If the planform has curvature and the aver- 
age chord is not easily determined, an 
alternate expression is: 
AR = b2/.S 
The aspect ratio is a fineness ratio of the 
wing and this quantity is very powerful in 
determing the aerodynamic characteristics 
and structural weight. Typical aspect ratios 
vary from 33 for a high performance sail- 
plane to 3.5 for a jet fighter to 1.28 for a 
flying saucer. 
(5) The raat chord, c,, is the chord at the 
wing centerline and the rip chord, c,, is 
measured at the tip. 
(6) Considering the wing planform to 
have straight lines for the leading and trail- 
ing edges, the taper ratio, A (lambda), is the 
ratio of the tip chord to the root chord. 
A=& 
The taper ratio affects the lift distribution 
and the structural weight of the wing. A 
rectangular wing has a taper ratio of 1.0 
while the pointed tip delta wing has a taper 
ratio of 0.0. 
(7) The sweep angle, A (cap lambda), is 
usually measured as the angle between the 
line of 25 percent chords and a perpendicular 
to the root chord. The sweep of a wing 
causes definite changes in compressibility, 
maximum lift, and stall characteristics. 
(8) The mean aerodynamic chord, MAC, 
is the chord drawn through the centroid 
(geographical center) of plan area. A rec- 
tangular wing of this chord and the same 
span would have identical pitching moment 
characteristics. The MAC is located on the 
reference axis of the airplane and is a primary 
reference for longitudinal stability considera- 
tions. Note that the MAC is not the average 
chord but is the chord through the centroid 
of area. As an example, the pointed-tip 
delta wing with a taper ratio of zero would 
have an average chord equal to one-half the 
NAVWEPS OO-BOT-BO 
BASIC AERODYNAMICS 
root chord but an MAC equal to two-thirds 
~‘of the root chord. 
The aspect ratio, taper ratio, and sweepback 
of a planform are the principal factors which 
determine the aerodynamic characteristics of a 
.wing. These same quantities also have a defi- 
nite influence on the structural weight and stiff- 
ness of a wing. 
DEVELOPMENT OF LIFT BY A WING. 
In order to appreciate the effect of the planform 
on the aerodynamic characteristics, it is neces- 
sary to study the manner in which a wing 
produces lift.’ Figure 1.29 illustrates the three- 
dimensional flow pattern which results when 
the rectangular wing creates lift. 
J.f a wing is producing lift, a pressure differ- 
ential will exist between the upper and lower 
surfaces, i.e., for positive lift, the static pres- 
sure on the upper surface will be less than on 
the lower surface. At the tips of the wing, 
the existence of this pressure differential creates 
the spanwise flow components shown in figure 
1.29: For the rectangular wing, the lateral 
flow developed at the tip is quite strong and a 
strong vortex is created at the tip. The lateral 
‘flow-and consequent vortex strength-reduces 
inboard from the tip until it is zero at the 
centerline. 
The existence of the tip vortex is described 
by the drawings of figure 1.29. The rotational 
pressure flow combines with the local airstream 
flow to produce the resultant flow of the 
trailing vortex. Also, the downwash flow 
field behind a delta wing is illustrated by the 
photographs of figure 1.29. A tuft-grid is 
mounted aft of the wing to visualize the local 
flow direction by deflection of th,e tuft ele- 
ments. This tuft-grid illustrates the existence 
of the tip vortices and the deflected airstream 
aft of the wing. Note that an increase in 
angle of attack increases lift and increases the 
flow deflection and strength of the tip vortices. 
Figure 1.30 illustrates the principal effect 
of the wing vortex system. The wing pro- 
ducing lift can be represented by a series of