NAVWEPS OO-UOT-80  STABILITY AND CONTROL  trolled in a sideslip to accomplish crosswind  takeoff and landing.  The lateral control dur-  ing crosswind takeoff and landing is a par-  ticular problem when the dihedral effect is  high.  Since the sweepback  contributes a large  dihedral effect at high lift coefficients, the  problem is most important for the airplane  with considerable sweepback. The limiting  crosswind components must be given due re-  spect especially when the airplane is at low  gross weight.  At low gross weight the speci-  fied takeoff and landing speeds  will be low and  the controlled angle of sideslip will be largest  for a given crosswind velocity.  twisting deflections which reduce the effec-  tiveness of the aileron, e.g., downward deflec-  tion of an aileron creates  a nose down twist of  the wing which reduces the rolling moment  due to aileron deflection. At very high speeds,  the torsional deflection of the wing may be  so great than a rolling moment is created  opposite to the direction controlled and “aile-  ron reversal” occurs.  Prior to the speed for  aileron reversal, a serious loss of roll helix  angle may be encountered. The effect of this  aeroelastic phenomenon on rolling  perform-  ance is illustrated in figure 4.30.  To counter the undesirable inceractiuo be-  tween aerodynamic forces and wing torsional  deflections, the trailing edge ailerons may be  moved inboard to reduce the portion of the  span subjected to twisting  moments.  Of  course, the short span, highly tapered wing  planform is favorable for providing relatively  high stiffness. In addition, various configura-  tions of spoilers may be capabIe of producing  the required rolling performance without the  development of large twisting moments.  There are several general problems of flying  which involve certain principles of stability as  well as specific areas of longitudinal,  direc-  tional and lateral stability.  Various condi-  tions of flight will  exist in which certain  problems of stability (or instability) are un-  avoidable for some reason or another.  any  of the following items deserve consideration  because  of the possible unsafe  condition of flight  and the contribution to an aircraft accident.  CRITICAL REQUIREMENTS, The critical  conditions for requiring adequate lateral con-  trol power may occur at either high speed or  low speed  depending on the airplane configura-  tion and intended use. In transonic and super-  sonic flight, compressibility effects tend to  reduce the effectiveness  of lateral control de-  vices to produce required roll helix angles.  These effects are most significant when com-  bined with a loss of control effectiveness  due to  aeroelastic effects. Airplanes designed for  high speed flight  must maintain suflicient  lateral control effectiveness  at the design dive  speed and this is usually the predominating  requirement.  There are three general configurations for the  aircraft landing gear: the tricycle, bicycle, and  “conventional”  tail wheel arrangement. At  low rolling speeds  where the airplane aerody-  namic forces  are negligible, the “control-fixed”  static stability of each of these configurations  is determined by the side force characteristics  of the tires and is not a significant problem.  The instability which allows ground loops  in an aircraft with a conventional tail wheel  landing gear is quite basic and can be appre-  ciated from the illustration of figure 4.31. Cen-  trifugal force produced by a turn must be  balanced and the aircraft placed in equilibrium.  The greatest side force is produced at the main  wheels but to achieve equilibrium with the  During landing and takeoff, the airplane  must have adequate lateral control power to  contend with the ordinary conditions of flight.  The lateral controls must be capable of achiev-  ing required roll helix angles and acceleration  through prescribed roll dispIacements. Also,  the airplane must be capable of being con-  LANDING GEAR CONFIGURATIONS  MISCELLANEOUS  STABILITY  PROBLEMS 