Free Turbine Turboprop
Figure 1.21 differs from Fig. 1.20 in that the turbine functions are
separated--one turbine drives a compressor and another the propeller. The
turbine that drives the compressor behaves as it does in a single-shaft
turbojet and the propeller turbine has the pressure ratio and flow characteristics
of a nozzle. Curve C-C of Fig. 1.16 again represents an operating line.
This engine offers better efficiency at reduced specific power than the
single-shaft engine running at constant speed. The speed of the gas generator
is reduced when the ETR is lowered; however, since there is no
propeller or large gear train on this shaft, the inertia is comparatively low.
The finite time required to respond to a demand for more power is small
and usually not objectionable.
The free turbine engine is the preferred choice for helicopters, which
require the main rotor to spin freely if the engine should stop. The load
imposed by the turbine, gears, and accessories is small enough to allow this
to happen without the use of a clutch or a free-wheeling device.
When the engine is idling, both the engine and propeller speeds are low.
This is a desirable quality for ground operations because of the low noise
level. Low engine speeds at part power are also useful when extended
periods of time for loitering are necessary.
Figure 1.21 differs from Fig. 1.20 in that the turbine functions are
separated--one turbine drives a compressor and another the propeller. The
turbine that drives the compressor behaves as it does in a single-shaft
turbojet and the propeller turbine has the pressure ratio and flow characteristics
of a nozzle. Curve C-C of Fig. 1.16 again represents an operating line.
This engine offers better efficiency at reduced specific power than the
single-shaft engine running at constant speed. The speed of the gas generator
is reduced when the ETR is lowered; however, since there is no
propeller or large gear train on this shaft, the inertia is comparatively low.
The finite time required to respond to a demand for more power is small
and usually not objectionable.
The free turbine engine is the preferred choice for helicopters, which
require the main rotor to spin freely if the engine should stop. The load
imposed by the turbine, gears, and accessories is small enough to allow this
to happen without the use of a clutch or a free-wheeling device.
When the engine is idling, both the engine and propeller speeds are low.
This is a desirable quality for ground operations because of the low noise
level. Low engine speeds at part power are also useful when extended
periods of time for loitering are necessary.
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