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The turboprop engine offers several advantages over
other types of engines such as:
• Light weight.
• Mechanical reliability due to relatively few
moving parts.
• Simplicity of operation.
• Minimum vibration.
• High power per unit of weight.
• Use of propeller for takeoff and landing.
Turboprop engines are most efficient at speeds
between 250 and 400 m.p.h. and altitudes between
18,000 and 30,000 feet. They also perform well at the
slow speeds required for takeoff and landing, and are
fuel efficient. The minimum specific fuel consumption
of the turboprop engine is normally available in the
altitude range of 25,000 feet up to the tropopause.
The power output of a piston engine is measured in
horsepower and is determined primarily by r.p.m. and
manifold pressure. The power of a turboprop engine,
however, is measured in shaft horsepower (shp). Shaft
horsepower is determined by the r.p.m. and the torque
(twisting moment) applied to the propeller shaft. Since
turboprop engines are gas turbine engines, some jet
thrust is produced by exhaust leaving the engine. This
thrust is added to the shaft horsepower to determine
the total engine power, or equivalent shaft horsepower
(eshp). Jet thrust usually accounts for less than
10 percent of the total engine power.
Although the turboprop engine is more complicated
and heavier than a turbojet engine of equivalent size
and power, it will deliver more thrust at low subsonic
airspeeds. However, the advantages decrease as flight
speed increases. In normal cruising speed ranges, the
propulsive efficiency (output divided by input) of a
turboprop decreases as speed increases.
The propeller of a typical turboprop engine is
responsible for roughly 90 percent of the total thrust
under sea level conditions on a standard day. The
excellent performance of a turboprop during takeoff
and climb is the result of the ability of the propeller to
accelerate a large mass of air while the airplane is
moving at a relatively low ground and flight speed.
“Turboprop,” however, should not be confused with
“turbosupercharged” or similar terminology. All
turbine engines have a similarity to normally aspirated
(non-supercharged) reciprocating engines in that
maximum available power decreases almost as a direct
function of increased altitude.
Although power will decrease as the airplane climbs
to higher altitudes, engine efficiency in terms of
specific fuel consumption (expressed as pounds of fuel
consumed per horsepower per hour) will be increased.
Decreased specific fuel consumption plus the
increased true airspeed at higher altitudes is a definite
advantage of a turboprop engine.
All turbine engines, turboprop or turbojet, are defined
by limiting temperatures, rotational speeds, and (in the
case of turboprops) torque. Depending on the
installation, the primary parameter for power setting
might be temperature, torque, fuel flow or r.p.m.
(either propeller r.p.m., gas generator (compressor)
r.p.m. or both). In cold weather conditions, torque
limits can be exceeded while temperature limits are
still within acceptable range. While in hot weather
conditions, temperature limits may be exceeded
without exceeding torque limits. In any weather, the
maximum power setting of a turbine engine is usually
obtained with the throttles positioned somewhat aft of
the full forward position. The transitioning pilot must
understand the importance of knowing and observing
limits on turbine engines. An overtemp or overtorque
condition that lasts for more than a very few seconds
can literally destroy internal engine components.
Ch 14.qxd 5/7/04 10:08 AM Page 14-2
14-3
TURBOPROP ENGINE TYPES
FIXED SHAFT
One type of turboprop engine is the fixed shaft
constant speed type such as the Garrett TPE331.
[Figure 14-2] In this type engine, ambient air is
directed to the compressor section through the engine
inlet. An acceleration/diffusion process in the twostage
compressor increases air pressure and directs it
rearward to a combustor. The combustor is made up of
a combustion chamber, a transition liner, and a turbine
plenum. Atomized fuel is added to the air in the
combustion chamber. Air also surrounds the
combustion chamber to provide for cooling and
insulation of the combustor.
The gas mixture is initially ignited by high-energy
igniter plugs, and the expanding combustion gases
flow to the turbine. The energy of the hot, high
velocity gases is converted to torque on the main shaft
　
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