be found on reciprocating engine aircraft.  [Figure 16-47]  A  variable ratio gear drive system is used to maintain a constant  rate of airflow during changes of engine rpm.  Near maximum operating altitude, the performance of any  reciprocating engine and the pressurization compressor  suffer. This is due to the reduced pressure of the air at altitude  that supplies the intake of each. The result is difficulty in  maintaining a sufficient volume of air to the engine intake  to produce power, as well as to allow enough air to the  fuselage for pressurization. These are the limiting factors for  determining the design ceiling of most reciprocating aircraft,  which typically does not exceed 25,000 feet. Turbine engine  aircraft overcome these shortcomings, permitting them to fly  at much higher altitudes. The main principle of operation of a turbine engine involves  the compression of large amounts of air to be mixed with  fuel and burned. Bleed air from the compressor section of the  engine is relatively free of contaminants. As such, it is a great  source of air for cabin pressurization. However, the volume of  air for engine power production is reduced. The amount of air  bled off for pressurization compared to the overall amount of  air compressed for combustion is relatively small but should  be minimized. Modern large-cabin turbofan engine aircraft  contain recirculation fans to reuse up to 50 percent of the air  in the cabin, maintaining high engine output. There are different ways hot, high-pressure bleed air can  be exploited. Smaller turbine aircraft, or sections of a large  aircraft, may make use of a jet pump flow multiplier. With  this device, bleed air is tapped off of the turbine engine’s  compressor section. It is ejected into a venturi jet pump  mounted in air ducting that has one end open to the ambient  air and the other end directed into the compartment to be  The roots blower is used on older, large reciprocating engine  aircraft.  [Figure 16-46]  The two lobes in this compressor  do not touch each other or the compressor housing. As  they rotate, air enters the space between the lobes and is  compressed and delivered to the cabin for pressurization.  Independent engine-driven centrifugal compressors can also  A third source of air for pressurizing the cabin in reciprocating  aircraft is an engine driven compressor. Either belt driven or  gear driven by the accessory drive, an independent, dedicated  compressor for pressurization avoids some of the potential  contamination issues of superchargers and turbochargers. The  compressor device does, however, add significant weight. It  also consumes engine output since it is engine driven. Ambient air intake To cabin and carburetor Overboard Exhaust gases Intake and pressurization  compressor Exhaust turbine 16-26 Figure 16-44.  A turbocharger used for pressurizing cabin air and engine intake air on a reciprocating engine aircraft.  Figure 16-43.  The radial engine supercharger cannot be used since  fuel is introduced before the supercharger impeller compresses the air.  Turbine Engine Aircraft 