_Mast_--A vertical post or strut giving angular altitude to guys or long stays. Also used (erroneously) to designate a spar reaching out laterally or longitudinally in a horizontal position. _Monoplane_--An aeroplane with one main supporting surface. A Double Monoplane has two of such surfaces set one behind the other (tandem) but on the same level. _Multiplane_--An aeroplane having several main planes, at least more than three (for which there is the special name of triplane). N _Nacelle_--The framework, or body, of a dirigible balloon or airship. _Negative Angle of Incidence_--An angle of incidence below the line of travel, and therefore expressed with a minus sign. Surfaces bent to certain curves fly successfully at negative angles of incidence, and exhibit a comparatively large lift. O _Ornithopter_--A type of flying machine with wing surfaces which are designed to raise and sustain the machine in the air by flapping. P _Panel_--Another name for Curtain--which see. _Parabola_--The form outlined when a cone is cut by a plane parallel to a line drawn on its surface from its apex to its base. Declared to be the correct scientific curve for aeroplane surfaces, but not so proven, as yet. _Pilot_--A term widely used for an operator, or driver, of any form of aircraft. _Pitch_--The distance which a propeller would progress during one revolution, if free to move in a medium which permitted no slip (which see); just as the thread of a bolt travels in the groove of its nut. _Plane_--Speaking with exactness, a flat spread of surface; but in aeronautics it includes also the curved sustaining surfaces of aeroplanes. _Polyplane_--Another term for Multiplane. _Port_--The left-hand side of an aircraft, as one faces forward. See Starboard. _Projected Area_--The total area of an irregular structure as projected upon a flat surface; like the total area of the shadow of an object cast by the sun upon a plane fixed at right angles to its rays. _Propeller Reaction_--A force produced by a single revolving propeller, which tends to revolve the machine which it is driving, in the contrary direction. This is neutralized in various ways in the machines driven by single propellers. Where two propellers are used it is escaped by arranging them to move in opposite directions. [Illustration: A pterygoid plane.] _Pterygoid_--That type of the wings of birds which is long and narrow--as distinguished from the apteroid type. _Pylon_--A tower-shaped structure used as a derrick (which see); also for displaying signals to aeronauts. R _Radial Spoke_--A wire spoke extending from the hub of an alighting wheel straight outward from the centre to the rim of the wheel. See Tangent Spoke.