Momentum.  A force caused by the inertia of a moving body  as it tries to keep the object moving in the same direction,  at the same speed. Mass.  A measure of the amount of matter in an object. For  the purpose of measuring the mass of air flowing through a  turbine engine, the weight of the air, in pounds per second,  is divided by the acceleration due to gravity (32.3 feet per  second). Matrix (advanced composites).  The material that bonds  the fibers together in an advanced composite structure. The  matrix carries the stresses into the fibers. Matter.  Something that has mass, takes up space, and exists  as a solid, liquid, or gas. Medium-bypass ratio engine.  A turbofan engine whose  bypass ratio is between 2:1 and 4:1. MEK.  Methul ethyl ketone. A volatile, water soluble, organic  chemical compound that is used as a solvent to remove oily  contaminants from ignition system components. Motor. (verb)  The act of rotating a turbine engine using the  starter, with the ignition system deactivated. An engine is  motored to force air through it to purge fuel fumes. Multiple-can combustor.  A combustor used in a gas turbine  engine that consists of a series of individual burner cans,  each made of an inner liner and an outer case. The individual  cans are arranged around the periphery of a centrifugal  compressor. Hot gases flow directly from the cans into the  turbine. N1.  A symbol representing the rotational speed of the low- pressure compressor in a dual-spool gas turbine engine. N2.  A symbol representing the rotational speed of the high- pressure compressor in a dual-spool gas turbine engine. NACA.  National Advisory Committee for Aeronautics.  This organization, dedicated to the technical development  of aviation, has been superseded by NASA. Methanol.  Alcohol made from wood. MFD.  Multifunction display. A liquid crystal or CRT display  that shows a number of parameters and replaces several  analog-type indicators. Microinches rms.  A measure used for cylinder wall surface  roughness. Twenty microinches rms means that the highest  and lowest deviation from the average surface is 20 millionths  of an inch. Micron.  A measurement used to identify the size of particles  trapped by filters. One micron is a micro meter, or one  millionth of a meter. It is 0.000039 inch. NACA cowling.  A long-chord cowling used over a  radial engine. The forward portion of this cowling has an  aerodynamic shape that produces a forward pull, and the  rear portion extends back to fair in with the fuselage. There  is a narrow peripheral gap between the rear of the cowling  and the fuselage for the cooling air to escape. Some NACA  cowlings have controllable flaps over this opening to control  the amount of cooling air that flows through the engine. Microprocessor.  A single silicon chip that contains the  arithmetic and logic functions of a computer. Nacelle.  An enclosed compartment, normally in the leading  edge of the wing, in which an aircraft engine is mounted. Naphtha.  A volatile, flammable liquid distilled from  petroleum. It is used as a cleaning agent and solvent, and is  present in some blended turbine-engine fuels. Milliammeter.  An instrument that measures electrical  current in units of thousandths of an ampere. NASA.  National Aeronautics and Space Administration. Millibar.  A unit of pressure in the metric system. One bar  is a pressure of 14.5 psi, or 29.52 in. Hg. One millibar is  one thousandth of a bar, or 0.01469 psi, or 0.02952 in. Hg. Naturally aspirated engine.  A reciprocating engine that  depends upon atmospheric pressure to force the fuel-air  mixture into the cylinders. Naturally aspirated engines are  neither supercharged nor turbocharged. Minor alteration.  Any alteration that does not fit the  definition of a major repair. See major repair. Module (modular engine construction).  The method of  construction for mast modern gas turbine engine. The engine  is made of several modules, or units, that can be removed and  replaced or serviced independent of the rest of the engine. Net thrust.  The thrust produced by a turbojet or turbofan  engine in which the acceleration factor is the difference  between the velocity of the incoming air and the velocity of  the exhaust gases leaving the engine.  G-18