Although the propeller driven airplane is not nearly as
efficient as the jet, particularly at the higher altitudes
and cruising speeds required in modern aviation, one
of the few advantages the propeller driven airplane has
over the jet is that maximum thrust is available almost
at the start of the takeoff roll. Initial thrust output of the
jet engine on takeoff is relatively lower and does not
reach peak efficiency until the higher speeds. The fanjet
or turbofan engine was developed to help compensate
for this problem and is, in effect, a compromise
between the pure jet engine (turbojet) and the propeller
engine.

Like other gas turbine engines, the heart of the turbofan
engine is the gas generator—the part of the engine
that produces the hot, high-velocity gases. Similar to
turboprops, turbofans have a low pressure turbine section
that uses most of the energy produced by the gas
generator. The low pressure turbine is mounted on a
concentric shaft that passes through the hollow shaft of
the gas generator, connecting it to a ducted fan at the
front of the engine. [Figure 15-2]

Air enters the engine, passes through the fan, and splits
into two separate paths. Some of it flows around—
bypasses—the engine core, hence its name, bypass
air. The air drawn into the engine for the gas generator
is the core airflow. The amount of air that bypasses
the core compared to the amount drawn into the gas
generator determines a turbofan's bypass ratio.
Turbofans efficiently convert fuel into thrust because
they produce low pressure energy spread over a large
fan disk area. While a turbojet engine uses all of the
gas generator's output to produce thrust in the form of
a high-velocity exhaust gas jet, cool, low-velocity
bypass air produces between 30 percent and 70 percent
of the thrust produced by a turbofan engine.

The fan-jet concept increases the total thrust of the jet
engine, particularly at the lower speeds and altitudes.
Although efficiency at the higher altitudes is lost (turbofan
engines are subject to a large lapse in thrust with
increasing altitude), the turbofan engine increases
acceleration, decreases the takeoff roll, improves initial
climb performance, and often has the effect of
decreasing specific fuel consumption.

OPERATING THE JET ENGINE

In a jet engine, thrust is determined by the amount of
fuel injected into the combustion chamber. The power
controls on most turbojet and turbofan powered airplanes
consist of just one thrust lever for each engine,
because most engine control functions are automatic.
The thrust lever is linked to a fuel control and/or electronic
engine computer that meters fuel flow based
upon r.p.m., internal temperatures, ambient conditions,
and other factors. [Figure 15-3]

In a jet engine, each major rotating section usually has
a separate gauge devoted to monitoring its speed of
rotation. Depending on the make and model, a jet
engine may have an N1 gauge that monitors the low
pressure compressor section and/or fan speed in
turbofan engines. The gas generator section may be
monitored by an N2 gauge, while triple spool engines
may have an N3 gauge as well. Each engine section
rotates at many thousands of r.p.m. Their gauges
therefore are calibrated in percent of r.p.m. rather than
actual r.p.m., for ease of display and interpretation.
[Figure 15-4]