56  delay the transition to some point farther aft  on the chord.  The subsequent reduction in  friction drag at the low angles of attack ac-  counts for the “drag bucket” shown on the  graphs of cd and cI for these sections. Of  course, the advantages of the laminar flow  airfoil are apparent only for the smooth airfoil  since surface roughness or waviness may pre-  clude extensive development of a laminar  boundary layer.  If the surface  of the plate is smooth and the  original airstream has no turbulence, the plate  at low Reynolds Numbers will exist with pure  laminar flow.  When the RN is increased to  approximately 530,000, transition occurs on  the plate and the flow is partly turbulent.  Once transition takes place, the drag coefficient  of the plate increases  from the laminar curve  to the turbulent curve. As the RN approaches  very high values (20 to 50 million) the drag  curve of the plate approaches  and nearly equals  the values for the turbulent curve. At such  high RN the boundary layer is predominantly  turbulent with very little laminar flow-the  transition point is very close to the leading  edge. While the smooth, flat plate is not ex-  actly representative  of the typical airfoil, basic  fluid friction phenomena are illustrated.  At  RN less than a half million the boundary layer  will be entirely laminar unless there is extreme  surface roughness or turbulence induced in the  airstream. Reynolds Numbers between one  and five million produce boundary layer flow  which is partly laminar and partly turbulent.  At RN above ten million the boundary layer  characteristics are predominantly turbulent.  In order to obtain low drag sections, the  transition from laminar to turbulent must be  delayed so that a greater portion of the sur-  face will be influenced by the laminar bound-  ary layer.  The conventional, low speed air-  foil  shapes are characterized, by minimum  pressure  points very close to the leading edge.  Since high  local velocities enhance early  transition, very little surface is covered by  the laminar boundary layer,  A comparison  of two 9 percent thick symmetrical airfoils is  presented in figure 1.25. One section is the  “conventional”  NACA C!UO~  section which  has a minimum pressure point at approxi-  mately 10 percent chord at zero lift.  The other  section is the NACA 66039 which has a  minimum pressure point at approximately 60  percent chord at zero lift.  The lower local  velocities at the leading edge and the favor-  able pressure gradient of the NACA 66-009  The action of the pressure gradient is such  that the favorable pressure gradient assists  the boundary layer while the adverse pressure  gradient impedes the flow of the boundary  layer. The effect of an adverse pressure  gradi-  ent is illustrated by the segment X-Y of figure  1.26. A corollary of the skin friction drag is  the continual reduction of boundary layer  energy as flow continues aft on a surface.  * The  velocity profiles of the boundary layer are  shown on segment X-Y of figure 1.26. In the  area of adverse pressure gradient the bound-  ary layer flow is impeded and tends to show a  reduction in velocity next to the surface. If  the boundary layer does not have sufhcient  kinetic energy in the presence  of the adverse  pressure gradient, the lower levels of the  boundary layer may stagnate prematurely.  AIRFLOW SEPARATION.  The character  of the boundary layer on an aerodynamic  surface is greatly influenced by the pressure  gradient.  In order to study this effect, the  pressure  distribution of a cylinder in a perfect  fluid is repeated in figure 1.26. The airflows  depict a local velocity of !zero at the forward  stagnation point and a maximum local velocity  at the extreme surface. The airflow moves  from the high positive pressure  to the minimum  pressure point-a  favorable pressure gradient  (high to low).  As the air moves from the  extreme surface  aft, the local velocity decreases  to zero at the aft stagnation point.  The static  pressure  increases  from the minimum (or max-  imum suction) to the high positive pressure  at the aft stagnation point-an  adverse pres-  sure gradient (low to high).  NAVWEPS 00-SOT-80  BASIC AERODYNAMICS 