plan area. The area of a surface as viewed from the top.
plane of rotation (helicopter rotor). The plane in which the rotor blades travel. The plane of rotation is perpendicular to the rotor shaft, which is the axis of rotation of the rotor blades.
plane of symmetry (aircraft layout). An imaginary vertical plane that passes through the longitudinal axis of an aircraft and divides the aircraft into two symmetrical halves.
planer. A woodworking power tool used to smooth the surfaces of a piece of wood.
planetary electrons. Electrons that circle around the nucleus of an atom in rings, or shells.
planetary gear system. A system of gears that changes the speed of rotation of a shaft, but does not change its direction of rotation. The sun gear is mounted on the crankshaft and is surrounded by and meshes with a set of planetary gears mounted on a carrier, or spider, that is part of the propeller shaft. The planetary gears also mesh with and rotate inside the fixed ring gear. When the sun gear turns, the planetary gear spider rotates inside the ring gear and turns the propeller shaft at a speed slower than that of the crankshaft.
The ratio between the speeds of the crankshaft and the propeller shaft is found by dividing the sum of the teeth on the ring gear and the sun gear by the number of teeth on the sun gear. Neither the number of teeth on the planetary gears nor the number of planetary gears rotating around the sun gear enters into the computation for the gear ratio.
plan position indicator. See PPI.
plan view. The top view of an object in an orthographic projection.
plaque (battery component). Powdered nickel used as a base for the active elements in both the positive and negative plates of an aircraft nickel-cadmium battery. Plaque is forced by heat and pressure to bond to a nickel wire mesh screen to form the plates for the battery.
plasma. A completely ionized gas, consisting of equal numbers of positive ions and free electrons. Plasma is electrically neutral.
plasma arc welding. A method of welding in which the metal is heated by an electric arc whose temperature has been greatly intensified by the injection of a gas. The plasma arc becomes a jet of high current density that produces temperatures of up to 60,000°F (33,316°C). One of the chief advantages of plasma arc welding is the concentration of this extreme heat which allows parts to be welded with a minimum of distortion.
plasma engine. An engine having applications for space travel. Neutral plasma, accelerated and directed by external magnetic fields, interacts with the magnetic fields produced by current flowing through the plasma.
plasma generator. A device, such as an electric arc chamber, that generates an extremely large amount of heat and changes neutral gases into plasma.