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are connected in such a way that the emitter of the input transistor is connected directly to the base of the
output transistor. The collectors of both transistors are connected together. A Darlington amplifier is a
double emitter-follower amplifier.
D’Arsonval meter (electrical measuring instrument). One of the most widely used types of
direct-current measuring instruments. In a D’Arsonval meter, a small coil carrying the current to be
measured is mounted on pivots and suspended in the field of a permanent magnet. A pointer is attached to
the coil, and the rotation of the coil is restrained by two calibrated hairsprings.
Current flowing through the coil produces a magnetic field that reacts with the field of the
permanent magnet, and the coil rotates against the opposition of the hairsprings.
The greater the current flowing through the coil, the stronger its magnetic field and the farther the
coil rotates. A pointer attached to the coil moves across a scale calibrated in amps, milliamps, or
microamps.
D’Arsonval meters can be connected into circuits with multiplier resistors or shunts and can be
used to measure voltage, current, or resistance.
dashpot. A pneumatic or hydraulic dampener used to cushion or slow down mechanical movements. A
loose-fitting piston moves back and forth in a cylinder which contains a fluid, either liquid or air. The
piston is connected to the component whose movement is to be damped.
The component can move slowly with little opposition if the fluid is given time to move from one
side of the piston to the other, but the dashpot restrains fast movement because of the time it takes to
transfer the fluid from one side of the piston to the other.
A shimmy damper used on the nose wheel of a tricycle-gear airplane is a form of dashpot.
data. A term that refers to all the facts available about a situation or project. Data can be accumulated and
changed into a form usable in a computer. The computer then processes the data to produce records and
projections. The word data is actually the plural form of datum, but it is often used as though it were
singular.
data base (computer operation). The body of information available on any particular subject.
All Airworthiness Directives issued by the Federal Aviation Administration are contained in a data
base. This allows a person performing an inspection on an aircraft to access, or tap into, this data base and
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 175
get a list of all of the ADs that pertain to the particular aircraft, its engines, and components.
data bus. A wire or group of wires used to move data within a computer system.
data-plate performance (gas turbine engine performance). Performance specifications of a gas turbine
engine that were determined in the manufacturer’s test cell when the engine was calibrated.
This information, recorded on the engine data plate includes the engine speed at which a specified
EPR is attained. When trimming the engine, the technician uses this data as the goal.
datum. A reference from which measurements are made. For example, mean sea level is a datum for
measuring true altitude, and the freezing point of pure water is a datum for measuring temperature.
datum (aircraft location). An imaginary vertical reference plane or line chosen by the aircraft
manufacturer from which all arms used for weight and balance computation are measured.
Davis wing (aerodynamics). A relatively thick, narrow-chord wing used on some airplanes. A Davis wing
has a relatively low drag and a stable center of pressure, and it develops lift at low angles of attack. The
B-24 Liberator bomber of World War II used the Davis wing.
DC electricity (direct current electricity). Electricity in which the electrons flow in the same direction all
of the time. Electricity produced by chemical cells, solar cells, and thermocouples is DC electricity.
DC alternator. The electrical generation device that has replaced the generator on practically all modern
automobiles and light airplanes.
Excitation current from the aircraft battery flows through the voltage regulator into the rotor coil
through brushes and slip rings to produce a multiple-pole rotating magnetic field. This rotating field cuts
across the three-phase output windings in the stator to produce an alternating current whose amount is
determined by the alternator speed and the amount of excitation current supplied by the voltage regulator.
A six-diode, three-phase rectifier changes the AC into DC before it leaves the alternator.
DC amplifier (electronic amplifier). An electronic amplifier which can amplify direct-current signals and
alternating current signals having a very low frequency.
　
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