text: the pressure forces created on an aerodynamic 
  surface can be studied in a simple form which 
  at first neglects the effect of friction and vis- 
  cosity of the airflow. The most appropriate 
  means of visualizing the effect of airflow and 
  the resulting aerodynamic pressures is to study 
  the fluid flow within a closed tube. 
  
  Suppose a stream of air is flowing through 
  the tube shown in figure 1.2. The airflow at 
  station 1 in the tube has a certain velocity, 
  static pressure, and density. As the airstream 
  approaches the constriction at station 2 certain 
  changes must take place. Since the airflow 
  the tube, the mass flow at 
  is enclosed within 
  any point along the tube must be the same and 
  the velocity, pressure, or density must change 
  to accommodate this continuity of flow. 
  
  distin- 
  A 
  is that 
  guishing feature of submnic airflow 
  changes in pressure and velocity 
  take place 
  with sniall and negligible changes in density. 
  For this reason the study of subsonic airflow 
  can be simplified by neglecting the variation 
  of density in the flow and assuming the flow 
  to be incomprmiblc. Of course, at high flow 
  speeds whjch approach the speed of sound, the 
  flow must be considered as compressible and 
  “compressibility effects” taken into account. 
  However, 
  the tube of 
  figure 1.2 is considered subsonic, the density of 
  the airstream is essentially constant at all sta- 
  tions along the length. 
  
  BERNOULLI’S  EQUATION. 