Sea-level boosted engine.  A reciprocating engine that has had  its sea-level rated horsepower increased by supercharging.  This is the same as a ground-boosted engine. Secondary winding.  The winding in a magneto or ignition  coil that connects to the distributor rotor. The secondary  winding is normally made of very small diameter wire and  has a large number of turns, typically about 20,000. Self-accelerating speed.  The speed attained by a gas turbine  engine during start-up that allows it to accelerate to its normal  idling speed without assistance from the starter.  Semiconductor transducer.  A piezoelectric crystal that  converts input energy of one form, such as pressure, into  output energy of another, such as an electrical signal. Series-wound motor.  An electric motor with field coils  connected in series with the armature. Serviceable limits.  Limits included in a reciprocating engine  overhaul manual. If a part measures outside of the new-parts  limits, but within the serviceable limits, it will not likely  wear to the point of causing engine failure within the next  TBO interval. Servo system.  A type of automatic control system in which  part of the output is fed back into the input. Shaft horsepower.  The horsepower actually available at a  rotating shaft.  Shielding.  The electrically conductive covering placed  around an electrical component to intercept and conduct to  ground any electromagnetic energy radiated from the device. Short circuit.  A low-resistance connection between two  points in an electric circuit. Shower of Sparks ignition system.  A patented ignition  system for reciprocating engines. An induction civrator  sends pulsating DC into a set of retard breaker points on one  of the magnetos. This provides a hot and retarded spark for  starting the engine. Single-shaft turbine engine.  A turboprop engine in which  the propeller reduction gears are driven by the same shaft  that drives the compressor for the gas generator. Single-spool gas-turbine engine.  A type of axial-flow- compressor gas turbine engine that has only one rotating  element. Skin radiator.  A type of radiator used on some early liquid- cooled racing airplanes. The radiator was made of two thin  sheets of brass, slightly separated so the heated coolant could  flow between them. Skin radiators were mounted on the  surface of the wing, on the sides of the fuselage, or on the  floats of seaplanes. Air flowing over the smooth surface of  the radiator removed heat from the coolant. Slip (propeller specification).  The difference between the  geometric and effective pitch of a propeller. Slip ring.  A smooth, continuous ring of brass or copper  mounted on the rotor shaft of an electrical generator or  alternator. Brushes riding on the smooth surface of the slip  ring carry current into and out of the rotor coil. Slow-blow ruse.  A special type of electrical circuit protection  device that allows a momentary flow of excess current, but  opens the circuit if the excessive flow is sustained. Sludge.  A heavy contaminant that forms in an aircraft  engine lubricating oil because of oxidation and chemical  decomposition of the oil. Sludge plugs.  Spool-shaped sheet metal plugs installed in  the hollow throws of some engine crankshafts. Slug.  The unit of mass equal to that which experiences an  acceleration of one foot per second, per second when a force  of one pound acts on it. It is equal to 32.174 pounds, or  14.5939 kilograms, of mass. Also called a G-pound. SOAP.  Spectrometric oil analysis program. An oil analysis  program in which a sample of oil is burned in an electric  arc and an analysis is made of the wavelength composition  of the resulting light. Each chemical element in the oil,  when burned, produces light containing a unique band of  frequencies. A computer analyzes the amount of each band  of frequencies and prints out the number of parts of the  element per million parts of the entire sample. SOAP can  predict engine problems by warning the engine operator of  an uncharacteristic increase of any elements in the oil. Sound suppressor.  The airframe component that replaces  the turbine engine tail pipe. It reduces the distance the sounds  made by the exhaust gases propagate by converting low- frequency vibrations. Specific gravity.  The ratio of the density of a material to the  density of pure water. Specific weight.  The ratio of the weight of an aircraft engine  to the brake horsepower it develops. G-25