Center of Gravity (CG) The Axes of an Aircraft  Static Stability Stability and Control  Dynamic Stability Longitudinal Stability Gravity is the pulling force that tends to draw all bodies  within the earth’s gravitational field to the center of the  earth. The CG may be considered the point at which all the  weight of the aircraft is concentrated. If the aircraft were  supported at its exact CG, it would balance in any position.  CG is of major importance in an aircraft, for its position has  a great bearing upon stability.  The CG is determined by the general design of the aircraft.  The designers estimate how far the CP travels. They then  fix the CG in front of the CP for the corresponding flight  speed in order to provide an adequate restoring moment for  flight equilibrium.  Three terms that appear in any discussion of stability and  control are: stability, maneuverability, and controllability.  Stability is the characteristic of an aircraft that tends to  cause it to fly (hands off) in a straight-and-level flightpath.  Maneuverability is the characteristic of an aircraft to be  directed along a desired flightpath and to withstand the  stresses imposed. Controllability is the quality of the response  of an aircraft to the pilot’s commands while maneuvering  the aircraft.  An aircraft is in a state of equilibrium when the sum of all the  forces acting on the aircraft and all the moments is equal to  zero. An aircraft in equilibrium experiences no accelerations,  and the aircraft continues in a steady condition of flight.  A gust of wind or a deflection of the controls disturbs the  equilibrium, and the aircraft experiences acceleration due to  the unbalance of moment or force.  Whenever an aircraft changes its attitude in flight, it must  turn about one or more of three axes.  Figure 2-10  shows the  three axes, which are imaginary lines passing through the  center of the aircraft.  The axes of an aircraft can be considered as imaginary axles  around which the aircraft turns like a wheel. At the center,  where all three axes intersect, each is perpendicular to the  other two. The axis that extends lengthwise through the  fuselage from the nose to the tail is called the longitudinal  axis. The axis that extends crosswise from wing tip to wing  tip is the lateral, or pitch, axis. The axis that passes through  the center, from top to bottom, is called the vertical, or yaw,  axis. Roll, pitch, and yaw are controlled by three control  surfaces. Roll is produced by the ailerons, which are located  at the trailing edges of the wings. Pitch is affected by the  elevators, the rear portion of the horizontal tail assembly.  Yaw is controlled by the rudder, the rear portion of the  vertical tail assembly.  An aircraft must have sufficient stability to maintain a  uniform flightpath and recover from the various upsetting  forces. Also, to achieve the best performance, the aircraft  must have the proper response to the movement of the  controls. Control is the pilot action of moving the flight  controls, providing the aerodynamic force that induces the  aircraft to follow a desired flightpath. When an aircraft is  said to be controllable, it means that the aircraft responds  easily and promptly to movement of the controls. Different  control surfaces are used to control the aircraft about each  of the three axes. Moving the control surfaces on an aircraft  changes the airflow over the aircraft’s surface. This, in turn,  creates changes in the balance of forces acting to keep the  aircraft flying straight and level. The three types of static stability are defined by the character  of movement following some disturbance from equilibrium.  Positive static stability exists when the disturbed object tends  to return to equilibrium. Negative static stability, or static  instability, exists when the disturbed object tends to continue  in the direction of disturbance. Neutral static stability exists  when the disturbed object has neither tendency, but remains  in equilibrium in the direction of disturbance. These three  types of stability are illustrated in  Figure 2-11 .  While static stability deals with the tendency of a displaced  body to return to equilibrium, dynamic stability deals with  the resulting motion with time. If an object is disturbed from  equilibrium, the time history of the resulting motion defines  the dynamic stability of the object. In general, an object  demonstrates positive dynamic stability if the amplitude  of motion decreases with time. If the amplitude of motion  increases with time, the object is said to possess dynamic  instability.  Any aircraft must demonstrate the required degrees of static  and dynamic stability. If an aircraft were designed with static  instability and a rapid rate of dynamic instability, the aircraft  would be very difficult, if not impossible, to fly. Usually,  positive dynamic stability is required in an aircraft design to  prevent objectionable continued oscillations of the aircraft.  When an aircraft has a tendency to keep a constant AOA  with reference to the relative wind (i.e., it does not tend to  put its nose down and dive or lift its nose and stall); it is said  to have longitudinal stability. Longitudinal stability refers  2-9