NAVWEPS OO-BOT-BO  BASIC AERODYNAMICS  loads on the structure and pitching moments  that must be controlled with the horizontal  tail.  Unfortunately, the flap types producing  the greatest increases  in c,,- usually cause the  greatest twisting moments. The Fowler flap  causes  the greatest change in twisting moment  while the split flap causes the least. This  factor-along with mechanical complexity of  the installation-may  complicate the choice  of a flap configuration.  The effectiveness  of flaps on a wing con-  figuration depend on many different factors.  One important factor is the amount of the  wing area affected by the flaps. Since a  certain amount of the span is reserved for  ailerons, the actual wing maximum lift prop-  erties will be less than that of the flapped  two-dimensional section. If the basic wing  has a low thickness, any type of flap will be  less effective than on a wing of greater thick-  ness. Sweepback of the wing can cause an  additional significant reduction in the effec-  tiveness of flaps.  The plainjap shown in figure 1.17 is a simple  hinged portion of the trailing edge. The effect  of the camber added well aft on the chord  causes  a significant increase in cbr.  In addi-  tion, the zero lift angle changes to a more  negative value and the drag increases  greatly.  The split flap shown in figure 1.17 consist of  plate deflected from the lower surface of the  section and produces a slightly greater change  in c ImoT  than the plain flap. However, a much  larger change in drag results from the great  turbulent wake produced by this type flap.  The greater drag ’ may not be such a disadvan-  rage when ir is realized that it may be advan-  tageous to accomplish steeper landing ap-  proaches  over obstacles  or require higher power  from the engine during approach (to minimize  engine acceleration time for waveoR).  The slottedPap  is similar to the plain flap but  the gap between the main section and flap  leading edge is given specific contours. High  energy air from the lower surface is ducted to  the flap upper surface. The high energy air  from the slot accelerates the upper surface  boundary layer and delays airflow separation  to some higher lift coefficient.  The slotted  flap can cause much greater increases in c,,,  than the plain or split flap and section drags  are much lower.  The Fowkr&zp arrangement is similar to the  slotted flap. The difference is that the de-  flected flap segment is moved aft along a set of  tracks which increases the chord and effects  an increase in wing area. The Fowler flap is  characterized by large increases in c,,, with  minimum changes in drag. ,.  High lift devices applied to the leading edge  of a section consist of slots, slats, and small  amounts of local camber. The fixed slot in a  wing conducts flow of high energy air into the  boundary layer on the upper surface  and delays  airflow separation to some higher angle of  attack and lift  coefficient. Since the slot  alone effects no change in camber, the higher  maximum lift coefficient will be obtained at a  higher angle of attack, i.e., the slot simply  delays stall to a higher angle of attack. An  automatic slot arrangement consists of a  leading edge segment (slat) which is free to  move on tracks. At low angles of attack the  slat is held flush against the leading edge by  the high positive local pressures.  When the  section is at high angles of attack, the high  local suction pressures at the leading edge  create a chordwise force forward to actuate  the slat.  The slot formed then allows the  section to continue to a higher angle of attack  and produce a clno. greater than that of the  One additional factor requiring consider-  ation in a comparison of flap types is the aero-  dynamic twisting  moments caused by the  flap. Positive camber produces a nose down  twisting moment-especially great when large  camber is used well aft on the chord (an  obvious implication is that flaps are not prac-  tical on a flying wing or tailless airplane).  The deflection of a flap causes  large nose down  moments which  create important twisting  41 