Gas Turbines Builders Association

Turbojet Engines

This is the most common model gas turbine arrangement, with a centrifugal compressor at the front, an axial flow or radial inflow turbine at the back and a vaporising combustor between the two wheels. Fuel can be either kerosene or, less commonly, liquid propane.

At full power, there is a considerable suction force at the intake, which can result in ingestion of loose items and clothing, leading to severe injury.

The jet exhaust from the thrust nozzle, at the rear of the engine, can reach over 600 deg C at a velocity close to 1000 mph. Although the gas heat and velocity diminish with increasing distance from the engine, no one must be allowed to stand in the direct path of the jet blast.


Due to the very high rpm of the rotating shaft, there is a risk of the compressor and/or turbine bursting under the enormous centrifugal forces exerted on these two components. There is also a risk that the lubrication flow to the bearings could be interrupted by contaminants in the fuel or lubrication system, causing the bearings to fail with little or no warning. This can also lead to subsequent compressor and/or turbine wheel failure. Such failures also present a significant risk of fire. Therefore, no one must be allowed to stand beside or behind this type of engine when it is operating.

The engine case and thrust nozzle of all gas turbines get very hot when running normally, leading to a risk of black metal burns if touched. Sufficient cooling time must be allowed before handling or allowing anyone to touch a gas turbine engine after it has been shutdown.

Turboprop Engines

Turboprop engines comprise of a turbojet (see above), referred to as a gas generator. In place of the turbojet’s thrust nozzle, there is an enclosed duct in which a second turbine, referred to as a Free Power Turbine, is located. The free power turbine converts the hot, high velocity gas, produced by the gas generator, into shaft power.

The most common arrangement for model turboprops is referred to as a back to back configuration, where the free power turbine and gearbox, which drives the propeller, is located immediately behind the primary turbine of the gas generator (turbojet). In this arrangement, the gas generator is back to front, in terms of direction of travel, as shown in the diagram below.

A less common and more complex arrangement is the concentric shaft arrangement, where the gas generator is arranged in the direction of travel, with the shaft driving the propeller passing through the centre of the gas generator shaft, as show below.


In both arrangements, at full power, there is a considerable suction force both at the gas generator intake in addition to the propeller suction / wash. Combined, these can result in ingestion of loose items and clothing, leading to severe injury.

The jet exhaust, usually from two nozzles, one on each side of the engine, in the case of the back to back arrangement and typically a single nozzle at the rear of the concentric arrangement, discharge the exhaust gas at a lower temperature and velocity, compared to a turbojet but still hot enough to cause burns.

Considerable care must be taken when installing the back to back arrangement of engine, as it is necessary to avoid the hot exhaust gases from being re-ingested by the intake. Re-ingestion will result in the engine overheating with an associated risk of fire. No such risk affects the concentric arrangement but means must be found of safely ducting the hot gasses from the exhaust, if the engine is mounted in an enclosed fuselage.

In addition to the risk of compressor and turbine burst, there is also a risk of injury from the propeller. Therefore, in the case of the back to back arrangement, no one must be allowed to stand beside or in front of this type of engine when it is operating. In the case of the concentric arrangement, no one must be allowed to stand beside or behind this type of engine when it is operating.

Because the free power turbine is not mechanically linked to the gas generator shaft, turboprops must never be run without a propeller, due to the risk of bursting the free power turbine.

To avoid the free power turbine being over sped and damaged, operators must follow the designer’s or manufacturer’s instructions when choosing the diameter and pitch of the propeller to use with the engine.

Turboshaft Engines - (Helicopters)

Turboshaft engines, intended for use in helicopters, comprise of a turbojet, referred to as a gas generator as mentioned above. Like the turboprop, in place of the turbojet’s thrust nozzle there is an enclosed duct in which the free power turbine is located.

The most common arrangement for model helicopter turboshafts is the same back to back configuration as described in the turboprop section above. However, the free power turbine connects to a 90 deg gearbox, the output of which drives the helicopter rotor head.


For clarity, the diagram above shows the exhaust and the output shaft being on the same plane. However, in practice, the exhaust and output shaft are typically arranged 90 degs to each other.

At full power, there is a considerable suction force at the gas generator intake, which can result in ingestion of loose items and clothing, leading to severe injury.

As with the turboprops, the jet exhaust, typically from two nozzles, one on each side of the engine, discharge the exhaust gas at a lower temperature and velocity, compared to a turbojet but still hot enough to cause burns.

In addition to the risk of injury from the helicopter rotors, there is a risk of compressor and turbine burst. Therefore, no one must be allowed to stand beside or behind this type of engine when it is operating.

Because the free power turbine is not mechanically linked to the gas generator shaft, turboshaft engines must never be run without a load on the output shaft, due to the risk of bursting the free power turbine. Operators must follow the designer’s or manufacturer’s instructions when choosing the diameter and head gearing of the helicopter rotor, to avoid the free power turbine being over sped and damaged.

