TM 1-320  4  AIRSHIP AERODYNAMICS  of symmetry, usually parallel to axis of the propeller, is called  the longitudinal axis; the axis perpendicular to this in the plane  of symmetry is called the normal axis; and the third axis perpen› dicular to the other two is called the lateral axis.  In mathe› matical discussions, the first of these axes, drawn from front to  rear, is called the  X  axis; the second, drawn upward, the  Z  axis;  and the third, running from right to left, the  Y  axis.  Ballast.-Any  substance, usually sand or water, carried in a balloon  or airship and intended to be thrown out, if necessary, for the pur› pose of reducing load carried and thus altering aerostatic rela› tions.  Ballonet.- Compartment  of variable volume constructed of fabric  or partitioned otf within the interior of a balloon or airship.  It  is  usually partially inflated with air under control of valves from a  blower or from an air scoop.  By blowing in or letting out air,  it serves to compensate for changes of volume in gas contained in  the envelope and to maintain gas pressure, thus preventing defor› mation or structural failure.  By means of two or niore ballonets,  often used in nonrigid airships, the trim can also be controlled.  The ballonet should not be confused with gas cell.  Blade back.-Si<le  of propeller blade which corresponds to upper  surface of an airfoil.  Blade face.-Surface  of propeller blade which corresponds to lower  surface of an airfoil.  Sometimes called "thrust face" or "driving  face."  Blade width ratio.-Ratio  of developed width of propeller blade at  any point to circumference of •a circle whose radius is the distance  of that point from the propeller axis.  Bow stiffener.-Rigid  member attached to bow of nonrigid or semi› rigid envelope to reinforce it against pressure caused by motion  of the airship.  Sometimes called "nose stiffener" or "nose batten."  Buoyancy.-Upward  air force on aerostat which is derived from  aerostatic conditions.  It  is equal to weight of air displaced.  Buoyancy, center of  (aerostat).-Center of gravity of volume of  contained gas.  Oamber.-Rise  in curve of an airfoil section from its chord, usually  expressed as ratio of departure of the curve from the chord to the  length of the chord.  "Upper camber" refers to the upper surface  of an airfoil and "lower camber" to the lower surface; "mean  camber" is the mean of these two.  Capacity.- Volume of the gas-containing portion of an aerostat.  Oar.- That portion of an airship intended to carry power unit or units,  6 