basic section. The effect of a fixed slot on 
the lift characteristics is shown in figure 1.18. 
.UO~J ana’ &Z~J can produce significant in- 
creases in cl, but the increased angle of 
attack for maximum lift can be a disadvantage. 
If slots were the only high lift device on the 
wing, the high take off and landing angles of 
attack may complicate the design of the 
landing gear. For this reason slots or slats 
are usually used in conjunction with flaps 
since the flaps provide reduction in the maxi- 
mum lift angle of attack. The use of a slot 
has two important advantages: there is only a 
negligible change in the pitching moment 
due to the slot and no significant change in 
section drag at low angles of attack. In fact, 
the slotted section will have less drag than 
the basic section near the maximum lift angle 
for the basic section. 
The slot-slat device finds great application 
in modern airplane configurations. The tail- 
less airplane configuration can utilize only the 
high lift devices which have negligible effect 
on the pitching moments. The slot and slat 
are often used to increase the cl- in high speed 
flight when compressibility effects are con- 
siderable. The small change in twisting mo- 
ment is a favorable feature for any high lift 
device to be used at high speed. Leading edge 
high lift devices are more effective on the 
highiy swept wing than trailing edge flaps 
since slats are quite powerful in controlling the 
flow pattern. Small amounts of local camber 
added to the leading edge as a high lift device 
is most effective on wings of very low thick- 
ness and sharp leading edges. Most usually 
the slope of the leading edge high lift device 
is used to control the spanwise lift distribution 
on the wing. 
‘Boundary larcr control devices are additional 
means of increasing the maximum lift coe&- 
cient of a section. The thin layer of airflow 
adjacent to the surface of an airfoil shows re- 
duced local velocities from the effect of skin 
friction. When at high angles of attack this 
boundary layer on the upper surface tends to 
NAVWEPS OO-BOT-RO 
BASIC AERODYNAMICS 
stagnate and come to a stop. If this happens 
the airflow will separate from the surface and 
stall occurs. Boundary layer control for high 
lift applications features various devices to 
maintain high velocity in the boundary layer 
to allay separation of the airflow. This con- 
trol of the boundary layer kinetic energy can 
be accomplished in two ways. One method is 
the application of a suction through ports to 
draw off low energy boundary layer and replace 
it with high velocity air from outside the 
boundary layer. The effect of surface suction 
boundary layer control on lift characteristics 
is typified by figure 1.18. Increasing surface 
suction produces greater maximum lift coe5- 
cients which occur at higher angles of attack. 
The effect is similar to that of a slot because 
the slot is essentially a boundary layer control 
device ducting high energy air to the upper 
surface. 
Another method of boundary layer control 
is accomplished by injecting a high speed jet 
of air into the boundary layer. This method 
produces essentially the same results as the 
suction method and is the more practical in- 
stallation. The suction type BLC requires the 
installation of a separate pump while the 
“blown” BLC system can utilize the high pres- 
sure source of a jet engine compressor. The 
typical installation of a high pressure BU 
system would be the augmentation of a de- 
flected flap. Since any boundary layer control 
tends to increase the angle of attack for maxi- 
mum lift, it is important to combine the bound- 
ary layer control with flaps since the flap de- 
flection tends to reduce the angIe of attack for 
maximum lift 
OPERATION OF HIGH LIFT DEVICES. 
The management of the high lift devices on an 
airplane is an important factor in flying opera- 
tions. The devices which are actuated auto-