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action is being taken. The attitude sensor progressively cancels its error signal as the deflected aileron
brings the wings back to a level attitude. By the time the wings are level, the error signal is eliminated and
the aileron is no longer deflected.
servomechanism. A type of control system in which a small signal or small force is used to control a much
larger force. In a hydraulic servomechanism, a sensor detects that some type of output movement is needed.
A control valve directs hydraulic fluid into a hydraulic actuator, and the movement takes place. A feedback
sensor compares the amount the actuator moves with the amount of movement the original sensor called for,
and when the actuator has moved the correct amount, the selector valve is shut off.
servo tab (aircraft controls). A small movable tab built into the trailing edge of an aircraft primary control
surface. A servo tab is actuated by the cockpit control, and it moves in the direction opposite to the
direction the primary surface is to move. The servo tab produces an aerodynamic force that moves the
surface on which it is mounted.
servo-type carburetor. A type of carburetor or continuous-flow fuel injection system that uses the
pressure drop across a servo bleed orifice (a metering jet) to control the amount of metered fuel allowed to
flow to the cylinders. The servo system adjusts for changes in inlet fuel pressure so the pressure of the fuel
sent to the cylinders is proportional only to the amount of air being drawn into the cylinders.
sesquiplane (type of airplane). A biplane in which one wing, usually the lower wing, has an area less than
one half the area of the other wing.
setback (sheet metal layout). A measurement used in sheet metal layout. Setback is the distance the jaws
of a brake must be set back from the mold line to form a bend. Setback for a 90° bend is equal to the inside
radius of the bend, plus the thickness of the metal being bent. For a bend of other than 90°, a factor called a
K-factor must be used. See K-factor.
setscrew. A headless screw used to prevent relative movement between a wheel and a shaft. A hole is
drilled through the hub boss of the wheel, and it is tapped so a setscrew can be screwed through it and
tightened against the shaft. The set screw prevents the wheel turning on the shaft.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 518
settling with power (helicopter operation). An operation of a helicopter in which the main rotor is
operating in its own downwash. The flow of air through the center portion of the rotor disk is upward, and
the flow through the outer portion of the disk is downward, as it should be. Settling with power is more
correctly called “operating in the vortex ring.”
severe weather avoidance plan. See SWAP.
sewed-in panel repair (aircraft fabric repair). A repair to a fabric-covered airplane wing in which a
section of fabric extending from the leading edge to the trailing edge between two or more ribs is replaced
by sewing the new fabric to the old. All the seams are reinforced with surface tape.
sextant (navigation instrument). An optical instrument used in celestial navigation to measure the angle
between the horizon and a line to one of the navigational stars. By finding this angle and referring to the
appropriate tables in a nautical almanac for the exact time the observation was made, the observer can
determine his latitude and longitude.
Seyboth fabric tester (aircraft fabric tester). A type of tester used to determine the condition of the
fabric installed on an aircraft. A specially shaped, spring-loaded, sharp-pointed plunger is pressed into the
fabric until a shoulder around the point contacts the fabric. The amount of force required to press the
shoulder to the fabric is measured on a colored scale in the handle of the tester. Weak fabric requires very
little force to press the shoulder against the fabric, and it shows up as a red band. Airworthy fabric requires
enough force to compress the spring that the indicator moves up to one of the green bands.
A Seyboth test does not indicate the strength of the fabric in pounds per inch, but it is sufficiently
accurate to distinguish between fabric that is either definitely good or definitely bad.
shaded-pole motor (AC electric motor). A type of alternating current induction motor used for light-duty,
constant-speed loads, such as fans, blowers, and tape players. A coil excited by single-phase alternating
current is wound around a laminated steel frame, and a squirrel-cage rotor is mounted in bearings attached
to the frame. Shading coils made of one or two bands of heavy copper are installed around diagonally
opposite pole shoes, the part of the frame around the rotor.
　
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