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alternating current throughout a conductor. AC at radio frequencies tends to have a greater current density
(more current flow) on the surface of a conductor than it does near the center. Because of skin effect, some
conductors used to carry high radio-frequency AC are hollow tubes.
skin friction (aerodynamic drag). A type of aerodynamic drag caused by air flowing over the surface of
the aircraft. Skin friction is a component of parasite drag.
skin radiator. A type of radiator used on some of the liquid-cooled racing airplanes. The radiator is made
of two thin sheets of brass, slightly separated so the heated coolant can flow between them. A skin radiator
may be mounted on the surface of the wing, the sides of the fuselage, or on the floats of seaplanes. Air
flowing over the smooth surface of the radiator removes heat from the coolant.
skip distance (radio transmission characteristic). The distance from a radio transmitting antenna to the
point on the surface of the earth the reflected sky wave first touches after it has been bounced off the
ionosphere.
ski plane. An airplane whose wheeled landing gear has been replaced with skis so it can operate from snow
or ice. Some types of skis require the wheels to be removed, and others can be installed without removing
the wheels. Retractable skis have a slot in their bottom surface so they can be pulled up enough for the
wheel to protrude below the ski, enabling the airplane to land on a hard surface on its wheels.
skip welding. A method of welding thin material. Skip welding is done in a series of short beads, with a
space between each bead about as long as one of the beads. After the first series of beads is made, a second
series is made between each of the first ones.
Skip-welded material is less likely to warp than material welded with one continuous bead. Skip
welding is sometimes called step welding.
skull-cap spinner (propeller spinner). A small, conical, streamlined cover retained by a bracket attached
to the hub of a fixed-pitch wood or metal propeller. The diameter of a skull-cap spinner is the same as that
of the propeller hub. The spinner serves as a cover over the propeller retaining bolts or nuts and provides a
degree of streamlining.
skunk works. A section of an aircraft factory set aside for research and development of new aircraft. No
one is allowed to visit the skunk works except those who are actually involved in the project being
developed there.
Skydrol hydraulic fluid. The registered trade name of a synthetic hydraulic fluid used in most modern
high-performance aircraft. Skydrol hydraulic fluid has a phosphate-ester base, is nonflammable, and is
usable over a wide range of temperatures. Skydrol hydraulic fluid meets Military Specifications
MIL-H-8446.
sky wave (radio transmission). The portion of an electromagnetic signal that radiates upward from the
antenna until it strikes the ionosphere, then bounces back to the earth.
slag (welding product). The hard, brittle, glass-like material that covers a weld bead that has been made
with a coated welding rod. The flux coating on the rod melts in the heat of the arc and flows out ahead of
the bead. The melted flux covers the metal where the weld is being made and keeps oxides from forming in
the weld. When the weld is made, the liquid flux hardens into slag that must be chipped off to allow the
bead to be inspected.
slam acceleration (gas turbine engine operation). Improper operation of the power control lever of a gas
turbine engine in which the control is moved forward too quickly to allow the engine to accelerate as it
should. There is danger of a rich flameout if the fuel is metered into the engine before the compressor is
able to pull enough air into the engine to furnish the correct fuel-air mixture ratio.
slant-line distance (aircraft navigation). The actual distance between an aircraft and a ground radio
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 532
facility. A distance measuring equipment (DME) indicator shows the slant-line distance between the
aircraft and the ground station. Slant-line distance is the hypotenuse of a right triangle, with the altitude of
the aircraft as one side, and the horizontal distance between the ground station and a point directly below
the aircraft as the other side.
slant range (radar measurement). The actual distance between a ground-based radar antenna and an
airborne target.
slant visibility (meteorology). The distance an airborne observer can see and distinguish objects on the
ground.
slash (radar indication). A radar beacon reply displayed on a radar scope as an elongated target.
slat (aircraft control). A secondary control on an airplane that allows the airplane to fly at a high angle of
　
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