decreases the engine rpm, reducing fuel consumption and  engine wear, and keeps thrust at a maximum. For climb after takeoff, the power output of the engine is  reduced to climb power by decreasing the manifold pressure  and increasing the blade angle to lower the rpm. Thus, the  torque (horsepower absorbed by the propeller) is reduced to  match the reduced power of the engine. The AOA is again  kept small by the increase in blade angle. The greater mass  of air handled per second, in this case, is more than offset by  the lower slipstream velocity and the increase in airspeed. At cruising altitude, when the airplane is in level flight and  less power is required than is used in takeoff or climb, engine  power is again reduced by lowering the manifold pressure  and increasing the blade angle to decrease the rpm. Again,  this reduces torque to match the reduced engine power; for,  although the mass of air handled per revolution is greater, it  is more than offset by a decrease in slipstream velocity and  an increase in airspeed. The AOA is still small because the  blade angle has been increased with an increase in airspeed.  Pitch distribution is the twist in the blade from the shank to  the blade tip, due to the variation in speeds that each section  of the blade is traveling. The tip of the blade is traveling  much faster than the inner portion of the blade. Fixed pitch propellers have no controls and require no  adjustments in flight. The constant-speed propeller has a  propeller control in the center pedestal between the throttle  and the mixture control.  [Figure 7-10]  The two positions for  the control are increase rpm (full forward) and decrease rpm  (pulled aft). This control is directly connected to the propeller  governor and, by moving the control, adjusts the tension on  the governor speeder spring. This control can also be used to  feather the propeller in some aircraft by moving the control  to the full decrease rpm position. The two main instruments  used with the constant-speed propeller are the engine  tachometer and the manifold pressure gauge. Rotations per  minute (rpm) is controlled by the propeller control and the  manifold pressure is adjusted by the throttle. Tractor propellers are those mounted on the upstream end of  a drive shaft in front of the supporting structure. Most aircraft  are equipped with this type of propeller. The tractor type of  propeller comes in all types of propellers. A major advantage  of the tractor propeller is that lower stresses are induced in  the propeller as it rotates in relatively undisturbed air. Pusher propellers are those mounted on the downstream  end of a drive shaft behind the supporting structure. Pusher  propellers are constructed as fixed- or variable-pitch  propellers. Seaplanes and amphibious aircraft have used a  greater percentage of pusher propellers than other kinds of  aircraft. On land planes, where propeller-to-ground clearance  usually is less than propeller-to-water clearance of watercraft,  pusher propellers are subject to more damage than tractor  propellers. Rocks, gravel, and small objects dislodged by  the wheels are quite often thrown or drawn into a pusher  propeller. Similarly, planes with pusher propellers are apt to  encounter propeller damage from water spray thrown up by  the hull during landing or takeoff airspeed. Consequently,  the pusher propeller is mounted above and behind the wings  to prevent such damage.  There are various types or classes of propellers, the simplest  of which are the fixed-pitch and ground-adjustable propellers.  The complexity of propeller systems increases from these  simpler forms to controllable-pitch and complex constant- speed systems (automatic systems). Various characteristics  of several propeller types are discussed in the following  paragraphs, but no attempt is made to cover all types of  propellers. As the name implies, a fixed-pitch propeller has the blade  pitch, or blade angle, built into the propeller.  [Figure 7-11] The blade angle cannot be changed after the propeller is  built. Generally, this type of propeller is one piece and is  constructed of wood or aluminum alloy. Fixed-pitch propellers are designed for best efficiency at  one rotational and forward speed. They are designed to fit  a set of conditions of both airplane and engine speeds and  any change in these conditions reduces the efficiency of  both the propeller and the engine. The fixed-pitch propeller  Propeller Controls and Instruments Propeller Location Tractor Propeller Pusher Propellers Types of Propellers Fixed-Pitch Propeller Figure 7-10.  Turboprop propeller controls.  7-6