momentum. A force caused by the inertia of a moving body trying to keep the body moving in the same direction, at the same speed or, if the body is at rest, trying to keep it at rest. The momentum of a body is the product of its mass times its velocity.
Monel. The registered trade name for a corrosion-resistant alloy made of 67% nickel, 30% copper, and 3% aluminum or silicon.
monitor (air traffic control). When used with communications transfer, means to listen on a specific frequency and stand by for instructions. Under normal circumstances, monitoring does not establish communications.
monkey wrench. A common name for a form of adjustable wrench. A monkey wrench has one fixed jaw and one movable jaw, with the opening between the jaws at right angles to the length of the handle. The jaws of a monkey wrench are smooth, rather than toothed like the jaws of a pipe wrench.
monocoque (aircraft structure). A single-shell type of aircraft structure in which all of the flight loads are carried in its outside skin. Modern aircraft have skins made of aluminum alloy, formed into compound curves, and riveted together into a structure that resembles an eggshell. There is a minimum of structure inside a monocoque skin.
monolithic casting. A type of casting in which the object is cast or formed as a single piece. A large and complex object cast with one pouring of the molten metal into a mold is a monolithic casting. Monolithic castings have no seams or joints.
monoplane (type of airplane). An airplane having only one main supporting wing. Up through the 1920s, most airplanes were biplanes (airplanes having two wings) because of the ease of bracing. But as engineering knowledge increased, engineers were able to make monoplanes of sufficient strength. The lower drag of a monoplane makes it more efficient than a biplane.
monopropellant (rocket fuel). A type of propellant used for rocket engines in which the fuel and the oxidizer are both parts of a single substance.
monorail. A type of transportation system using a single rail to carry the cars or other objects moved along the system. Many industrial factories use a monorail above the production line to move heavy equipment along the line.
monospar wing (aircraft structure). A form of airplane wing structure which uses only one spar, or main spanwise structural member, to carry the loads.
monostable. Having only one stable mode of operation. Any time a monostable device is disturbed from its stable condition, it automatically returns to it.