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transonic speed range. In the transonic range, some air flowing over the aircraft surface is moving faster
than the speed of sound, while other air is moving at a speed slower than the speed of sound.
When a shock wave forms on the upper surface of a wing, it gets in the way of the air flowing
behind it, and this air breaks away from the wing surface. The shock wave causes the wing to stall. Shock
stalls can cause control problems as an airplane is passing from subsonic flight into supersonic flight.
shock strut. See shock absorber.
shock wave. A pressure wave formed in the air as an object, such as a flight vehicle, passes through the air
at a speed greater than the speed at which sound can travel. As the vehicle moves through the air, it creates
disturbances, and sound waves spread out in all directions from the disturbance. Since the vehicle is flying
faster than the sound waves are moving, they build up and form a shock wave at the front and the rear of
the vehicle.
As air passes through a shock wave, it slows down and its static pressure increases. The energy in
the air is decreased. See oblique shock wave and normal shock wave.
shop head (rivet head). The head of a solid rivet which is formed when the shank is upset.
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Shore Scleroscope. A form of hardness tester used to determine the hardness of such material such as
rubber, metal, or plastic. A metal weight is dropped through a glass tube onto the surface of the material
being tested, and the height of the rebound of the weight is measured. The harder the material, the higher
the weight will bounce.
short circuit (electrical circuit). An electrical circuit in which there is a fault that causes electrons to flow
across a portion of the circuit that should act as an insulator. The electrons find a short path to ground.
Fuses and circuit breakers protect the circuit from damage that could be caused by excess current flowing
through a short circuit.
shorting switch. A type of make-before-break switch. A make-before-break switch is a multipole switch in
which one circuit is completed before another circuit is opened. A shorting switch is used in circuits that
must never operate, even for a microsecond, without an electrical load being connected.
short stacks (reciprocating engine exhaust system). The exhaust system of an aircraft reciprocating
engine made of short pipes that direct the exhaust gases from each individual cylinder of the engine away
from the aircraft.
short wave (radio waves). Waves of electromagnetic radiation that are shorter than 60 meters, or about
197 feet. The frequencies used for short-wave radio communications are above about five megahertz.
shot peening (metal treatment). A method of strengthening the surface of a metal part by blasting it with
steel shot. The blows from the steel shot produce a compressive stress on the surface of the metal which
must be overcome by a tensile stress of a greater intensity before the tensile stress can cause the surface of
the metal to crack.
shoulder bolt. A special type of fastener used to prevent damage to a structure that could be caused by
squeezing the surfaces together when the nut is tightened on the bolt. The diameter of the shank is larger
than the diameter of the threaded portion of the bolt. The length of a shoulder bolt is chosen so its shank is
the same length as the thickness of the material being fastened together.
shoulder-wing airplane. A monoplane in which the wing is mounted near the top of the fuselage, but not
on the top.
The classifications of a monoplane with regard to the placement of the wing are: low-wing,
midwing, shoulder-wing, high-wing, and parasol-wing.
show. A term meaning to demonstrate, confirm, validate, or substantiate something. In legal documents it
means to plead or allege. 14 CFR Part 1: “Unless the context otherwise requires, means to show to the
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satisfaction of the Administrator.”
shower (meteorology). Precipitation from a cumuliform cloud. Showers are characterized by the
suddenness of beginning and ending, by the rapid change of intensity, and usually by the rapid change in
the appearance of the sky. Showery precipitation may be in the form of rain, ice pellets, or snow.
Shower of Sparks ignition system (aircraft reciprocating engine ignition system). The registered trade
name for a type of magneto ignition system. When the ignition switch in a Shower of Sparks ignition
system is placed in the START position, the primary circuit of the right magneto is grounded, and pulsating
direct current from a vibrator flows into the primary winding of the left magneto to ground through a set of
　
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