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taper, normally 12:1, and fastened together by gluing. A glued scarf joint makes a strong splice, because the
joint is made along the side of the wood fibers rather than along their ends.
scavenge oil system. The subsystem in the lubrication system of a gas turbine engine that collects the oil
after it has lubricated the bearings and gears and returns it to the oil tank.
scavenger pump (lubrication system). A pump in a dry sump lubrication system that picks up the oil after
it passes through the engine and returns it to the oil tank. Scavenger pumps have a larger capacity than
pressure pumps, because the oil being returned is hot and contains air. Both conditions increase its volume.
schematic diagram. A diagram of an electric or fluid power system in which the components are
represented by symbols, rather than drawings or pictures of the actual devices. Schematic diagrams are
used for troubleshooting.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 504
Schrader valve. A spring-loaded service valve used in an air conditioning system. A Schrader valve is
much like the valve used to put air into a tire.
scientific notation (mathematics). A mathematical time saver used when working with very large and
very small numbers. When using scientific notation, all numbers are changed into a positive number
between one and 10 by moving the decimal point and attaching the number ten, raised to the power
showing the number of places the decimal has been moved.
A negative (minus) number is used if the decimal was moved to the right, and a positive number is
used if it is moved to the left. Using this system, 1,000 = 1 · 103, and 0.001 = 1 · 10-3.
scimitar shape. The shape of the blades of the propellers mounted on an unducted-fan engine. The name is
taken from the shape of a curved Asian sword that has its edge on the convex side.
scissors (landing gear component). A name commonly used for landing gear torque links. See torque
links.
score (mark). A scratch across a piece of material made to cause the material to break along the line. When
a piece of glass is to be cut with a glass cutter, it is scored by scratching through its surface with the cutter.
When the glass is bent, all of the stresses concentrate in the scored line, and the glass breaks along the line.
SCR (silicon controlled rectifier). A reverse-blocking triode thyristor that can be triggered into
conduction in only one direction. The three terminals of an SCR are the anode, the cathode, and the gate.
An SCR blocks electron flow in both directions until a pulse of electrical energy of the proper
polarity is applied to the gate. The SCR will then conduct electrons in its forward direction, between its
cathode and its anode. A conducting SCR will continue to conduct until the voltage across the anode and
cathode is removed or until its polarity is reversed. See thyristor.
scramjet (supersonic combustion ramjet). A special type of ramjet engine whose fuel can be ignited
while the vehicle is moving at a supersonic speed. See ramjet engine.
scraper ring (reciprocating engine component). The bottom ring on a piston of a reciprocating engine.
The scraper ring, also called a wiper ring, scrapes the oil from the cylinder wall and pumps it into the space
between the piston and the cylinder.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 505
screeching (gas turbine engine operation). A condition of combustion instability in a gas turbine engine
or afterburner. The combustion produces a shrill, high-pitched noise.
screeding tool. A tool used when working with plastic resins, to spread the resin evenly over a surface and
level it. Screeding tools are often made of some form of polyvinylchloride plastic material that releases the
resin, making the tool easy to clean.
screen grid (electron tube component). The grid in an electron tube between the control grid and the plate
(anode). The screen grid has a positive voltage on it slightly lower than the voltage on the plate. The steady
positive voltage on the screen grid minimizes voltage feedback from the plate to the control grid through
the interelectrode capacitance.
screwdriver. A hand tool used to install or remove screws. The bit of a screwdriver may be in the form of a
straight, flat-edged blade, or it may be fluted to fit into a recessed-head screw. A screwdriver has a handle
on its shank in line with the blade, but on the opposite end. The bit of the screwdriver fits into the slot or
recess in the head of the screw, and the handle is twisted to turn the screw.
screw-pitch gage. A set of thin metal blades with a series of V-shaped notches cut along one edge of each
blade. The notched edge of each gage is compared with the thread being checked until one is found that
　
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