Four-Stroke Engine Warming Propeller Gearboxes Fixed-Pitch Propeller Figure 4-9. Engine rpm is indicated on the analog gauge (top) and  the digital gauge (bottom). The pitch of the fixed-pitch propeller is set by the  manufacturer and cannot be changed. Refer to the Pilot’s  Handbook of Aeronautical Knowledge for basic propeller  principles. 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-stroke engines can generate a great  deal of vibration at low rpm when loaded. As the engine  speeds up, the centrifugal clutch engages and smoothly starts  Four-stroke propeller reduction gearboxes use oil from the  engine oil system for lubrication. Some gearboxes have a built-in electric starter motor. When  activated, the motor turns the gearing which cranks the  engine.  A two-cycle engine gearbox is kept lubricated with its own  built-in reservoir of heavy gearbox oil. The reservoir is  actually part of the gearbox case itself. The gearbox oil has  to be changed periodically since the meshing of the gears  will cause them to wear and will deposit steel fi lings into  the oil. If the oil is not changed, the abrasive fi lings cause  even more wear. Some examples are a two-stroke rpm reduction from 6,500  engine rpm with a 3.47 to 1 reduction, resulting in 1,873  propeller rpm. A four-stroke rpm reduction could be from  5,500 engine rpm with a 2.43 to 1 reduction, resulting in  2,263 propeller rpm. A gearbox is a simple device that bolts  directly to the engine and, in turn, has the propeller bolted  directly to it. Gearboxes are used on most WSC reciprocating engines to  take the rotational output of an internal combustion engine  which is turning at a high rpm and convert it to a slower (and  more useful) rpm to turn the propeller. Gearboxes come in  different gear ratios depending on the output speed of the  engine and the needed propeller turning speeds. The propeller provides the necessary thrust to push the WSC  aircraft through the air. The engine power is used to rotate the  propeller, which 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 (AOA), and the engine rpm.  [Figure 4-9]  Light-sport  aircraft (LSA) are equipped with either a fixed-pitch or a  ground adjustable-pitch propeller.  the propeller spinning. When the engine is brought back  to idle, the clutch disengages and allows the engine to idle  smoothly again; the propeller stops when on the ground and  windmills when fl ying.  A four-stroke engine must also be warmed up. The four- stroke engine has a pressurized oil system that provides  more uniform engine temperatures to all of its components.  Takeoff power can be applied as soon as the water, cylinder  head temperature (CHT), oil temperatures, and oil pressure  are within the manufacturer’s recommended tolerances for  takeoff power applications. Any engine restart is an example in which it would be  appropriate to warm the engine up until the gauges reach  operating temperatures. The lower end of the engine is warm  and now a pilot needs to be concerned only with preventing  the pistons from seizing. 4-8 rpm x1000 0 1 2 3 4 5 6 7 8 ENGINE HOURS Shown when engine is shut off ENGINE RPM 6500MAX CON 6800MAX