20
thrust only at a single airspeed. By varying the pitch of the propeller, the best possible efficiency
can be realized throughout a range of airspeeds. To be efficient throughout the aircraft’s
envelope, the C-130 propeller is variable pitch.
To simplify the compressor aerodynamics of the gas generator, most turboprops, including the
T56, are designed to operate in flight at a constant RPM. To increase thrust, fuel is added to the
gas generator, and the increase in power is absorbed by increasing the blade angle of the
propeller.
Running the gas generator all the time at 100% in flight provides excellent go-around
performance, as the there is no spool-up time as is typical with a turbo-fan engine that must
accelerate turbo-machinary with a big rotational inertia. All that is required to increase thrust is
additional fuel to the gas generator and a small blade angle change. Since a large portion of the
wing is bathed in the propeller wash, not only does the thrust increase quickly, the addition of
power increases the lift coefficient at a constant angle of attack.
Ground and flight modes: For ground operations, a low pitch angle is required to minimize
thrust. Additionally, the ability to operate at negative blade angles provides reverse thrust and the
ability to back the aircraft on the ground without creating a FOD hazard. Too high a blade angle
makes it difficult to control taxi speed and creates a hazard during engine running on and off
loads. For flight operations, higher blade angles are required to ensure the engine propeller
combination is producing positive thrust. As a result of the different requirements for blade
angle on the ground and in-flight, the C-130 propeller control mechanism has two modes, an
“alpha” mode intended for flight or ground operation, and a “beta” mode intended only for
ground operation. On the C-130, the position of the throttle within the throttle quadrant
determines if the propeller is in alpha or beta mode.
