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Chapter 3 - Engines: Knowledge of Engines Is Power

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             During a descent, your job is to maintain stable
          cylinder head temperatures (CHT) and oil temper-
          atures (i.e., keep their temperature indications in
          the green). On some airplanes, gear extension or
          even partial flap extension at high speeds can be
          used in lieu of large power reductions to start a
          descent (check your POH). While momentary power
          reductions aren’t as harmful if the power is
          immediately restored, large ones over long
          periods can be damaging. Try planning your
          descents so engine temperatures change
          slowly from their previous cruise values.


                       The Propeller
             Propellers come in all sizes and colors, but they
          are of two basic types: fixed pitch and constant
          speed. In an airplane with a fixed pitch prop, one
          lever—the throttle—controls both power and pro-
                                                               Fig. 46
          peller blade RPM (revolutions per minute). In a
          constant speed prop, there are separate con-
          trols for power and RPM.
             When you start your flight training, you’ll
          probably fly an airplane with a fixed pitch
          propeller. Fixed pitch propellers have their
          pitch (angle of attack) fixed during the forg-
          ing process. The angle is set in stone (actu-
          ally, aluminum). This pitch can’t be changed
          except by replacing the propeller, which pret-
          ty much prevents you from changing the pro-
          peller’s pitch in flight. Fixed pitch props are not
          ideal for any one thing, yet they’re in many ways
          best for everything. They represent a compromise
                                            between the best
                                            angle of attack for climb and the best angle for cruise. They are simple to oper-
                                            ate, and easier (thus less expensive) to maintain.
                                              On fixed pitch propeller airplanes, engine power and engine RPM are both con-
                                          trolled by the throttle. One lever does it all, power equals RPM, and that’s the end.
                                                 As you move up into higher performance airplanes, you’ll soon encounter
                                            constant speed (controllable pitch) propellers. Airplanes with these propellers
                                            usually have both a throttle and a propeller control, so you manage engine power
                                           and propeller RPM separately (Figure 45).
                                            On airplanes with constant speed propellers, movement of the throttle determines
                                       the amount of fuel and air reaching the cylinders. Simply stated, the throttle deter-
                                           mines how much power the engine can develop. Movement of the propeller control
                                             changes the propeller’s pitch (its angle of attack). This directly controls how
                                              fast the propeller rotates (its speed or RPM) as shown in Figure 46. While
                                               throttle determines engine power, propeller pitch determines how efficiently
                                               that power is used. Let’s examine how the controllable propeller works. Then
                                                we’ll examine why changing the propeller’s pitch is useful.
                                                     Forward movement of the propeller control causes both halves of the
                                                propeller to rotate about their axes and attack the wind at a smaller angle
                                               (i.e., take a smaller bite of air) as shown in Figure 46A. From aerodynamics,
                                              you know that a smaller angle of attack means less drag and less resistance to
           Fig. 45                          forward motion. Therefore, moving the propeller control forward increases propeller
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