porcelain. A high-grade, translucent ceramic material having a hard, glazed finish. Porcelain is used for dishes, vases, and high-strength electrical insulators. It was used as the insulator in some early aircraft spark plugs.
porosity. The condition of a material that has many tiny openings, or pores, in its surface. These pores allow liquids or gases to seep (pass slowly) through the material. Cast iron and certain types of wood are porous, while glass and hard steel are not. They have very little porosity.
porous chrome plating. A method of treating the walls of aircraft reciprocating engine cylinders that hardens them and increases their ability to hold lubricants. Worn cylinder barrels may be ground so their bore is straight and round. Then hard chromium is electroplated on the walls to a depth that brings the diameter of the cylinder bore back to its original dimension.
The surface of the hard chrome plating resembles a maze of spiderwebs, with thousands of tiny, interconnected cracks. After the correct depth of chromium has been deposited, the electroplating current is reversed, and the tiny cracks enlarge enough to hold oil.
Porous chrome plating provides a hard, wear-resistant surface over which the piston rings ride. The oil trapped in the tiny grooves furnishes lubrication to minimize piston ring and cylinder wall wear and helps the rings seal.
porpoising (flight condition). An unstable flight condition in which an aircraft continually oscillates along its longitudinal axis. Porpoising is caused by the aircraft possessing neutral dynamic longitudinal stability.