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10 REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS [Column 1] blade element - A portion of a propeller blade contained between the surfaces of two cylinders coaxial with the propeller cutting the propeller blades. blade face - The surface of a propeller blade that corresponds to the lower surface of an airfoil. Sometimes called "thrust face" or "driving face." blade section - A cross section of a propeller blade made at any point by a plane parallel to the axis of rotation of the propeller and tangent at the centroid of the section to an arc drawn with the axis of rotation as its center. blade-width ratio - The ratio of the chord of a propeller section to the diameter of the propeller. mean blade-width ratio - The ration of the mean blade with to the diameter of the propeller. blast gate (supercharger) - A device for controlling the pressure in the nozzle box of a turbosupercharger by discharging into the free atmosphere a portion of the exhaust gases that would otherwise pass through the turbine wheel. blimp - A colloquial term for a nonrigid airship. blind flying - See INSTRUMENT FLYING. blinker light - See LIGHT, BLINKER. blister - A sheet of clear water raised by the motion of a float or hull and separated from the free-water surface by an air space. boat, flying - See FLYING BOAT. boom, tail - See TAIL BOOM. boost - To supply an engine with more air or mixture than it would normally induct at sea level. boost control, automatic - An automatic regulator of boost pressure. booster magneto - See MAGNETO, BOOSTER. boundary layer - A layer of fluid, close to the surface of a body placed in a moving stream, in which the impact pressure is reduced as a result of the viscosity of the fluid. boundary light - See LIGHT, BOUNDARY. bow cap - See CAP, BOW. bowheavy - The condition in which, in normal flight, the forward end of an airship tends to sink, and which requires correction by means of the horizontal controls. It may be due to either aerodynamic or static conditions, or to both (cf. sternheavy). bow-weighing device - An instrument for measuring the horizontal and vertical forces between an airships and its mooring mast (cf. stern-weighing device). box girder - See GIRDER, BOX. breathing - The passage of air into or out of an aerostat, due to changing volume. bridle - (a) A sling of cordage or cable which has its ends fixed at two different points, to the bight of which a single line may be attached, either removable or fixed, thus distributing the pull of the single line to two points or more in the case of a multiple bridle. (b) A towing or mooring line having two legs and intended to reduce yawing when towing or mooring. [Column 2] buffeting - The repeated aerodynamic forces experienced by any part of an aircraft, caused and maintained by unsteady flow arising from a disturbance set up by any other part of the aircraft (cf. flutter). bump - A sudden acceleration of an aircraft caused by a region of unstable atmosphere characterized by marked local vertical components in the air currents. bumper bag - A cushion secured to the bottom of an airship to prevent damage when in contact with the ground. (See figs. 4 and 6.) buoyancy: center of buoyancy (aerostat) - The center of gravity of the air displaced by a balloon of airship. It is approximately the center of gravity of the contented gas. center of buoyancy (seaplane) - the center of gravity of the fluid displaced. reserve buoyancy (excess buoyancy) - The difference between the buoyancy of a completely submerged float and the buoyancy of the float when submerged to the normal-load water line, usually expressed as a percentage of the normal- load buoyancy. burble - A term designating the breakdown of the streamline flow about a body. cabane - An arrangement of struts used for bracing on an aircraft. cable: axial cable - The axial member sometimes fitted in a rigid airship. It is attached to the central fitting of the radial or diametral wires of each main transverse and to the hull structure at bow and stern. control cable - The line of wire or stranded cable leading from the control levers to the control surfaces or interconnecting the control surfaces. (See fig. 5.) main mooring cable - The wire cable by which an airship is hauled in to a mooring mast. camber - The rise of the curve of an airfoil section, usually expressed as the ratio of the departure of the curve from a straight line joining the extremities of the curve to the length of this straight line. "Upper camber" refers to the upper surfaces; "lower camber" to the lower surface; and "mean camber" to the mean line of the section. Camber is positive when the departure is upward, and negative when it is downward. (See fig. 2.) canard airplane - See AIRPLACE, CANARD. canopy, parachute - The main supporting surface of a parachute. cap: bow cap - (1) A cap of metal or fabric used to reinforce the extreme forward end of a bow stiffener of a nonrigid or semirigid airship. (See figs. 3 and 4.) (2) The conical or cap-shaped structure at the extreme bow of rigid