NAVWEPS oOgOT-8O 
BASIC AERODYNAMICS 
(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. 
(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. 
(7) The Iift produced by an airfoil is the 
net force produced perpendicular to the n&a- 
tive wind. 
(8) The drag incurred by an airfoil is the 
net force produced parallel to the relative wind. 
(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- 
i ferentiate between pitch attitude angle and 
22 
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. 
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: 
(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 
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