NAVWEPS OO-BOT-RO  BASIC AERODYNAMICS  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  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-  matically-such as automatic slats and slots-  are usually of little concern and cause little  complication since relatively small changes in  drag and pitching moments take place.  How-  ever, the flaps must be properly managed by  the pilot to take advantage of the capability  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  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. 