spoiler (aircraft control). A small deflector that can be raised into the air flowing over an airfoil. When a 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 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.