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and it ignites and burns.
compression molding (plastics manufacturing process). A method of molding thermosetting resins. A
mold is opened and heated, and resin is put in it. The mold is then closed, and pressure is applied. The resin
flows under pressure and completely fills the cavity of the mold. The mold is kept closed and heated until
the resin is completely cured.
compression ratio (reciprocating engine parameter). The ratio of the volume of a cylinder of a
reciprocating engine with the piston at the bottom of its stroke to the volume of the cylinder with the piston
at the top of its stroke.
compression ratio (turbine engine parameter). The ratio of the pressure of the air at the discharge of a
turbine engine compressor to the pressure of the air at the inlet.
compression rib (aircraft structural member). A special rib used in a Pratt truss airplane wing to take
the place of a separate compression strut. A compression rib has strong cap strips and webs designed to
withstand compressive loads.
compression rings (reciprocating engine piston rings). The top rings used on reciprocating engine
pistons to seal the piston in the cylinder so the upward-moving piston can compress the air in the cylinder.
Compression rings also transfer heat from the piston into the cylinder wall. See illustration for scraper
rings.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 145
compression spring. A form of spring which resists a force trying to compress it, or squeeze it together.
Open coil springs are often used as compression springs.
compression stress. A type of stress that tries to squeeze the ends of an object closer together. A
compression stress is also called a compressive stress.
compression strut (airplane wing structural member). A heavy structural member, often in the form of a
steel tube, used to hold the spars of a Pratt truss airplane wing apart. A compression strut opposes
compressive loads between the spars put there by the tensile loads from the drag and anti-drag wires.
compression test (reciprocating engine maintenance test). A maintenance test performed on a
reciprocating engine to determine the ability of the cylinders to hold air pressure. A cylinder will fail a
compression test if an excessive amount of air leaks past the piston rings, or if any air leaks past the intake
or exhaust valve.
compression tester (differential). A maintenance tool used to determine the functional condition of a
reciprocating engine cylinder.
A regulated air pressure of 80 psi is directed into the cylinder through a calibrated orifice, and the
pressure is measured on both sides of the orifice. The difference between the indications on the two gages
is the pressure drop across the orifice. This drop is caused by air leaking past the piston rings or the valves.
Typically, a cylinder should hold 75% of the air pressure applied to it. Therefore the cylinder
pressure gage should read at least 60 psi when the input pressure gage reads 80 psi.
compression wave (high-speed aerodynamics). A shock wave which forms on the surface of an airfoil
moving through the air at a speed greater than the speed of sound.
compression wood. The defect in wood which causes it to have a high specific gravity and the appearance
of an excessive growth of summerwood. In most species, there is little difference between the color of the
springwood and summerwood. Any material containing compression wood is unsuited for aircraft
structural use and must be rejected.
compressive load (compressive stress). A physical load, or stress, which tries to squeeze together the ends
of the object on which it acts.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 146
compressive strength. The ability of an object to resist a compressive load. Compressive strength resists
the force that tries to squeeze the ends of an object together.
compressor (air conditioning system component). The component in a vapor-cycle cooling system in
which the low-pressure refrigerant vapors, after they leave the evaporator, are compressed to increase both
their temperature and pressure before they pass into the condenser. Increasing the temperature of the
refrigerant allows it to give up some of its heat energy to the air flowing through the condenser.
Some compressors are driven by electric motors, others by hydraulic motors and, in the case of
many light airplanes, are belt driven from the engine.
compressor (gas turbine engine component). The section of the gas turbine engine whose components
increase the pressure of the air flowing through it.
compressor blade (gas turbine engine component). A rotating airfoil installed on the disk of a
　
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