Thus, wirh rhe higher lift coefficienr available, 
less dynamic pressure is required to provide 
the necessary lift. 
Because of the stated variation of stall speed 
with C-, large changes in CL- are necessary 
to produce significant changes in stall speed. 
This effect is illustrated by the graph in figure 
1.16 and certain typical values are shown 
below: 
Percent increase in CL. .~. 2 10 so loo 300 
Percent reduction in stall speed 1 5 18 29 50 
The contribution of the high lift devices must 
be considerable to cause large reduction in 
stall speed. The most elaborate combination 
of flaps, slots, slats, and boundary layer con- 
trol throughout the span of the wing would 
be required to increase C,- by 300 percent. 
A common case is that of a typical propeller 
driven transport which experiences a 70 per- 
cent increase in CzIM1 by full flap deflection. 
A typical single engine jet fighter with a thin 
swept wing obtains a 20 percent increase in 
CL- by full flap deflection. Thin airfoil sec- 
tions with sweepback impose distinct limita- 
tions on the effectiveness of flaps and the 20 
percent increase in CL- by flaps is a typical- 
if not high-value for such a configuration. 
One factor common to maximum lift condi- 
tion is the angle of attack and pressure distri- 
bution. The maximum lift coefficient of a 
particular wing configuration is obtained at 
one angle of attack and one pressure distribu- 
tion. Weight, bank angle, load factor, density 
altitude, and airspeed have no direct effect on 
the stall angle of attack. This fact is sufficient 
justification for the use of angle of attack indi- 
cators and stall warning devices which sense 
pressure distribution on the wing. During 
flight maneuvers, landing approach, takeoff, 
turns, etc. the airplani will stall if the critical 
angle of attack is cxcccdcd. The airspeed ar 
which stall occurs will be determined by 
weight, load factor, and altitude but the stall 
NAVWEPS OO-EOT-RO 
BASIC AERODYNAMICS 
angle of attack is unaffected. At any parricu- 
lar altitude, the indicated stall speed is a func- 
tion of weight and load factor. An increase 
in altitude will produce a decrease in density 
and increase the true airspeed at stall. Also, 
an increase in altitude will alter compressibility 
and viscosity effects and, generally speaking, 
cause the in,&ztcd stall speed to increase. 
This parti&lar consideration is usually sig- 
nificant only above altitudes of 20,000 ft. 
Recovery from stall involves a very simple 
concept. Since stall is precipitated by an 
excessive angle of attack, the angle of attack 
must be dccmmd. This is a fundamental princi- 
ple which is common to any airplane. 
An airplane may be designed to be “stall- 
proof” simply by reducing the effectiveness of 
the elevators. If the elevators are not power- 
ful enough to hold the airplane to high angles 
of attack, the airplane cannot be stalled in any 
condition of flight. Such a requirement for a 
tactical military airplane would seriously re- 
duce performance. High lift coefficients near 
the maximum are required for high maneuver- 
ability and low landing and takeoff speeds. 
Hence, the Naval Aviator must appreciate the 
effect of the many variables affecting the stall 
speed and regard “attitude flying,” angle of 
attack indicators, and stall warning devices 
as techniques which allow more precise control 
of the airplane at high lift conditions. 
HIGH LIFT DEVICES 
There are many different types of high lift 
devices used to increase the maximum lift co- 
efficient for low speed flight. The high lift 
devices applied to the trailing edge of a section 
consist of a flap which is usually 15 to 25 per- 
cent of the chord. The deflection of a flap 
produces the effect of a large amount of camber 
added well aft on the chord. The principal 
types of flaps are shown applied to a basic sec- 
tion of airfoil. The effect of a 30’ deflection of 
a 25 percent chord flap is shown on the lift