Increase RPM with altitude

Discussion in 'Technical Corner' started by Grum.Man, Nov 21, 2018.

  1. Grum.Man

    Grum.Man En-Route

    Joined:
    Jul 17, 2014
    Messages:
    2,634
    Location:
    Statesville NC
    Display Name:

    Display name:
    Grum.Man
    Just curious who here will increase the rpm on their airplane as altitude goes up to maintain 65% power or greater? As an example, at a PA of 2K SA 65% power in a Cirrus SR22 is set to roughly 2500 rpm and 23.4". At 14K the book says to maintain 65% power you need 2700 and 17.1". While cylinder pressures aren't excessive it seems like it's turning awful hard. I know in theory that shouldn't matter as long as there is a film of oil protecting the bearings.

    Thinking about turning my 550G to an N. Don't really need the take off performance but would like the extra speed by maintaining HP higher. Just don't know if I would want to run it that hard...
     
  2. dell30rb

    dell30rb Final Approach PoA Supporter

    Joined:
    May 18, 2011
    Messages:
    7,079
    Location:
    Raleigh NC
    Display Name:

    Display name:
    Ren
    I don't think you'll hurt anything by doing that, assuming that operation at 2700 is not time limited for some reason. However, even with the latest and greatest lightspeed or bose products, I would not like to run around at 2700 RPM for any extended length of time.

    Depending on the airplane, I normally run 2400 RPM, 30F lean of peak and cruise 6,000-9000 ft. Above 10k i'll turn it up to 2500 RPM and run at peak EGT or slightly ROP to eek as much power out of the engine as possible (poor man's turbocharger?) Its 60% power or less, and CHT's/EGT's are nice and cool.
     
    Grum.Man likes this.
  3. Grum.Man

    Grum.Man En-Route

    Joined:
    Jul 17, 2014
    Messages:
    2,634
    Location:
    Statesville NC
    Display Name:

    Display name:
    Grum.Man
    In the Venture you pretty much just leave it at 2500 all the time unless I am trying to go slow. The prop is so short even at 2700 it will not be very loud.
     
    SoCal RV Flyer likes this.
  4. eman1200

    eman1200 Touchdown! Greaser! PoA Supporter

    Joined:
    Mar 10, 2013
    Messages:
    11,178
    Display Name:

    Display name:
    eman1200
    I'll just consult my power setting chart which will give me options for setting different RPM/MP at given altitudes and SA. but I typically keep mine right at 2500 and adjust mp accordingly. down low maybe 2400-2500/22-23". up high twenty five hundo and WOT puts me at 1/2 Venture cruise speeds.
     
    Eric Stoltz and Grum.Man like this.
  5. Ryanb

    Ryanb Touchdown! Greaser! PoA Supporter

    Joined:
    Jul 21, 2010
    Messages:
    10,397
    Location:
    Tennessee
    Display Name:

    Display name:
    Ryan
    WOT puts me at 1/16 venture cruise speeds.
     
  6. Grum.Man

    Grum.Man En-Route

    Joined:
    Jul 17, 2014
    Messages:
    2,634
    Location:
    Statesville NC
    Display Name:

    Display name:
    Grum.Man
    If I could get 250 in the mid teens lop I would be happy.
     
  7. Dan Thomas

    Dan Thomas Final Approach

    Joined:
    Jun 16, 2008
    Messages:
    5,648
    Display Name:

    Display name:
    Dan Thomas
    [​IMG]

    Look at that, willya? Got to run clear up to 2650 at 7500' to get 77%. I've encountered so many that simply set the RPM on a fixed-pitch airplane to 2500 or 2400 by rote, and wonder why the thing mushes at altitude.

    Note that those are cruise numbers. Long-term settings, though the manufacturer does say that RPM is "normally limited to 2500 RPM." They don't forbid higher settings. All the way to 2700 if you want some speed. Note, too, that you can't get 2700 at 7500 or higher; the air's too thin to let the engine develop that much power. 2650 at 7500' would be wide-open throttle.

