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Turboprop

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A turboprop is a specialized gas-turbine engine designed to drive an aircraft propeller. While jet engines like turbojets use exhaust gases for thrust, a turboprop uses most of its energy to turn a shaft. This shaft connects to a propeller, which moves a large volume of air to create propulsion. Because the propeller moves so much air at lower speeds, the engine is highly efficient for certain types of flight. In fact, the exhaust gas in a turboprop provides only about 10% of the total thrust. The rest of the power is extracted from the turbine to drive the propeller.

Turboprop operation-en
Turboprop operation-en

The mechanism of a turboprop follows a specific sequence of physical processes. First, air enters through an intake and moves into a compressor. The compressor squeezes the air to increase its pressure. Next, jet fuel is injected into this compressed air within the combustor. The fuel-air mixture ignites, creating high-energy combustion gases. These hot gases expand rapidly as they pass through the turbine stages. This expansion generates power at the point of exhaust. Some of this power drives the compressor and an electric generator, while the rest is sent to the propeller via a gearbox.

A critical component is the reduction gearbox. The turbine spins at very high revolutions per minute (RPM) but with low torque. A propeller, however, requires low RPM and high torque to function effectively. The gearbox converts these high-speed, low-torque outputs into the low-speed, high-torque rotation needed for the propeller. There are two primary designs for this connection: fixed-shaft and free-turbine. In a fixed-shaft engine, such as the Honeywell TPE331, the gearbox, turbine, and gas generator are all connected on a single shared shaft. In a free-turbine engine, like the Pratt & Whitney Canada PT6, the turbine is connected to the propeller while the rest of the gas generator spins independently. This design allows the power section to be replaced easily if the propeller is damaged.

Turboprop operation-en
Turboprop operation-en

Propellers used in these engines are typically constant-speed, or variable-pitch, types. This means the angle of the blades, known as the pitch, can change during flight. This adjustment is necessary because turbine engines can respond slowly to power changes at low speeds. Pilots use two main modes for the propeller: Alpha and Beta. The Alpha mode is used for all standard flight operations, including takeoff. The Beta mode is used for ground operations and can produce zero or even negative thrust. Beta is further divided into "Beta for taxi," which produces very little thrust, and "Beta plus power," which produces reverse thrust. Reverse thrust is helpful for stopping quickly on short runways.

Propulsive efficiency for different engine types and Mach numbers
Propulsive efficiency for different engine types and Mach numbers

The history of the turboprop began with early research into compressor and turbine designs. In 1928, Hungarian engineer György Jendrassik published the first turboprop idea. He patented his invention in 1929 and built a small 100 Hp experimental turbine in 1938. His larger Cs-1 engine was tested in Budapest between 1937 and 1941. Although it was intended for the Varga RMI-1 X/H fighter-bomber, the aircraft was destroyed in a bombing raid before its first flight.

Varga RMI-1 3-view (2)
Varga RMI-1 3-view (2)

Later, the Rolls-Royce RB.50 Trent became the first British turboprop to fly in September 1945. This led to the development of the highly reliable Rolls-Royce Dart engine. The Dart powered the Vickers Viscount, which was the first turboprop aircraft to enter mass production.

Some turboprops are designed for extreme performance and speed. The Soviet Union developed the Kuznetsov NK-12, a massive engine used on the Tupolev Tu-95 Bear. This aircraft used eight contra-rotating propellers to achieve cruise speeds exceeding 575 mph. This speed is comparable to many early jet aircraft. While most turboprops are not used for speeds above 0.6 or 0.7 Mach, the Tu-95 proved that specialized designs could push these limits.

Kuznetsov NK-12M turboprop on Tu-95
Kuznetsov NK-12M turboprop on Tu-95

Modern turboprops continue to serve various roles, from heavy military transport to small civilian aircraft. For example, the Lockheed C-130 Hercules uses them for heavy lifting.

Lockheed C-130 Hercules
Lockheed C-130 Hercules

Meanwhile, the Beech King Air uses them for efficient regional travel.

Svensk flygambulans97
Svensk flygambulans97

Understanding the turboprop requires looking at the relationship between air velocity and thrust. A large-diameter propeller accelerates a massive volume of air by a small amount. This is more efficient at low speeds than a small fan accelerating a small amount of air by a large amount. This concept is known as low disc loading, which helps reduce fuel consumption. However, as an aircraft approaches the speed of sound at the blade tips, propeller efficiency drops significantly. To manage this, pilots can use the "feathering" function. Feathering turns the propeller blades edge-on into the wind. This minimizes drag if an engine fails during flight.

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🖼️ Images & Media (8)
File:JMSDF P-2J T64-IHI-10E turboprop engine left side view at Kanoya Naval Air Base Museum April 29, 2017.jpg
JMSDF P-2J T64-IHI-10E turboprop engine...
File:Turboprop operation-en.svg
Turboprop operation-en.svg
File:Propulsive efficiency for different engine types and Mach numbers.png
Propulsive efficiency for different...
File:Varga RMI-1 3-view (2).svg
Varga RMI-1 3-view (2).svg
File:Rolls-Royce RB50 Trent Turboprop On Test Rig At Hucknall.jpg
Rolls-Royce RB50 Trent Turboprop On Test...
File:Kuznetsov NK-12M turboprop on Tu-95.jpg
Kuznetsov NK-12M turboprop on Tu-95.jpg
File:Lockheed C-130 Hercules.jpg
Lockheed C-130 Hercules.jpg
File:Svensk flygambulans97.jpg
Svensk flygambulans97.jpg
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