Updraft carburetor. A carburetor that mounts on the bottom of a reciprocating engine. Air entering the engine flows upward through the carburetor. 

Upper-deck pressure.  The absolute pressure of air at the inlet to the fuel metering system of a turbocharged engine. Upper-deck pressure is the same as the turbocharger discharge pressure. 

V-blocks. A fixture that allows a shaft to be centered and rotated to measure any out-of-round condition. 

V-engine.  A form of reciprocating engine in which the cylinders are arranged in two banks. The banks are separated by an angle of between 45º and 90º. Pistons in two cylinders, one in each bank, are connected to each throw of the crankshaft. 

Valence electrons.  Electrons in the outer shell, or ring, around the nucleus of an atom. It is the valence electrons that give an atom its electrical characteristics and are the electrons that may be pulled loose from an atom to cause electrical current. 

Valve overlap.  The portion of the operating cycle of a four- stroke-cycle reciprocating engine during which both the intake and exhaust valves are off of their seats at the same time. 

Vapor lock. A condition of fuel starvation that can occur in a reciprocating engine fuel system. If the fuel in the line between the tank and carburetor is heated enough for the fuel to vaporize, a bubble will form in the line. If the vapor pressure of the bubble is high enough, it will block the fuel and keep it from flowing to the engine. 

Vapor pressure. The amount of pressure needed above a liquid to prevent it from evaporating. 

Vaporize.  The changing of a liquid into a vapor. 

Vectored-thrust engine.  A turbojet or turbofan engine with the fan and/or exhaust nozzles mounted in such a way that they may be rotated in flight to produce forward, vertically upward, or rearward thrust. 

Velocity.  A vector quantity that expresses both the speed an object is moving and the direction in which it is moving. 

Velocity turbine.  A turbine driven by forces produced by the velocity, rather than the pressure, of gases flowing through the vanes. 

Venture.  A specially shaped restrictor in a tube designed to speed up the flow of fluid passing through it. According to Bernoulli’s principal, any time the flow of fluid speeds up without losing or gaining any energy from the outside, the pressure of the fluid decreases. 

Vernier coupling. A timing coupling used with base- mounted magnetos. The vernier coupling allows the timing to be adjusted in increments of considerably less than one degree. 

Vertical tape instrument.  A tall rectangular instrument that displays the quantity of the parameter being measured by a movable strip of colored tape. The presentation resembles a vertical bar graph. 

Vibration loop.  A loop in a rigid fluid line used to prevent vibration from concentrating stresses that could cause the line to break. 

VIFF.  Vectoring in forward flight. A method of enhancing the maneuverability of an airplane by vectoring the exhaust gases and/or fan-discharge air to produce thrust components not parallel to the longitudinal axis of the aircraft. 

Viscosimeter.  An instrument used to measure the viscosity of a liquid. The time required for a given volume of liquid at a specified temperature to flow through a calibrated orifice is used to indicate the viscosity of the liquid. 

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. 

