Page 92 - Planning and Design of Airports

64 Airp o r t Pl anning

Jet engines can be classified into two general categories, turbojet
and turbofan. A turbojet engine consists of a compressor, a combus-
tion chamber, and a turbine at the rear of the engine. The early jet
airline aircraft, particularly the Boeing 707 and the DC-8, were pow-
ered by turbojet engines, but these were discarded in favor of turbo-
fan engines principally because the latter are far more economical.
A turbofan is essentially a turbojet engine to which has been
added large-diameter blades, usually located in front of the compres-
sor. These blades are normally referred to as the fan. A single row of
blades is referred to as single stage, two rows of blades as multistage.
In dealing with turbofan engines reference is made to the bypass
ratio. This is the ratio of the mass airflow through the fan to the mass
airflow through the core of the engine or the turbojet portion. In a
turbofan engines the air flow through the core of the engine, the inner
flow, is hot and very compressed and is burned in it. The air flow
through the fan, the outer flow, is compressed much less and exits
from the engine without burning into an annulus around the inner
core. Fan engines are quieter than turbojet engines and the develop-
ment of quiet propulsive integrated power plants in modern turbo-
fan has included extensive acoustic lining development both in the
inlet and the fan exhaust [38].
Most fans are installed in front of the main engine. A fan can be
thought of as a small diameter propeller driven by the turbine of the
main engine. Nearly all airline transport aircraft are now powered by
turbofan. Current technological advances in engines are concentrated
toward the development of propfan engines for short and medium
haul aircraft and ultrahigh bypass ratio turbofan engines for long
haul aircraft. These engine technologies reduce fuel consumption by
25 to 35 percent. These engines, which are variously termed unducted
fan (UDF) engines and ultrahigh bypass ratio (UHB) turbofan engines,
have brought on the emergence of very light jet aircraft.
Jet engine performance is made in measured both in terms of
power and efficiency. The power of an aircraft engine is typically
measured in pounds of forward moving force, or “thrust.” Table 2-4
lists a sample of jet engines, and their measurements of thrust installed
on historical and current transport category aircraft.
Aircraft engine power efficiency is measured in terms of the
thrust-to-weight ratio, defined simply as the pounds of thrust pro-
vided by the engine, divided by the weight of the engine. Early jet
engines were produced with thrust-to-weight ratios of approximately
3:1. In the early part of the twenty-first century, new light but power-
ful jet engines with thrust to weight ratios nearing 5:1 have signifi-
cantly improved the operating efficiency of air transport aircraft and
have made the emergence of the very light jet market feasible.
One important measure of engine performance efficiency is that
of specific fuel consumption, expressed in terms of pounds of fuel per