of each blade tend  to drive the blades to high pitch, low rpm. Inertia, or  apparent force (called centrifugal force) acting through  Completely securing a failed engine may not be necessary  or even desirable depending upon the failure mode, altitude,  and time available. The position of the fuel controls, ignition,  and alternator/generator switches of the failed engine has no  effect on aircraft performance. There is always the distinct  possibility of manipulating the incorrect switch under  conditions of haste or pressure. To unfeather a propeller, the engine must be rotated so that  oil pressure can be generated to move the propeller blades  Feathering a propeller only alters blade angle and stops  engine rotation. To completely secure the engine, the pilot  must still turn off the fuel (mixture, electric boost pump,  and fuel selector), ignition, alternator/generator, and close  the cowl flaps. If the airplane is pressurized, there may also  be an air bleed to close for the failed engine. Some airplanes  are equipped with firewall shutoff valves that secure several  of these systems with a single switch. To feather the propeller, the propeller control is brought fully  aft. All oil pressure is dumped from the governor, and the  counterweights drive the propeller blades towards feather. As  centrifugal force acting on the counterweights decays from  decreasing rpm, additional forces are needed to completely  feather the blades. This additional force comes from either  a spring or high-pressure air stored in the propeller dome,  which forces the blades into the feathered position. The entire  process may take up to 10 seconds. the counterweights, is generally slightly greater than the  aerodynamic forces. Oil pressure from the propeller governor  is used to counteract the counterweights and drives the blade  angles to low pitch, high rpm. A reduction in oil pressure  causes the rpm to be reduced from the influence of the  counterweights.  [Figure 12-4] Figure 12-3.  Propeller drag contribution. Low Pitch High Pitch Full Feathered 90° Change in Equivalent Parasite Drag Propeller Blade Angle 0 15 30 45 60 90 W i n d m i l l i n g p r o p e l l e r S t a t i$_{o}$$_{n}$ a r y p r o p e l e r Feathered position Flat blade position 12-4