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devices as carburetors and in the large venturi tubes used to provide the low pressure for some
vacuum-operated flight instruments.
boosted brake (aircraft hydraulic brake). A form of power brake which uses hydraulic system pressure
to help the pilot apply force to the brake master cylinder. When the brake pedal is depressed, hydraulic
system pressure acts on the piston in the brake master cylinder to apply the brakes.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 84
Boosted brakes and power brakes are not the same. Power brake systems use hydraulic system
pressure in the brake wheel cylinders themselves.
boosted control system (aircraft flight controls). Flight controls of large, high-performance aircraft
actuated by hydraulic cylinders. Hydraulic fluid under pressure is directed into the cylinders by valves
actuated by the cockpit flight controls.
booster coil (ignition system component). An induction coil excited by pulsating DC supplied by the
aircraft battery and a vibrator.
Booster coils produce a high voltage, which is directed to an auxiliary trailing finger on the rotor
of the magneto distributor. When this high voltage jumps to ground across the electrodes of the spark plug,
it produces a hot and late spark for starting the engine.
booster magneto. A small, auxiliary, hand-cranked magneto used with some of the older aircraft engines
to provide a hot, retarded spark for starting the engine.
bootstrapping. A self-initiating or self-sustaining action. In a turbocharger system, bootstrapping describes
a transient increase in engine power that causes the turbocharger to speed up, which in turn causes the
engine to produce more power.
The word bootstrap comes from the figure of speech “a person lifts himself by his own
bootstraps.”
bore (dimension). The diameter of a round hole. The distance across the hole at its widest point.
bore (engine dimension). The diameter of the inside of the cylinder of a reciprocating engine.
borescope (inspection instrument). An instrument used to examine the inside of a structure through a
very small hole. The inside of a turbine engine and the inside of the cylinder of a reciprocating engine, for
example, can be examined by inserting the probe of the borescope through a small inspection hole or
through a spark plug hole.
A borescope furnishes its own light, and some borescopes have different ranges of magnification.
Some borescopes are fitted to a camera to photograph the inside of the structure. A modern trend in
borescopes is to place a video pickup on the borescope tube and display the results on a small television
screen.
boric acid. A white crystal (H3BO3) that can be dissolved in water to make a weak acid solution. A boric
acid solution can be used to neutralize spilled electrolyte from nickel-cadmium batteries.
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.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 85
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.
　
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