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, 

