NAVWEPS DD-BDT-80  STABILITY AND CONTROL  center of gravity aft of the main wheels a bal-  ancing load on the tail wheel must be produced  toward the center of turn.  When the tail  wheel is free to swivel, the equilibrium of the  turn requires a control force opposite to the  direction of turn-i.e..  control force insta-  bility.  The inherent stability problem exists  because  the center of gravity is aft of the point  where the main side forces are developed. This  condition is analogous to the case of static  longitudinal  stability  with  the center of  gravity aft of the neutral point.  The bicycle configuration of landing gear  has stability  characteristics more like the  automobile.  If  directional control is ac-  complished with the front wheels operated  by power controls, no stability problem exists  at low speeds.  A problem can exist when the  airplane is at high speeds  because  of a distribu-  tion of normal force being different from the  ordinary static weight distribution.  If the  airplane is held onto the runway at speeds  well above the normal takeoff and landing  speeds, the front wheels carry a greater than  ordinary amount of normal force and a tend-  ency for instability exists. However, at these  same high speeds the rudder is quite powerful  and the condition is usually well within  control.  The conventional tail wheel configuration  has this basic instability or ground loop tend-  ency which must be stabilized by the pilot.  At high rolling  speeds where aerodynamic  forces are significant, the aerodynamic direc-  tional stability of the airplane resists the  ground looping tendency. The most likely  times for a ground loop exist when rolling  speeds  are not high enough to provide a con-  tribution of the aerodyhamic forces. When the  tail wheel is free to swivel or when the normal  force on the tail wheel is small, lack of pilot  attention can allow the ground loop to take  place.  The basically stable nature of the tricycle  and bicycle landing gear configurations is best  appreciated by the ease of control and ground  maneuvering of the airplane.  Operation of  a conventional tail wheel configuration after  considerable experience with tricycle cohfigu-  rations requires careful consideration af the  stability that must be furnished by the pilot  during ground maneuvering.  The tricycle landing gear configuration has  an inherent stability d,ue to the relative posi-  tion of the main wheels and the center of  gravity.  Centrifugal force produced by a  turn is balanced by the side force on the main  wheels and a side force on the nose wheel in  the direction of turn.  Note that the freeing  the nose wheel to swivel produces moments  which bring the aircraft out of the turn. Thus,  the tricycle configuration has a basic stability  which.is given evidence by control displace-  ment and a wheel side force in the direction  of turn.  Because  of the contrast in stability,  the tricycle configuration is much less difficult  to maneuver than the tail wheel configuration  and does not provide an inherent ground loop  tendency. However, a steerable nose wheel  is usually necessary to provide satisfactory  maneuvering capabilities.  The motion of an airplane in a spin can  involve many complex aerodynamic and in-  ertia forces and moments. However, there are  certain fundamental relationships regarding  spins and spin recoveries with  which  all  aviators should be familiar.  The spin differs  from a spiral dive in that the spin always  involves flight at high angle of attack while  the spiral dive involves a spiral motion of  the airplane at relatively low angle of attack.  The stall characteristics and stability  of  the airplane at high lift coefficients are im-  portant in the initial tendencies  of the airplane.  As previously mentioned, it is desirable to  have the wing initiate stall at the root first  rather than tip first.  Such a stall pattern  prevents the undesirable rolling moments at  high lift coeGients, provides suitable stall  SPINS AND PROBLEMS OP SPIN  RECOVERY 