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.
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.
hydrostatic testing (gas cylinder test). A type of test required for cylinders used to carry compressed gases. The cylinder is filled with water under a pressure higher than that of the gas it is to carry, and this pressure is held for a specified length of time. Cylinders are hydrostatically tested rather than being tested with compressed air because, if the cylinder should fail, the incompressible water will not cause an explosion, as would occur if a cylinder filled with compressed air should fail.
When oxygen cylinders are hydrostatically tested for installation in an aircraft, they are filled with water and pressurized to 5/3 of their working pressure. Standard-weight cylinders (DOT 3AA) must be hydrostatically tested every five years, and lightweight cylinders (DOT 3HT) must be tested every three years.
hydroxide. A chemical compound consisting of a metal, usually a base, or a non-metal, usually an acid, and a negative hydroxyl ion (OH). Hydroxides are found in both organic and inorganic chemicals. Some of the more familiar hydroxides are sodium hydroxide (NaOH) and potassium hydroxide (KOH).
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