changed, the filings themselves are abrasive and will  cause even more wear. Some gearboxes have the electric starter motor built  into it. When activated, the motor turns the gearing,  which in turn cranks the engine itself. Four-stroke propeller reduction gearboxes use oil  from the engine oil system for lubrication. Some gearboxes come with a built-in centrifugal  clutch, and others have allowances for installation. A  centrifugal clutch is very useful in a two-stroke engine  because it allows the engine to idle at a lower speed  without the load of the propeller. Otherwise, two- strokes can generate a lot of vibration at low RPM  when loaded. As the engine speeds up, the centrifugal  clutch engages the rest of the gearbox and smooth- ly starts the propeller spinning. When the engine is  brought back to idle, the clutch disengages and allows  the engine to again idle smoothly; the propeller stops  when on the ground and windmills when flying. The propeller provides the necessary thrust to push  the powered parachute through the air. The engine  power is used to rotate the propeller, which in turn  generates thrust very similar to the manner in which  a wing produces lift. The amount of thrust produced  depends on the airfoil shape, the propeller blade angle  of attack, and the engine RPM. [Figure 4-5] Powered  parachutes are equipped with either a fixed-pitch or  ground adjustable pitch propeller.  The pitch of this propeller is set by the manufactur- er and cannot be changed. Refer to Chapter 5 of the Pilot’s Handbook of Aeronautical Knowledge  for ba- sic propeller principles. Adjustable-pitch propellers for PPCs can only be ad- justed on the ground with hand tools. If an engine is  over-revving, more pitch can be added to the propeller.  If the engine is not developing the full recommended  RPM during flight, then some pitch can be taken out  of the blades. This should be done per the PPC’s POH  and by a qualified technician. The induction system brings air in from the atmo- sphere, mixes it with fuel, and delivers the fuel-air  When the throttle is closed, for engine idling, the throt- tle valve is closed and the fuel is supplied through the  idle (pilot) jet and idle (pilot) air passage. The fuel/ Modern two- and four-stroke carburetors operate with  one of three jetting systems, depending on engine  power. [Figure 4-6] PPCs use float-type carburetors. Reference the  Pilot’s  Operating Handbook of Aeronautical Knowledge  for  basics on float carburetor operation.  mixture to the cylinder where combustion occurs.  Outside air enters the induction system through an air  filter on the engine. The air filter inhibits the entry of  dust and other foreign objects. Two types of induction  systems are used in powered parachute engines: 1. The carburetor system is most common; it mixes the fuel and air in the carburetor before this  mixture enters the engine intake, and 2. The fuel injection system, which injects the fuel  into the air just before entry into each cylinder. Figure 4-5. Engine RPM is indicated on the gauge. Carburetor Systems Induction Systems Ground Adjustable-Pitch Propeller Fixed-Pitch Propeller Propeller Centrifugal Clutch 4-6