of such a device. To illustrate a few principles 
of flap management, figure 1.19 presents the 
lift and drag curves of a typical airplane in the 
clean and flap down configurations. 
In order to appreciate some of the factors 
involved in flap management, assume that the 
airpIane has just taken off and the flaps are 
extended. The pilot should not completely 
retract the flaps until the airplane has sufficient 
speed. If the flaps are retracted prematurely 
at insufhcient airspeed, maximum lift coefi- 
cient of the clean configuration may not be 
able to support the airplane and the airplane 
will sink or stall. Of course, this same factor 
must be considered for intermediate flap posi- 
tions between fully retracted and fully ex- 
tended. Assume that the airplane is allowed 
to gain speed and reduce the flight lift coefii- 
cient to the point of flap retraction indicated 
on figure 1.19. As the configuration is altered 
from the “cluttered” to the clean configura- 
tion, three important changes take place: 
(1) The reduction in camber by flap re- 
traction changes the wing pitching moment 
and-for the majority of airplanes-requires 
retrimming to balance the nose up moment 
change. Some airplanes feature an automat- 
ic retrimming which is programmed with 
flap deflection. 
(2) The retraction of flaps shown on 
figure 1.19 causes a reduction of drag coeffi- 
cient at that lift coefficient. This drag 
reduction improves the acceleration of the 
airplane. 
(3) The retraction of flaps requires an 
increase in angle of attack to maintain the 
same lift coefficient. Thus, if airplane accel- 
eration is low through the flap retraction 
speed range, angle of attack must be in- 
creased to prevent the airplane from sinking. 
This situation is typical after takeoff when 
gross weight, density altitude, and tempera- 
ture are high. However, some aircraft have 
such high acceleration through the flap re- 
traction speed that the rapid gain in air- 
speed requtres much less noticeable attitude 
change. 
NAVWEPS OO-EOT-SO 
BASIC AERODYNAMICS 
When the flaps are lowered for landing essen- 
tially the same items must be considered. Ex- 
tending the flaps will cause these. changes to 
take place: 
(1) Lowering the flaps requires retrim- 
ming to balance the nose down moment 
change. 
(2) The increase in drag requires a higher 
power setting to maintain airspeed and 
altitude. 
(3) The angle of attack required to pro- 
duce the same lift coefficient is less, e.g., 
flap extension tends to cause the airplane to 
“balloon.” 
An additional factor which must be consid- 
ered when rapidly accelerating after takeoff, 
or when lowering the flaps for landing, is the 
limit airspeed for flap extension. Excessive 
airspeeds in the flap down configuration may 
cause structural damage. 
In many aircraft the effect of intermediate 
flap deflection is of primary importance in 
certain critical operating conditions. Small 
initial deflections of the flap cause noticeable 
changes in C’s,, without large changes in drag 
coefficient. This feature is especially true of 
the airplane equipped with slotted or Fowler 
flaps (refer to fig. 1.17). Large flap deflections 
past 30’ to 33’ do not create the same rate of 
change of Cs- but do cause greater changes in 
CD. A fact true of most airplanes is that the 
first 50 percent of flap deflection causes mwc 
than half of the total change in Cr.- and the 
last 50 percent of flap deflection causes mo~c 
than half of the total change in Cs. 
The effect of power on the stall speed of an 
airplane is determined by many factors. The 
most important factors affecting this relation- 
ship are powerplant type (prop or jet), thrust- 
to-weight ratio, and inclination of the thrust 
vector at maximum lift. The effect of the 
propeller is illustrated in figure 1.20. The 
slisstream velocity behind the propeller is 
different from the free stream velocity depend- 
ing on the thrust developed. Thus, when the