News: Added Links For Twin Commander and Facebook Pages

Login  |  Register

Author Topic: Manifold pressure vs RPM  (Read 13554 times)

ghancock

  • Administrator
  • Hero Member
  • *****
  • Posts: 1152
Manifold pressure vs RPM
« on: November 23, 2015, 07:38:04 am »
So we were flying yesterday and my mechanic pushed the manifold pressure up to around 30" and RPM to 2200.  In my world of flying airplanes I was taught to never have the manifold higher than the RPM other than on take off and even then only for a few minutes.  My mechanic said that because the engines are supercharged it was ok to do this but I wanted to check with you guys as I've had more than one situation with my mechanic that causes me to question pretty much everything now.

So is it cool to do this and if so by how much?

Thanks,

Glenn
--glenn
You can't win an argument with an ignorant person,  they'll just drag you down to their level and beat you with experience.

EricDapp

  • Full Member
  • ***
  • Posts: 186
Re: Manifold pressure vs RPM
« Reply #1 on: November 23, 2015, 11:09:36 am »
In normally aspirated engines you are generally correct. The term I learned was "never go over square". 

In my turbo charged ( exhaust driven)T-210 most cruise power settings are over squared. Most of the time I run 26" and I always run 2300 rpm.  I am allowed to run 30" continuos but I never do. It burns a lot of fuel. 28" if I want to push a little bit.

The term "super charging" is usually referred to as a gear driven blower to boost manifold pressure. I would assume you are allowed to go over square.

I would look at your poh or performance chart to verify. Top of green on manifold gauge is usually for continuous power settings. Your panel might also have some placarding which might limit Certain rpm's with power settings.

I'm glad your in the air. Congratulations.

Adam Frisch

  • Administrator
  • Hero Member
  • *****
  • Posts: 1656
    • Adam Frisch FSF
Re: Manifold pressure vs RPM
« Reply #2 on: November 23, 2015, 12:48:09 pm »
It's generally considered an old wives tale. The Aerostars boost up to 42" and the prop only goes to 2350rpm, so there it's standard procedure.

Also, the gearing on yours - does the Rpm gauge the prop rpm or the engine? My 520 was the engine, so that also skewed any numbers. I saw 3400rpm on takeoff (as I recall).
Slumming it in the turboprop world - so you don't have to.

ghancock

  • Administrator
  • Hero Member
  • *****
  • Posts: 1152
Re: Manifold pressure vs RPM
« Reply #3 on: November 23, 2015, 01:00:34 pm »
Mine is 47" and 3400RPM on takeoff for 2 min  then back to 45" and 3200RPM for climb.  So I guess it's good if it can do that for extended climbs, just wanted to make sure.  Will have to get the old wives tail out of my head. :-)

And yes, it's awesome to at least be able to say I sat right seat.  Just ready for this craziness to be done.

Thanks,

Glenn
--glenn
You can't win an argument with an ignorant person,  they'll just drag you down to their level and beat you with experience.

Adam Frisch

  • Administrator
  • Hero Member
  • *****
  • Posts: 1656
    • Adam Frisch FSF
Re: Manifold pressure vs RPM
« Reply #4 on: November 23, 2015, 01:10:52 pm »
Do you have a 5 minute limit on max rpm?

I had on the 520. The great thing about geared engines is that due to the high engine rpm, low prop rpm, they have much more torque. And you can really tell during takeoff - geared engines are unbeatable there. Also, they hum so nicely when you do reduce rpm. It's like a low, throaty sound I love.
Slumming it in the turboprop world - so you don't have to.

EricDapp

  • Full Member
  • ***
  • Posts: 186
Re: Manifold pressure vs RPM
« Reply #5 on: November 23, 2015, 04:02:26 pm »
It's not an old wives tail in normally aspirated ( non turbo and non supercharged)small motors. Running them over square lugs them.

Think of it like this. Putting your geo metro in 5th gear and running at 1000 rpm around town. It lugs it and is not good on the drive train.

Your obviously moving into a new category airplane and the rules are different. But don't forget the "Ol wives tail" when you go fly a hawk xp or Cardinal.

That's all I'm saying.

ghancock

  • Administrator
  • Hero Member
  • *****
  • Posts: 1152
Re: Manifold pressure vs RPM
« Reply #6 on: November 23, 2015, 04:51:46 pm »
Yes, I understand and won't forget.  Your explanation was perfect and put it into perspective.

Thanks,

Glenn
--glenn
You can't win an argument with an ignorant person,  they'll just drag you down to their level and beat you with experience.

Willis

  • Full Member
  • ***
  • Posts: 156
  • Retired: Engineer; PE, PhD, Professor Emeritus
Re: Manifold pressure vs RPM
« Reply #7 on: November 23, 2015, 05:59:11 pm »
It's not an old wives tail in normally aspirated ( non turbo and non supercharged)small motors. Running them over square lugs them.

Think of it like this. Putting your geo metro in 5th gear and running at 1000 rpm around town. It lugs it and is not good on the drive train.

Your obviously moving into a new category airplane and the rules are different. But don't forget the "Ol wives tail" when you go fly a hawk xp or Cardinal.

That's all I'm saying.

For the most part, the "Square-ness" factor for NA aircraft comes straight out of most POH's and thier recommendations for cruise power settings.  Almost all makes have these cruise restrictions:  Essentially  <75% power.  however you determine that to be.  The maximum RPM and MP regardless of FF is the  "TOP of the GREEN ARC"  which equates to 75% power.

I also have an NA 210D and if you want to apply maximum HP, then the top of the RPM arc is 2500 and the top of the MP is 25 inches......cant go any higher.....according to Hoyle.

Wise engine operations also dictate this from a ROP/LOP point of view.  cant get any higher anyway.

