Flying Handbook Menu > Transition to Turbopropeller Powered Airplanes > Turboprop Engines
The turbojet engine excels the reciprocating
engine in top speed and altitude performance. On the other hand,
the turbojet engine has limited takeoff and initial climb performance,
as compared to that of a reciprocating engine. In the matter
of takeoff and initial climb performance, the reciprocating
engine is superior to the turbojet engine. Turbojet engines
are most efficient at high speeds and high altitudes, while
propellers are most efficient at slow and medium speeds (less
than 400 m.p.h.). Propellers also improve takeoff and climb
performance. The development of the turboprop engine was an
attempt to combine in one engine the best characteristics of
both the turbojet, and propeller driven reciprocating engine.
The turboprop engine offers several advantages
over other types of engines such as:
• Light weight.
• Mechanical reliability due to relatively few moving
• 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 overtorquecondition that lasts for more than
a very few seconds can literally destroy internal engine components.
figure14-2. Fixed shaft turboprop