NAVWEPS 00-8OT-80 
BASIC AERODYNAMICS 
EFFECT OF HIGH LIET DEVICES. The 
primary purpose of high lift devices (flaps, 
slots, slats, etc.) is to increase the CLn, of the 
airplane and reduce the stall speed. The take- 
off and landing speeds are consequently re- 
duced. The effect of a typical high lift device 
is shown by the airplane lift curves of figure 
1.15 and is summarized here: 
weight of the airplane and the horizontal com- 
ponent of lift be equal to the centrifugal force. 
Thus, the aircraft in a steady turn develops a 
lift greater than weight and experiences in- 
creased stall speeds. 
Trigonometric ‘relationships allow deter- 
mination of the effect of bank angle on stall 
speed and load factor. The load factor, B, is 
the proportion between lift and weight and is 
determined by: 
L fizz-- W 
1 n=- cos I$ 
where 
n=load factor (or “G”) 
cos 6 = cosine of the bank angle, + (phi) 
Typical values of load factor determined by 
this relationship are: 
.+.- 00 130 300 450 600 759 
n-l.00 1.035 1.154 1.414 z.ooo 4.ooo 
The stall speed in a turn can be determined by: 
where 
v,+= stall speed at some bank angle + 
V,= stall speed for wing level, lift-equal- 
weight flight 
n=load factor corresponding to the 
bank angle 
The percent increase in stall speed in a turn is 
shown on figure l.i6. Since this chart is predi- 
cated on a steady turn and constant CL,, the 
figures a!e valid for any airplane. The chart 
shows that no appreciable change in load fac- 
tor or stall speed occurs at bank angles less than 
30“. Above 4S” of bank the increase in load 
factor and stall speed is quite rapid. This fact 
emphasizes the need for avoiding steep turns at 
low airspeeds-a flight condition common to 
stall-spin accidents. 
37 
c.mip~tion L. (II far C‘, 
clun(tla~Up) . . . . . . . . . . . . . 1.5 200 
Php down. 2.0 IS.9 
The principal effect of the extension of flaps is 
to increase the C,, and reduce the angle of 
attack for any given lift coefficient. The in- 
crease in CL,, afforded by flap deflection re- 
duces the stall speed in a certain proportion, 
the effect described by the equation: 
- 
v,=v, z% J Ch, 
where 
V,,= stall speed with flaps down 
v,=stall speed without flaps 
C,= maximum lift coefficient of 
the clean configuration 
C&,= maximum lift coefficient 
with flaps down 
For example, assume the airplane described by 
the lift curves of figure 1.15 has a stall speed of 
100 knots at the landing weight in the clean 
configuration. If the flaps are lowered the 
reduced stall speed is reduced to: 
=86.5 knots