NAVWEPS 00-ROT-80  HIGH SPEED AERODYNAMICS  marginal control during crosswind takeoff  and landing where the aircraft must move in  a controlled sideslip. Therefore, it is not  unusual to find swept wing aircraft with  negative dihedral and lateral control de-  vices designed principally to meet cross  wind  takeoff and landing requirements.  and landing speeds  usually will be accepted.  While the reduction of lift curve slope may  be an advantage for gust considerations,  the reduced sensitivity to changes in angle  of attack has certain undesirable effects in  subsonic flight.  The reduced wing lift  curve slope tends to increase maximum lift  angles of attack and complicate the problem  of landing gear design and cockpit visi-  bility.  Also, the lower lift curve slope  would reduce the contribution to stability  of a given tail surface area.  (5) The structural complexity and aero-  elastic problems created by sweepback  are of  great importance. First, there is the effect  shown in figure 3.17 that swept wing has a  greater structural span than a straight wing  of the same area and aspect  ratio.  This effect  increases wing  structural weight  since  greater bending and shear material must be  distributed in the wing to produce the same  design strength. An additional problem is  created near the wing root and “carry-  through” structure due to the large twisting  loads and the tendency of the bending stress  distribution to concentrate  toward the trail-  ing edge. Also shown in figure 3.17 is the  influence of wing deflection on the spanwise  lift distribution.  Wing bending produces  tip rotation which tends to unload the tip  and move the center of pressure forward.  Thus, the same effect which tends to allay  divergence can make an undesirable contri-  bution to longitudinal stability.  (3) The use of sweepback will  reduce  the effectiveness of trailing  edge control  surfaces and high lift devices. A typical  example of this effect is the application of  a single slotted flap over the inboard 60  percent span to both a straight wing and a  wing with 35” sweepback. The flap applied  to the straight wing produces an increase  in maximum lift  coefficient of approxi-  mately 50 percent. The same type flap  applied to the swept wing produces an  increase in maximum lift  coefficient of  approximately 20 percent.  To produce some  reasonable maximum lift coefficient one a  swept wing may require unsweeping the  flap hinge line, application of leading edge  high lift devices such as slots or slats, and  possibly boundary layer control.  EFFECT OF ASPECT RATIO AND TIP  SHAPE. In addition to wing sweep, plan-  form properties such as aspect ratio, and tip  shape, can produce significant effects on the  aerodynamic characteristics at high speeds.  There is no particular effect of aspect ratio on  critical Mach number at high or medium  aspect ratios. The aspect ratio must be less  than four or five to produce any apparent  change in critical Mach number. This effect  is shown for a typical 9 percent thick sym-  metrical airfoil in the graph of figure 3.18.  Note that very low aspect ratios are required  to cause a significant increase in critical Mach  number. Very low aspect ratios create the  extremes of three dimensional flow and sub-  sequent increase in free stream speed to create  (4) As described previously, sweepback  contributes to lateral stability by producing  stable rolling moments with sideslip. The  lateral stability contribution of sweepback  varies with the amount of wing sweepback  and wing lift coefficient-large sweepback  and high lift coefficients producing large  contribution to lateral stability.  While sta-  bility is desirable, any excess  of stability will  reduce controllability.  For the majority of  airplane configurations, high  lateral sta-  bility is neither necessary  nor desirable, but  adequate control in roll is absolutely neces-  sary for good flying qualities.  An excess  of  lateral stability from sweepback  can aggra-  vate “Dutch  roll”  problems and produce  134 