Density altitude . The altitude in standard air at which the density is the same as that of the existing air.

Detergent oil . A type of mineral oil with metallic-ashforming additives that protects the inside of an engine from sludge and varnish buildup. Used in automotive engines, it has proven unsuitable for use in aircraft engines.

Detonation . An uncontrolled explosion inside the cylinder of a reciprocating engine. Detonation occurs when the pressure and temperature of the fuel inside the cylinder exceeds the critical pressure and temperature of the fuel. Detonation may be caused by using fuel that has a lower octane rating or performance number than is specified for the engine.

Dewar bottle . A special container used to store liquid oxygen and liquid nitrogen. A Dewar bottle has an inner and an outer container, and the space between them forms a vacuum. The two surfaces within the vacuum are silvered to reflect heat away from the container walls.

Differential pressure . A single pressure that is the difference between two opposing pressures.

Diffuser . A component in a gas turbine engine that decreases the velocity of air flowing through it and increases its pressure.

Digitized image . A modified image picked up by the miniature TV camera in the end of a fiber-optic probe. This image is converted into a digital electronic signal that eliminates unwanted portions of the viewed area and allows the desired image to be enhanced for a clearer view of the inside of a turbine engine.

Dipstick . A gage, in the form of a thin metal rod, used to measure the level of liquid in a reservoir. The dipstick is pushed into the reservoir until it contacts a built-in stop; then it is removed and visually inspected. The level of liquid in the reservoir is indicated by the amount of the dipstick wet by the liquid.

Dirigible . A large, cigar shaped, lighter-than-air flying machine. Dirigibles differ from balloons in that they are powered and can be steered.

Distributed pole stator winding . Alternator stator windings wound in a series of slots in the stator frame. A distributed pole stator is distinguished from a salient pole stator whose coils are wound around separate pole shoes that project inward from the field frame toward the rotor.

Distributor . A high-voltage selector switch that is geardriven from the shaft of the rotating magnet in a magneto. The distributor rotor picks up the high voltage from the secondary winding of the coil and directs it to high-voltage terminals. From here, it is carried by high-tension ignition leads to the spark plugs.

Divergent duct . A duct that has an increased cross-sectional area in the direction of flow.

Downdraft carburetor . A carburetor that mounts on the top of a reciprocating engine. Air entering the engine flows downward through the carburetor.

∆P (delta P ). Differential pressure.

Droop . A progressive decrease in RPOM with load in a gas turbine engine whose speed is governed with a fly-weighttype governor in the fuel control. As the load increases, 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 on-speed position at a lower rpm.

Dry-sump engine . An engine that carries its lubricating oil supply in a tank external to the engine.

Dual ignition . An ignition system of an aircraft reciprocating engine that has two of every critical unit, including two spark plugs in each cylinder. Dual ignition provides safety in the event of one system malfunctioning, but more important, igniting the air/fuel mixture inside the cylinder at two locations provides more efficient combustion of the air/fuel mixture in the cylinder.

Dual-spool gas turbine engine . An axial-flow turbine engine that has two compressors, each driven by its own stage or stages of turbines.

Duct heater . A thrust augmentation system, similar to an afterburner, where fuel is added to the fan-discharge air and burned.

Duct losses . A decrease in pressure of the air flowing into a gas turbine engine caused by friction.