10  Streamline.-Path  of a small portion of a fluid relative to a solid  body with respect  to  which the fluid is moving.  The term is coin- Static.-Stability of such a cha-racter that, if the airship is dis› placed slightly £rom its normal attitude by rotation about an  axis through its center of gravity (as may be done in wind  tunnel experiments), moments come into play which tend  to  return the airship toward its original attitude.  Longitudinal.- Stability  with reference to disturbances in the  plane of symmetry, that is, disturbances involving pitching  .and variation of longitudinal and normal velocities.  Lateral.-Stability  with reference to disturbances involving  rolling, yawing, or side slipping, that is, disturbances in which  position of the plane o£ symmetry of the aircraft is affected.  Inherent.-Stability  of an aircraft due solely to disposition and  arrangement of its fixed parts, that is, that property which  causes it when disturbed to return to its normal attitude of  flight without use of controls or interposition of any mechani› cal devices.  Directional.-Stability  with reference to rotations about the nor› mal axis, that is, an airship possesses directional stability in  its simplest form  if  a restoring moment comes into action  when it is given a small angle of yaw.  Owing to symmetry,  directional stability is closely associated with lateral stability.  .Automatic.-Stability  dependent upon movable control surfaces  automatically operated by mechanical means.  Stability.-That  property of a body which causes it, when disturbed •  from a condition of equilibrium or steady motion, to develop .forces  or moments which tend to restore the body to its original condi› tion .  Speed,  grouou.l .-Hor~zontal  c•mponent  of velocity of aircraft rela› tive to the earth.  Slipstream.- Stream  of air driven astern by propeller.  (The indraft  ~- is sometimes included also.)  •  SliiJ  junction.- Ratio  of speed of advance through undisturbed a:ir  to the product of propeller diameter by number of revolutions in  unit time,_  that is,  Jv•  Slip function is the primary factor con› trolling propeller performance. It is  1r  times ratio of forward speed  to tip speed of propeller.  Slip.-Diiference between mean geometrical pitch and effective pitch.  Slip maY’ be expressed as a percentage of the mean geometrical pitch’  or as a linear dimension.  •  AIR CORPS  TM 1-320  4 