How do turboprop jets reverse thrust?

What is the difference between a turbofan and a turboprop engine?

Both engines use a turbine for power supply. This is where the "turbo" part of the name comes from. In a turbine engine, air is compressed and then fuel in that compressed air is ignited. The energy generated by the ignition turns the turbine. The turbine can then drive both the compressor at the front of the engine and a useful load. In airplanes it creates thrust.

The first jet engine was a Turbo jet . This is a simple turbine engine that generates all of its thrust from the exhaust of the turbine section. However, since all of the air flows through the entire turbine, all of the fuel must be burned. This means that it is inefficient and the solution is the turbofan.


In one Turbofan the turbine primarily drives a fan on the front of the engine. Most engines drive the fan directly from the turbine. There are usually at least two separate shafts to allow the fan to spin slower than the inner core of the motor. The fan is surrounded by a hood that directs air to and from the fan. Some of the air enters the turbine section of the engine and the remainder is bypassed around the engine. In high-bypass engines, most of the air just goes through the fan and bypasses the rest of the engine, providing most of the thrust.


In one Turboprop the turbine primarily drives a propeller on the front of the engine. There is no hood around the prop. Some air goes into the turbine, the rest not. The propeller is designed to turn slower than the turbine. Although this diagram shows only a single shaft, many turboprops have two, with a high pressure shaft driving the compressor and a low pressure shaft driving the propeller. Some motors, such as the popular PT6, also reverse the direction of flow several times.


Turboprops are more efficient at lower speeds because the prop, with a smaller turbine, can move much more air than the fan on a turbofan engine. The hood around the turbofan's large fan allows for better performance than an open propeller at high speeds, but limits the practical size of the fan.

Turbojets tend to have a performance advantage at supersonic speeds. They develop all of their thrust from the high-speed turbine exhaust, while turbofans supplement that with the lower-speed air from the fan. Also, since the air from the fan is not compressed nearly as much as the core turbine flow, it is more difficult to prevent the flow from generating supersonic and causing losses.

The Concorde used turbo jets because it was designed for long periods of time at supersonic speeds. Modern fighter jets are turbo fans who offer a compromise between efficiency and speed.

There are other pros and cons between turbojets, turbo fans, and turbo props, but I think they are beyond the scope of this question.

Work has been done to develop a "propfan" engine to achieve the efficiency of a turbo-prop and the speed of a turbo-fan. You haven't developed a viable design yet.

Turbine engines are used elsewhere in aviation

  • Helicopter as a turbo shaft motor that drives the rotors instead of a propeller, and with an overrunning clutch to enable autorotations

  • APUs in jet aircraft and large turboprop aircraft

Turbines are also used outside of aviation in power plants (to generate electricity) and even in vehicles (such as the Abrams tank).

Turbocharged piston engines use a turbine that is very different from the examples above. Instead of being the primary source of energy, the turbine only supports the piston engine. A turbocharger uses a turbine to compress air that is directed to the engine intake. The increased compression helps the engine produce more power. A turbocharger's turbine is powered by engine exhaust, and a supercharger is similar but is powered directly by the engine. See the Wikipedia page for more information.

Short story

Good answer. Are turbo jets better than turbo fans for high altitude supersonic flights? I see most of the 1960s fighter jets and the Concorde used turbojets ...

Garrison Neely

@fooot Great, informative answer. You might want to mention turbo-charged piston engines. That was a confusion for me when I started researching the different types of aircraft engines.

RoboKaren

Why is the exhaust air discharged in the example of the turboprop? Wouldn't it make sense to have it shot right through?

Slebetman

@RoboKaren: Don't consider it headed down. Think of this as an exhaust. Turboprops, like piston engines, are usually installed in front of the aircraft. Hence, you have to divert the exhaust somewhere to keep it from blowing up the cockpit and passenger compartment. If the aircraft does not have this diversion (e.g. twin-engined engine on the wings), it does indeed make sense to simply let the jet through. In fact, some real world designs do just that.

Meower68

@RoboKaren: The PT-6A (an ordinary turboprop engine) is "backwards". Air enters the rear, works its way forward, and exits the exhaust from the side. The main turbine is on the front of a gearbox and then on the propeller. The nature of turbojets and turbofans is that the exhaust gases have a significant velocity. With a turboprop, the speed in the exhaust gas is relatively low. Most of the energy in the exhaust gas was absorbed by the main turbine and converted into rotational power.