NAVWEK OD-BOT-BO  BASIC AERODYNAMICS  be appreciated. The general effect of altitude  is expressed  by:  impression of the “barn door” size. Hence,  parasite drag can be appreciated as the result  of the dynamic pressure, 4, acting on the  equivalent parasite area, j.  The “equivalent”  parasite area is defmed by this relationship as  a hypothetical surface with a C,=l.O which  produces the same parasite drag as the air-  plane. An analogy would be a barn door in  the airstream which is equivalent to the air-  plane.  Typical values for the equivalent para-  site area range from 4 sq. ft. for a clean fighter  type airplane to 40 sq. ft. for a large transport  type airplane.  Of course, when any airplane  is changed from the clean configuration to the  landing configuration, the equivalent parasite  area increases.  EFFECT OF CONFIGURATION.  The par-  asite drag, D,,  is unaffected by lift, but is  variable with dynamic pressure  and equivalent  parasite area. This principle furnishes the  basis for illustrating the variation of parasite  drag with the various conditions of flight.  If all other factors are held constant, the para-  site drag varies directly with the equivalent  parasite area.  This relationship implies that parasite drag  would decrease  at altitude, e.g., a given air-  plane in flight at a given T.4.Y at 40,COO  ft.  (e=O.29 would have one-fourth the parasite  drag when at sea level (u=l.OO).  This effect  results when the lower air density produces  less dynamic pressure. However, if the air-  plane is flown at a constant EAS, the dynamic  pressure  and, thus, parasite drag do not vary.  In this case,  the TASwould be higher at altitude  to provide the same EAS.  where  D,, = parasite drag corresponding to some orig-  inal altitude density ratio, 0,  D,,=parasite drag corresponding to some new  altitude density ratio, (ra  (and f, V are constant)  EFFECT OF SPEED. The effect of speed  alone on parasite drag is the most important.  If all other factors are held constant, the effect  of velocity on parasite drag is expressed  as:  where  D,,= parasite drag corresponding to some orig-  inal parasite area, fi  D,,==parasite drag corresponding to some new  parasite area, fi  D,,=parasite drag corresponding to some orig-  inal speed, Vi  D,,=parasite drag corresponding to some new  speed, VS  &,  V, *  -=-  (3  D,,  V  D,,= b  C)  D,,  I  (j and o are constant)  (V and (r are constant)  where  This relationship expresses  a powerful effect  of speed on parasite drag.  As an example, a  given airplane in flight at some altitude would  have four times as much parasite drag at twice  As an example, the lowering of the landing  gear and flaps may increase the parasite area  80 percent. At any given speed and altitude  this airplane would experience an 80 percent  increase in parasite drag.  EFFECT OF ALTITUDE.  In a similar man-  ner the effect of altitude on parasite drag may  91 