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spoiler is raised into the airstream, it disturbs the smooth flow of air and destroys part of the lift the airfoil
is producing. Spoilers are used on high-performance sailplanes to decrease the lift so they will not float
during landing.
spokeshave (wood-cutting tool). A short drawknife with handles on each end. The spokeshave cuts as the
blade is drawn toward you. A spokeshave is used for rounding the edges of strips of wood. The name
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 547
comes from its use in shaping the wooden spokes used in the wheels of horse-drawn wagons.
spongy brakes (hydraulic brake condition). The feel of the brake pedal when the hydraulic brake fluid
contains air. In normal operation, the brake system is filled with noncompressible brake fluid, and the pedal
has a firm feel. But if there is any air in the fluid, the air compresses when the pedal is depressed and gives
the pedal a spongy feel.
sponson (aircraft structure). A surface much like a short stub wing that extends out on the side of a flying
boat hull near the waterline. Sponsons keep the flying boat level when it is not moving through the water,
and they also give it stability and provide some aerodynamic lift.
spontaneous combustion. Self-ignition of a material caused by heat produced in the material as it
combines with oxygen from the air. If oily rags are left in a bundle, the heat produced when oxygen in the
air reacts with the oil cannot be carried away. The rags get hot enough to burst into flames. They are ignited
by spontaneous combustion.
spot check. A method of manufacturing inspection in which only a certain percentage of the products are
checked. The products to be checked are selected from the production line in a random fashion, so those
checked are typical of the entire production run.
spot-facing. A smooth, flat, circular area around a bolt hole that provides a good seat for the bolt head or
for a washer under the bolt head.
spotlight. A lighting device that produces a brilliant, concentrated beam of light. Spotlights are used for
searching an area at night, because they concentrate the light energy in a small area that allows the beam to
extend out for a great distance.
spot welding. A form of electrical resistance welding used for thin sheets of metal. The metal parts are
assembled and clamped between the electrodes of the spot welding machine. A timed pulse of current flows
between the electrodes and melts a spot of the metal. Pressure on the electrodes forces the molten metal
from the two sheets together to form a single spot which joins the sheets. Fuel tanks can be welded with a
continuous, leakproof seam by allowing the spots to overlap each other.
spout. A pipe or similar device used to carry a liquid from a container in a stream that is easy to control.
sprag clutch. A form of freewheeling clutch used in the rotor-drive system of a helicopter. The sprag
clutch allows the engine to drive the rotor, but, when the engine power is reduced, the clutch allows the
rotor to freewheel, and prevents it trying to drive the engine.
spray gun (cleaning tool). A type of maintenance tool used for cleaning parts. Compressed air is blown
through the spray gun, and as it passes through a venturi, it produces a low pressure. This low pressure
picks up a cleaning solution, such as varsol or naphtha, and sprays it out in the form of tiny drops onto the
surface being cleaned. The combined solvent action of the cleaning solution and the force of the
high-pressure air removes dirt, grease, or other contaminants from the surface.
spray gun (paint spray gun). A device that breaks a liquid finishing material, such as paint or lacquer, into
tiny drops and blows them onto the surface being painted. Two basic types of paint spray guns are:
suction-feed and pressure-feed guns. A suction-feed gun uses suction produced by venturi action inside the
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 548
gun to pick up the finishing material from a cup attached to the gun. The material sprayed by a
pressure-feed gun is forced into the gun by air pressure on the fluid stored in a pressure cup or pressure pot.
spray strip (water craft). Small strips of metal mounted on the side of a flying boat hull. Spray strips
throw a spray of water out away from the aircraft.
spreader bar (aircraft floats). Horizontal structural members between the two floats mounted on an
aircraft.
springback (aircraft control system). A method used to assure that the control arm of a device being
controlled actually reaches its full-travel stop. There are stops on both ends of an aircraft throttle control.
The stop on the carburetor must be contacted first, then the throttle control is moved farther until its stop in
　
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