..  TM 1-320  2Q-21  AIRSHIP AERODYNAMICS  c. Problem UJrU1 solution.  (1)  Problem.-An  airship of 195,000-cubic-foot capacity is to be  equipped with two .engines developing a total of 300 horsepower.  What speed can be expected using the following  data~  (a) Standard atmospheric density.  (b) Shape coefficient,  0  n is 0.0136.  (e) Propeller efficiency,  E,  is 60 percent.  (d)  Envelope resistance is 40 percent  of  total resistance of com› pletely rigged airship.  (2)  Solution.-Using  Prandtl coefficient.  d.  Experience has shown the lower figure, as determined by Prandtl  coefficients, to be more generally correct than the higher figure as  determined by the Burgess formula.  .  21. Summary.-a. From study of the formulas it appears that  the speed of an airship is proportional to the cube root of the horse› power, or vice versa the horsepower varies directly as the cube of  the speed.  Since power plant weights vary directly as the horse› power, the weight of the power plant varies also as the cube of the  speed.  A point is readily reached therefore beyond which it is not  economical to increase the speed due to the excessive weight"s involved.  b.  In still air the higher the speed the less economical the fuel  consumption and the shorter the radius of action.  This is not true  when the airship is traveling against adverse winds.  The study of  just which air speed is the most economical will not be discussed in  this manual as it properly belongs to the subject of navigation.  8EariON IV  STABILITY  Paragraph  Variation of pressur-e distribution on airship bull________________________  22  Specific stability and center of gravity of airshiP-------------- -------- ---- 23  Center of buoyanCY- -------------------- - --------- --------------------- 24  Description of major axis of airshiP- ------------ ----------------- ------ 25  Types of stability---------------- --------- - ---------•--- ------------- 26  Forces and moments acting on airshiP----------------------------------- 27  Damping moment------------------------- ------ ---------------------- 28  Longitudinal stabi I i  ty ---------------- --- ----------- --------------------- 29  Directional stabilitY----------------------- ------------ - -------------- 30  Lateral stability---------------------- - - - ---- -------------------------- 31  S1JmJuarr-----•-----•------------- ..... .  ••••- ----------------... -•fl"’---•-•- ~  35  (  300X550X0.60X0.40 ) 0 • 86  V=  0.0136X0.00237X3,376.4  =88.4 feet per second=60.3 miles per hour. 