NAVWEPS oOgOT-8O  BASIC AERODYNAMICS  angle of attack.  Regardless of the condi-  tion of flight, the instantaneous flight path  of the surface  determines the direction of the  oncoming relative wind and the angle of  attack is the angle between the instantaneous  relative wind and the chord line. To respect  the definition of angle of attack, visualize  the flight path of the aircraft during a loop  and appreciate that the relative wind is  defined by the flight path at any point dur-  ing the maneuver.  (2) The chord  is the characteristic dimen-  sion of the airfoil.  (3) The mean-camber  line is a line drawn  halfway between the upper and lower sur-  faces. Actually, the chord line connects the  ends of the mean-camber  line.  (4) The shape of the mean-camber  line is  very important in determining the aerody-  namic characteristics of an airfoil section.  The maximum camber  (displacement of the  mean line from the chord line) and the Ioca-  tion of the maximum camber help to define  the shape of the mean-camber line.  These  quantities are expressed  as fractions or per-  cent of the basic chord dimension.  A typi-  cal iow speed airfoil may have a maximum  camber of 4 percent located 40 percent aft of  the leading edge.  Notice that the description of an airfoil  profile is by dimensions which are fractions or  percent of the basic chord dimension. Thus,  when an airfoil. profile is specified a relative  shape is described. (NOTB: A numerical sys-  tem of designating airfoil profiles originated  by the National ~Advisory  Committee for Aero-  nautics [NACA] is used to describe the main  geometric features and certain aerodynamic  properties. NACA Report Nol 824 wi!! pro-  vide the detail of this system.)  AERODYNAMIC FORCE COEFFICIENT.  The aerodynamic forces  of lift and drag depend  on the combined effect of many different vari-  ables. The important single variables could  IX:  (5) The thickness and thickness distribu-  tion of the profile are important properties  of a section. The maximum tbicknus and  location of maximum thickness define thick-  ness and distribution of thickness and are  expressed  as fractions or percent of the chord.  A typical low speed airfoil may have a.  maximum thickness of 12 percent located  30 percent aft of the leading edge.  (6) The leading edge  radius of the airfoil is  the radius of curvature given the leading edge  shape. It is the radius of the circle centered  on a line tangent to the leading edge camber  and connecting tangency pcints of upper and  lower surfaces  with the leading edge. Typi-  cal leading edge radii are zero (knife edge)  to 1 or 2 percent.  If the effects of viscosity and compressibility  are not of immediate importance, the remain-  ing items can be combined for consideration.  Since the major aerodynamic forces are the  result of various pressures distributed on a  surface, the surface  area will be a major factor.  Dynamic prcssurc  of the airstream is another  common denominator of aerodynamic forces  and is a major factor since the magnitude  of a  pressure distribution depends on the source  energy of the free stream. The remaining  major factor is the relative peJJ#re  dittribution  (9) The angle of attack  is the angle between  the chord line and the relative wind.  Angle  of attack is given the shorthand notation  a (alpha). Of course, it is important to dif-  ferentiate between pitch attitude angle and  (8) The drag incurred by an airfoil is the  net force produced parallel to the relative  wind.  (7) The Iift produced by an airfoil is the  net force produced perpendicular  to the n&a-  tive wind.  22  (1) Airstream velocity  (2) Air density  (3) Shape or profile of the surface  (4) Angle of attack  (5) Surface  area  (6) Compressibility effects  (7) Viscosity effects 