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. 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. Yield strength.  The amount of stress needed to permanently  deform a material. 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. Yaw.  Rotation of an aircraft about its vertical axis. Work.  The product of force times distance. Woof threads.  See fill threads. 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. Wing heavy.  An out-of-trim flight condition in which an  airplane flies hands off, with one wing low. Wing fences.  Vertical vanes that extend chordwise across the  upper surface of an airplane wing to prevent spanwise airflow. 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. Weft threads.  See fill threads. Web of a spar.  The part of a spar between the caps. 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. G-39