TM 1-320  87-38  AIR CORPS  approach altitude the speed of the airship is reduced to 15 miles per  hour plus the wind velocity.  This is an excellent approach speed.  (2) The pilot now wishes to check behavior of the airship at this  speed.  Controls are placed in neutral and if the trim is correct the  airship will maintain constant altitude in a manner described  in  b  above.  If  it does not, it is necessary to adjust further the trim to  effect that result.  The principle is exactly the same whether the air.  ship is statically light or heavy.  During  remainde~  of the approach  •controls are useu to overcome gusts or changes in static conditions,  care being taken to observe principles of reverse control should the  speed fall below reversing speed or should the airship be placed in  danger by loss of static lift.  g.  When the airship arrives within 50 to.200 yards of the landing  party it is brought to landing height.  This depends on type of air› ship and length of handling guys used.  Large nonrigids usually land  about 60 feet off the ground, rigids at a much higher altitude, while  the motorized observation balloon must be landed at an altitude of  25 feet or less.  In this conn.ection it should be borne in mind that the  lower a statically  h~avy  landing can be made the smaller the drop  after aerodynamic control ceases.  In that case,• also, care should be  t aken to level the airship by use of elevators as it falls into the hands  of the party, as otherwise the tail would be injured.  h.  The landing described above is the usual type of landing. The  description is not at all complete since it omits nearly all the static  principles involved.  The other types of landings, such as turn land› ings,  will not be discussed, since the dynamic principles involved  therein are similar to those already explained.  38. Assumption as to condition of maximum stress.-a. For  airships designed prior to the World War the air speeds were quite  slow.  The aerodynamic forces acting on these airships were conse- 56  Paragraph  Assumption as to condition of maximum stress------ ------------- --- -- - 38  Transverse forces acting on airship flying at constant angle of pitch______  39  Transverse forces acting on airship in steady turn_____________________  40  Forces caused by gusts--------- ---- - - ------ -- ------------------------- 41  Empirical formulas for maximum aerodynamic bending moment on hull  and for forces on tail surfaces------ ---- - - ----- --------- - ------------- 42  Method of calculating shear and bending moment on hnll_________________  4g  Conclusion____________________________________________________________  44  SECTION  VI  AERODYNAMIC STRESS 