boring (machining operation). A method of increasing the size of a hole in a piece of material by cutting it with a rotary cutting tool.
boron. A soft, brown, nonmetallic, trivalent chemical element. Boron’s symbol is B, its atomic number is 5, and its atomic weight is 10.811.
Boron is used to add stiffness and strength to some of the modern composite structural materials, and to dope silicon or germanium to make a P-type semiconductor material.
boss. An enlarged area in a casting or machined part. A boss provides additional material to strengthen the part where holes are drilled for mounting or attaching parts.
bottled gas. Any gas stored in heavy steel containers, with the gas kept under pressure. Acetylene, propane, oxygen, and nitrogen are commonly used bottled gases.
bottom. See cone bottoming.
bottom dead center (reciprocating engine piston position). The position of the piston in a reciprocating engine cylinder when the piston is at the bottom of its stroke and the wrist pin, crankpin, and center of the crankshaft are all in line.
bottoming reamer (metalworking tool). A reamer used to smooth and enlarge blind holes. The blades of a bottoming reamer are parallel; they have no taper.
bottoming tap (metalworking tool). A tool used to cut threads all the way to the bottom of a hole. A bottoming tap has a flat end and no taper. It is used after the hole has been partially tapped with a plug tap.
boundary layer (aerodynamics). The layer of air that flows next to an aerodynamic surface.
Because of the design of the surface and local surface roughness, the boundary layer often has a random flow pattern, sometimes even flowing in a direction opposite to the direction of flight. A turbulent boundary layer causes a great deal of aerodynamic drag.
boundary-layer control. A method of decreasing aerodynamic drag caused by the turbulent flow of the boundary layer. Boundary-layer control can use either a high-velocity blast of air to blow the random-flowing air off the surface, or it can use a low pressure inside the structure to suck the boundary layer air off the surface through tiny holes or slots.
Bourdon tube. The measuring element used in certain types of pressure-measuring instruments.
A thin-walled metal tube, having an elliptical cross section, is formed into a curve. One end of the tube is sealed and is connected to an arm that moves a pointer. The other end of the tube is open and is secured to the instrument case. The pressure to be measured is connected to the open end of the tube, and this pressure causes the elliptical cross section to try to become round and thus begin to straighten the tube.
The amount the tube straightens is proportional to the pressure inside the tube, and as it straightens, it moves a pointer across the calibrated instrument dial.
BOW (basic operating weight). The weight of an aircraft, including the crew, ready for flight but without payload and fuel. This term applies only to transport category aircraft.
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