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20 REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS marker, circle - A circular band marking the approximate center of the landing area or the intersection the principal landing strips on an airport or landing field. mast, mooring - See MOORING MAST. mast, radio - See RADIO MAST. mean line (of an airfoil profile) - An intermediate line between the upper and lower contours of the profile. (See fig. 2.) mechanical pilot - See AUTOMATIC PILOT. meteorograph - A recording instrument for obtaining meteorological information above the earth's surface. It contains elements to record temperature, pressure, and humidity. (Also called aerograph.) mixture control, altitude - A device on the carburetor for regulating the weight proportions of air and fuel supplied to the engine at different altitudes. moment, trimming - See TRIMMING MOMENT. monocoque fuselage - See FUSELAGE, MONOCOQUE. onoplane [[monoplane]] - An airplane with but one main supporting surface, sometimes divided into two parts by the fuselage. high-wing monoplane - A monoplane in which the wing is located at, or near, the top of the fuselage. low-wing monoplane - A monoplane in which the wing is located at, or near, the bottom of the fuselage. midwing monoplane - A monoplane in which the wing is located approximately midway between the top and bottom of the fuselage. parasol monoplane - A monoplane in which the wing is above the fuselage. monoplane, equivalent - See EQUIVALENT MONOPLANE. mooring band - See BAND, MOORING. mooring cone - See CONE, MOORING. mooring-cone outrigger - See OUTRIGGER, MOORING-CONE. mooring drag - Same as TAIL DRAG. mooring harness - A system of webbing bands, fitted over the top of a balloon, to which the mooring ropes are attached; usually found only on kite or observation balloons. mooring mast - A mast or tower at the top of which there is a fitting to which the bow of an airship may be secured. multiplane - An airplane with two or more main supporting surfaces placed one above another. nacelle - An enclosed shelter for personnel or for a power plant. A nacelle is usually shorter than a fuselage, and does not carry the tail unit. navigation light - See LIGHT, NAVIGATION. net: free balloon net - A rigging made of ropes and twine shaped to fit the upper surface of the envelope, which supports the weight of the basket, etc., and distributes the load over the entire upper surface of the envelope. gas-cell net (rigid airship) - A small-mesh netting of cord, intended to assist the fabric of the gas net - continued. cells in transmitting the gas force to a wire netting of coarser mesh and to the longitudinals, both being fitted between the longitudinals. It may be compared to the net of a free balloon. Sometimes called "gas-cell netting" or "cord netting." (See fig. 6.) inflation net - A rectangular net of cordage, used to restrain the envelope of a kite balloon or nonrigid airship during inflation. Also applied to a free-balloon net designed to be removed after inflation. net weight - See WEIGHT, NET. nonrigid airship - See AIRSHIP, NONRIGID. nose-down - To depress the nose of an airplane in flight. noseheavy - The condition of an airplane in which the nose tends to sink when the longitudinal control is released in any given attitude of normal flight (cf. tailheavy). nose-over - A colloquial expression referring to the accidental turning over of an airplane on its nose when landing. nose-up - To elevate the nose of an airplane in flight. Number, Reynolds - See REYNOLDS NUMBER. obstruction light - See LIGHT, OBSTRUCTION. octant - A variation of the aircraft sextant which measures angles up to 90[[degrees]]. Its artificial horizon is usually the bubble type. oleo gear - A type of oil-damping device that depends on the flow of oil through an orifice for its shock-absorbing effect in a landing gear. ornithopter - A form of aircraft heavier than air, deriving its chief support and propelling force from flapping wings. oscillation: phugoid oscillation - A long-period oscillation characteristic of the disturbed longitudinal motion of an aircraft. stable oscillation - An oscillation whose amplitude does not increase. unstable oscillation - An oscillation whose amplitude increases continuously until an attitude is reached from which there is no tendency to return toward the original attitude, the motion becoming a steady divergence. outrigger, mooring-cone - The member, usually tubular, which supports the mooring cone at the bow of an airship; sometimes referred to as the "mooring spindle." (See fig. 6.) over-all length - The distance from the extreme front to the extreme rear of an aircraft, including the propeller and the tail unit. overhang - (1) One half the difference in span of any two main supporting surfaces of an airplane. The overhang is positive when the upper of the two main supporting surfaces has the larger span. (See fig. 5.) (2) The distance from the outer strut attachment to the tip of a wing.