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Author Topic: RPM recomendations  (Read 1653 times)

appleseed

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RPM recomendations
« on: February 02, 2023, 01:56:54 pm »
Is the advice to operate as much as possible at “cruise” rpm (96ish percent) new? And by new I mean like this century, since the Honeywell takeover.

When I first started flying 331 powered planes it seemed like the mantra was “sure, you can pull it back a few percent - if you can’t handle the noise, and like going slower.  But the engine was designed to run at 100% and it is most efficient at that rpm.”

I actually got a call from chief pilot for reducing rpm on short flights that only justified a climb to 16.5/17.5.

I never heard about any problems with 100% until about 10-15 yrs back at recurrent, there was a Honeywell handout (I still have it somewhere if anyone needs it) that said basically the opposite.

What I thought they left out was actual objective data. Like how much wear, hot section expense etc. I wish there was some data along the lines of “If you run the engine 3% slower and 30 degrees hotter you’ll lose 10 percent of your climb and 3 percent of your cruise, but save $100,00 in T blades over the life of the engine.

Bruce Byerly

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Re: RPM recomendations
« Reply #1 on: February 02, 2023, 04:22:12 pm »
What I know is that Honeywell claimed and explained the inlet design etc was setup for max efficiency at 96%.  Blade elongation and other factors favoring the lower RPM. I recall the presentation quoting the forces involved on the blades and thought it made sense. But who knows if it makes a difference.  The other thing that stuck with me from those Eggling presentations was the benefit of the cool down spin in terms of the reduction in fuel nozzle coking which reduces erosion in the hot section.  They seemed to have science behind that but yet many think you do it to reduce “shaft bow.” It does help with that but I digress …

donv

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Re: RPM recomendations
« Reply #2 on: February 02, 2023, 06:39:20 pm »
I run 100% all the way up to cruise, and then pull it back to 96-97 (my RPM gauges don't quite line up perfectly).

I have tried pulling it back after takeoff, but somehow that just doesn't feel right to me.

I have become quite religious about both the 3 minute cooldown after landing, and pulling the props through. Barry Lane and I have agreed to disagree on the pulling the props through thing...

I learned about it in the late 1980s when I was flying a Merlin III. We had a couple of Metro guys who flew with us for a while, and their airline made a big deal about pulling the props through after shutdown. I believe that had to be based on data, as the airline would have had a lot of it.