NAVWEPS DD-RDT-80  BASIC AERODYNAMICS  boundary layer obtained may reduce the form  drag due to separation.  In each instance, the  forced transition will be beneficial if the reduc-  tion in form drag is greater than the increase  in skin friction.  Of course, this possibility  exists only at low Reynolds Numbers.  1,n a similar sense, “trip”  wires or small  surface protuberances  on a wind tunnel model  may be used to force transition of the boundary  layer and simulate the effect  of higher Reynolds  Numbers.  camber at 40 percent chord, 12 percent thick-  ness at 30 percent chord)--a fairly typical  “conventionaal” airfoil section.  The lift curve  show a steady increase in cl  with increasing  RN.  However, note that a>maller change in  cr  occurs between Reynolds Numbers of 6.0  ad 9.0 million than occurs between 0.1 and  3.0 million.  In other words, greater changes  in CI  occur in the range of Reynolds Num-  bers zhere the laminar (low energy) boundary  layer predominates. The drag curves for the  section show essentially the same feature-the  greatest variations occur at very low Reynolds  Numbers. Typical full scale Reynolds Num-  bers for aircraft in flight may be 3 to 5@O  million  where the boundary layer is predominately  turbulent.  Scale model tests may involve  Reynolds Numbers of 0.1 to 5 million where  the boundary layer be predominately laminar.  Hence, the “scale”  corrections are very neces-  sary to correlate the principal aerodynamic  characteristics.  The previous discussion of aerodynamic  forces concerned the properties of airfoil sec-  tions in two-dimensional flow with no consid-  eration given to the influence of the planform.  When the effects of wing planform are intro-  duced, attention must be directed to the ex-  istence of flow components in the spanwise  direction.  In other words, airfoil  section  properties deal with flow in two dimensions  I  while plonform properties consider flow in  three dimensions.  EFFECT OF WING PLANFORM  PLANFORM  EFFECTS AND  AIRPLANE  DRAG  The very large changes in aerodynamic  characteristics at low Reynolds Numbers are  due in great part to the low energy laminar  boundary layer typical of low Reynolds Num-  bers. Low Reynolds Numbers are the result  of some combination of low velocity, small  size, and high kinematic viscosity RN=  (  3  Thus, small surfaces,  low flight speeds,  or very  high altitudes can provide the regime of low  Reynolds Numbers. One interesting phenom-  enon associated  with low BN is the high form  drag due to separation of the low energy  boundary layer.  The ordinary golf ball oper-  ates at low RN and would have very high  form drag without  dimpling.  The surface  roughness from dimpling disturbs the laminar  boundary layer forcing a premature transition  to turbulent.  The forced turbulence in the  boundary layer reduces the form drag by pro-  viding a higher energy boundary layer to  allay separation. Essentially the same effect  can be produced on a model airplane wing by  roughening the leading edge-the  turbulent  In order to fully describe the planform of a  wing, several terms are required. The terms  having the greatest influence on the aerody-  namic characteristics are illustrated in figure  1.28.  (1) The wing r?rc11,  S, is simply the plan  surface area of the wing.  Although a por-  tion of the area may be covered by fuselage  or nacelles, the pressure carryover on these  surfaces allows legitimate consideration of  the entire plan area.  (2) The wing ~ptia, 6, is measured tip to  tip.  (3) The avcragc  chord, c, is the geometric  average.  The product of the span and the  average chord is the wing area (6X6=$).  (4) The aspect  ratio, AR, is the proportion  of the span and the average chord.  AR=b/c 