TM 1-320  3-4  AffiSHIP AERODYNAMICS  3.  Importance.-Airships are controlled in two ways, stati<-ally  a.nd dynamically.  The former method is discussed in TM 1-325 and  will be mentioned but incidentally in this manual.  Because of the  existence of static means of control, the study of aerodynamics may"  appear of minor importance to the operation of airships.  This is  untrue.  Stability and control are constantly effected by a combina› tion of static and dynamic forces.  To insure safety of the airship  and to preclude possibility of exposing it to dangerous conditiOJ:!.S, the  pilot must be aware of existing dynamic forces and their effects on  the airship itself and on its flight path.  F requently airships, due  tJ  unavoidable causes such as leakage of gas or accumulation of mois› ture, have become statically uncontrollable but have been sayed by the  intelligent application of dynamic means of control.  4.  Glossary of t erm s.-During recent years many terms have  been introduced into the English language covering various aspects of  aeronautical science.  Report No. 240, National Advisory Commit› tee for Aeronautics, defines the meaning of the most common of these  expressions, from which most of the following definitions have been  abstracted :  .A.erodynamics.-Branch  of dynamics which treats of the motion of  air and other gaseous fluids and of the forces acting on solids  in  motion relative to such fluids .  .A.eronautics.-Science  and art pertaining  to  the flight of aircr aft  .  .A.ero3tat.-Generic  term for aircraft whose support•is chiefly due to  buoyancy derived from . aerostatic forces.  The immersed body  consists of one or more bags, cells, or other containers filled with  a gas which is lighter than air.  Airfoil.-Any  surface designed to be projected through the air  in  order to produce a useful dynamic reaction.  Airfoil section  (or profile) .- Cross section of an airfoil made by a  plane parallel to a specified reference plane.  A line perpendicular  to this plane is called the axis of the airfoil.  Air  scoop.-Projecting scoop which uses the wind or slipstream to  maintain air pressure in the interior of the ball<:met of an aerostat.  Airship.-Aerostat  provided with a propelling. system and with  means of controlling the direction of motion.  When its power  plant is not operating it acts like a free balloon.  Nonrigid.-Airship whose form is maintained by the internal  pressure in the gas bags and ballonets (fig. 1) .  Rigid.-Airship whose form is maintained by a rigid structure  (fig. 3). 