Single-Shaft Turboprop
Simplified analysis. This type of engine is represented by the sketch of
Fig. 1.20. The design differs from the turbojet in that the engine speed can
be independently set by adjusting the propeller pitch. The magnitude of
(N/x/~)C.l is then known when the compressor inlet temperature is given.
The speed parameter of a compressor map (i.e., Fig. 1.12) is thus established
as an independent variable. A match point is defined on a compressor
map by the intersection of the given corrected speed line with a line for
a given ETR such as curve A-A in Fig. 1.16. The corrected speed of the
turbine is then determined. If we assume that the pressure ratio through the
engine inlet to be about equal to that through the exhaust nozzle, we can
say that turbine pressure ratio is approximately equal to that of the
compressor. A match point on the turbine map is thus located and the
estimate of the powers developed by the turbine, compressor, and engine
are routine.
Performance trends. Adjustable propeller pitch provides for good
propulsion efficiency at a variety of flight speeds and engine powers.
Constant engine speed is the preferred mode of operation, however, even at
reduced power levels. The engine then quickly responds to demands for
higher power. (If rapid increases in engine speed were necessary at lower
speeds, the large polar moment of inertia of the propeller and gears would
delay the response. This can be dangerous.) Engine efficiency at reduced
power with constant speed is inferior to that obtainable along curve C-C of
Fig I. 16, but this situation is seldom necessary and usually has only a small
effect on most missions. Optimum altitudes are usually selected for cruise to
get good efficiency.
A single-shaft turboprop engine has a characteristic that is useful when
steep descents into tight airports are made. The propeller can be used as a
windmill to absorb power from the velocity of the airstream to drive the
engine. In this way, the flight speed is reduced to low levels in spite of a
steep angle of descent. This drag force can also cause problems in a
two-engine airplane when one engine fails.
Simplified analysis. This type of engine is represented by the sketch of
Fig. 1.20. The design differs from the turbojet in that the engine speed can
be independently set by adjusting the propeller pitch. The magnitude of
(N/x/~)C.l is then known when the compressor inlet temperature is given.
The speed parameter of a compressor map (i.e., Fig. 1.12) is thus established
as an independent variable. A match point is defined on a compressor
map by the intersection of the given corrected speed line with a line for
a given ETR such as curve A-A in Fig. 1.16. The corrected speed of the
turbine is then determined. If we assume that the pressure ratio through the
engine inlet to be about equal to that through the exhaust nozzle, we can
say that turbine pressure ratio is approximately equal to that of the
compressor. A match point on the turbine map is thus located and the
estimate of the powers developed by the turbine, compressor, and engine
are routine.
Performance trends. Adjustable propeller pitch provides for good
propulsion efficiency at a variety of flight speeds and engine powers.
Constant engine speed is the preferred mode of operation, however, even at
reduced power levels. The engine then quickly responds to demands for
higher power. (If rapid increases in engine speed were necessary at lower
speeds, the large polar moment of inertia of the propeller and gears would
delay the response. This can be dangerous.) Engine efficiency at reduced
power with constant speed is inferior to that obtainable along curve C-C of
Fig I. 16, but this situation is seldom necessary and usually has only a small
effect on most missions. Optimum altitudes are usually selected for cruise to
get good efficiency.
A single-shaft turboprop engine has a characteristic that is useful when
steep descents into tight airports are made. The propeller can be used as a
windmill to absorb power from the velocity of the airstream to drive the
engine. In this way, the flight speed is reduced to low levels in spite of a
steep angle of descent. This drag force can also cause problems in a
two-engine airplane when one engine fails.
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