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On July 20, 1969, the bulkily space-suited figure of astronaut Neil Armstrong descended the ladder of Apollo 11’s lunar lander, planted his boot in the Moon's gray dust, and declared: "That's one small step for a man, one giant leap for mankind."…

A material of extreme hardness that is used to shape other materials by a grinding or abrading action. Abrasive materials may be used either as loose grains, as grinding wheels, or as coatings on cloth or paper. They may be formed into ceramic…

Either the taking up of energy from radiation by the medium through which radiation is passing, or the taking up of matter in bulk by other matter. A simple example of the absorption of energy is how sunlight warms a tree's leaves (see…

A mathematical technique, often done graphically, by which accelerations of parts of a mechanism are determined. In high-speed machines, particularly those that include cam mechanisms, inertial forces may be the controlling factor in the design of…

The technique of measuring the magnitude and direction of acceleration. Measurement of acceleration includes translational acceleration with tangential and radial components and angular acceleration. Acceleration Translation acceleration The…

A mechanical or electromechanical instrument that measures acceleration. The two general types of accelerometers measure either the components of translational acceleration or angular acceleration. Acceleration measurement Translational…

An impurity atom in a semiconductor which can accept or take up one or more electrons from the crystal and become negatively charged. An atom which substitutes for a regular atom of the material but has one less valence electron may be expected to…

A method of nondestructive testing and materials characterization that uses mechanical waves moving through materials. It is similar to seismology, except in being concerned with the scale of engineering structures, such as aircraft, bridges, and…

A powdered, granular, or pelleted form of amorphous carbon characterized by very large surface area per unit volume because of an enormous number of fine pores. Activated carbon is capable of collecting gases, liquids, or dissolved substances on the…

The modification of sound fields by using additional sound transmitted from loudspeakers. The signals from the loudspeakers are controlled electronically by using digital signal processing techniques. Physical principles The physical principle…

A specific system or procedure for determining accurate manufacturing costs in order to achieve profitability goals. Traditional accounting systems allocate overhead costs by using a volume-oriented base, such as direct labor hours or direct…

A special type of nonlinear control system which can alter its parameters to adapt to a changing environment. The changes in environment can represent variations in process dynamics or changes in the characteristics of the disturbances.Nonlinear…

An approach to management of natural resources that emphasizes how little is known about the dynamics of ecosystems and that, as more is learned, management will evolve and improve. Adaptive management is a systematic strategy that seeks to improve…

A wing is the primary lift-generating surface of an aircraft. Adaptive wings—also known as smart, compliant, intelligent, morphing, controllable, and reactive wings—are lifting surfaces that can change their shape in flight to achieve optimal…

A material capable of fastening together two other materials by means of surface attachment. The terms glue, mucilage, mastic, and cement are synonymous with adhesive. In a generic sense, an adhesive is any material capable of fastening by means of…

The process of using an adhesive to manufacture an assembly. In this article, the adhesive-bonded assembly will be called an adhesive joint, and the materials to which the adhesive adheres will be called the adherends. The adhesive bonding process…

The ratio of the current to the voltage in an alternating-current circuit.In terms of complex current I and voltageV, the admittance of a circuit is given by Eq. (1), and is related to the impedance of the circuit Z by Eq. (2). Y is a complex number…

Processes for separation of gases based on the adsorption effect. When a pure gas or a gas mixture is contacted with a solid surface, some of the gas molecules are concentrated at the surface due to gas-solid attractive forces, in a phenomenon known…

The force exerted on a body whenever there is a relative velocity between the body and the air. There are only two basic sources of aerodynamic force: the pressure distribution and the frictional shear stress distribution exerted by the airflow on…

The force retarding an airplane, especially in supersonic flight, as a consequence of the formation of shock waves. Although the physical laws governing flight at speeds in excess of the speed of sound are the same as those for subsonic flight, the…

The applied science that deals with the dynamics of airflow and the resulting interactions between this airflow and solid boundaries. The solid boundaries may be a body immersed in the airflow (Fig. 1), or a duct of some shape through which the air…

A branch of engineering concerned primarily with the special problems of flight and other modes of transportation involving a heavy reliance on aerodynamics or fluid mechanics. The main emphasis in aeronautical engineering is on airplane and missile…

Aeronomy concentrates on the regions above the tropopause or upper part of the atmosphere. The region of the atmosphere below the tropopause is the site of most of the weather phenomena that so directly affect all life on the planet; this region has…

A suspension of small particles in a gas. The particles may be solid or liquid or a mixture of both. Aerosols are formed by the conversion of gases to particles, the disintegration of liquids or solids, or the resuspension of powdered material.…

A major constituent of the cell walls of certain red algae, especially members of the families Gelidiaceae and Gracilariaceae. Extracted for its gelling properties, it is one of three algal polysaccharides of major economic importance, the others…

A variety of chalcedonic quartz that is distinguished by the presence of color banding in curved or irregular patterns (Fig. 1). Most agate used for ornamental purposes is composed of two or more tones or intensities of brownish-red, often…

Aircraft specifically designed or adapted from a general utility airframe for use in agriculture and forestry and for control of insect vectors of human, animal, and plant diseases. Agricultural aircraft (Fig. 1) have become an indispensable tool…

The hinged rear portion of an aircraft wing, moved differentially on each side of the aircraft to obtain lateral or roll control moments. The angular settings of the ailerons are controlled by the human or automatic pilot through the flight control…

The control of certain environmental conditions including air temperature, air motion, moisture level, radiant heat energy level, dust, various pollutants, and microorganisms.Comfort air conditioning refers to control of spaces to promote the…

A device attached to an air-distributing duct for the purpose of discharging air into the space to be heated or cooled. These openings are referred to as registers, diffusers, supply outlets, or grills. By common acceptance, a register is an opening…

Separation of atmospheric air into its primary constituents. Nitrogen, oxygen, and argon are the primary constituents of air. Small quantities of neon, helium, krypton, and xenon are present at constant concentrations and can be separated as…

A sodium-rich plagioclase feldspar mineral whose composition extends over the range Ab100An0 to Ab90An10, where Ab (= albite) is NaAlSi3O8 and An (= anorthite) is CaAl2Si2O8 (see illustration). Albite occurs in crustal igneous rocks as a major…

A major constituent (10–47% dry weight) of the cell walls of brown algae. Extracted for its suspending, emulsifying, and gelling properties, it is one of three algal polysaccharides of major economic importance, the others being agar and…

Any compound having highly basic properties, strong acrid taste, and ability to neutralize acids. Aqueous solutions of alkalies are high in hydroxyl ions, have a pH above 7, and turn litmus paper from red to blue. Caustic alkalies include sodium…

Light emissions in the upper atmosphere from elemental lithium, potassium, and especially sodium. These alkali metals are present in the upper atmosphere at altitudes from about 50 to 62 mi (80 to 100 km) and are very efficient in resonant…

In the petroleum industry, a chemical process in which an olefin (ethylene, propylene, and so forth) and a hydrocarbon, usually 2-methylpropane, are combined to produce a higher-molecular-weight and higher-carbon-number product. The product has a…

An intentional difference in sizes of two mating parts. With running or sliding fits, in which mating parts move relative to each other, allowance is a clearance, usually for a film of oil. In this sense, allowance is the space “allowed” for motion…