Controllable pitch propeller on a Cessna 180.


Controllable pitch propeller:

The main difference between a simple Cessna 172 and the Cessna 180 is the use of a propeller control and manifold pressure to set power. The controllable pitch propeller can also be called a constant speed propeller since the propeller knob (often colored blue) controls the engine RPM at the normal cruise power settings. Once this RPM is set, the prop will continually rotate at this constant speed. The throttle controls manifold pressure (MP) and pitch of the prop. Increasing throttle increases the manifold pressure (the pressure of the flow of fuel to the engine) and the prop tries to turn faster. The prop controller then resists faster RPM by increasing the "bite" of the blades in the air. This increased pitch and angle of attack of the blades causes more thrust, but also causes more drag, resisting an increase in RPM. In a sense, throttle only increases the torque of the engine, and then the prop governor increases the pitch of the blades to produce more thrust and drag. This happens quickly and automatically, so no change in RPM is noticed. So the throttle controls the MP/engine torque/prop pitch, and the prop knob regulates engine/prop RPM. Many modern high-performance aircraft are very noisy on takeoff and landing. Experienced pilots mitigate this problem by avoiding high RPM and oversquare conditions whenever possible. At high RPM on takeoff or in preparation for anding, the prop tips may reach supersonic speeds, causing quite a bit of noise and inefficiency. Some airplane owners replace their two-bladed props with three blades to reduce noise and increase efficiency, since each of the three blades now has an angle-of-attack which is less than for the two-bladed prop. This is also why pilots avoid oversquare conditions (MP greater than RPM, such as 27"MP and 2200 RPM). At such high torque and low RPM, the blades take a very large "bite", and this high blade angle-of-attack is inefficient or may even cause the blades to stall. Imagine the blades are little wings. At squared power(27" and 2700RPM) the blades are exactly at Vy, best efficiency for climb for time. At oversquare (27" and 2400RPM) the blades may be at Vx, still efficient, but less so than squared. Finally, at severe oversquare (27" and 2200RPM) the blades are in a stall, and they produce negligible thrust (analogous to zero lift of the wing). 

In review of the controllable pitch propeller: 

• the RPM control (blue knob) sets prop/engine RPM 

• the throttle (black knob) controls prop pitch/manifold pressure

• unlike a fixed-pitch prop, produces max RPM/power for takeoff 

• loss of oil pressure causes flat pitch to stops (high RPM) 

• lower cruise RPM causes less noise, smoother through turbulence 

• prop oil freezes at altitude, alter the prop control every 15 min 

• idle throttle causes flat pitch, high drag "plate" airbrake 

• 12" causes zero thrust condition, no drag and no thrust 

• higher MP with same RPM causes more torque and p-factor 

• oversquare settings are inefficient and overtorque the engine

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