TM 1-320  2~27  AIR CORPS  ship is statically stable if it tends to return toward initial condition  of steady motion whenever slightly disturbed from that motion.  This  requirement is not dependent upon the plane  in  which deviation from  steady motion occurs, and, as will be shown later, an airship is  statically unstable in yaw.  (2) Dynamic stability is the stability effected by action of the air  ~tream  upon controlled surfaces.  Were it not for these surfaces air› ships would become unmanageable at very slow speeds.  c.  Stability may be classified further.  An airship in steady flight  has three types of stability, pitch or longitudinal, yaw or directional,  and roll about the longitudinal axis.  While these stabilities are all  correlated in the case of an airplane, this is not the case with an air› ship, the three types of stability being independent of each other.  w  Airs/lip  l'rtll'~h/1~  hDri'zonl"t~l!y  in  Sl"t~flc  ~9uiliiJri11m.  Lon~ifudintJ!  t~xis  coinicidenr  with direction o/  mot-ion  FIGURE 20.- Forces on airship in horizontal flight.  d.  The following discussion will be based upon the assumptions  for each situation that- 27 .. Forces and moments acting on airship.-a. Suppose an  airship flies along a horizontal right-line trajectory .while its longi› tudinal axis makes an angle of  oo with the flight path, then the  airship will be acted on by the following forces and moments (see  fig. 20).  (1)  Forces:  (a) L$_{0}$=Lift of inflating gas acting through center of buoyancy,  G.  40  (1)  Ascensional force remains constant.  (2) Total weight remains constant.  (3) Speed remains the same.  (4) Form of airship remains unchanged.  (5) Center of gravity and center of buoyancy remain fixed.  (6) Controls remain in neutral. 