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Transition to SETPs
by Thomas P. Turner
Congratulations! You’re about to transition into a single- engine turboprop (SETP)! You’ll need to use most of the skills you’ve developed as a pilot of a piston-engine
airplane. But there are significant differences in the way you’ll f ly a turboprop – and only some of them are related to the engine itself. Let’s take a broad overview of what you’ll need to study and master in order to fly a SETP airplane.
Piston/Turbopropeller Differences
A turbopropeller engine (“turboprop”) is similar to the piston engines with which you’re likely familiar, in that both types are internal combustion engines. Where the process of combustion in a piston engine occurs in individual, distinct ignition events, a turbine engine is ingesting and compressing air, accepting fuel, and igniting that mixture continually. A piston engine might be considered a “rapid explosion” device, while a turbine is a “continually explod- ing” power source.
In a piston engine, exhaust is a waste product that is vented overboard (although some of that waste power is captured in supercharged or turbocharged piston engines). In a turboprop, by contrast, exhaust is the primary product. It spins turbines that, in turn, drive gears spinning the
6 • TWIN & TURBINE / January 2021
PHOTO COURTESY OF CLINT GOFF
propeller (to generate thrust) and the engine’s compressors (for combustion). A piston engine’s ignition system operates the entire time the engine is running, providing a spark to the cylinders. Once a turbine engine starts the igniters are turned off. Ignition is constant as air and fuel are continually fed to the engine and the fire continually burns.
Piston engine power is measured in horsepower (HP) and is determined by manifold pressure, propeller speed and fuel flow. The power of a turboprop is measured in shaft horsepower (SHP). Shaft horsepower is a function of propeller RPM and the force (torque) applied to the propeller shaft. Some jet thrust is also produced by exhaust leaving the turboprop engine. This is added to the shaft horsepower to determine the equivalent shaft horsepower (ESHP). Jet thrust usually accounts for less than 10 percent of a turbo- prop’s total power output.
A turbine engine compresses air, but the amount of compression is usually fixed. Therefore, as the airplane goes up in altitude and the ambient air pressure decreases, the turboprop’s power output decreases as well – turbines are normally aspirated engines. Since turboprops gener- ally have much more power than piston engines, they still