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28 REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS transverse-continued. main transverse-One of the main transverse strengthening frames of a rigid airship provided with wire or girder bracing and spaced at regular intervals throughout the length of the airship. trim: (airplane) trim-The attitude with respect to wind axes at which balance occurs in rectilinear flight with free controls. (airship) trim-The attitude of the longitudinal axis of an airship with respect to the horizontal. (seaplane) trim-The angle with the horizontal surface of the water assumed by the float or hull under given conditions. trimming moment-The moment about a reference point, usually the center of gravity, exerted by the seaplane hull or float when held at a fixed trim angle. It is considered positive when the bow tends to rise. tunnel, water-See WATER TUNNEL. tunnel, wind-See WIND TUNNEL. turbulent flow-See FLOW, TURBULENT. turn, Immelman-See IMMELMAN TURN. turn-and-bank indicator-An instrument combining in one case a turn indicator and a lateral inclinometer. turn indicator-An instrument for indicating the existence and approximate magnitude of angular velocity about the normal axis of an aircraft. turnmeter-An instrument that measures the rate of turn of an aircraft about any predetermined axis. ultimate load-See LOAD, ULTIMATE. undercarriage-See LANDING GEAR. undock-To remove an airship from its dock. useful load-See LOAD, USEFUL. valve-To release air or supporting gas from an aerostat into the atmosphere. valve: automatic valve-(1) A spring-loaded relief valve fitted to the envelope, ballonet, or gas cell of an aerostat and set to open at a predetermined pressure for the purpose of preventing excessive internal pressure. (See figs. 4 and 6.) (2) A type of valve, used on some aerostats, which opens at a predetermined volume or hull dimension. maneuvering valve-A manually operated valve fitted to the envelope, ballonet, or gas cell of an aerostat for the purpose of releasing gas or air from within the envelope or gas cell as desired. valve petticoat-A special sleeve betwen the valve and gas container making it possible to tie off the sleeve and change valves without loss of gas. valve seal-A fabric cover used to seal the automatic valves of a rigid airship while it is docked. vane-type supercharger-See SUPERCHARGER, VANE TYPE. velocity, terminal-The hypothetical maximum speed that an airplane could attain along a specified straight flight path under given conditions of weight and propeller operation, if diving an unlimited distance in air of specified uniform density. If the term is not qualified, a vertical path angle, normal gross weight, zero thrust, and standard sea-level air density are assumed. vent: fuel-tank vent-A small tube used to conduct surplus fuel from a fuel tank, overboard clear of the airplane, and to equalize pressures. oil-tank vent-A large tube used to conduct oil vapors from the engine to the oil tank. parachute vent-A distendable opening in the apex of the canopy of a parachute designed to relieve excess pressure and to stabilize the parachute in descent. venturi tube (or venturi)-A short tube of varying cross section. The flow through the venturi causes a pressure drop in the smallest section, the amount of the drop being a function of the velocity of flow. visibility-The greatest distance at which conspicuous objects can be seen and identified. volume, aerodynamic-See AERODYNAMIC VOLUME. volume, gas (airship)-The volume of the contained gas (cf. capacity). walkway girder-See GIRDER, WALKWAY. warp-To change the form of a wing by twisting it. Warping was formerly used to perform the function now performed by ailerons. wash-The disturbance in the air produced by the passage of an airfoil. Also called the "wake" in the general case for any solid body. washin-A warp of an airplane wing giving an increase of the angle of attack toward the tip. washout-A warp of an airplane wing giving a decrease of the angle of attack toward the tip. water recovery-The process and the equipment of an airship for condensing and recovering the water contained in the exhaust gases of internal-combustion engines, in order to avoid the valving of gas as the fuel is consumed. water tunnel-A device similar to a wind tunnel, but using water as the working fluid. weight: dischargeable (or consumable) weight (airship)- All weight that can be consumed or discharged and still leave the airship in safe operating condition with a specified reserve of fuel, oil, water ballast, and provisions, and with her normal crew. empty weight-The structure, power plant, and fixed equipment of an aircraft. Included in this fixed equipment are the water in the radiator and cooling system, all essential instruments and furnishings, fixed electric wiring for lighting, heating, etc. In the case of an aerostat, it also includes the amount of ballast that must be carried to assist in making a safe landing.
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