TM 1-320  7-8  AIR CORPS  (2)  Pressure-differenoe  theory.-Figure 5 shows the motion of the  particles of an air stream passing a flat plate held at right angles  to the flow.  The air is deflected from its course some distance in front  of the plate and has a complex eddying motion in rear o.f it.  In  front  of the plate the air is under an increased pressure, while behind the  plate there is an area of reduced pressure.  The drag can be con› sidered as due to the difference between the pressures in front of and  behind the plate.  8. Shape coefficients.-a. The two systems in common use for ex-.  pressing air resistance are the engineering and the absolute.  (1) Under the engineering system the formula is- where Rv=air resistance due to pressure difference.  A =  cross sectional area normal to the air stream  in  square feet.  V  =velocity of motion in miles per hour.  K IJ)=an  empirically determined constant depending on the  shape of the solid and the mass density of the air.  In lighter than air practice the letter  "K,"  minus subscript, is used to  denote  K IJ)  when the mass density of the air is standard (0.00237  pound per cubic foot, which is the •value when the pressure is 29.92  inches and the temperature is 60°  F.).  (2} The absolute system, adopted by the National Advisory Com› mittee for Aeronautics, uses the formula:  where p= mass density of the air.  2  ’;’ .is the dynamic pressure per unit of area or the velocity head of  the air stream.  This formula has more definite physical interpreta› tion than the engineering formula from both the momentum and  pressure-difference theories.  Before studying aerodynamic data, the  system which is being used should always be determined.  b.  Some of the first practical tests made  to  determine the effect of  shape upon the resistance offered the motion of solids through  the air were .conducted by Eiffel.  Since then studies have been  conducted by various investigators until at present the store of in› formation on this subject is quite elaborate.  Figures 5 to 15 give  the action of the air on various shapes together with the values of  K.  16  .  u2  Ro = K DAP2  v= velocity of motion in feet per second.  K v=an  empirical shape coefficient.  Rv=K a:AV2  . 