Principle of a Gas Turbine Engine
Gas Turbine Engines
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, 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, 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 models, which translates in much longer 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 they consists of.
Jet Engine Design
Gas turbines are available in a number of different types: turbojet, -prop, -shaft and turbofan. We will concentrate on the turboprop and turbofan models 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.
They consist of a multistage compressor fan, combustion chambers and exhaust turbine which is connected through a shaft to the compressor. The core design can use one or multiple shafts (N1, N2 and so on), these shafts are also called spools.
Normally one of the shafts directly drives the fan, but in the geared turbofan design the shaft is coupled to a gearbox and so 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 of this.
The propulsive efficiency of a propeller is much higher than the reactive thrust of a turbojet in the lower subsonic speed range. Propellers peak at about 300 KTAS. The combination of a propeller with a gas turbine engine has a number of advantages in reliability, parts 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 installed and driven at the accessory side.
High and Low Bypass Turbofan
The compressor fan directs air around (bypasses it so to speak) and through the core and this air is used to cool the engine and generate a very large amount of bypass thrust. This cooler air surrounds the very hot exhaust and this will result in lower noise emissions by the engine. Pure turbojets do not have this bypass air and they sort of scream at high thrust settings.
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 passes around it (different from bypass air), carried through ducts to cool the engine and is ejected at the rear or around the engine, in case of a front mounted ducted fan.
The ratio of air around the combustion zone compared to the amount of air entering the engine is called the bypass ratio. Usually about 8:1 or higher to improve its Specific Fuel Consumption.
Not every homebuilt aircraft builder is going to be able to afford 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. But as these engines operate in aircraft flying at flight levels due to much higher true airspeeds fuel consumption per kilometer or mile is lower than would be with a piston engine.
Anyway, if you like the sound of a starting turbine and can afford it, then by all means go for it!