prevent this, slowly add power well before you get  close to the ground where you will need power. This  will give the system a chance to gradually open the  thermostat and warm up the radiator water. The cylinders have steel cylinder walls that expand  slowly compared to aluminum pistons that expand  quickly. If an engine is revved too quickly during  takeoff before warming up, a lot of heat is generated  on top of the piston. That quickly expands the piston,  which can then seize in the cylinder. A piston seizure  will stop the engine abruptly. The second area of concern is lower in the engine  around the engine crankshaft. This is an area where  things may get too loose with heat, rather than seizing  up. Additionally, the crankcase has steel bearings set  into the aluminum which need to expand together or  the bearings could slip. Just as it takes a while for the engine crankcase and  bearings to warm up, it also takes those steel parts a  long time to cool down. If you land, refuel and want  to take off again quickly, there is no need to warm up  again for 5 minutes. The lower end of the engine will  stay warmed up after being shut down for short peri- ods. An engine restart is an example where 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 you only need to be con- cerned with preventing the pistons from seizing. Many two-stroke engines have steel bearings that nor- mally hug the walls of the aluminum engine case. The  crank spins within the donuts of those steel bearings.  If you heat up the engine two quickly, the aluminum  case will out-expand those steel bearings and the  crank will cause the bearings to start spinning along  with it. If those steel bearings start spinning, they can  ruin the soft aluminum walls of the case, which is  very expensive. 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 its  components. You can apply takeoff power as soon  as the water, cylinder head temperature (CHT), oil  temperatures and oil pressure are within the manu- facturer’s recommended tolerances for takeoff power  applications. If heat is slowly added to an engine, all the parts will  expand more evenly. This is done through a proper  warm-up procedure. Many two-stroke engines are  best warmed up by running the engine at a set RPM  for a set amount of time. Follow the instructions in  your POH; however, a good rule of thumb is to ini- tially start the engine at idle RPM, get it operating  smoothly, and then warm the engine at 3,000 RPM  for 5 minutes. Gearboxes are used on all powered parachute recip- rocating engines to take the rotational output of an  internal combustion engine which is turning at a very  high RPM and convert it to a slower (and more useful)  RPM to turn the propeller. Gearboxes come in differ- ent gear ratios depending on the output speed of the  engine and the needed propeller turning speeds.  A typical two-stroke RPM reduction is from 6,500  engine RPM with a 3.47 to 1 reduction, resulting in  1,873 propeller RPM. A typical four-stroke RPM re- duction is from 5,500 engine RPM with a 2.43 to 1  reduction, resulting in 2,263 propeller RPM. Once the engine is warmed up and the powered para- chute is flying, it is still possible to cool down the engine too much. This will happen when the engine is  idled back for an extended period of time. Even though  the engine is running, it is not generating as much heat  as the cooling system is efficiently dumping into the  atmosphere. An immediate power application with a  cooled engine can seize the engine just as if the engine  had not been warmed in the first place.  A gearbox is a simple device that bolts directly to the  engine and in turn has the propeller bolted directly  to it.  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 filings into the oil. If the oil is not  In water-cooled engines, on a long descent at idle, the  coolant cools until the thermostat closes and the en- gine is not circulating the radiator fluid through the  engine. The engine temperature remains at this ther- mostat closed temperature while the radiator coolant  continues to cool further. If full throttle is applied, the  thermostat can open allowing a blast of coolant into  the warm engine. The piston is expanding because of  the added heat and the cylinder is cooling with the  cold radiator water resulting in a piston seizure. To  Gearboxes Four-Stroke Engine Warming 4-5