Principle of a Gas Turbine Engine

Gas Turbine Engine Types

In WW-II and shortly thereafter, piston powered aircraft peaked in power, performance and complexity wise. Power went up to over 4000 bhp for large multi-row radial engines. Only to be defeated by the jet engine, which was developed by (among others) Germany's Dr. Hans von Ohain and separately in the UK by Sir Frank Whittle.

Its principles are based on the "Aeolipile" of the ancient Greek scientist Hero and other great thinkers like Leonardo da Vinci and the laws of Isaac Newton.

Compared to a piston engine, the gas turbine has less parts and the moving parts rotate in only one motion without stopping and accelerating as the pistons do in a common engine.

Thus, a running gas turbine is basically free of the vibrations normally found in piston engines, which translates in much longer engine service life and higher reliability.

Here we will take a look at the different types of gas turbine engines used in aviation and discuss its operating principles and basic components the engine consists of.

Engine Design

Gas turbine engines are available in a number of different types: turbojet, -prop, -shaft and turbofan. We will concentrate on the turboprop and turbofan engines as you will see these types more than the other two in our part of the aviation community.

Bypass Turbofan

High and Low Bypass Turbofan

The bypass principle is used because it improves the specific fuel consumption (SFC) and the propulsive efficiency of the engine. Air taken in by the engine is compressed and partially bypassed around it, carried through ducts and ejected at the rear or around the engine in case of a front mounted ducted fan.
The ratio of air bypassing the combustion zone compared to the amount of air entering the engine is called the bypass ratio. Usually around 8:1 or higher to improve the SFC.

GTE Manufacturers

A short list of GTE manufacturers is below to give you an impression of what is available today.

Pratt & Whitney

Pratt & Whitney PT6 series engine A widely known turboprop engine from Pratt & Whitney Canada is the PT6A series engines. It is a multi fuel, reverse flow, free turbine and has proven to be a very reliable engine with millions of flight hours and is installed on a multitude of aircraft like the Cessna Caravan (675 hp), Quest Kodiak (750 hp, PT6A-34) and Pacific Aerospace 750 XL (750 hp, PT6A-34AG).

To learn more about this remarkable engine read these PT6A technical specifications.

Rolls Royce RR500

Mooney Aircraft is also bringing out a new model aircraft with a turbine engine, the new Rolls Royce RR-500. This engine could also be used by experimental aircraft builders in their designs. Click the following link if you want to read about the RR500 technical specifications.

Quote from Mooney: "The benefits of developing the RR500 to power a four place aircraft include improved global fuel availability in emerging markets where 100LL is scarce to non-existent".
I think that this hits the nail exactly, as time goes by AVgas will become more a thing of the past. Especially when aviation diesel engines will finally lift off.

Walter M601

This commonly known turboprop engine (from the Czech Republic) is used in numerous aircraft commercial and experimental, for example, the Turbine Legend uses the Walter M601-D. It has 675 bhp. Lately the Honeywell Garrett TPE331-10 engine is installed for extra climb performance and speed at altitude, this engine has about 1070 bhp. See below.

The Walter M601 is a dual-shaft reverse-flow free turbine turboprop engine made by Walter Engines a.s.. The gas generator section consists of two axial and one centrifugal compressor stages, an annular combustion chamber, and a single stage axial compressor turbine. The power section consists of a single stage axial power turbine, exhaust system, and a two stage planetary reduction gearbox (15:1) with torque meter.
You will see this engine in a number of aircraft like the Lancair IV-P, CompAir turbo models, Murphy T-Moose and many more.

Honeywell Garret

In contrary to the Walter and the PT6, the Garret is not a free turbine engine. The core shaft is direct connected to the propeller through a gearbox. When shutdown you can see that easily as the propeller blades are in flat (fine pitch) position, the free turbine engine has the blades in full coarse pitch when shutdown.

Sources claim that an aircraft with the Garret engine runs with less fuel than one with a Walter or Pratt & Whitney PT-6 turboprop and this is with the same speed/power setting. Noise level seems to be higher on the ground but lower when flying.

CompAir uses a 1650 hp version in their turboprop aircraft.

Innodyn (1997-2009)

Was under development since 1997, the Innodyn turboprop engine has a direct spooled and a centrifugal compressor and radial inflow turbine. The fuel system is FADEC controlled and accepts diesel, JET, biodiesel and kerosene. Full installed weight is 188 Lbs and maintenance was suppose to be really low: change gear oil every 500 hours, the spark plugs bianually and inspect the turbine for damage. Propeller is electric controlled type.
The engines have a power range between 165 bhp and 255 bhp, fuel flows over 15 USG/h