Viscosity . The resistance of a fluid to flow. Viscosity is the stiffness of the fluid, or its internal friction.

Viscosity index (VI ). A measure of change in viscosity of an oil as it changes temperature. The higher the viscosity index, the less the viscosity changes.

Viscosity index improver . An additive used to produce a multi-viscosity lubricating oil. The polymer additive expands as temperature increases and contracts as temperature decreases. VI improvers cause viscosity to increase as oil heats and decrease as it cools.

Volatile memory . Computer memory that is lost when the power to the computer is turned off.

Volatility . The characteristic of a liquid that relates to its ability to vaporize or change into a gas.

Volumetric efficiency . The ratio of the volume of the charge of the fuel and air inside the cylinder of a reciprocating engine to the total physical volume of the cylinder.

Von Ohain, Dr. Hans Pabst . The designer and developer of the first turbojet engine to power an airplane. His HeS3b engine was built in Germany by the Heinkel Company and it flew in a Heinkel He178 airplane on August 27, 1939.

Vortex . A whirling mass of air that sucks everything near it toward its center.

Vortex dissipator . A high-velocity stream of compressor bleed air blown from a nozzle into an area where vortices are likely to form. Vortex dissipaters destroy the vortices that would otherwise suck debris from the ground into engines mounted in pods that are low to the ground.

Wake . The high-velocity stream of turbulent air behind an operating aircraft engine.

Wankel engine . See rotating combustion (RC) engine.

Waste gate . A controllable butterfly valve in the exhaust pipe of a reciprocating engine equipped with an exhaust-driven turbocharger. When the waste gate is open, exhaust gases leave the engine through the exhaust pipe, and when it is closed, they leave through the turbine.

Watt . The basic unit of power in the metric system. One watt is the amount of power needed to do one joule (0.7376 footpound of work) in one second. One watt is 1⁄ horsepower. 746

Wet-sump engine . An engine that carries its lubricating oil supply in a reservoir that is part of the engine itself.

Wet-sump lubrication system . A lubrication system in which the oil supply is carried within the engine itself. Return oil drains into the oil reservoir by gravity.

Whittle, Sir Frank . The British Royal Air Force flying officer who in 1929 filed a patent application for a turbojet engine. Whittle’s engine first flew in a Gloster E.28 on May 15, 1941. The first jet flight in America was made on October 2, 1942, in a Bell XP-59A that was powered by two Whittletype General Electric I-A engines.

Windmilling propeller . A propeller that is rotated by air flowing over the blades rather than powered by the engine.

Work . The product of a force times the distance the force is moved.

Worm gear . A helical gear mounted on a shaft. The worm meshes with a spur gear whose teeth are cut at an angle to its face. A worm gear is an irreversible mechanism. The rotation of the shaft, on which the worm gear locks the spur gear so its shaft cannot be rotated.