 G-26 Precession.  The characteristic of a gyroscope that causes a  force to be felt, not at the point of application, but at a point  90° in the direction of rotation from that point. PNP transistor.  A bipolar transistor made of a thin base of  N-type silicon or germanium sandwiched between a collector  and an emitter, both of which are made of P-type material. Power control valve.  A hand-operated hydraulic pump  unloading valve. When the valve is open, fluid flows from the  pump to the reservoir with little opposition. To actuate a unit, turn  the selector valve, and manually close the power control valve.  Pressurized fluid flows to the unit, and when it is completely  actuated, the power control valve automatically opens. Pneumatics.  The system of fluid power which transmits  force by the use of a compressible fluid. Plywood.  A wood product made by gluing several pieces  of thin wood veneer together. The grain of the wood in each  layer runs at 90° or 45° to the grain of the layer next to it. Power brakes.  Aircraft brakes that use the main hydraulic  system to supply fluid for the brake actuation. Aircraft that  require a large amount of fluid for their brake actuation  normally use power brakes, and the volume of fluid sent to  the brakes is increased by the use of deboosters. Ply rating.  The rating of an aircraft tire that indicates its  relative strength. The ply rating does not indicate the actual  number of plies of fabric in the tire; it indicates the number  of piles of cotton fabric needed to produce the same strength  as the actual piles. Power.  The time rate of doing work. Power is force  multiplied by distance (work), divided by time. Pot life.  The length of time a resin will remain workable  after the catalyst has been added. If a catalyzed material is  not used within its usable pot life, it must be discarded and  a new batch mixed up. Plenum.  An enclosed chamber in which air can be held at a  pressure higher than that of the surrounding air. Plastics.  The generic name for any of the organic materials  produced by polymerization. Plastics can be shaped by  molding or drawing. Potentiometer.  A variable resistor having connections to  both ends of the resistance element and to the wiper that  moves across the resistance. Plastic media blasting (PMB).  A method of removing paint  from an aircraft surface by dry-blasting it with tiny plastic beads. Plasticizer.  A constituent in dope or lacquer that gives its  film flexibility and resilience. Potential energy.  Energy possessed in an object because of  its position, chemical composition, shape, or configuration. Position error.  The error in pitot-static instruments caused by  the static ports not sensing true static air pressure. Position error  changes with airspeed and is usually greatest at low airspeeds. Planer.  A woodworking power tool used to smooth the  surfaces of a piece of wood. Plan position indicator (PPI).  A type of radar scope that  shows both the direction and distance of the target from the  radar antenna. Some radar antenna rotate and their PPI scopes  are circular. Other antenna oscillate and their PPI scopes are  fan shaped. Polyvinyl chloride.  A thermoplastic resin used in the  manufacture of transparent tubing for electrical insulation  and fluid lines which are subject to low pressures. Polyurethane enamel.  A hard, chemically resistant finish  used on aircraft. Polyurethane enamel is resistant to damage  from all types of hydraulic fluid. Plain-weave fabric.  Fabric in which each warp thread passes  over one fill thread and under the next. Plain-weave fabric  typically has the same strength in both warp and fill directions. Polyester resin.  A thermosetting resin used as a matrix for  much of the fiberglass used in composite construction. Polyester fibers.  A synthetic fiber made by the polymerization  process in which tiny molecules are united to form a long chain  of molecules. Polyester fibers are woven into fabrics that are  known by their trade names of Dacron, Fortrel, and Kodel.  Polyester film and sheet are known as Mylar and Celenar. Pitch pocket (wood defect).  Pockets of pitch that appear in  the growth rings of a piece of wood. Pitot pressure.  Ram air pressure used to measure airspeed.  The pitot tube faces directly into the air flowing around the  aircraft. It stops the air and measures its pressure.