NAVWE,PS OD-80T-80 
APPLICATION OF AERODYNAMICS 
TO SPECIFIC PROBLEMS OF FLYING 
the forces acting on the airplane, i.e., lift, 
thrust, weight, and drag. 
ANGLE OF ATTACK VERSUS AIRSPEED. 
In order to achieve equilibrium in the vertical 
direction, the net lift must equal the airplane 
weight. This is a contingency of steady, level 
flight or steady climbing and descending flight 
when the flight path inclination is slight. A 
refinement of the basic lift equation defines the 
relationship of speed, weight, lift coefficient, 
etc., for the condition of lift equal to weight. 
V=17.2 y J TP 
or 
where 
V=velocity, knots (TAX) 
VE=equivalent airspeed, knots (EAS) 
W=gross weight, lbs. 
S= wing surface area, sq. ft. 
W/S= wing loading, psf 
g=altitudc density ratio 
C,= lift coefficient 
From this relationship it is appreciated that a 
given configuration of airplane with a specific 
wing loading, W/S, will achieve lift equal to 
weight at particular combinations of velocity, 
V, and lift coefficient, C,. In steady flight, each 
equivalent airspeed demands a particular vaIue 
of C,, and each value of C, demands a particular 
equivalent airspeed to provide lift equal to 
weight. Figure 6.1 illustrates a typical lift 
curve for an airplane and shows the relation- 
ship between C, and OL, angle of attack. For 
this relationship, some specific value of a will 
create a certain value of C, for any given aero- 
dynamic configuration. 
For the conditions of steady flight with 
a given airplane, each angle of attack corre- 
sponds to a specific airspeed. Each angle of 
attack produces a specific value of CL and each 
value of C, requires a specific value of equiva- 
lent airspeed to provide lift equal to weight. 
Hence, angle of attack is the primary .control of 
airspeed in mad3 fright. If an airplane is es- 
tablished in steady, level flight at a particular 
airspeed, any increase in angle of attack will 
result in some reduced airspeed common to the 
increased C,. A decrease in angle of attack 
will result in some increased airspeed com- 
mon to the decreased CL. As a result of the 
change in airspeed, the airplane may climb or 
descend if there is no change in powet setting 
but the change in airspeed was provided by 
the change in angle of attack. The state of 
the airplane during the change in speed will 
be some transient condition between the 
original and final steady state conditions. 
Primary control of airspeed in steady flight 
by angle of attack is an important principle. 
With some configurations of airplanes, low 
speed flight will bring about a low level of 
longitudinal stick force stability and possi- 
bility of low airplane static longitudi- 
nal stability. In such a case, the “feel” for 
airspeed will be light and may not furnish 
a ready reference for easy control of the air- 
plane. In addition, the high angles of attack 
common to low speed flight are likely to pro- 
vide large position errors to the airspeed indi- 
cating system. Thus, proper control of air- 
speed will be enhanced by good “attitude” 
flying or-when the visual t;eference field is 
poor-an angle of attack indicator. 
RATE OF CLIMB AND .DESCENT. In 
order for an airplane to achieve ‘equilibrium at 
constant altitude, lift must be equal to weight 
and thrust must be equal to drag. Steady, 
level flight requires equilibrium in both the 
vertical and horizontal directions. For the 
case of climbing or descending flight condi- 
tions, a component of weight is inclined along 
the flight path direction and equilibrium is 
achieved when thrust is not equal to the drag. 
When the airplane is in a steady climb or 
descent, the rate of climb is related by the 
following expression: 
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