So, in economy cruise;  reduce the RPM and run WOT at any altitude higher than one that produces 25" MAP and then reduce FF accordingly.  Find the fastest speed for the FF/MP/RPM in the tables for that altitude.  If you want good LOP, drop down to the settings that produce less than 65% power.

For my 210 this happens to be 10,500 msl for 20" mp and 2300 rpm.  Fuel flow is about 12gph and I get 175kTAS.  Higher I get lower fuel burn, but also less speed.  Lower, I get more speed, but more fuel burn.   Every flight is a trade-off of ground-speed, economy and the above settings.

Overall costs dictate this and Ive trimmed it down (at 5$/gal)to about  60-70$/hr (in gas alone) or 14mpg or $0.35/mi.

Top of the MP arc will NOT be the same on all aircraft, owing to each having different losses in the induction system.  This will result in different FF of each and........so on.

Still, if the OWT is still alive, its because of the general recommendation of not to be running higher than 75% power in cruise.

Thats just what Ive gleaned from some time in this business...  ust sayin'   ;)


-Bud

Richard Thompson

  • Newbie
  • *
  • Posts: 2
Re: Manifold pressure vs RPM
« Reply #8 on: November 27, 2015, 03:47:48 pm »
Glenn. Go and find someone who has experience with Supercharged Geared engines and get some instruction. The rpm is on the engine and not prop. MP will always be higher than RPM in most cases. Never close the throttles and let the props drive the engines. Always keep the MAP in the green until touchdown . Never push the props into fine pitch on approach. Leave them in cruise setting. If you need to go round just push the power up. You will have plenty of power. Flying the aeroplane is easy. You have to forget everything you learned about engine operation. Geared engines are a completely different animal to direct drive engines. Treat them right and you will have no grief. Treat them wrong and you will destroy them very very quickly. These things its all about flying the engine.

ghancock

  • Administrator
  • Hero Member
  • *****
  • Posts: 1152
Re: Manifold pressure vs RPM
« Reply #9 on: November 27, 2015, 06:38:55 pm »
Thanks, I plan to get with an Instructor as soon as it's flying again.  I've also been reading a good deal about the engines so it's starting to sink in.

Glenn
--glenn
You can't win an argument with an ignorant person,  they'll just drag you down to their level and beat you with experience.

Sergio Vega

  • Jr. Member
  • **
  • Posts: 75
Re: Manifold pressure vs RPM
« Reply #10 on: November 02, 2016, 02:03:45 am »
Can somebody help me with this:

On both engines after some time without being started (maybe about a couple of weeks or so) they start rough with low rpm (1100 aprox) and high MP (ambient pressure, between 29-30 inches). They run smooth above 30"  but failed to do the magneto check (drop about 350 rpm each). We shut down the engines waited for 30 minutes approx  and on a second start both engines corrected to normal parameters and checks. This has happened on two occasions, one on each engine. Any ideas?

Adam Frisch

  • Administrator
  • Hero Member
  • *****
  • Posts: 1656
    • Adam Frisch FSF
Re: Manifold pressure vs RPM
« Reply #11 on: November 02, 2016, 02:36:24 am »
Can somebody help me with this:

On both engines after some time without being started (maybe about a couple of weeks or so) they start rough with low rpm (1100 aprox) and high MP (ambient pressure, between 29-30 inches). They run smooth above 30"  but failed to do the magneto check (drop about 350 rpm each). We shut down the engines waited for 30 minutes approx  and on a second start both engines corrected to normal parameters and checks. This has happened on two occasions, one on each engine. Any ideas?

Sure you just don't have some oil on the bottom plugs where it tends to collect? With a leaking valve or ring that's quite common.
Slumming it in the turboprop world - so you don't have to.

ghancock

  • Administrator
  • Hero Member
  • *****
  • Posts: 1152
Re: Manifold pressure vs RPM
« Reply #12 on: November 02, 2016, 08:53:57 am »
Here is what I'd try on next start.  If you wait more than a couple weeks between flying it is possible you'll get oil build up in the cylinders.  I'd suggest turning the props through 3-5 times by hand before you start the engines.  This will work the oil out of the cylinders and should let it start normally.

A few things to note:  1) I am assuming your engines have a dry sump where the oil is up in the wing and pumps into the engine.  2) If this is the case it is possible you have a bad shutoff valve and it is allowing more oil than normal to seap into the engine.  3) As adam suggested, if you remove the bottom plugs and see a lot of oil draining out of the hole then it would confirm the issue.

I'm sorry but I'm not sure what engines or plane we're talking about here so not sure if any of these conditions fit your situation.  No matter what though if  the plane sits a while it is always a good idea to manually pull it through a number of times.

Hope that helps,

Glenn
--glenn
You can't win an argument with an ignorant person,  they'll just drag you down to their level and beat you with experience.

Sergio Vega

  • Jr. Member
  • **
  • Posts: 75
Re: Manifold pressure vs RPM
« Reply #13 on: November 02, 2016, 11:39:22 am »

The question is pertaining to the Commander 685, it's got the GTSIO 520 k engines. I'll try turning the props, see what happens and I'll try and check for oil in the bottom plugs. Thanks a lot!!!

Adam Frisch

  • Administrator
  • Hero Member
  • *****
  • Posts: 1656
    • Adam Frisch FSF
Re: Manifold pressure vs RPM
« Reply #14 on: November 02, 2016, 11:53:05 am »
If you do have oil on the bottom plugs, you have either a bad ring (most likely) or leaking valve. Oil tends to collect in rocker area and then seep down into cylinder. Sometimes the rings set themselves after awhile if you haven't been flying, but most likely it's the ring or cylinder that's a little off.
Slumming it in the turboprop world - so you don't have to.