Turbines, Turbofans, Turboprops, oh my!

[wookiehasbeen]

hot soak??

ok if this is true about garrett's why is it not true for a pt-6 or a cj-610, cf700 or a garrett 731. why dont you see kingair pilots or lear drivers or falcon drivers or anyone else but garrett pilots doing this retarded act. i guess it is because garrett turbo props are the only turbines that get hot?

 

[bugchaser]

Good explanation Timebuilder. On the TPE-331 engines it is possible to have what is called a shaft sag if this procedure is not carried out. Remember that even though you are turning the prop slowly, the turbine wheels are turning relatively fast because of the reduction gearbox. It only takes a few seconds to do this and it certainly can't hurt anything.

 

[bugchaser]

First off, it would not really do much good to pull a PT6 through because you would only be turning the gearbox and the power turbine. On the Garrett 331's the entire engine is directly connected to the prop. This is just a characteristic of the 331 engine and the other engines are of a different basic design. The pilots you have seen pulling these engines through after shutdown are not doing this for the fun of it. It is a required procedure on these engines.

 

[Timebuilder]

why dont you see kingair pilots or lear drivers or falcon drivers or anyone else but garrett pilots doing this retarded act. i guess it is because garrett turbo props are the only turbines that get hot?

The simple answer is that those manufactures haven't determined that this activity (turning the props by hand after a flight) is required. In the case of the PT-6, as mentioned, you are only spinning the free turbine wheel, and that won't draw a gram's worth of air into the first stage wheel or the combustor or the turbine.

In the case of the Lear, if my engines are HOT, as in a quick turn situation, I motor the engines until the ITT comes down to about 125 C before I take the lever up to the idle detent for the start. Not exactly the same thing, but is shows another Garrett engine (731) that doesn't like to be too hot when it isn't running, or in this case, during starting.

And yes, "shaft sag" was the term I heard used for the condition you want to avoid.

Good discussion.

 

[jergar999]

The reason you rotate the props (at least 16 blades through, per the manufacturer) is because the Garretts put everything on one shaft which must either be cool or spinning to stay straight. For example, if you shut the engine down (insert timebuilders lecture here about radiant heat) the single shaft that connects everything tends to sag or bow between the bearings. This can have the same effect as an unbalanced prop could, causing bearing failure due to the shaft exerting more stress on one side than the other as it spins. Also, if you don't spin the props the passengers wouldn't be able to make winding the rubber bands up again comments. Pratts are exempt because they use two separate shafts so they are short enough that there is not a lot of space between bearings. They are also Canadian, so they stay colder.

While we are on the topic, does anyone know if there are Pratts that have Equivalent Horse Power Ratings to account for the exhaust, or is it just negligible? I know when I flew Garretts they figured about 300lbs of thrust from the TPE-331.

 

[Timebuilder]

They are also Canadian, so they stay colder.

While this applies to my dad, it does not apply to my first girlfriend.

She was from Toronto. Good DAY!

 

[bugchaser]

I have heard that on the small PT6's there is about a 25 hp "push" from the exhaust.

 

[80/20]

Originally posted by chawbein:
quote:
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Originally posted by LR25
You know there has always has to be someone to contradict you.

Really though, the 707 and DC-9 types actually had meduim/ low bypass turbofan. Pratt & Whitney JT3's (707) and JT8's(DC-9).

I believe there was an engine on some 707's and DC-8's that were actuall turbojets. I think there were called Conways's, not sure which one it was, some had JT4's, which I'm not sure if that was a turbojet either.

But like I said, I aint trying to bust your chops on the little stuff.
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Make sure you push your taped glasses further up on your face while you type....
"Weeeeellll, actually the 707..." (In a very nerdly voice)

Weeeeellll, actually, the old 707s & DC-8s (sorry I just had to) had Rolls-Royce Conway 43 engines with low-bypass TURBOFAN and JT3C-6/7 TURBOJET engines. Followed by later series with JT3D-7 low-bypass TURBOFAN engines.....and my glasses are not taped - (yet)!

 

[msw]

jergar999:

The PT6A-42's on a King Air B200 are listed at 850 SHP each, and 903 ESHP each. So it looks like each side gets a 53 SHP "push" from the exhaust.

 

[Deftone45075]

Just to chime in with my .02 cents about shaft bow ( or sag), since i've been flying the Garretts.

10,000 degrees is a bit high. It's closer to around 1200 - 1400C.

Because of the mechanical connection between the turbine and the propeller gearbox, the turbine slows down pretty quick (all that weight from the prop and gear ruduction). This leaves a fair amount of residual heat which rises to the top of the combuster, therefore, the bottom of the shaft cools off faster than the top, and when metal cools off at different rates, it warps.

Spinning the prop gets rid of the residual heat, and IS necessary for the long term health of the engine. If you make a habbit out of not spinning the props, over a period of time, the shaft will warp.

Furthermore, the AFM tells you to.

Free flow turbo-props don't get spun for three reasons.

It wouldn't do a d@mn thing since there is no connection between the prop and the turbine.

Since the mechanical connection isn't there, the turbine isn't loaded down during shutdown so it continues to spin longer than a Garrett will.

The AFM doesn't tell you to.