function as diffusers, decreasing the velocity and increasing the  static pressure of the air passing through them.  [Figure 3-26] The choice of configuration of the entrance to the duct is  dictated by the location of the engine within the aircraft and  the airspeed, altitude, and attitude at which the aircraft is  designed to operate.  As with military supersonic aircraft, a diffuser progressively  decreases in area in the downstream direction. Therefore, a  supersonic inlet duct follows this general configuration until  the velocity of the incoming air is reduced to Mach 1. The  aft section of the duct then increases in area, since this part  must act as a subsonic diffuser.  [Figure 3-27]  In practice, inlet  ducts for supersonic aircraft follows this general design only  as much as practical, depending upon the design features of  the aircraft. For very high speed aircraft, the inside area of  configuration of the duct is changed by a mechanical device  as the speed of the aircraft increases or decreases. A duct of  this type is usually known as a variable-geometry inlet duct. Military aircraft use the three methods described above to  diffuse the inlet air and slow the inlet airflow at supersonic  flight speeds. One is to vary the area, or geometry, of the inlet  The requirements of high-speed, single- or twin-engine  military aircraft, in which the pilot sits low in the fuselage and  close to the nose, render it difficult to employ the older type  single-entrance duct, which is not used on modern aircraft.  Some form of a divided duct, which takes air from either side  of the fuselage, has become fairly widely used. This divided  duct can be either a wing-root inlet or a scoop at each side of  the fuselage.  [Figure 3-25]  Either type of duct presents more  problems to the aircraft designer than a single-entrance duct  because of the difficulty of obtaining sufficient airscoop area  without imposing prohibitive amounts of drag. Internally,  the problem is the same as that encountered with the single- entrance duct: to construct a duct of reasonable length with  as few bends as possible. Scoops at the sides of the fuselage  are often used. These side scoops are placed as far forward  as possible to permit a gradual bend toward the compressor  inlet, making the airflow characteristics approach those of a  single-entrance duct. A series of turning vanes is sometimes  placed in the side-scoop inlet to assist in straightening the  incoming airflow and to prevent turbulence. The main function of an inlet duct is to furnish the proper  amount of air to the engine inlet. In a typical military aircraft  using a turbojet or low bypass turbofan engine, the maximum  airflow requirements are such that the Mach number of the  airflow directly ahead of the face of the engine is less than  Mach 1. Airflow through the engine must be less than Mach 1  at all times. Therefore, under all flight conditions, the velocity  of the airflow as it enters the air-inlet duct must be reduced  through the duct before the airflow is ready to enter the  compressor. To accomplish this, inlet ducts are designed to  3-19 Figure 3-27.  The aft section of an inlet duct acting as a subsonic  diffuser.  Figure 3-25.  An example of a divided-entrance duct. Figure 3-26.  An inlet duct acts as a diffuser to decrease the airflow  velocity and to increase the static pressure of air.  Divided-Entrance Duct Variable-Geometry Duct Convergent supersonic section Divergent subsonic section