_Double-Surfaced_--Planes which are covered with fabric on both their upper and lower surfaces, thus completely inclosing their frames. _Down-Wind_--Along with the wind; in the direction in which the wind is blowing. _Drift_--The recoil of an aeroplane surface forced through the air: also the tendency to float in the same direction as the wind. E _Elevator_--A shorter name for the elevating planes or elevating rudder, used for directing the aeroplane upward or downward. _Ellipse_--An oval figure outlined by cutting a cone through from side to side on a plane not parallel to its base. Some inventors use the curves of the ellipse in forming the wings of aeroplanes. See Hyperbola and Parabola. _Entry_--The penetration of the air by the forward edge of aircraft surfaces. See Angle of Entry. _Equivalent Head Area_--Such an area of flat surface as will encounter head resistance equal to the total of that of the construction members of the framework--struts, braces, spars, diagonals, etc., of the aerial craft. F _Feathering_--A form of construction in which mounting on hinges, or pivots, permits the surfaces to engage the air flatwise in one direction and to pass edgewise through it in other directions. _Fin_--A fixed vertical stabilizing surface, similar in form to the fin on the back of a fish. _Fish Section_--A term applied to the lengthwise section of an aircraft when the outline resembles the general shape of a fish--blunted in front and tapering toward the rear. This form is believed to encounter less resistance than any other, in passing through the air. _Fixed Wheel_--A wheel in a fixed mounting, so that it does not swivel as does a caster wheel. _Flapping Flight_--Flight by the up-and-down beating of wings, similar to the common flight of pigeons. _Flexible Propeller_--A propeller in which the blades are frames covered more or less loosely with a fabric which is in a measure free to adjust its form to the compression of the air behind it as it revolves. _Flying Angle_--The angle of incidence of the main surface of an aeroplane when in flight. See Ground Angle. _Footpound_--The amount of force required to raise one pound to a height of one foot. _Fore-and-aft_--From front to rear: lengthwise: longitudinal. _Fuselage_--The framework of the body of an aeroplane. G _Glider_--A structure similar to an aeroplane, but without motive power. _Gliding_--Flying down a slope of air with a glider, or with an aeroplane in which the propelling power is cut off. _Gliding Angle_--The flattest angle at which a given machine will make a perfect glide. This angle differs with different machines. The flatter the gliding angle the safer the machine.