    Read the instructions. They work.
     
  8. flyingron

    flyingron Touchdown! Greaser!

    Joined:
    Jul 31, 2007
    Messages:
    17,201
    Location:
    Catawba, NC
    Display Name:

    Display name:
    FlyingRon
    Let me know if your 172 actually gets 2600 RPM at 12,5.
     
    imwithtuxedo likes this.
  9. Pilawt

    Pilawt Final Approach PoA Supporter

    Joined:
    Sep 19, 2005
    Messages:
    7,378
    Display Name:

    Display name:
    Pilawt
    That's why I like having a manifold pressure gauge, even with a fixed-pitch prop. You get a better idea what the engine is really doing.

    C-172N, O-360-A4M, 13,500' MSL (about 14,000' DA), full throttle, 17.3"/2590 rpm:

    Screen Shot 2018-11-21 at 9.24.32 AM.jpeg
     
  10. SoCal RV Flyer

    SoCal RV Flyer En-Route

    Joined:
    Apr 27, 2016
    Messages:
    2,543
    Display Name:

    Display name:
    SoCal RV Flyer
    2700 rpm all day long on trips to Osh (FP prop rated to 3200 rpm). Actually, it's a sweet speed with very little vibration.
     
  11. flyingcheesehead

    flyingcheesehead Touchdown! Greaser!

    Joined:
    Feb 23, 2005
    Messages:
    22,909
    Location:
    UQACY, WI
    Display Name:

    Display name:
    iMooniac
    If you think about it in terms of % power... Are you merely going to run a lower % power on the N?

    My Mooney has a G, and an available STC to go to the N. I don't think I'll ever bother, because it really only changes the altitude at which you'll have the same amount of power by 2500 feet or so. The STC paperwork costs money, but the prop is also a big additional cost.

    Plus, many of my legs are short enough that it makes no sense to climb higher than maybe 6500 feet. That means that I might not be able to run at the most efficient WOT/2300 setting that I like and not just make more power and burn more gas. If you run either engine at 65% LOP, the 280hp will burn 12.2 gph, while the 310hp will burn 13.5 gph. Running at the same altitude, that extra few hp will get me a whopping six knots of extra speed.

    But to really answer your question about RPM... No, I don't run it up higher at higher altitudes. I run 65% at WOT/2300 in the range of altitudes that that's possible, and I keep it at 65% mostly with the mixture. Once I'm high enough that I can no longer maintain 65% even with a best power mixture, I just accept the power loss with altitude because it also means reduced fuel flow. I peak around 175 KTAS at around 9 or 10 thousand feet, but I've had a long leg at 13,000 where I was still truing 172 knots on 10.1 gph.
     
    Grum.Man likes this.
  12. AggieMike88

    AggieMike88 Touchdown! Greaser!

    Joined:
    Jan 13, 2010
    Messages:
    17,346
    Location:
    Denton, TX
    Display Name:

    Display name:
    The original "I don't know it all" of aviation.
    He do the webinar script o the EAA website and find h recent webinars from Mike Busch. That might have the answers that can help.
     
  13. Kenny Phillips

    Kenny Phillips Pattern Altitude

    Joined:
    Jul 29, 2018
    Messages:
    2,491
    Display Name:

    Display name:
    Kenny Phillips
    Our F model has that same chart; at 11K feet, I got bored with the minuscule climb rate; I'm pretty sure that 12.5K wasn't happening. However, on a pretty close to standard day at 1,500 feet, it ran about 135 mph.
     
  14. Clip4

    Clip4 En-Route

    Joined:
    Jun 27, 2013
    Messages:
    4,889
    Location:
    A Rubber Room
    Display Name:

    Display name:
    Cli4ord
    If Hp loss = elevation x 0.03 x hp @ sea level/1000, how is 65% at 14K possible in a normally aspirated SR22.