TM 1-320  87  AIRSHIP AERODYNAMICS  elevators and gives the tail a positive lift. At the same time the trim  of the airship will keep its nose elevated so that there will be the  familiar dynamic lift on the hull surface and the vertical component  of the propeller thrust to assist the ascent. In this cas:e-e - Lu+ Le+L t+Ls> W  (1) From the altitude of weigh-off the airship is brought quickly  to the altitude of approach.  This varies from 150 feet for a nonrigid  to 500 feet for a large rigid, depending in some measure on gustiness  of the atmosphere. During the descent the pilot arranges the trim  he estimates to be necessary to make the landing. On arrival at the  f.  The principal object of the approach is to determine in advance  of the arrival at the party the behavior of the airship at landing speed.  Figure 26 gives a graphical picture of the approach.  placed in neutral and air speed reduced to as low a speed as possible.  The airship will quickly assume an attitude determined by the trim,  which can be read from the inclinometer.  At the same time the pilot  can notice whether the airship is rising or descending statically.  It is  useless to bring the airship to an even keel to eliminate dynamic  lift  caused by unavoidable residual speed incident  to  idling propellers, as  this would, in reality create a dynamic thrust on the tail surfaces.  As a result of the knowledge of condition of the airship derived from  weigh-off and after due consideration of existing meteorological condi› tions, the pilot is ready to make the approach.  FIGURE 26.-Approach of an a.irship to a landing.  ~~~W<tl9h  oil  horo  _,.•  NJ  •  ......,.-JtOO  ~---------- --- 2Mi-- ------ ----------~  55  d.  The most diffic•lit  maneuver which confronts the pilot is the  landing.  This operation may  b~  divided into three parts, as• follows:  (1) Weigh-off.  (2) Approach.  (3) Arrival at the landing party.  e.  Weigh-off is made at a safe altitude (1,000 feet for large airships,  250 to 500 feet for smaller ones).  For this maneuver controls are 