NAVWEPS 00-801-80  BASIC AERODYNAMICS  If the potential energy is represented  by the  static pressure,  p, the sum of the potential and  kinetic energy is the total pressure of the air-  stream.  where H=total  pressure, psf (sometimes re-  ferred to as “head ’ pressure)  p=static pressure,  psf.  p=density, siugs per cu. ft.  V= velocity, ft./set.  This equation is the Bernoulli equation for  ‘ incompressible flow.  It is important to ap-  preciate that the term >$pV2  has the units of  pressure, psf. This term is one of the most  important in all aerodynamics and appears so  frequently t&it  is given the name “dynamic  pressure” and the shorthand notation “4”.  H=p+% P V’  q= dynamic pressure,  psf  = jgpv2  With this definition it could be said that the  sum of static and dynamic pressure  in the flow  tube remains constant.  Figure 1.3 illustrates the variation of static,  dynamic, and total pressure of air flowing  through a closed tube. Note that the total  pressure is con,stant throughout the length  and any change in dynamic pressure  produces  the same magnitude change in static pressure.  The dynamic pressure of a free airstream is  the one  ‘ common denominator of all aero-  dynamic forces and moments. Dynamic pres-  sure represents the kinetic energy of the free  airstream and is a factor relating the capability  for producing changes in static pressure on a  surface. As defined, the dynamic, pressure  varies directly as the density and the square of  the velocity.  Typical values of dynamic pres-  sure, 4, are shown in table l-1 for various true  airspeeds  in the standard atmosphere.  Notice  that the dynamic pressure  at some fixed veloc-  ity varies directly with the density ratio at any  altitude.  Also, appreciate the fact that at an  altitude of 40,oM) feet (where the density ratio,  b, is 0.2462) it is necessary  to have a true air  velocity twice that at sea level in order to  product the same dynamic pressure.  Around the surface of the object the airflow  will divide and the local velocity will increase  from zero at the stagnation point to some  maximum on the sides of the object.  If fric-  tion and viscosity effects are neglected, the  AIRSPEED MEASUREMENT.  If a sym-  metrically shaped object were placed in a  moving airstream, the flow pattern typical of  figure 1.4 would result.  The airstream at the  very nose of the object would stagnate and the  relative flow velocity at this point would be  zero. The airflow ahead of the object pos-  sesses some certain dynamic pressure and  ambient static pressure. At the very nose of  the object the local velocity will drop to zero  and the airstream dynamic pressure will  be  converted into an increase in static pressure  at  the stagnation point.  In other words, there  will exist a static pressure at the stagnation  point which is equal to the airstream total  pressure-ambient static pressure  plus dynamic  pressure.  9  TABLE  l-l.  Effect of Speed and Altitvde on Dwzmnic Prerrure  True air  speed  (fr./scc.)  m=  169  338  507  616  845  I, 013  -  ,I I  c  _- 