NAVWEPS OO-BOT-BO  BASIC AE,RODYMAMlCS  sections have zero lift at zero angle of attack,  the sections with positive camber have nega-  tive angles for zero lift.  attack would produce an approximate 0.5  change in lift coefficient. Evidently, lift,~curve  slope is not a factor important in the selection  of an airfoil.  An important lift property affected by the  airfoil shape is the section maximum lift co-  efficient, ci-.  The effect of airfoil shape on  ci-  can be appreciated by comparison of the  lift curves for the five airfoils of figure 1.12.  The NACA airfoils 63X06,63-009, and 63i-012  ate symmetrical sections of a basic thickness  distribution but maximum thicknesses of 6,  9, and 12 percent respectively. The effect of  thickness on ~1%  is obvious from an inspec-  tion of these curves  :  The importance of maximum lift coefficient  is obvious.  If the maximum lift coefficient is  high, the stall speed will be low.  However,  the high thickness and camber necessary  for  high section maximum lift coefficients may  produce low critical Mach numbers and large  twisting moments at high speed. In other  words, a high maximum lift coefficient is just  one of the many features desired of an airfoil  section.  DRAG CHARACTERISTICS. Drag is the  net aerodynamic force parallel to the relative  wind and its source is the pressure  distribution  and skin friction on the surface.  Large, thick  bluff bodies in an airstream show a predomi-  nance of form drag due to the unbalanced pres-  sure  distribution.  However,  streamlined  bodies with smooth contours show a ptedomi-  nance of drag due to skin friction.  In a  fashion similar to other aerodynamic forces,  drag forces may be considered in the form of a  coefficient which is independent of dynamic  pressure and surface area. The basic drag  equation is as follows:  D=GqS  where  D=drag, lbs.  C,= drag coefficient  q= dynamic pressure,  psf  UP  =z  (V in knots, TAS)  S= wing surface  area, sq. ft.  The force of drag is shown as the product of  dynamic pressure, surface area, and drag co-  efficient, C,.  The drag coefficient in this  equation is similar to any other aerodynamic  force coefficient-it is the ratio of drag pres-  sure to dynamic pressure. If the drag co-  efficient of a conventional airplane were plotted  versus angle of attack, the result would be  typical of the graph shown in figure 1.13.  At  low angles of attack the drag coefficient is  low and small changes in angle of attack create  only slight changes in drag coefficient. At  The 12-percent section has a cr-  approxi-  mately 70 percent greater than the 6-percent  thick section. In addition, the thicker airfoils  have greater benefit from the use of various  high lift devices.  The effect of camber is illustrated by the lift  curves of the NACA 4412 and 631-412 sections.  The NACA 4412 section is a 12 percent thick  airfoil which has 4 percent maximum camber  located at 40 percent of the chord.  The  NACA 63i-412 airfoil has the same thickness  and thickness distribution as the 631-012 but  camber added to give a “design”’  lift coefficient  (c, for minimum section drag) of 0.4.  The  lift curves for these two airfoils show that  camber has a beneficial e&t  on cl-.  29  An additional effect of camber is the change  in zero lift angle. While the symmetrical  ScCdO”  %.I  a0 for “&*  NACA 6h-312  (symmctricd)  :.  1.40  13.e  NACA 631-412  Whmd).  1.73  IS. z”  NACA  63-005  .~.  :.  Cl.82  6Mo9.  1.10  10.5~  NACA 63 ‘ -01?,.  1.40 