Page 148 - Aeronautical Engineer Data Book

Principles of propulsion 123
on the low pressure rotor and an axial
compressor plus turbine on the high pressure
rotor. Many turbines are fitted with thrust
reversing cowls that act to reverse the direc­
tion of the slipstream of the fan bypass air.

8.2.3 Turboprop
The turboprop configuration is typically used
for smaller aircraft. Data for commercial
models are shown in Table 8.2. The engine (see
Figure 8.3) uses a separate power turbine stage
to provide torque to a forward-mounted
propeller. The propeller thrust is augmented by
gas thrust from the exhaust. Although often
overshadowed by the turbofan, recent devel­
opments in propeller technology mean that
smaller airliners such as the SAAB 2000 (2
4152 hp (3096 kW) turboprops) can compete
on speed and fuel cost with comparably sized
turbofan aircraft. The most common turboprop
configuration is a single shaft with centrifugal
compressor and integral gearbox. Commuter
airliners often use a two- or three-shaft ‘free
turbine’ layout.

8.2.4 Propfans
Propfans are a modern engine arrangement
specifically designed to achieve low fuel
consumption. They are sometimes referred to
as inducted fan engines. The most common
arrangement is a two-spool gas generator and
aft-located gearbox driving a ‘pusher’ fan.
Historically, low fuel prices have reduced the
drive to develop propfans as commercially
viable mainstream engines. Some Russian
aircraft such as the Anotov An-70 transport
have been designed with propfans.

8.2.5 Turboshafts
Turboshaft engines are used predominantly for
helicopters. A typical example such as the
Rolls-Royce Turbomeca RTM 32201 has a
three-stage axial compressor direct-coupled to a
two-stage compressor turbine, and a two-stage