spring tab (aircraft controls). A small auxiliary tab, set into the trailing edge of a primary control surface on a high-speed airplane. The control surface is attached to the control horn through a torsion rod. Under normal flight loads, the spring tab remains fixed to the control surface and serves no purpose. But, when the air loads are high and a large amount of force is needed to move the control surface, the torsion rod twists, and the control rod moves the spring tab in a direction opposite to that of the surface on which it is mounted. It then acts as a servo tab and aids the pilot in moving the control surface. See servo tab.
springwood. The lighter, softer part of the grain of wood. The heavier and darker part of the wood grain is the summerwood. The cross section of a tree trunk contains a series of concentric rings, called annual rings. Each ring is made up of a light and a dark area. The light area is the part of the tree that grew at a fast rate in the spring of the year and is called the springwood. The dark area is the part of the tree that grew more slowly during the summer of the year and is called summerwood.
sprue (casting). A hole in a mold through which liquid plastic or molten metal is poured to fill the mold. When the molded material becomes solid, the mold is opened, and the metal which was in the sprue (this metal is also called a sprue) is broken from the casting and discarded.
spur and pinion reduction gear system. A type of reduction gear system that allows one shaft to turn at a speed slower than that of another shaft. A pinion is a small-diameter spur gear, whose teeth mesh with those on the larger-diameter gear. The pinion turns faster than the larger gear, and it turns in the direction opposite to that of the larger gear.
spur gear. A gear wheel with teeth radiating outward around its periphery. The term spur gear comes from the fact that this type of gear looks much like the rowel, or toothed wheel, that was a part of the spur worn by many of the early-day cowboys.
spur gear pump. A form of constant-displacement fluid pump that uses two meshing spur gears mounted in a close-fitting housing. Fluid is taken into the housing where it fills the space between the teeth of the gears and is carried around the housing as the gears rotate. On the discharge side of the pump, the teeth of the two gears mesh, and the fluid is forced out of the pump.
squall (meteorology). A sudden increase in wind speed by at least 15 knots to a peak of 20 knots or more, and lasting for at least one minute.
The essential difference between a gust, lasting less than one minute, and a squall is the duration of the peak speed.
squall line (meteorology). Any nonfrontal line or narrow band of active thunderstorms with or without squalls.
square (geometric figure). A closed, plane (flat), four-sided figure. Each angle in a square is a right angle (90°), all sides have the same length, and the opposite sides of a square are parallel.
square engine (reciprocating engine). A reciprocating engine whose bore and stroke have the same dimensions.
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