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 gear- driven 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-weight- type 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 counterweights 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 fuel-air mixture inside the cylinder at two locations provides more efficient combustion of the fuel-air 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. 

Durability. A measure of engine life. Durability is usually measured in TBO hours. 

Duty cycle. A schedule that allows a device to operate for a given period of time, followed by a cooling down period before the device can be operated again. 

Dwell chamber.  A chamber in a turbine engine into which the scavenged oil is returned. Entrained air separates from the oil in the dwell chamber before it is picked up by the pressure pump. 

Dynamometer.  A device used to measure the amount of torque being produced by an engine. The drive shaft of the engine is loaded with either an electric generator or a fluid pump, and the output of the generator or pump is measured and converted into units of torque. Torque at a specific rpm can be converted into brake horsepower. 

Dyne.  The unit of force that imparts an acceleration of one centimeter per second, per second to a mass of one gram. 

One dyne is equal to 2.248 ∙ 10-6 pounds. 

Eddy current. Current induced into a conductor due to a mobbing or non-uniform magnetic field. 

EEC. Electronic engine control. An electronic fuel control for a gas turbine engine. The EEC senses the power-lever angle (PLA), engine RPM, bleed valve, and variable stator vane position, and the various engine pressures and temperatures. 

It meters the correct amount of fuel to the nozzles for all flight conditions, to prevent turbine over-speed and over- temperature. 

Effective pitch.  The actual distance a propeller advances in one revolution through the air. 

E-gap angle.  The position of the rotating magnet in a magneto when the breaker points are timed to open. The E-gap (efficiency gap) angle is several degrees of magnet rotation beyond the magnet’s neutral position. At this point, the magnetic field stress is the greatest, and the change in flux is the greatest, inducing the maximum voltage in the secondary winding. 

EGT. Exhaust gas temperature. The temperature of the gases as they leave the cylinder of a reciprocating engine or the turbine of a gas turbine engine. 

