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Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 307
hydraulic valve lifter (reciprocating engine component). A hydraulic device in the valve train of certain
aircraft reciprocating engines. When the cam follower is on top of a cam lobe, the intake or exhaust valve is
held open by a cushion of oil inside the valve lifter. A specific amount of this oil leaks out while the valve
is open, and when the valve closes, a spring pushes the piston up and oil is pulled into the lifter to replace
that which was lost.
The continual leakage and replenishment of the oil in the hydraulic valve lifter keeps all clearance
out of the valve operating system.
hydrocarbon. An organic compound which contains only carbon and hydrogen. The vast majority of fossil
fuels such as gasoline and turbine-engine fuel are hydrocarbons.
hydrodynamics. The branch of science or technology that deals with forces produced by incompressible
fluids in motion.
hydrofoils. Flat or airfoil-shaped plates attached to struts below the hull of a hydrofoil watercraft.
hydrofoil vessel. A watercraft with a conventional hull which supports it when it is still or moving at a
slow speed, but when traveling at a high speed, hydrodynamic lift produced by hydrofoils below the hull
lifts the vessel out of the water, greatly decreasing the drag and allowing the vessel to travel at a higher
speed.
hydrogen. A colorless, highly flammable, lighter-than-air, gaseous chemical element. Hydrogen’s symbol
is H, its atomic number is 1, and its atomic weight is 1.00797. Hydrogen is the lightest of all of the
chemical elements.
An atom of hydrogen has one proton and no neutrons in its nucleus, and spinning around this
nucleus is only one electron. A molecule of hydrogen gas is made up of two atoms of hydrogen (H2).
Hydrogen gas is colorless, tasteless and lighter than air. It is used as a fuel gas for welding.
hydrogen bomb. A nuclear weapon in which two light elements such as deuterium and tritium are forced
together under extreme conditions of heat and pressure. When two of the light nuclei join together to form a
heavier nucleus (this is called atomic, or nuclear, fusion), a tremendous amount of heat is released. A
hydrogen bomb is far more powerful than the original atomic bomb.
hydrogen brazing. A form of brazing in which hydrogen is used as the fuel gas.
hydrogen embrittlement. A condition in which certain types of metal become brittle from absorbing
hydrogen while they are being electroplated.
hydrogen peroxide. A heavy, colorless, unstable liquid used as a bleaching agent, an antiseptic, and a fuel
for liquid-propellant rocket engines.
hydrolysis. The change in a chemical substance caused by water.
hydromechanical. Any device that combines fluid pressures with mechanical actions to achieve a desired
result. In a hydromechanical fuel control used for a turbine engine, hydraulic servos are used in conjunction
with mechanical linkages.
hydrometeor (meteorology). A general term for particles of liquid water or ice, such as rain, fog, frost,
etc., formed by modification of water vapor in the atmosphere. The term hydrometeor also applies to sea
spray or blowing snow, which are water or ice particles lifted from the earth by the wind.
hydrometer. An instrument used to measure the specific gravity of a liquid. A float-type hydrometer
consists of a specially weighted float with a graduated stem above the weight. The specific gravity, or
density, of the liquid determines the amount the float sinks, and the number on the stem even with the
surface of the liquid is the specific gravity of the liquid.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 308
hydroplaning. A condition that can exist when a high-speed airplane is landed on a water-covered runway.
High-speed airplanes use relatively narrow, high-pressure tires that, when the brakes are applied, try to lock
up and skid on the surface of the water in much the same way a water ski rides on the surface. A
hydroplaning tire can develop enough heat to ruin it. Effective anti-skid brake systems prevent
hydroplaning.
hydropneumatic. Equipment that uses both hydraulic and pneumatic forces. Certain types of control
systems use pneumatic inputs to operate hydraulic valves. A pneumatic input requires very little power, and
it can control a hydraulic output that produces a great deal of force.
hydro-ski. A hydrofoil mounted below the hull of a flying boat. As the flying boat begins to move through
the water, hydrodynamic lift produced by the hydro-ski lifts the hull out of the water. When the hull is out
of the water, the drag is decreased, and the flying boat can quickly accelerate to takeoff speed.
　
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