NAVWEPS, OO-ROT-80  STABILITY AND CONTROL  (2) Yawing moment due to sideslip or  static directional stability.  (3) Yawing moment due to rolling veloc-  ity or the adverse (or proverse) yaw.  (4) Rolling moment due to yawing ve-  locity-a  cross effect similar to (3).  If the  aircraft has a yawing motion to the right,  the left wing will move forward faster and  momentarily develop more lift  than the  right and cause a rolling moment to the  right.  effective and these sections have a small  spanwise  moment arm. Therefore, the changes  in wing lift due to sideslip occur closer in-  board and the dihedral effect is reduced. The  effect of power on dihedral effect is negligible  for the jet airplane but considerable for the  propeller driven airplane. The propeller slip-  stream at high power and low airspeed makes  the inboard wing sections much more effective  and reduces  the dihedral effect. The reduction  in dihedral effect is most critical when the  flap and power effects are combined, e.g., the  propeller driven airplane in the power approach  or waveoff.  (3) Aerodynamic side force due to side-  slip.  (6) Rolling moment due to rolling ve-  locity or damping in roll.  (7) Yawing moment due yawing velocity  or damping in yaw.  With certain exceptions during the condi-  tions of landing and takeoff, the dihedral  effect or lateral stability should be positive  but light.  The problems created by excessive  dihedral effect are considerable and difficult  to contend with.  Lateral stability will  be  evident to a pilot by stick forces and displace-  ments required to maintain sideslip. Positive  stick force stability will be evident by stick  forces  required in the direction of the controlled  sideslip.  (8) The moments of inertia of the air-  plane about the roll and yaw axes.  The complex interaction of these effects pro-  duces three possible types of motion of the  airplane: (a) a directional  divergence, (b)  a spiral divergence, and (c) an oscillatory  mode termed Dutch roll.  Directional divergence  is a condition which  cannot be tolerated. If the reaction to a small  initial sideslip is such as to create moments  which tend to increase  the sideslip, directional  divergence will exist. The sideslip would in-  crease until the airplane is broadside to the  wind or structural failure occurs.  Of course,  increasing the static directional stability re-  duces the tendency for directional divergence.  Spiral divergence  will  exist when the static  directional stability is very large when com-  pared with the dihedral effect. The character  of spiral divergence is by no means violent,  The airplane, when disturbed from the equilib-  rium of level flight, begins a slow spiral which  gradually increases  to a spiral dive.  When a  small sideslip is introduced, the strong direc-  tional stability tends to restore the nose into  the wind while the relatively weak dihedral  effect lags in restoring the airplane laterally,  In the usual case, the rate of divergence in the  Previous discussion has separated  the lateral  and directional response of the airplane to  sideslip.  This separation is convenient for  detailed study of each the airplane static  lateral stability and the airplane static direc-  tional stability.  However, when the airplane  in free flight is placed in a sideslip, the lateral  and directional response  will be coupled, i.e.,  simultaneously the airplane produces rolling  moment due to sideslip and yawing moment  due to sideslip.  Thus, the lateral dynamic  motion of the airplane in free flight must  consider the coupling or interaction of the  lateral and directional effects.  The principal effects which determine the  lateral dynamic characteristics of an airplane  are :  (1) Rolling moment due to sideslip or  dihedral effect (lateral stability).  LATERAL DYNAMIC EFFECTS  299 