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turbulence. A fish is an excellent example of a streamlined body. The fish can move through the water with
a minimum of opposition. Fluid flowing around a flat plate or circular object has a turbulent area behind it.
This turbulence causes drag. There is a minimum area of turbulence in the flow of fluid behind a
streamlined body.
streamline flow. The flow of a fluid in which there is no turbulence. All particles of the fluid move in
continuous smooth lines.
strength. The ability of a material to resist distortion or deformation caused by an external force acting on
it. Strength is the ability of a material to resist stresses that try to break it.
stress. A force within an object that tries to prevent any outside force changing its shape.
stress corrosion. A form of intergranular corrosion that forms in metals that are subject to a continuous
tensile stress. The tensile stress separates the metal along the internal grain boundaries, and the corrosion
acts at the apex of the cracks that form.
stressed-skin structure. A type of aircraft structure that has a minimum of internal structure and carries all,
or most, of the stresses in its outside skin. An egg shell is a perfect example of a natural stressed-skin. All
of the stresses acting on an egg are carried in its shell.
stress relieve (steel heat treatment). A type of heat treatment of steel. When steel parts are forged,
machined, or welded, stresses are often locked in them that cause the parts to warp or break. These stresses
can be relieved by heating the part until it is red-hot and then allowing it to cool slowly in still air to room
temperature. This type of stress relieving is called normalizing.
stress riser. A location on a structure where the cross-sectional area of the part changes drastically.
Stresses concentrated at such locations are likely to cause failure. A scratch, gouge, or tool mark in the
surface of a highly stressed part can change the area enough to concentrate the stresses, and it becomes a
stress riser.
stringer (aircraft structure). A part of an aircraft fuselage structure used to give the fuselage its shape and,
in some types of structure, to provide a small portion of the fuselage strength. Formers give the fuselage its
cross-sectional shape, and stringers, which are channels or angles of sheet metal or metal extrusions, fill in
the shape between the formers. Stringers in a semimonocoque fuselage are covered with sheet metal, and in
a truss-type fuselage, with fabric.
stroboscope. A special light that flashes with a brilliant, short-duration flash. A variable-frequency
oscillator controls the stroboscope so it flashes at any required interval. Stroboscopes are used to study the
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
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motion of rotating or vibrating bodies. The stroboscope is shone on the object, and the frequency of the
flashes is adjusted until the object appears to stand still or move in slow motion.
stroboscopic tachometer. A device used to measure rotational speed by the use of a stroboscopic light. A
brilliant light which flashes at a controlled rate is shone on the rotating object. When the light is adjusted to
flash at the same rate the object is rotating, the object appears to stand still. The flash rate of the light is
calibrated in terms of revolutions per minute (RPM).
stroke (reciprocating engine parameter). The linear distance the piston moves inside the cylinder from
the top to the bottom.
strontium. A soft, silvery, easily oxidized, metallic chemical element. Strontium’s symbol is Sr, its atomic
number is 38, and its atomic weight is 87.62. Finely divided strontium ignites spontaneously in air.
Strontium is used in the manufacture of pyrotechnic devices.
stud (fastening device). A threaded fastener used to attach components to an engine crankcase or other
type of casting. A stud used to fasten a cylinder to a cast aluminum alloy crankcase has threads on both
ends. The threads that screw into the soft aluminum crankcase are coarse, and the threads on which a nut is
screwed to clamp the cylinder base to the crankcase are fine.
stuffing box. A box through which a sliding or rotating shaft passes. The stuffing box contains packing
material that can be compressed around the shaft to prevent liquid or gas leaking past the moving shaft.
Styrofoam. The registered trade name for an expanded (full of bubbles), rigid polystyrene plastic material.
subassembly. An assembly of components that is part of a larger device. A wing is a subassembly of an
airplane.
subatomic particles. Particles smaller than an atom. Electrons, protons, and neutrons are sub-atomic
particles.
subfreezing temperature. Temperature lower than that needed to cause water to freeze.
　
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