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 direction without stopping and accelerating as the pistons normally do in a 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 (TBO) 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.
Jet 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. Turbojets were used in military fighters although these aircraft use turbofans nowadays.
Turbofan engines
These engines consist of a multistage compressor fan, combustion chambers and exhaust turbine which is connected through a shaft to the compressor. Engine core design can be one or multiple shafts (N1, N2 and so on).
Geared Turbofan
Normally one of the shafts drives the fan, but in the geared fan design the shaft is coupled to a gearbox and the fan rotates even slower improving fuel efficiency by a whopping 20% and lowering noise at the same time. The Pratt & Whitney PW1000G is a good example.
Turboprop engines
The propulsive efficiency of a propeller is much higher than the reactive thrust of a turbojet in the lower subsonic speed range, they peak at about 300 KTAS. The combination of a propeller with a gas turbine engine has a number of advantages in reliability, part count, power developed and specific fuel consumption.
The turboprop engine has the same basic parts as the turbofan less the fan in front. There is a gearbox connected which drives a propeller and propeller rpm control. The generator/starter, oil pump, air conditioning, hydraulic pump etc are driven at the accessory side.
Bypass Turbofan
High and Low Bypass Turbofan
The compressor fan directs air around and through the engine (bypass) and this bypass air is used to cool the engine and generate a large amount of bypass thrust. This cool air surrounds the very hot exhaust and results in lower noise emissions by the engine. Pure turbojet engines do not have this bypass air.
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 then compressed and partially bypassed around it (different from secondary air), 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.
Not every homebuilt aircraft builder is going to be able to affort a gas turbine in his or her aircraft. Fuel consumption is higher (for example: a P&W PT6A-34 in Cessna Caravan will consume some 50 USG/h) but you will get awesome performance from the aircraft.
Anyway, if you like the sound of a starting turbine and can afford it, then by all means go for it!
