Gimbal.  A support that allows a gyroscope to remain in an  upright condition when its base is tilted. Glass cockpit.  An aircraft instrument system that uses a  few cathode-ray-tube displays to replace a large number of  mechanically actuated instruments. Glaze ice.  Ice that forms when large drops of water strike  a surface whose temperature is below freezing. Glaze ice is  clear and heavy. Glide slope.  The portion of an ILS (Instrument Landing  System) that provides the vertical path along which an aircraft  descends on an instrument landing. Goniometer.  Electronic circuitry in an ADF system that  uses the output of a fixed loop antenna to sense the angle  between a fixed reference, usually the nose of the aircraft, and  the direction from which the radio signal is being received. Gram.  The basic unit of weight or mass in the metric system.  One gram equals approximately 0.035 ounce. Graphite.  A form of carbon. Structural graphite is used in  composite structure because of its strength and stiffness. Greige (pronounced “gray”).  The unshrunk condition of a  polyester fabric as it is removed from the loom. Ground effect.  The increased aerodynamic lift produced  when an airplane or helicopter is flown nearer than half  wing span or rotor span to the ground. This additional lift is  caused by an effective increase in angle of attack without the  accompanying increase in induced drag, which is caused by  the deflection of the downwashed air. Ground.  The voltage reference point in an aircraft electrical  system. Ground has zero electrical potential. Voltage values,  both positive and negative, are measured from ground. In the  United Kingdom, ground is spoken of as “earth.” Ground-power unit (GPU).  A service component used to  supply electrical power to an aircraft when it is being operated  on the ground. Guncotton.  A highly explosive material made by treating  cotton fibers with nitric and sulfuric acids. Guncotton is used  in making the film base of nitrate dope. Gusset.  A small plate attached to two or more members of  a truss structure. A gusset strengthens the truss. Homebuilt aircraft.  Aircraft that are built by individuals  as a hobby rather than by factories as commercial products.  Homebuilt, or amateur-built, aircraft are not required to meet  the stringent requirements imposed on the manufacture of  FAA-certified aircraft.  G-18 Holding relay.  An electrical relay that is closed by sending  a pulse of current through the coil. It remains closed until the  current flowing through its contacts is interrupted. Hertz.  One cycle per second. Helix.  A screw-like, or spiral, curve. Heat exchanger.  A device used to exchange heat from one  medium to another. Radiators, condensers, and evaporators  are all examples of heat exchangers. Heat always moves from  the object or medium having the greatest level of heat energy  to a medium or object having a lower level. Heading indicator.  A gyroscopic flight instrument that gives  the pilot an indication of the heading of the aircraft. Hardwood.  Wood from a broadleaf tree that sheds its leaves  each year. Hangar rash.  Scrapes, bends, and dents in an aircraft  structure caused by careless handling. Halon 1301.  A halogenated hydrocarbon fire-extinguishing  agent that is one of the best for extinguishing cabin and  powerplant fires. It is highly effective and is the least toxic  of the extinguishing agents available. The technical name for  Halon 1301 is bromotrifluoromethane. Halon 1211.  A halogenated hydrocarbon fire-extinguishing  agent used in many HRD fire-extinguishing systems for  powerplant protection. The technical name for Halon 1211  is bromochlorodifluoromethane. Gyroscopic precession.  The characteristic of a gyroscope  that causes it to react to an applied force as though the force  were applied at a point 90° in the direction of rotation from  the actual point of application. The rotor of a helicopter acts  in much the same way as a gyroscope and is affected by  gyroscopic precession. Gyro (gyroscope).  The sensing device in an autopilot system.  A gyroscope is a rapidly spinning wheel with its weight  concentrated around its rim. Gyroscopes have two basic  characteristics that make them useful in aircraft instruments:  rigidity in space and precession. See rigidity in space and  precession.