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National Advisory Committee for Aeronautics, Technical Notes - Theoretical Evaluation of the Ducted Fan Turbojet Engine

Elie ducted-fan type of turboJet engine represents an attempt
to combine the fuel economy of a propeller-type engine with the light
weight of the turbojet engine. A ducted—fan turboaet engine my be
considered as a mdification of the tu'rboJet engine, which requires
the installation of a more powerful turbine to drive a relatively
small-diameter multibladed propeller in addition to the normal oom-
pressor. All or part of the air (depending on the configuration)
that is handled by the propeller, or fan, is passed through a sep-
arate duct. Burners are installed in the sepwate duct to augnent
the thrust when necessary. These engines are hereinafter designated
ducted-fan engines. A schematic diagram of this type of engine is
presented in figure 1; a turboJet-‘lwpe engine is also shown for com:-
parison.

For economy operation, the ducted-fan engine is intended to
handle a greater mes of air at lower Jet velocities (unheated outer-
duct air) and hence attain a higher propulsive efficiency than a
turbojet engine of equal thrust. The higher propulsive efficiency
appears in the form of a lower specific fuel consunmtion. If the
thrust per unit frontal area of the ducted-fan engine were mde equal
to that of the turboJet engine, it would be necessary to increase the
air-handling capacity per unit frontal area of the ducted-fan engine
beyond that of the turboaet engine.

The present investigation, which was conducted at the NACA Cleveland
laboratory, is based, however, on the fact that if an increased air
flow per unit area were possible for a ducted-fan engine it would also
be possible for the turbojet engine. The ducted-fan engine should
therefore have a lower specific fuel ooneunmtion and a lower thrust
per unit frontal area than a comparable turboaet engine.

A survey of available literature reveals only incomplete infor-
nation on the performance of ducted-fan engines. An American inves-
tigtion (reference 1) indicates that at 400 to 500 miles per hour
at altitudes of 50,000 to 55,000 feet the fuel economy of a ducted-
fan engine is substantially better than that of a turboJet engine.