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16 REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS gap- The distance separating two adjacent wings of a multiplane. (See fig. 5.) gas- To replenish a balloon with fresh gas, in order to increase the purity, or to make up for a loss of gas. gas-cell fabric- See FABRIC, GAS-CELL gas-cell net- See NET, GAS-CELL gassing factor- The quantity of aerostatic gas required to maintain an aerostat for one year. It is ordinarily expressed as a percentage of the gas volume. geared propeller- See PROPELLER, GEARED. geometrical pitch of a propeller- See PITCH OF A PROPELLER, GEOMETERICAL. get-away speed- See SPEED, GET-AWAY. girder: box girder- Any built-up girder of rectangular section. Frequently used for the rectangular longitudinal members in the keel of a rigid airship from which the fuel tanks and gas cells are suspended. cruciform girder- The structure, consisting of vertical and horizontal transverse girders, fitted at the stern of a rigid airship for the purpose of supporting the inboard ends of the sternposts of the fins or of the rudder posts. walkway girder- A girder supporting a walkway along the keel or other part of a rigid or semirigid airship gland- A short tube fitted to an envelope or gas bag so that a rope or line may slip through it without leakage of gas or air. (See fig. 4.) glide- To descend at a normal angle of attack with little or no thrust. glide landing- See LANDING, GLIDE. glider- An aircraft heavier than air, similar to an airplane but without a power plant. primary-type glider- A ruggedly built glider designed for use in elementary training of student glider pilots. secondary-type glider- A gilder designed to have better aerodynamic performance than the primary type, but rugged enough for the use of pilots with limited training. performance-type glider- A glider having a high degree of aerodynamic refinement and low minimum sinking speed. gilded angle- See ANGLE, GLIDING. goldbeaters-skin fabric- See FABRIC, GOLDBEATERS-SKIN. ground angle- Same as LANDING ANGLE. ground gear- The gear, or equipment, necessary for the landing and handling of an airship on the ground. ground loop- An uncomfortable violent turn of an airplane while taxying, or during the landing or take-off run. ground speed- See SPEED, GROUND. guide rope- See DRAG ROPE. gyro, directional - See DIRECTIONAL GYRO. gyro horizon- A gyroscopic instrument that indicates the lateral and longitudinal attitude of the airplane by simulating the natural horizon. gyro pilot- See AUTOMATIC PILOT. gyroplane- A type of rotor plane whose support in the air is chiefly derived from airfoils rotated about ian approximately vertical axis by aerodynamic forces, and in which the lift on opposite sides of the plane of symmetry is equalized by rotation of the blades about the blades' axes. hangar- A shelter for housing airplanes. head, air-speed- See AIR-SPEED HEAD. heaviness, nose- See NOSEHEAVY. heaviness, stern- See STERNHEAVY. heaviness, tail- See TAILHEAVY. heaviness, wing- SEE WINGHEAVY. height, pressure- The altitude at which the gas cells of a rigid airship or the gas bag of a nonrigid airship are completely full of gas. helicopter- A type of rotor plane whose support in the air is normally derived from airfoils mechanically rotated about an approximately vertical axis. honeycomb- A grid of intersecting surfaces used to check lateral disturbances in a fluid stream. (See fig. 7.) hood: gas-shaft hood- A hood, or cowl, located on the outer cover of a rigid airship at the outer end of a gas shaft. maneuvering-valve hood- A hood, or cowl, located on the outer cover of a rigid airship just over a maneuvering valve. It is usually made of light wood or fabric and is faced to facilitate the escape of gas. valve hood- The appliance, having the form of a hood or parasol, which protects the valve of an airship or balloon against rain; also called "valve cover" or "bonnet." (See fig. 4.) hood, N.A.C.A.- The ring portion of an N.A.C.A. cowling. hook, arresting- A hook attached to an airplane which engages the arresting gear in landing. horizon, artificial- See ARTIFICIAL HORIZON. horn- A short lever attached to a control surface of an aircraft, to which the operating wire or rod is connected. (See fig. 5.) horsepower of an engine, rated- The average horsepower developed by a given type of engine at the rated speed when operating at full throttle, or at a specified altitude or manifold pressure. hub dynamometer- See DYNAMOMETER, HUB. hull: airship hull- The main structure of a rigid airship, consisting of a covered elongated framework which incloses the gas cells and supports the cars and other equipment. seaplane hull- That portion of a flying boat which furnished buoyancy when in contact with the