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attack without stalling. A slat is a section of the leading edge of the wing mounted on curved tracks. These
tracks move into and out of the wing on rollers. In flight at low angles of attack, the air pressure on the
wing holds the slat in, and it forms the leading edge of the wing. At a high angle of attack, aerodynamic
forces pull the slat forward on its tracks, and it forms a duct that forces the air down onto the top of the
wing to keep it from stalling.
slaved gyro (aircraft flight instrument). A gyro-stabilized direction indicator whose directional signals
are produced by a flux valve. See flux valve.
slide rule (mathematics). A mechanical calculating device consisting of two side-by-side logarithmically
graduated scales. The graduations are spaced according to the logarithm of the number, rather than the
number itself. Two numbers are multiplied by adding the lengths of the logarithmic scales representing the
numbers.
Slide rules, which are available in both straight and circular forms, are used to make fast
calculations involving multiplication and division and the use of logarithms and trigonometric functions.
Mechanical slide rules have been almost totally replaced by electronic calculators that can work all of the
problems a slide rule can work, as well as many other types, much faster and with a far greater degree of
accuracy.
slide switch. A type of electrical switch in which connections are made by sliding a conductor over fixed
contacts.
slide valve. A valve that opens or closes a port by sliding back and forth over it. Slide valves are used in
reciprocating steam engines to open and close the passage into the cylinder to allow steam to enter and
leave.
slide-wire potentiometer. A form of variable resistor in which the amount of resistance used in a circuit is
determined by the position of a contact along a length of resistance wire. The amount of resistance is
directly proportional to the length of wire in the circuit.
slinger ring (propeller anti-icing system component). A hollow metal ring mounted around the hub of a
propeller so that it turns with it. A mixture of alcohol and ethylene glycol is pumped into the slinger ring,
and centrifugal force throws the fluid out through short discharge tubes along the leading edges of the
propeller blades. The fluid prevents ice from forming on the blades.
sling psychrometer. An instrument used to measure the relative humidity of the air. A sling psychrometer
consists of two mercury thermometers mounted on a frame with a handle that allows it to be slung around,
so air moves across the thermometers. The bulb of one thermometer is covered with a cloth wick saturated
with water. As the air blows over it, the water evaporates and lowers the temperature.
A chart showing the dry-bulb temperature and the difference between the wet-bulb and dry-bulb
temperatures is used to find the relative humidity, in percentage, for these conditions.
slip (aircraft flight maneuver). A maneuver in which the aircraft moves through the air sideways, rather
than straight ahead. Slipping produces a large amount of drag so the airplane can descend at a steep angle
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 533
without gaining excessive speed. An airplane is slipped by crossing its controls, using right rudder and left
aileron.
A forward slip is one in which the aircraft turns its side into the wind, but continues in forward
flight. A slide slip is one in which the aircraft continues to point straight ahead, but slips to the side.
slip (aircraft propeller specification). Slip, with regard to an aircraft propeller, is the difference between
the geometric pitch and the effective pitch. Geometric pitch is the distance the propeller would move
forward if it were moving through a solid, and effective pitch is the distance the propeller actually moves
through the air.
slip (induction motor specification). The difference between the synchronous speed of an induction motor
(the speed of the rotating magnetic field in the stator) and the actual speed the rotor is turning. An induction
motor tries to turn at its synchronous speed, but friction and the mechanical load connected to the rotor
prevent it turning at this speed.
The amount the rotor lags behind the rotating field is called slip. It is slip that gives the motor its
torque. The greater the slip, the greater the pull between the magnetic fields in the rotor and stator. Slip is
measured in percentage of the synchronous speed.
slip clutch (aircraft starter component). A heavy spring-loaded mechanism built into an aircraft
reciprocating-engine starter that allows the starter drive to slip enough to take up the initial shock when the
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