14th Aug 2012, 17:32
I have PPL and always watch whatever aviation related program is on tv.
One thing I have noticed is when turboprop aircraft start up or shut down, the props appeared to be fully feathered.
Now if I tried that in a piper arrow, it would shake the engine to bits..
Why are turbines different?
14th Aug 2012, 17:46
You can't feather the prop on an Arrow!
14th Aug 2012, 17:56
Only some have them feathered.
Free turbine such as the PT6 does. Because the Prop isn't driectly connected to the turbine.
And direct shaft engines (garettes) get put on whats called start locks by selecting reverse as the engine is shut down. This is because as you start the engine the prop turns as well because the whole lot is connected on a shaft through the engine. The requires a huge amounts of power if feathered.
14th Aug 2012, 18:17
Ours are started in feather because the prop is connected to the gear box not the drive shaft directly, we put them to feather prior to the 1 minute cool down on shutdown.
14th Aug 2012, 18:40
I normal PT-6 will allow it's propeller to go to the feathered position when there is not oil pressure. Thus when the engine is not running, it'll be feathered. There are a few exceptions to this. Therefore, though you can select fine pitch for a start, the prop won;t actually get there until the engine is running. The prop is feathered prior to shutdown simply to slow it down more quickly. I once out of curiosity shut down a PT-6 in fine pitch. It took about 6 minutes for the prop to stop. Not safe for people on the ground, and not good for the propeller gearbox, and there is no engine oil pressure, just some splash lubrication.
It can also be reassuring during testing I do, that I can feather a propeller without shutting the engine down, when single engine testing is required. I also do this for glide testing a Caravan. You gotta be very careful bringing it out of feather though, as a prop overspeed would be possible.
14th Aug 2012, 19:09
Just always intrigued me :ok:
14th Aug 2012, 22:15
When we started the Proteus turbo props on RAF Britannias, a brave ground crewman had to hold the prop blade stationary. He released it once the torque built up and it was able to rotate in the correct sense. Apparently once the prop was free on start up it could windmill backwards, risking a hot start. H & S would have had a fit in those days!! :eek:
14th Aug 2012, 22:34
Y'know vee-tail, I always wondered about that from when, back in about '71, I loaded some cadets in my car and we went to Brize to scrounge. A fresh crew took over after our flight and we stood and watched as an airman leaned against each propellor, and suddenly stepped away just as the prop started turning, so that explains it.
Would it I wonder, explain why a couple of years later at Glasgow, I witnessed numerous Monarch Brits starting at night with a huge jet of flame from the exhaust. I certainly never noticed anyone leaning against their props.
14th Aug 2012, 22:55
That's because they were vapourised in the exhaust blast........
15th Aug 2012, 00:42
That's probably due to avoiding decapitation of a trade union member :hmm: