NAVWEPS 00-801-80  HIGH SPEED AERODYNAMICS  (4) Sweepback  contributes to lateral sta-  bility in rhe same sense as dihedral.  When  the swept wing aircraft is placed in a side-  slip, the wing into the wind experiences  an  increase in lift since the sweep is less and  the wing away from the wind produces less  lift since rhe sweep is greater. As shown in  figure 3.15, the swept wing aircraft in a  sideslip experiences  lift changes and a sub-  sequent rolling  moment which  tends to  right the aircraft.  This lateral stability  conrribution depends on the sweepback and  the lift coefficient of the wing.  A highly  swept wing operating at high lift coeflicient  usually experiences such an excess of this  lateral stability contribution that adequate  controllability may be a significant problem.  As shown, the swept wing has certain im-  arated.  The combined effect of taper and  sweep present a considerable problem of tip  stall and this is illustrated by the flow pat-  terns of figure 3.16.  Design for high speed  performance may dictate high sweepback,  while structural efficiency may demand a  highly tapered planform.  When such is the  case, the wing may require extensive aero-  dynamic tailoring to provide a suitable stall  pattern and a lift distribution at cruise condi-  tion which reduces drag due to lift.  Wash-  out of the tip, variation of section camber  throughout span, flow fences, slats, leading  edge extension, etc., are typical devices used  to modify the stall pattern and minimize  drag due to lift at cruise condition.  (2) As shown by the lift curve of figure  3.15 the use of sweepback  will reduce the lift  curve slope and the subsonic maximum lift  coefficient. It is important to note this  case is definitely subsonic since sweepback  may be used to improve the transonic ma-  neuvering capability. Various sweep angles  applied to wings of moderate aspect ratio  produce these approximate effects on the  subsonic lift characteristics:  portant advantages. However, the use of  sweepback produces certain inevitable disad-  vantages which are important from the stand-  point of both airplane design and flight oper-  ations. The most important of these disad-  vantages are as follows:  (1) When sweepback is combined with  taper there is an extremely powerful tendency  for the wing to stall tip first.  This pattern  of stall is very undesirable since there would  be little stall warning, a serious reduction  in lateral control effectiveness,  and the for-  ward shift of the center of pressure would  contribute to a nose up moment (“pitch  up”  or “stick force lightening”).  Taper has its  own effect of producing higher local lift  coefhcients toward the tip and one of the  effects of sweepback is very similar.  All  outboard wing sections are affected by the  upwash of the preceding inboard sections  and the lift distribution resulting from sweep-  back alone is similar to that of high taper.  The reduction of the low speed maximum  lift coefficient (which is in addition to that  lost due to tip stall) has very important  implications in design. If wing loading is  not reduced, stall speeds increase and sub-  sonic maneuverability decreases. On the  other hand, if wing loading is reduced, the  increase in wing surface area may reduce  the anticipated benefit of sweepback in the  transonic flight regime. Since the require-  ments of performance predominate, certain  increases of stall speeds, takeoff speeds,  An additional effect is the tendency to  develop a strong spanwise flow of the bound-  ary layer toward the tip when the wing is at  high lift coefficients. This spanwise flow  produces a relatively low energy boundary  layer near the tip which can be easily sep-  251  sweep  Angle  (A):  O”.................................  0  w................................  4  300.  14  450..........  30  M)Q................................  yl 