TM 1-320  22-24  AffiSHIP AERODYNAMICS  {3) Areas of reduced pressure are not symmetrical.  Their maxi› mum values occur beneath the stern and above the bow.  23. Specific stability and center of gravity of airship.-a.  By specific stability is meant the property of the airship itself to  maintain the relative position of its various parts unaltered in any  contingency.  b.  Conditions necessary for specific stability are the invariahility  of- (1) Shape of envelope whether airship is in motion or not.  (2) Relative positions of envelope and cars and surfaces.  c.  Methods used to maintain envelope shape are discussed in sec› tion I. Invariability of suspension of the car from the envelope is in› sured by a rectangular system of suspensions braced by diagonal cables  lengthwise and crosswise.  These cables prevent any very appreciable  motion of the car in regard to the envelope in case of oscillations of  the airship in vertical longitudinal plane or in transverse plane.  As  . will be shown later specific stability is absolutely essential to static  stability of airships.  d.  W’hen invariability of suspensions has been assured, the position  • of the center of gravity of the airship may be determined.  The center  of  gravity is the point at which may be aE:3umed to be applied the total  resultant of the various weights which oppose the lifting power of  the gas.  The position of the center of gravity is naturally not invari› able since the live load of the airship is variable.  Usually for ’non› rigid airships the center of grayity,  M ,  falls above the car and either  slightly above or slightly below the bottom of the envelope (see  fig. 18).  24. Center of buoyancy.- The center of gravity of the ascen› sional force of the gas contained in the envelope is called the center  of buoyancy.  For an envelope which is not moving this point should  FIGURE 17 .- Pressure distribution on nlrsbip hull (longitudinal axis Inclined to direction  of motion).  ------ - ...  -