Static.  Still, not moving.  Static air pressure.  Pressure of the ambient air surrounding  the aircraft. Static pressure does not take into consideration  any air movement. Static dischargers.  Devices connected to the trailing edges  of control surfaces to discharge static electricity harmlessly  into the air. They discharge the static charges before they can  build up high enough to cause radio receiver interference. Static stability.  The characteristic of an aircraft that causes it  to return to straight and level flight after it has been disturbed  from that condition. Stoddard solvent.  A petroleum product, similar to naphtha,  used as a solvent and a cleaning fluid. STOL.  Short takeoff and landing. Stop drilling.  A method of stopping the growth of a crack  in a piece of metal or transparent plastic by drilling a small  hole at the end of the crack. The stresses are spread out all  around the circumference of the hole rather than concentrated  at the end of the crack. Straight polarity welding.  DC-electric arc welding in which  the electrode is negative with respect to the work. Strain.  A deformation or physical change in a material  caused by a stress. Stress.  A force set up within an object that tries to prevent  an outside force from changing its shape. Stressed skin structure.  A type of aircraft structure in which  all or most of the stresses are carried in the outside skin. A  stressed skin structure has a minimum of internal structure. Stress riser.  A location where the cross-sectional area of the  part changes abruptly. Stresses concentrate at such a location  and failure is likely. A scratch, gouge, or tool mark in the  surface of a highly stressed part can change the area enough  to concentrate the stresses and become a stress riser. Stringer.  A part of an aircraft structure used to give the  fuselage its shape and, in some types of structure, to provide  a small part of fuselage strength. Formers give the fuselage its  cross-sectional shape and stringers fill in the shape between  the formers. Surface tape.  Strips of aircraft fabric that are doped over all  seams and places where the fabric is stitched to the aircraft  structure. Surface tape is also doped over the wing leading  edges where abrasive wear occurs. The edges of surface tape  are pink, or notched, to keep them from raveling before the  dope is applied. Supplemental Type Certificate (STC).  An approval issued  by the FAA for a modification to a type certificated airframe,  engine, or component. More than one STC can be issued for  the same basic alteration, but each holder must prove to the  FAA that the alteration meets all the requirements of the  original type certificate. Supersonic flight.  Flight at an airspeed in which all air  flowing over the aircraft is moving at a speed greater than  the speed of sound. Super heterodyne circuit.  A sensitive radio receiver circuit  in which a local oscillator produces a frequency that is a  specific difference from the received signal frequency. The  desired signal and the output from the oscillator are mixed,  and they produce a single, constant intermediate frequency.  This IF is amplified, demodulated, and detected to produce  the audio frequency that is used to drive the speaker. Superheat.  Heat energy that is added to a refrigerant after  it changes from a liquid to a vapor. Supercooled water.  Water in its liquid form at a temperature  well below its natural freezing temperature. When  supercooled water is disturbed, it immediately freezes. Sump.  A low point in an aircraft fuel tank in which water  and other contaminants can collect and be held until they  can be drained out. Summerwood.  The less porous, usually harder portion of  an annual ring that forms in the latter part of the growing  season, the summer of the year. Stroboscopic tachometer.  A tachometer used to measure  the speed of any rotating device without physical contact.  A highly accurate variable-frequency oscillator triggers a  high-intensity strobe light. Sublimation.  A process in which a solid material changes  directly into a vapor without passing through the liquid stage. Subsonic flight.  Flight at an airspeed in which all air flowing  over the aircraft is moving at a speed below the speed of sound.  G-34