NAVWEPS OO-BOT-80  STABILITY AND CONTROL  Since wing dihedral is so powerful in pro-  ducing lateral stability it is taken as a common  denominator of the lateral stability contribu-  tion of all other components. Generally, the  contribution of wing position, flaps, power,  etc., is expressed  as an equivalent amount of  “effective dihedral” or “dihedral effect.”  there is no wing lift to change.  Thus, the  dihedral effect due to sweepback  is zero at zero  lift  and increases directly with  wing  lift  coefficient. When the demands of high speed  flight require a large amount of sweepback,  the  resulting configuration may have an excessive-  ly high dihedral effect  at low speeds  (high CL)  while the dihedral effect may be satisfactory  in normal flight (low or medium C,).  The contribution of the fadage  alone is  usually quite small depending on the location  of the resultant aerodynamic side force on the  fuselage. However, the effect of the wing-  fuselage-tail combination is significant since  the vertical placement of the wing on the fuse-  lage can greatly affect the stability of the com-  bination.  A wing located at the mid wing  position will generally exhibit a dihedral effect  no different from that of the wing alone. A  low wing location on the fuselage may con-  tribute an effect equivalent to 3’ or 4’ of nega-  tive dihedral while a high wing location may  contribute a positive dihedral of 2’ or 3 ’ .  The  magnitude of dihedral effect contributed by  vertical position of the wing is large and may  necessitate  a noticeable dihedral angle for the  low wing configuration.  The vertical tail of modern configurations  can provide a sign&ant-and,  at times, un-  desirable-contribution to the effective dihe-  dral. If the vertical tail is large, the side force  produced by sideslip may produce a noticeable  rolling moment as well as the important yaw-  ing moment contribution.  Such an effect is  usually small for the conventional airplane  configuration but the modern high  speed  airplane configuration induces this effect to a  great magnitude. It is difficult then to obtain  a large vertical tail contribution to directional  stability without incurring an additional con-  tribution to dihedral effect.  The contribution of wccpback  to dihedral ef-  fect is important because  of the nature of the  contribution.  As shown in figure 4.28, the  swept wing in a sideslip has the wing into  wind operating with an effective decrease  in  sweepback while the wing out of the wind  is operating with  an effective increase in  sweepback. If the wing is at a positive lift  coefficient, the wing into the wind has less  sweep and an increase  in lift and the wing out  of the wind has more sweep and a decrease  in  lift.  In this manner the swept back wing  would contribute a positive dihedral effect and  the swept forward wing would contribute a  negative dihedral effect.  The amount of effective dihedral necessary  to produce satisfactory flying qualities varies  greatly with the type and purpose of the air-  plane. Generally, the effective  dihedral should  not be too great since high roll due to side-  slip can create certain problems.  Excessive  dihedral effect can lead to “Dutch  roll,”  difficult rudder coordination in rolling maneu-  vers, or place extreme demands for lateral  control power during crosswind takeoff and  landing.  Of course, the effective dihedral  should not be negative during the predominat-  ing conditions of flight, e.g., cruise, high  speed, etc. If the airplane demonstrates  satis-  factory dihedral effect for these conditions of  flight, certain exceptions can be considered  when the airplane is in the takeoff and landing  configuration. Since the effects of flaps and  power are destablizing and reduce the dihedral  effect, a certain amount of negative dihedral  effect  may be possible due to these sources.  The unusual nature of the contribution of  sweepback to dihedral effect is that the con-  tribution  is proportional to the wing lift  coefficient as well as the angle of sweepback.  It should be clear that the swept wing at zero  lift will provide no roll due to sideslip since  The deflection of flaps causes the inboard  sections  of the wing to become  relatively more  298 