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22 REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS position light-See LIGHT POSITION. power loading-See LOADING, POWER. pressure, altitude-See ALTITUDE, PRESSURE. pressure, dynamic-See DYNAMIC PRESSURE. pressure, impact-See IMPACT PRESSURE. pressure, manometer (aerostat)-The excess pressure inside the envelope of an aerostat over the atmospheric pressure at a standard reference point. The point of reference for the excess pressure is usually the bottom of the envelope or gas cell on airships and the level of the basket on the kite balloons. pressure flap-A flap valve fitted on the outer cover or envelope of a rigid airship and arranged to permit the rapid flow of air in and out-particularly inward. The purpose is to facilitate the rapid equalisation of the pressure of the air in the envelope with that of the surrounding air. pressure height-See HEIGHT, PRESSURE. primary structure (stress analysis)-The main framework, including fittings and attachments. Any structural member, the failure of which would seriously impair the safety of the airplane, is a part of the primary structure. profile drag-See DRAG, PROFILE. profile thickness-The maximum distance between the upper and lower contours of an airfoil, measured perpendicularly to the mean line of the profile. (See fig. 2.) projected propeller-blade area-See AREA, PROJECTED PROPELLER-BLADE. projector-A device for projecting a beam of light, as a searchlight projector. projector, bearing-See BEARING PROJECTOR. projector, ceiling-See CEILING PROJECTOR. projector, traffic-control-See TRAFFIC-CONTROL PROJECTOR. propeller-Any device for propelling a craft through a fluid, such as water or air; especially a device having blades which, when mounted on a power-driven shaft, produce a thrust by their action on the fluid. adjustable propeller-A propeller whose blades are so attached to the hub that the pitch may be changed while the propeller is at rest. automatic propeller-A propeller whose blades are attached to a mechanism that automatically sets them at their optimum pitch for various flight conditions. controllable propeller-A propeller whose blades are so mounted that the pitch may be changed while the propeller is rotating. geared propeller-A propeller driven through gearing, generally at some speed other than the engine speed. pusher propeller-A propeller mounted on the rear end of the engine or propeller shaft. tractor propeller-A propeller mounted on the forward end fo the engine or propeller shaft. propeller area-See AREA, PROPELLER. propeller-blade area-See AREA, PROPELLER-BLADE. propeller-disk area-See AREA, PROPELLER-DISK. propeller efficiency-The ratio of the thrust power to the input power of a propeller. propeller radius-See TIP RADIUS. propeller rake-The mean angle which the line joining the centroids of the sections of a propeller blade makes with a plane perpendicular to the axis. propeller root-That part of the propeller blade near the hub. (See fig. 5.) propeller thrust-The component of the total Air Force on the propeller which is parallel to the direction of advance. propeller thrust, effective-The net driving force developed by a propeller when mounted on an aircraft, i.e., the actual thrust exerted by the propeller, as mounted on an airplane, minus any increase in the resistance of the airplane due to the action of the propeller. propeller thrust, static-The thrust developed by a propeller when rotating without translation. propeller tipping-A protective covering of the blade of a propeller near the tip. (See fig. 5.) propulsive efficiency-The ratio of the product of the effective thrust and flight speed to the actual power input into the propeller as mounted on the airplane. pull-out-The maneuver of transition from a dive to horizontal flight. pull-up-A maneuver, in the vertical plane, in which the airplane is forced into a short climb, usually from approximately level flight. (cf. zoom). sudden pull-up (or sudden pull-out) (stress analysis)-A loading condition for the tail surfaces resulting from a sudden application of up-elevator (cf. dive). purity (of gas)-The ratio of the partial pressure of the aerostatics gas in the container to the total pressure of all the contained gases. push-down-The opposite of pull-up. pusher airplane-See AIRPLANE, PUSHER. pusher propeller-See PROPELLER, PUSHER. radial engine-See ENGINE, RADIAL. radio-directive devices: landing beam-A beam projected from the field to indicate to the pilot his height above the ground and the position of the airplane on the proper path for a glide landing. radio-marker beacon-A radio transmitter of low power emitting a characteristic aural signal to indicate course positions with respect to a landing field or an airway. radio-range beacon-A radio transmitter supplying directive radio waves that provide a means of keeping an aircraft on its proper course. runway localizing beacon-A small radio-range beacon giving accurate lateral direction along the runway of an airport or landing field and a few miles beyond.