text: (2) At the surface, the laminar boundary 
  layer has the less rapid change of velocity 
  with distance above the plate. Since the 
  shearing stress is proportional to the velocity 
  gradient, the lower velocity gradient of the 
  laminar boundary layer is evidence of a 
  lower friction drag on the surface. 
  If the 
  conditions of flow were such that either a 
  turbulent or a laminar boundary layer could 
  exist, the laminar skin friction would be 
  about one-third that for turbulent flow. 
  The low friction drag of the laminar bound- 
  ary layer makes it quite desirable. However, 
  the transition tends to take place in a natural 
  fashion and limit 
  the extensive development 
  of the laminar boundary layer. 
  
  The most direct use of Reynolds Number is 
  the indexing or correlating the skin friction 
  drag of a surface. Figure 1.25 illustrates the 
  variation of the friction drag of a smooth, 
  flat plate with a Reynolds Number which is 
  based on the length or chord of the plate. 
  The graph shows separate lines of drag coeffi- 
  cient if the flow should be entirely laminar or 
  entirely turbulent. The two curves for lam- 
  inar and turbulent friction drag illustrate the 
  relative magnitude of friction drag coefficient 
  if either type of boundary layer could exist. 
  The drag coefficients for either laminar or tur- 
  bulent flow decrease with 
  increasing RN since 
  the velocity gradient decreases as the boundary 
  layer thickens.  REYNOLDS NUMBER. Whether a lam- 
  inar or turbulent boundary layer exists depends 
  on the combined effects of velocity, viscosity, 
  distance from the leading edge, density, etc. 
  The effect of the most important 
  factors is 
  combined in a dimensionless parameter called 
  “Reynolds Number, RN.” 
  The Reynolds 
  Number is a dimensionless ratio which por- 