[woofa.express]

this follows on from yesterdays story about Russ.

My observation of aero engines.


Yesterday I wrote on the PT6. Today I will speak about some aspects of them and other aeroplane engines.
The PT6 comes in varying sizes, the smallest that I’ve flown is 550hp and the biggest is the 1400hp variant, all Airtractors. I do read the Fokker 50 runs out at 5000hp.
The engine is known as a free turbine. The shaft that runs the compressor turbines is not connected to the power turbine wheel. (two shafts) It is possible and indeed a practice to hold the propeller on start-up. In very cold weather it allows oil to warm prior to lubricating the gearbox which is a planetary system. It is however not possible to catch it once you let it loose, without injury that is.
The other type of popular gas turbine is the Garratt. The turbines, compressor and power are on two shafts, one within the other. They cannot run independently as does the PT6. I can’t tell you alot because I am not familiar with them. They do have an advantage on the PT6 in the fact they are slightly more fuel efficient. Popular with computer and executive planes because better fuel efficiency gives more range. Power is instant because thrust is achieved by prop pitch. Need more power, increase the pitch. They are terribly noisy and most uncomfortable to work around. Surprisingly they are quieter in flight.
All American piston aero engines rotate clockwise unlike the motor car engines. The PT6 likewise. The Garratt, like your motor car, clockwise. English aero pistons turnclockwise. Some aeroplanes swing left (anti clockwise) and others swing right (clockwise rotating). Swing is corrected by rudder and if one runs out of rudder the break will hold straight. However one runs out of correction as cross wind reaches max and coincides with torque then it is not possible to hold straight.
If you are interested in reading all about the PT6 the link below will take you to the engine manual. If you read and understand you will know more about it than I. What matters to me is – more power, the trees get smaller. Close throttle and the trees get bigger.
http://mautone.eng.br/apostilas/propulsao1/PT6%20Training%20Manual.pdf

The schematic drawings are of both PT6 and Garrett. The PT6 has numerous axel compressors,depending on the engine size plus one centrifugal compressor. The Garrett has 2 centrifugal compressors and I guess this is what make them very noisy.
Hope you enjoyed reading what was not intended to be so detailed. The schematics are, from left to right are PT6 airflow, PT6 shafts and the Garratt.