AIR CORPS  TJ4 1-320  84-86  be very careful to turn a heavy airship slowly when at low altitudes.  On the other hand, he can very materially assist the altitude pilot  in  holding a light airship down by making abrupt turns.  35. Altitude.-a.  Methods.-(l)  Altitude control of airships is  effected by two means, static and dynamic.  Tha former method  is  discussed in TM 1-325.  {2) Static means of control must always be augmented by dy› namic means.  Even though an airship takes off in perfect equi› librium it will not remain so.  Changes occur in the static lift due  to changes in meteorological conditions and loading is being varied  constantly by consumption of fuel.  To balance inequalities between  loading and lift, dynamic means must be used.  b. Trim of airship.-(1)  In the study of stability, to simplify the  discussion the subject of trim of the airship was omitted. A thor› ough know ledge of trim is however essential to intelligent control of  the airship.  (2) Under action of the static righting moment, the center of  gravity of the airship will lie directly below the center of buoyancy.  If  the line joinil!-g these two points is at right angles to the longitu› dinal axis, this axis is horizontal, and the airship is said to be  trimmed in neutral.  If, on the other hand, due to the manner of  loading or to location of the air  i.n  the ballonets of a pressure airship,  the longitudinal axis is inclined to the horizontal when the center of  gravity is directly below the center of buoyancy, the airship is said  to be trimmed nose heavy or tail heavy, as the case may be.  The  application of trim to dynamic control of airships  is  discussed in  paragraph 37.  c. Olimbing  fJifiA.l  descending.-(l)  Change in altitude is accom› plished dynamically by use of elevators  in  conjunction with thrust of  propellers.  To simplify the following discussion the airship  is  assumed to be flying with neutral trim and in static equilibrium.  If  it is desired to climb, the altitude pilot raises the elevators which causes  an action in the vertical plane similar to that described in paragraph  34 for turning in a horizontal plane. However,  in  this case, the ele› vators must be held in the raised position to prevent the static righting  moment bringing the longitudinal axis back to the horizontal.  (2) It should be especially noted that when the elevator is raised  the tail of the airship actually descends.  For this reason extreme  caution should be used  in  use of the elevator when the airship is near  the ground.  36. Reverse.-a. There is one curious paradox  in  control of air› ships at very low speeds.  It  the speed falls below a certain definite  lSO 