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which preservative oil has been pumped. Drivescrews are installed by driving them in place with a
hammer.
drizzle (meteorology). Very small droplets of water that appear to float with the air currents while falling
in an irregular path. Drizzle differs from rain, which falls in a comparatively straight path, and fog, whose
droplets remain suspended in the air.
drogue. Any of several types of devices used to slow or stabilize a body moving through water or air. A
drogue parachute is a small parachute which can be opened behind an airplane to slow or stabilize it in
flight.
A drogue is also the funnel-shaped device on the end of an inflight refueling hose. The drogue
stabilizes the hose and allows the aircraft being refueled to connect its probe into the fueling line.
A sea anchor, which is a canvas bucket with a small opening in its bottom, is also a drogue. A sea
anchor is used to stabilize and control the movement of a boat or seaplane.
drone. A pilotless airplane used for target practice or for research in conditions that would be dangerous
for a manned aircraft. Drones are remotely controlled by radio from a mother aircraft or from a ground
station.
droop (turbine engine fuel control condition). A progressive decrease in RPM with load in an engine
whose speed is governed with a flyweight-type governor.
As the load is increased, the pilot valve drops down to meter more fuel. The lower position of the
valve decreases the compression of the speeder spring and allows the flyweights to assume an onspeed
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 205
position at a lower RPM.
droop compensator (helicopter turbine engine control). A linkage between the collective pitch control
and the turbine speed governor that adjusts for the increased load the rotor places on the engine, as
collective pitch is increased. The droop compensator prevents the rotor speed decreasing as the load is
applied.
droop restraint (helicopter rotor system). A mechanical device that limits the amount the blades of a
helicopter rotor can droop when the rotor is turning at a slow speed, and there is not enough centrifugal
force to overcome the effect of gravity on the blades.
drop cloth. A piece of canvas or plastic used to cover the floor or objects on the floor when walls and
ceilings are being painted. The drop cloth prevents paint from dripping on areas where it should not be.
drop forging. A method of forging steel parts by forcing semi-molten metal into a die by a heavy blow
from a drop hammer. The grain structure of a drop-forged part gives it more strength than a cast part.
drop hammer. A large metalworking machine that uses heavy matched dies to form compound curves in
sheet metal.
The sheet of metal to be formed is placed over a female die, and a heavy, matching male die is
raised above it and dropped into the female die, forming the metal into the shape of the dies. Drop hammers
are also used to forge semimolten metal in a die.
drop-out voltage (electrical relays). The minimum voltage needed to keep an electrical relay energized.
When the voltage drops below this value, the relay automatically de-energizes.
Electromagnetism holds the relay energized, and a spring de-energizes it when the voltage
producing the electromagnetism is below the drop-out voltage.
dropping resistor (electric circuit). A resistor in an electrical circuit used to drop voltage. For example, if
a 12-volt light bulb is used in a 24-volt electrical system, a dropping resistor must be installed in series with
the bulb to drop 12 volts. This allows only 12 volts across the bulb.
dropsonde (meteorology). An instrument pack and radio transmitter dropped by parachute from an aircraft
to measure conditions of the atmosphere through which it passes. This information is continuously
transmitted to meteorologists on the ground.
drop tank (fuel tank for military aircraft). An externally mounted fuel tank used on some military
aircraft. Fuel is used from the drop tank first, and then the entire tank is dropped from the aircraft to get rid
of the weight and the drag.
dross. The impurity that forms on the surface of molten metal. Dross is composed of the oxides produced
by the reaction of oxygen in the air with the hot metal.
drum brake. A form of friction brake used on some airplanes. A drum brake is composed of two curved
shoes which have a heat-absorbing friction material bonded to their outer surface. These shoes are pressed
outward against the inside of a cylindrical drum that rotates with the wheel.
The brake is applied by hydraulic pressure or mechanical leverage forcing the shoes out against
the drum, and a spring pulls them away from the drum when the brake pedal or handle is released.
　
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