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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.
Bowden cable (aircraft control system). A form of control system which uses a spring steel wire,
enclosed inside a helically wound wire casing. A Bowden cable transmits both pushing and pulling motion
to the device being actuated.
Bowden cable systems are often used for moving such aircraft engine controls as the throttle and
the mixture control.
bowline knot. A handy knot used for such aeronautical purposes as securing an aircraft to prevent wind
damage.
A properly tied bowline knot will not slip, and it is easy to untie.
bow wave (supersonic aircraft flight). A shock wave which forms when an aircraft is flying at a speed
faster than the speed of sound, a speed greater than Mach one. A bow wave either forms immediately ahead
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 86
of the aircraft or is attached to the nose of the aircraft.
box beam (aircraft structural member). The main spanwise structural member of a wing made in the
form of a box.
Tensile and compressive stresses are carried in the top and the bottom of the box beam, and the
sides act as webs to give the beam rigidity. The top and bottom of the box beam oppose the greatest loads
and are the strongest parts of the beam. A box beam is also called a box spar.
box brake (metal-forming shop tool). A metal-forming machine similar to a leaf, or cornice, brake. It is
used to make straight bends across a piece of sheet metal.
The holding jaws of a box brake are made in the form of fingers. All four sides of a box can be
formed on a box brake by allowing the sides that have been bent to fit between the fingers of the clamp
while the last bends are being made. A box brake is also called a finger brake.
boxing of paint. A mixing procedure in which the pigment in a can of paint is thoroughly mixed with the
vehicle. The paint is poured back and forth between two containers until the pigment and the vehicle are
completely and uniformly mixed.
box wrench (mechanic’s hand tool). A wrench with an enclosed end shaped with six, eight, or twelve
points to fit a square- or hexagonal-head bolt or nut.
Box wrenches can be used in close spaces, and they can be used to apply a greater force than can
be applied with an open-end wrench.
Boyle’s law. One of the basic gas laws, which states that the product found by multiplying the pressure of a
gas by its volume is always constant. If the volume of a container of gas is decreased without changing the
absolute temperature of the gas, the pressure of the gas will increase.
brace. A part of a structure that supports or strengthens the main load-carrying portion of the structure.
brad. A thin wire nail with a small-diameter, barrel-shaped head.
brad-point drill. A special drill used for drilling cured Kevlar-reinforced composite materials. The
tendency of Kevlar fibers to pull and stretch when they are cut with a drill is minimized by using a
brad-point drill.
braid (electrical shielding and bonding component). A woven metal tube used to encase a wire carrying
alternating current. This braid, called shielding, intercepts the electromagnetic field produced by the AC
and prevents the field causing radio interference.
Braid can also be flattened and used as a bonding strap, to conduct static electricity away from a
component insulated by shock mounts. Bonding helps reduce static electricity that causes radio
interference.
brake (aircraft landing gear component). A mechanism inside an aircraft wheel used to apply friction to
　
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