CHAPTER 4. Powerplants Ground Adjustable-Pitch Propeller Adjustable-pitch propellers for WSC aircraft can be adjusted only on the ground with hand tools. If an engine is overrevving, more pitch can be added to the propeller. If the engine is not developing the full recommended rpm during fl ight, then some pitch can be taken out of the blades. This should be done according to the WSC aircraft’s POH and by a qualifi ed technician. Induction Systems The induction system brings air in from the atmosphere, mixes it with fuel, and delivers the fuel/air mixture (fuel/oil/ air mixture for two stroke engines) to the engine intake and to the cylinders where combustion occurs. Outside air enters the induction system through an air fi lter on the engine. The air fi lter inhibits the entry of dust and other foreign objects. Two types of induction systems are used in WSC engines: The carburetor system is most common. It mixes the fuel and air in the carburetor before this mixture enters the engine intake. The fuel injection system injects the fuel into the air just before entry into each cylinder. Carburetor Systems WSC aircraft use fl oat-type carburetors. The “fl oat-type carburetor” acquires its name from a fl oat that rests on fuel within the carburetor fl oat chamber, commonly known as the fuel bowls. The fl oat maintains the fuel level in the fuel bowls. As fuel is used by the engine, the fuel and fl oat levels drop, opening the valve letting more fuel into the fuel bowls until the proper level of fuel in the fuel bowls is achieved and the valve is closed. Reference the Pilot’s Handbook of Aeronautical Knowledge for basic information on fl oat carburetor operation. Modern two- and four-stroke carburetors operate with three separate jetting systems depending on engine power. [Figure 4-10] When the throttle is closed for engine idling, the throttle valve is closed and the fuel/air mixture is supplied through the idle (pilot) jet and idle (pilot) air passage. The fuel/air mixture is supplied to the cylinders through the bypass hole. [Figure 4-11] As the throttle is advanced and the throttle valve is raised, the fuel is sucked up through the main jet but is controlled by the opening and taper of the jet needle and needle jet. This is effective throughout most of the midrange operation. About half throttle, the main jet size starts to infl uence the amount of fuel mixed with the air and this effect continues until it is the main infl uence at the highest throttle settings. [Figures 4-10 and 4-12]

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