Turboshaft Engines - (Other Application)

In addition to helicopters, turboshaft engines can be used to drive almost anything. Other examples of turboshaft applications include electric power generators, direct drive of land based vehicles, such as cars, trucks and trains and tanks. Also waterborne vessels, such as powerboats and hydroplanes. The list of applications is wide and varied.

As described in the above turboprop and helicopter turboshaft sections, the arrangement is the same, comprising of a turbojet (gas generator). As detailed, in place of the turbojet’s thrust nozzle is a duct, in which the free power turbine is located.

The most common arrangement for model turboshafts is the same back to back design as described previously. However, the output shaft can be used to drive any suitable load.

At full power, there is a considerable suction force at the gas generator intake, which can result in ingestion of loose items and clothing, leading to severe injury.

The exhaust, which, in a static application, could be a single upright stack, or, in a mobile application, one or more exhaust ports suitably located on the vehicle, discharge the exhaust gas at a lower temperature and velocity, compared to a turbojet but still hot enough to cause burns.

Considerable care must be taken when installing this arrangement of engine, as it is necessary to avoid the hot exhaust gases being ingested by the intake.


In addition to the risk of compressor and turbine burst, the operator must be aware of the risks associated with the application in which the engine is to be used. However, as a standalone machine, no one must be allowed to stand beside or down stream of the exhaust path of this type of engine when it is operating.

Because the free power turbine is not mechanically linked to the gas generator shaft, turboshafts must never be run without a load being applied to the output shaft, due to the risk of bursting the free power turbine. Operators must follow the designer’s or manufacturer’s instructions when choosing the load to be applied to this type of engine, to avoid the free power turbine being over sped and damaged.

Turbofan Engines (Back to Back, Puller)

Like turboshaft engines, turbofans comprise of a gas generator (turbojet) with an enclosed duct in which a second (free power) turbine, is located. The free power turbine converts the hot, high velocity gas from the gas generator into shaft power.

There are three alternative arrangements of model turbofan. The one shown below is the back to back puller design, where the free power turbine, which drives the fan through a gearbox, is located immediately behind the primary turbine of the gas generator. In this arrangement, the gas generator is back to front.

At full power, there is a considerable suction force at the gas generator intake, which can result in ingestion of loose items and clothing, leading to severe injury.

The jet exhaust, typically from two nozzles, one on each side of the engine, discharge the exhaust gas at a lower temperature and velocity, compared to a turbojet but still hot enough to cause burns.


Considerable care must be taken when installing this arrangement of engine, as it is necessary to avoid the hot exhaust gases being re-ingested by the intake.

In addition to the risk of compressor and turbine burst, there is also a risk of injury from the fan. Therefore, no one must be allowed to stand beside or in front of this type of engine when it is operating.

Because the free power turbine is not mechanically linked to the gas generator shaft, turbofans must never be run without the fan in place, due to the risk of bursting the free power turbine. Operators must follow the designer’s or manufacturer’s instructions when choosing the diameter and pitch of the fan, to avoid the free power turbine being over sped and damaged.

Turbofan Engines (Concentric Shaft, Puller)

In this second alternative arrangement of model turbofan, the free power turbine shaft passes concentrically through the centre of the gas generator shaft. The free power turbine drives the fan through a gearbox, which in this case is located in front of the gas generator intake (as shown in the diagram below).

The gas generator intake is located immediately behind the fan, enhancing the gas generator compression ratio.

The exhaust, typically from a single thrust nozzle, similar to that of the turbojet, is located at the back of the engine and the cold air stream, from the fan, passes around the gas generator and hot exhaust.

In addition to the risk of compressor and turbine burst, there is also a risk of injury from the fan. Therefore, no one must be allowed to stand beside or behind this type of engine when it is operating.

Because the free power turbine is not mechanically linked to the gas generator shaft, turbofans must never be run without a fan in place, due to the risk of bursting the free power turbine. Operators must follow the designer’s or manufacturer’s instructions when choosing the diameter and pitch of the fan, to avoid the free power turbine being over sped and damaged.

Turbofan Engines (Pusher)

The third alternative arrangement, is the back to back pusher design, where the free power turbine that drives the fan, through a gearbox, is located immediately behind the primary turbine of the gas generator. In this arrangement, the gas generator is at the front, facing the direction of travel (as shown in the diagram below).

At full power, there is a considerable suction force at the gas generator intake, not only from the compressor but also the fan, which can result in ingestion of loose items and clothing, leading to severe injury.


The exhaust, typically from two nozzles, one on each side of the engine, discharge the exhaust gas at a lower temperature and velocity, compared to a turbojet but still hot enough to cause burns. It is also essential that the exhaust be ducted around the outside of the fan shroud.

In addition to the risk of compressor and turbine burst, there is also a risk of injury from the fan. Therefore, no one must be allowed to stand beside or behind this type of engine when it is operating.

Because the free power turbine is not mechanically linked to the gas generator shaft, turbofans must never be run without a fan in place, due to the risk of bursting the free power turbine. Operators must follow the designer’s or manufacturer’s instructions when choosing the diameter and pitch of the fan, to avoid the free power turbine being over sped and damaged.