NAVWEPS W-ROT-80 
APPLICATION OF AERODYNAMICS 
TO SPECIFIC PROBLEMS OF FLYING 
speed will not cause any significant reduction 
of glide ratio. This is fortunate and allows the 
specifying of convenient glide speeds which 
will be appropriate for a range of gross weights 
at which power-off gliding may be encoun- 
tered, e.g., small quantities of fuel remaining. 
An attempt to stretch a glide by flying at 
speeds above or below the best glide speed will 
prove futile. As shown by the illustration of 
figure 6.6, any C, above or below the optimum 
will produce a lift-drag ratio less than the 
maximum. If the airplane angle of attack is 
increased above the value for (L/D),,, a tran- 
sient reduction in rate of descent will take place 
but this process must be reserved for the land- 
ing phase. Eventually, the steady-state condi- 
tions would be achieved and the increased 
angle of attack would incur a lower airspeed 
and a reduction in (L/D) and glide ratio. 
The effect of gross weight on glide performance 
may be difficult to appreciate. Since (L/D)- 
of a given airplane configuration will occur at a 
specific value of C,, the gross weight of the air- 
plane will not affect the glide ratio if the air- 
plane is operated at the optimum C,. Thus, 
two airplanes of identical aerodynamic con- 
figuration but different gross weight could 
glide the same distance from the same altitude. 
Of course, this fact would be true only if both 
airplanes are flown at the specific C, to produce 
(L/D),,. The principal difference would be 
that the heavier airplane must fly at a higher 
airspeed to support the greater weight at the 
optimum C,. In addition, the heavier airplane 
flying at the greater speed along the same flight 
path would develop a greater rate of descent. 
The relationship which exists between gross 
weight and velocity for a particular C, is as 
follows: 
In order to obtain maximum glide ratio, the 
airplane must be operated at the angle of at- 
tack and lift coefficient which provide maxi- 
mum lift-drag ratio. The illustration of figure 
6.6 depicts a variation of lift-drag ratio, L/D, 
with lift coeficient, C,, for a typical airplane 
in the clean and landing configurations. Note 
that (LID),,, for each configuration will occur 
at a specihc value of lift coefficient and, hence, 
a specific angle of attack. Thus, the maximum 
glide performance of a given airplane configu- 
ration will be unaffected by gross weight and 
altitude when the airplane is operated at 
(L/D),az. Of course, an exception occurs at 
very high altitudes where compressibility ef- 
fects may alter the aerodynamic characteristics. 
The highest value of (L/D) will occur with the 
airplane in the clean configuration. As the 
airplane is changed to the landing configura- 
tion, the added parasite drag reduces (L/D)_nz 
and the C, which produces (L/D),, will be in- 
creased. Thus, the best glide speed for the 
landingconhguration generallywill be lessthan 
the best glidespeed .for theclean configuration. 
The power-off glide performance may be 
appreciated also by the graph of rate of descent 
versus velocity shown in figure 6.6. When a 
straight line is drawn from the origin tangent 
to the curve, a point is located which produces 
the maximum proportion of velocity to rate 
of descent. Obviously, this condition provides 
maximum glide ratio. Since the rate of descent 
is proportional to the power required, the 
points of tangency define the aerodynamic 
condition of (L/D)m.z. 
FACTORS AFFECTING GLIDE PER- 
FORMANCE. In order to obtain the mini- 
mum glide angle through the air, the airplane 
must be operated at (L/D)mor. The subsonic 
(LIDL of a given airplane configuration will 
occur at a specific value of lift coefficient and 
angle of attack. However, as can be noted 
from the curves of figure 6.6, small deviations 
from the optimum C, will not cause a drastic 
reduction of (L/D) and glide ratio. In fact, a