NAVWEPS Oe8OT-80 
BASIC AERODYNAMICS 
higher angles of attack the drag coefficient is 
much greater and small changes in angle of 
attack cause significant changes in drag. As 
stall occurs, a large increase in drag takes 
place. 
A factor more important in airplane per- 
formance considerations is the lift-drag ratio, 
L/D. With the lift and drag data available for 
the airplane, the proportions of CL and CD can 
be calculated for each specific angle of attack. 
The resulting plot of lift-drag ratio with angle 
of attack shows that L/D increases to some 
maximum then decreases at the higher lift 
coefficients and angles of attack. Note that 
the maximum lift-drag ratio, (L/D),,,, occurs 
at one specific angle of attack and lift coefIi- 
cient. If the airplane is operated in steady 
flight at (L/D),,,, the total drag is at a mini: 
mum. Any angle of attack lower or higher 
than that for (L/D),,, reduces the lift-drag 
ratio and consequently increases -the total 
drag for a given airpiane iift. 
The airplane depicted by the curves of Figure 
1.13 has a maximum lift-drag ratio of 12.5 at 
an angle of attack of 6”. Suppose this airplane 
is operated in steady flight at a gross weight 
of 12,500 lbs. If flown at the airspeed and 
angle of attack corresponding to (L/D),.., 
the drag would be 1,000 lbs. Any higher or 
lower airspeed would produce a drag greater 
than 1,000 lbs. Of course, this same airplane 
could be operated at higher or lower gross 
weights and the same maximum lift-drag ratio 
of 12.5 could be obtained at the same angle of 
attack of 6”. However, a change’ in gross 
weight would require a change in airspeed to 
support the new weight at the same lift co- 
efficient and angle of attack. 
Type airplane: (L/D) emz 
High performance sailplane. 25-40 
Typical patrol or transport.. 12-20 
High Performance bomber. 2~25 
Propeller powered trainer.. 1~15 
J et trainer.. 14-16 
Transonic fighter or attack.. lo-13 
Supersonic fighter or attack. 4-9 (subsonic) 
32 
Revised Januay 1965 
The configuration of an airplane has a great 
effect on the lift-drag ratio. Typical values 
of (L/D),.. are listed for various types of 
airplanes. While the high performance sail- 
plane may have. extremely high lift-drag 
ratios, such an aircraft has no real economic 
or tactical purpose. The supersonic fighter 
may have seemingly low lift-drag ratios in 
subsonic flight but the airplane configurations 
required for supersonic flight (and high [L/D]‘* 
at high Mach numbers) precipitate this situa- 
tion. 
Many important items of airplane perform- 
ance are obtained in flight at (L/D),... Typi- 
cal performance conditions which occur at 
(L/D),., are: 
maximum endurance of jet powered air- 
planes 
maximum range of propeller driven air- 
planes 
maximum climb angle for jet powered air- 
planes 
maximum power-off glide range, jet or 
Prop 
The most immediately interesting of these 
items is the power-off glide range of an air- 
plane. By examining the forces acting on an 
airplane during a glide, it can be shown that 
the glide ratio is numerically equal to the 
lift-drag ratio. For example, if the airplane 
in a glide has an (L/D) of 15, each mile of alti- 
tude is traded for 15 miles of horizontal dis- 
tance. Such a fact implies that the airplane 
should be flown at (L/D)- to obtain the 
greatest glide distance. 
An unbelievable feature of gliding perform- 
ance is the effect of airplane gross weight. 
Since the maximum lift-drag ratio of a given 
airplane is an intrinsic property of the aero- 
dynamic configuration, gross weight will not 
affect the gliding performance. If a typical 
jet trainer has an (L/@- of 15, the aircraft 1 
can obtain a maximum of 15 miles horizontal 
distance for each mile of altitude. This would 
be true of this particular airplane at any gross