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REPORT No. 474 NOMENCLATURE FOR AERONAUTICS By the NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS absolute altitude-See ALTITUDE, ABSOLUTE. absolute ceiling-See CEILING, ABSOLUTE. accelerometer-An instrument that measures the accelerations of an aircraft in a defined direction. acrobatics- Evolutions voluntarily performed with an aircraft other than those required for normal flight. adjustable propeller- See PROPELLER, ADJUSTABLE. aerodynamic center, wing section - A point located on or near the chord of the mean line approximately one quarter of the chord length aft of the leading edge and about which the moment coefficient is practically constant. (See fig. 2) aerodynamics-The branch of dynamics that treats of the motion of air and other gaseous fluids and of the forces acting on solids in motion relative to such fluids. aerodynamic volume (or air volume)- The total volume of an aerostat, including its projecting parts. aerodyne- A generic term for aircraft that derive their lift in flight chiefly from aerodynamic forces. aerograph- Same as METEOROGRAPH. aeronaut- The pilot of an aerostat. aeronautics- The science and art of flight. aerostat- (a) A generic term for an aircraft whose support is chiefly due to buoyancy derived from aerostatic forces. The immersed body consists of one or more containers filled with a gas that is lighter than air. (b) A balloon or airship. aerostatics- The science that treats of the equilibrium of gaseous fluids and of bodies immersed in them. aerostation- The art of operating aerostats. aileron- A hinged or movable portion of an airplane wing, the primary function of which is to impress a rolling motion on the airplane. It is usually part of the trailing edge of a wing (see fig. 5) external aileron - A separate airfoil mounted clear of the wing surfaces of an airplane but usually attached to them and deflected for lateral control. Frise aileron - An aileron having the nose portion projecting ahead of the hinge axis, the lower surface being in line with the lower surface of the wing. When the trailing edge of the aileron is raised, the nose portion protrudes below the lower surface of the wing, increasing the drag. (see fig. 1.) aileron- continued slotted aileron - An aileron having a nose and axis arrangement somewhat similar to a Frise aileron but having a smooth air passage between the nose portion of the aileron and the wing for the purpose of maintaining a smooth airflow over the upper surface of the aileron when its trailing edge is deflected downward. (see fig. 1.) upper-surface aileron- A split flap forming the rear upper surface of a wing, deflected for lateral control. aileron angle- See ANGLE, AILERON. aileron linkage arrangements: differential aileron linkage arrangement - Ailerons so interconnected that a given movement of the control stick results in the upward displacement one aileron being greater than the downward displacement of the other. floating aileron linkage arrangement- Ailerons so linked together and to the control stick as to "float" freely in the air stream except when displaced by the lateral motion of the control stick. aircraft- Any weight-carrying device designed to be supported by the air, either by the buoyancy or by dynamic action. aircraft carrier- A ship designed to carry aircraft and to permit their landing and take-off. air duct- A tube, usually of fabric, supplying air for filling or for maintaining pressure in the air-filled parts of an aerostat. (see fig. 4.) airfoil- Any surface, such as an airplane wing, aileron, or rudder, designed to obtain reaction from the air through which it moves. airfoil profile- the outline of an airfoil section (fig. 2). airfoil section- A cross section of an airfoil parallel to the plane of symmetry or to a specified reference plane. airline- The great circle route between two points. air line- An established system of aerial transportation, its equipment, or the company owing or operating it. airplane- A mechanically driven fixed-wing aircraft, heavier than air, which is supported by the dynamic reaction of the air against its wings (fig.5). canard airplane- A type of airplane having the horizontal stabilizing and control surfaces in front of the main supporting surfaces. 5