NAVWEPS 00-801-80  BASIC AERODYNAMICS  The effect of free stream density and velocity  is a necessary  consideration when studying the  development of the various aerodynamic forces.  Suppose that a particular shape of airfoil is  fixed at a particular angle to the airstream.  The relative velocity and pressure distribution  will be determined by the shape of the airfoil  and the angle to the airstream.  The effect of  varying the airfoil size, air density and air-  speed is shown in figure 1.9.  If the same air-  foil shape is placed at the same angle to an  airstream with twice as great a dynamic pres-  sure the magnitude  of the pressure  distribution  will be twice as great but the r&rive shape of  the pressure distribution will  be the same.  With twice as great a pressure existing over  the surface, all aerodynamic forces and mo-  ments will ~double. If a half-size airfoil ib  placed at the same angle to the original air-  stream, the magnitude of the pressure distri-  bution is the same as the origina! airfoi! and  again the relative shape of the pressure dis-  tribution is identical.  The same pressure  act-  ing on the half-size surface would reduce all  aerodynamic forces to one-half that of the  original.  This similarity  of flow patterns  means that the stagnation point occurs at the  same place, the peak suction pressure occurs  at the same place, and the actual magnitude  of  the aerodynamic forces and moments depends  upon the airstream dynamic pressure and the  surface area. This concept is extremely im-  portant when attempting to separate  and ana-  lyze the most important factors affecting the  development of aerodynamic forces.  rotation will be quite a “curve ball artist”  the golfer that cannot control the lateral mo-  tion of the club face striking the golf ball will  impart an uncontrollable spin and have trouble  with a “hook”  or “slice.”  While a rotating cylinder can produce a net  lift from the circulatory flow, the method is  relatively inefficient and only serves to point  out the relationship between lift and circula-,  tion.  An airfoil is capable of producing lift  with relatively high efficiency and the process  is illustrated in figure 1.8. If a symmetrical  airfoil is placed at zero angle of attack to the  airstream, the streamline pattern and pressure  distribution give evidence of zero lift.  HOW-  ever, if the airfoil is given a positive angle of  attack, changes occur in the streamline pattern  and pressure distribution similar to changes  caused by the addition of circulation to the  cylinder.  The positive angle of attack causes  increased  velocity  on the upper surface with  an increase in upper surface suction while the  decreased  velocity on the lower surface causes  a decrease in lower surface suction. Also,  upwash is generated ahead of the airfoil, the  forward stagnation point moves under the  leading edge, and a downwash is evident aft  of the airfoil.  The pressure distribution 0”  the airfoil now provides a net force perpendicu-  lar to the airstream-lift.  The generation of lift by an airfoil is depend-  ent upon the airfoil being able to create circu-  lation in the airstream and develop the lifting,  pressure distribution on the surface. In all  cases, the generated lift will be the net force  caused by the distribution of pressure  over the  upper and lower surfaces of the airfoil.  At  low angles of attack, suction pressures  usually  will exist on both upper and lower surfaces.  but the upper surface suction must be greater  for positive lift.  At high angles of attack  near that for maximum lift, a positive pressure  will exist on the lower surface but this will  account for approximately one-third the net  lift.  AIRFOIL  TERMINOLOGY.  Since the  shape of an airfoil and the inclination to the  airstream are so important in determining the  pressure  distribution, it is necessary  to properly  define the airfoil terminology.  Figure 1.10  shows a typical airfoil and illustrates the  various items of airfoil terminology  (1) The chord  line is a straight line connect-  ing the leading and trailing edges of the  airfoil.  20 