Viscosity cup.  A specially shaped cup with an accurately sized hole in its bottom. The cup is submerged in the liquid to completely fill it. It is then lifted from the liquid and the time in seconds is measured from the beginning of the flow through the hole until the first break in this flow. The viscosity of the liquid relates to this time. 

Vixen file.  A metal-cutting hand file that has curved teeth across its faces. Vixen files are used to remove large amounts of soft metal. 

V$_{NE}$.  Never-exceed speed. The maximum speed the aircraft is allowed to attain in any conditions of flight. 

Volatile liquid. A liquid that easily changes into a vapor. 

Voltmeter multiplier. A precision resistor in series with a voltmeter mechanism used to extend the range of the basic meter or to allow a single meter to measure several ranges of voltage. 

VOR.  Very high frequency Omni Range navigation. 

VORTAC.  An electronic navigation system that contains both a VOR and a TACAN facility. 

Vortex (plural vortices).  A whirling motion in a fluid. 

Vortex generator.  Small, low-aspect-ratio airfoils installed in pairs on the upper surface of a wing, on both sides of the vertical fin just ahead of the rudder, and on the underside of the vertical stabilizers of some airplanes. Their function is to pull high-energy air down to the surface to energize the boundary layer and prevent airflow separation until the surface reaches a higher angle of attack. 

Warp clock.  An alignment indicator included in a structural repair manual to show the orientation of the piles of a composite material. The ply direction is shown in relation to a reference direction. 

Warp threads.  Threads that run the length of the roll of fabric, parallel to the selvage edge. Warp threads are often stronger than fill threads. 

Warp tracers.  Threads of a different color from the warp threads that are woven into a material to identify the direction of the warp threads. 

Wash in.  A twist in an airplane wing that increases its angle of incidence near the tip. 

Wash out.  A twist in an airplane wing that decreases its angle of incidence near the tip. 

Watt.  The basic unit of electrical power. One watt is equal to 1 ⁄ 746 horsepower. 

Way point. A phantom location created in certain electronic navigation systems by measuring direction and distance from a VORTAC station or by latitude and longitude coordinates from Loran or GPS. 

Web of a spar.  The part of a spar between the caps. 

Weft threads.  See fill threads. 

Wet-type vacuum pump.  An engine-driven air pump that uses steel vanes. These pumps are lubricated by engine oil drawn in through holes in the pump base. The oil passes through the pump and is exhausted with the air. Wet-type pumps must have oil separators in their discharge line to trap the oil and return it to the engine crankcase. 

Wing fences.  Vertical vanes that extend chordwise across the upper surface of an airplane wing to prevent spanwise airflow. 

Wing heavy.  An out-of-trim flight condition in which an airplane flies hands off, with one wing low. 

Wire bundle.  A compact group of electrical wires held together with special wrapping devices or with waxed string. These bundles are secured to the aircraft structure with special clamps. 

Woof threads.  See fill threads. 

Work. The product of force times distance. 

Yaw.  Rotation of an aircraft about its vertical axis. 

Yaw damper.  An automatic flight control system that counteracts the rolling and yawing produced by Dutch roll. See Dutch roll. A yaw damper senses yaw with a rate gyro and moves the rudder an amount proportional to the rate of yaw, but in the opposite direction. 

Yield strength.  The amount of stress needed to permanently deform a material. 

Zener diode. A special type of solid-state diode designed to have a specific breakdown voltage and to operate with current flowing through it in its reverse direction. 

