19
Turboprops are rated using power, in units of horsepower (Hp) or kilowatts (KW). For the C-130
with 54H60 propellers under static, standard day, sea level conditions:
ESHP = 4,910 horsepower
BHP = 4,591 horsepower
T
J
*V/325*
η
P
= 319 horsepower
During flight, the T-56 runs at a constant 100% RPM (13,820 RPM), so torque (force times
distance) is proportional to horsepower. The C-130 T-56 installation uses torque as the primary
indication of engine output. The most important operational limitations for the C-130 engine are:
Torque:
19,600 in-lbs. maximum
Turbine inlet temperature (“TIT”):
1083º maximum
.
The T56-A-15 engine is flat rated (meaning it gives out a constant 19,600 in-lbs. of torque, until
the ambient temperature reaches a “break” temperature, and then available torque decreases as
temperature increases. Stated another way,
at low ambient temperatures the engine is torque
limited by 19,600 in-lbs of torque, and at high ambient temperatures, the engine is TIT
limited
.
Reduction Gear Box
In order to obtain the mass flows with a reasonable cross section, the compressor of the gas
generator is required to operate at high RPM (13,820 RPM for the T56). The propeller sees both
the forward velocity of the aircraft and its rotational velocities, therefore the propeller tips
approach the speed of sound well before the aircraft itself. Once the propeller tips approach the
speed of sound, wave drag increases substantially. In order for a turboprop to be efficient over a
practical range of aircraft speeds, the propeller must turn slower than the gas generator. As a
result, there is a need for a reduction gear assembly to reduce the rotation speed of the propeller
relative to the gas generator. The reduction gear box for the C-130 has a ratio of 13.54 to 1, so
the propeller spins at 1,021 RPM at 100% RPM.
Variable Pitch Propeller
The angle of attack of a fixed-pitch propeller, and thus its thrust, depends on the forward speed
of the aircraft and the rotational velocity. A fixed pitch propeller will provide the maximum