Figure 3-32.  An example of a ducted arrangement on a turboprop  engine. Figure 3-31.  Probes within a bellmouth inlet used to measure intake  temperature and pressure.  Figure 3-33.  Deflector doors used to deflect ice or dirt away from  the intake. 3-21 The air inlet on a turboprop is more of a problem than some  other gas turbine engines because the propeller drive shaft,  the hub, and the spinner must be considered in addition to  other inlet design factors. The ducted arrangement is generally  considered the best inlet design of the turboprop engine as  far as airflow and aerodynamic characteristics are concerned.  [Figure 3-32]  The inlet for many types of turboprops are  anti-iced by using electrical elements in the lip opening of  the intake. Ducting either part of the engine or nacelle directs  the airflow to the intake of the engine. Deflector doors are  sometimes used to deflect ice or dirt away from the intake.  [Figure 3-33]  The air then passes through a screen and into  the engine on some models. A conical spinner, which does  not allow ice to build up on the surface, is sometimes used  with turboprop and turbofan engines. In either event, the  arrangement of the spinner and the inlet duct plays an important  function in the operation and performance of the engine. movable part of the test stand and moves with the engine. The  thrust stand is made up of two components, one nonmoving  and one moving. This is so the moving component can push  against a load cell and measure thrust during the testing  of the engine. The bellmouth is designed with the single  objective of obtaining very high aerodynamic efficiency.  Essentially, the inlet is a bell-shaped funnel having carefully  rounded shoulders which offer practically no air resistance.  [Figure 3-30]  Duct loss is so slight that it is considered  zero. The engine can, therefore, be operated without the  complications resulting from losses common to an installed  aircraft inlet duct. Engine performance data, such as rated  thrust and thrust specific fuel consumption, are obtained  while using a bellmouth inlet. Usually, the inlets are fitted  with protective screening. In this case, the efficiency lost as  the air passes through the screen must be taken into account  when very accurate engine data are necessary. High-bypass turbofan engines are usually constructed with  the fan at the forward end of the compressor. A typical  turbofan intake section is shown in  Figure 3-34.  Sometimes,  the inlet cowl is bolted to the front of the engine and provides  the airflow path into the engine. In dual compressor (dual  spool) engines, the fan is integral with the relatively slow- turning, low-pressure compressor, which allows the fan  blades to rotate at low tip speed for best fan efficiency. The  fan permits the use of a conventional air inlet duct, resulting  in low inlet duct loss. The fan reduces engine damage from  ingested foreign material because much of any material  that may be ingested is thrown radially outward and passes  through the fan discharge rather than through the core of  the engine. Warm bleed air is drawn from the engine and  circulated on the inside of the inlet lip for anti-icing. The  fan hub or spinner is either heated by warm air or is conical  Turboprop and Turboshaft Compressor Inlets Turbofan Engine Inlet Sections