viscosity index. See VI.
viscosity index improver. See VI improver.
viscosimeter. An instrument used to measure the viscosity of a fluid. A cup-type viscosimeter is a specially shaped cup with an accurately sized hole in its bottom.
The cup is filled with the fluid whose viscosity is to be measured, and the hole is opened so fluid can flow from it. The number of seconds is measured between the beginning of the flow and the time the first break in the flow occurs. This time relates to the viscosity of the fluid.
viscous damping. A method of damping vibration or oscillation by converting some of the vibrational energy into heat. Viscous damping is done by allowing the vibrating object to force a fluid, either a liquid or gas, through a small orifice. The amount of damping is determined by the viscosity of the fluid and the size of the orifice.
An aircraft nose wheel shimmy damper operates on the principle of viscous damping. See shimmy damper.
vise. A bench-mounted clamp used to hold material while it is being worked on. One jaw of the vise is fixed, and the other is mounted on a screw that allows it to be moved away from or close to the fixed jaw.
Vise-grip pliers. A patented type of pliers in which the jaws can be locked together by the over-center compound action of a toggle. A screw in one handle of the pliers allows the basic size of the jaw opening to be adjusted, and when the handles are squeezed together, the jaws lock tightly on the object between them. Vise-grip pliers are made with many different types and shapes of jaws for many different uses.
visibility. The ability, as determined by atmospheric conditions and expressed in units of distance, to see and identify prominent unlighted objects by day, and prominent lighted objects by night. Visibility is reported in statute miles, hundreds of feet, or meters.
visible light. The very small range of electromagnetic radiation that produces sensations the human eye and brain can accept. Vibrations making up visible light have wavelengths of between approximately 4,000 and 7,700 angstroms. One angstrom is equivalent to one billionth of a centimeter.
The different wavelengths of electromagnetic energy give light its color. Visible light with the shortest wavelength has the color of violet, and that with the longest wavelength has the color of red.
visual approach (air traffic control). An approach conducted on an instrument flight rules (IFR) flight plan which authorizes the pilot to proceed visually and clear of clouds to the airport. The pilot must, at all times, have either the airport or the preceding aircraft in sight. This approach must be authorized and under the control of the appropriate air traffic control facility, and the reported weather at the airport must be: ceiling at or above 1,000 feet and visibility of three miles or greater.