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area, measurement of—continued.
    vertical tail area—The area of the actual outline of the rudder and the fin projected in the vertical plane, the fairings and fillets being ignored.
    wing area—Wing area is measured from the projection of the actual outline on the plane of the chords, without deduction for area blanketed by fuselage or nacelles. That part of the area, so detemined, which lies within the fuselage or nacelles is bounded by two lateral lines that connect the intersections of the leading and trailing edges with the fuselage or nacelle, ignoring fairings and fillets. For the purpose of calculating area, a wing is considered to extend without interruption through the fuselage and nacelles. Unless otherwise stated, wing area always refers to toal area including ailerons.

area, projected propeller—Projected blade area times the number of blades.

area, projected propeller-blade—The projection of the propeller-blade area on a plane perpendicular to the axis of rotation of the propeller.

area, propeller—Blade area times the number of blades.

area, propeller-blade—The developed area of the blade face exclusive of the boss and the root; i.e., exclusive of that portion the thrust of which is negligible in comparison with the total thrust of the blade.

area, propeller-disk—The total area swept by a propeller; i.e., the area of a circle having the same diameter as the propeller.

arresting gear—The gear incorporated in aircraft and in the landing area to facilitate landing in a limited space.

artificial horizon—(1) A device that indicates the attitude of an aircraft with respect to the true horizon. (2) A substitute for a natural horizon, such as a liquid level, pendulum, or gyroscope, incorporated in a navigating instrument. 

aspect ratio—The ratio of the span to the mean chord of an airfoil; i.e., the ratio of the square of the span to the total area of an airfoil.
    effective aspect ratio—The aspect ratio of an airfoil of elliptical plan form that, for the same lift coefficient, has the same induced-drag coefficient as the airfoil, or the combination of airfoils, in question.

aspect ratio, propeller-blade—The ratio of the tip radius to the maximum blade width. (Obsolete.)

    altimeter-calibration standard atmosphere—A standard atmosphere used in calibrating aeronautic instruments. The standard now in use in the United States is completely defined in N.A.C.A. Report No. 246.
    standard atmosphere—An arbitrary atmosphere used in comparing the performance of aircraft. The standard atmosphere in use in the United States at present represents very nearly the average conditions found at latitude 40° and is completely defined in N.A.C.A. Report No. 218
    standard international atmosphere—The atmosphere used as an international standard presumes for mean sea level and a temperature of 15° C., a pressure of 1,013.2 millibars, lapse rate of 6.5° C. per kilometer from sea level to 11 kilometers, and thereafter a constant temperature of -56.5° C.

attack, angle of—See ANGLE OF ATTACK.

attitude—The position of an aircraft as determined by the inclination of its axes to some frame of reference. If not otherwise specified, this frame of reference is fixed to the earth. 

attitude of flight—Inclination of the three principal airplane axes to the relative wing.

autogiro—A type of rotor plane whose support in the air is chiefly derived from airfoils rotated about an approximately vertical axis by aerodynamic forces, and in which the lift on opposite sides of the plane of symmetry is equalized by the vertical oscillation of the blades.

automatic pilot—An automatic control mechanism for keeping an aircraft in level flight and on a set course. Sometimes called "gyro pilot," "mechanical pilot," or "robot pilot."

automatic propeller—See PROPELLER, AUTOMATIC.

aviation—The operation of aircraft heavier than air.

axes of an aircraft—Three fixed lines of reference, usually centroidal and mutually perpendicular. The horizontal axis in the plane of symmetry, usually parallel to the axis of the propeller, is called the longitudinal axis; the axis perpendicular to this in the plane of symmetry is called the normal axis; and the third axis perpendicular to the other two is called the lateral axis. In mathematical discussions, the first of these axes, drawn from rear to front, is called the X axis; the second, drawn downward, the Z axis; and the third, running from left to right, the Y axis. (See page 32.)

axial cone—See CONE, AXIAL.

axis, elastic (stress analysis)—The locus of all points through which a force may be applied to a structure without causing torsional deflection.

axis, wing—The locus of the aerodynamic centers of all the wing sections.

backswept—See SWEEPBACK.

balance—A condition of steady flight in which the resultant force and moment on the airplane are zero.

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