Power settings (RPM VS MP)

Power settings (RPM VS MP)

<HTML>I've had a 172XP (Continental 6 Cyl) for less than 2 months (Flown it 5 times). It  has been on static display in the parking area under class B airspace since 11 Sept. 

Anyway, while trying to learn all I can about it, I recently read an article which I interpert as saying in some cases MP can exceed RPM. It said Continental authorized cruise operations at 1 to 3 inches "oversquare" for most normal-aspirated engines. It said to check the cruise chart in the POH and operating at minimum RPM and maximum MP (within the allowable envelope) actually help your engine last longer.

I don't even know what oversquare means - nor do I know what aspirated engines means.

Does this mean if the POH says Cruise 2200 -2600 RPM and 15-25 IN HG that you should actually set 2200 RPM and 25 IN HG?

I would have assumed this would be tearing your engine up -- not prolonging it?

What's the opinion of you more knowledgebale folks?</HTML>

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Re: Power settings (RPM VS MP)

<HTML>"normally-Aspirated" means an engine that is not supercharged or turbocharged.  (Aspiration refers to "breathing",  in other words, if your engine is normally-breathing, and doesn't have it's intake pressurized mechanically.)
"Oversquare" is a term used to indicate a power setting in which the M.P. exceeds RPM/100, i.e., 25 in. M.P. and 2500 RPM would be a "square" power setting but 29 in. M.P. and 2500 RPM would be an "oversquare" one.
Traditionally flight instructors taught students that following takeoff the power settings should be reduced from full power (say, for example, 29" and 2750 RPM) to 25"/2500.   They were always cautioned to reduce MP first to 25, THEN to reduce RPM to 2500 (so that an oversquare condition would not occur, even temporarily.) 
  This technique continued to be taught throughout the power-setting regime.  For example, when reducing power still further, to perhaps 23/2300, then the MP should first be reduced before the prop RPM was brought down.  Students were taught that when INCREASING power that the reverse procedure was necessary,...first increasing RPM then increasing MP, and never to allow an oversquare condition to happen.
  The entire idea of teaching pilots to never allow oversquare conditions was a combination of factors, most of which no longer apply.  One was that, during the period of time it was taught, several quality-levels of fuel were dispensed and sold at airports each of which carried a different octane rating.  Many times, to save costs, aircraft main fuel tanks might be filled with a higher octane rated fuel (for use during takeoff only), and the aux tanks might be filled with a lower octane fuel (for operations at reduced power).  A higher MP during operation on lower octane fuel could cause destructive detonation to occur.
  Another reason for teaching such power-setting techniques was that many pilots were being trained in several different aircraft types over short periods of time, and it was faster, safer, and simpler to teach only one technique that would apply to all types of aircraft.  This occured during the late '30's/early '40's when USAAF cadets were being trained in PT-22's, then transitioned into PT17/18/19 aircraft (each of which was a Stearman but with differing engines), next into BT-13's and on into possibly a myriad of fighter, escort, bomber, or transport aircraft.  By teaching one technique that could safely be utilized in all the type aircraft substantial savings in both money, time, and maintenance could be achieved.
  Keep in mind that many higher performance aircraft these 300-hour wonder pilots were supposed to operate in combat conditions were even more complicated that many modern airliners and had no ergonometric (human) engineering factors or standardization.  (The cockpit of a Lockheed Hudson looks like a nightmarish steam-driven plumbing factory complete with water valves for which to manage fuel.)  To top off the nightmare, many of the engines were not only turbo-compound supercharged, but were sometimes also equipped with prop-shaft gear-reduction units,...the prop and the engine turned different RPM's!
  The bottom line was that, for simplicity, pilots were taught conservative power management techniques.  Fast-Forward to modern times.........
  Many instructors today were trained by instructors who were trained by instructors who were trained by.......you get the idea.
  Meanwhile, the same cylinder that fits a turbocharged engine may actually be installed on a normally aspirated engine.  (This is for inventory and certification simplicity.)  Not always, but sometimes this is the case.  That cylinder may expect to see 45" of MP at 2700 RPM on a Piper Chieftan, but only 29" at 2700 RPM on an Aztec.  Does that cylinder care whether it observes 29" at 2500 RPM?  The consideration is:  Am I installed on a Chieftain on climbout?  Or in an Aztec at cruise?  The answer is:  No. It doesn't matter.
  In fact the oversquare power setting can actually benefit the engine more than the supposedly conservative "square" power setting, in many cases.  An example of this is in a carbureted engine with a "power enrichment" valve.  (Now that's an entirely different course of instruction, so don't expect me to go there without you sending me some money.;Þ Just let it suffice to say that if your engine has a carburetor, and in most cases even if it's fuel injected, that during wide-open throttle, the engine gets a little more rich fuel mixture than normal, for various important and common reasons.) 
  In such an engine, if the pilot reduces throttle to 25" after takeoff, ...the mixture goes from a beneficial over-rich condition to a leaner condition, while the engine is still churning out BTU's that the cylinders/pistons/valves have to absorb and dissipate.  It's a rough time to treat your engine that way.  (Ever hear how 99% of all engine failures occur after the first power reduction of a flight?  Ever wonder why?)  High MP is also useful in sealing piston rings against the cylinder walls and reducing blow-by.  This not only serves to efficiently produce power, it also reduces hot combustion gases from entering the crankcase where it heats up lower units, runs oil temps up, and introduces corrosive gases to the rest of the engine components.  On top of which, that suddenly leaner mixture serves to increase cylinder head temps and exhaust-valve-eating exhaust valves.
  On top of all this, after the pilot reduces the power to 25/2500, ...in only 2 or 3 thousand feet of climb the aircraft is running only 22/2500 and the pilot then has to add that MP back.   So an entirely unecessary heat-cycle has been added to the takeoff/climb routine.  Why not simplify your life, and be nicer to your engine, by leaving the MP at 29" and just reduce the RPM to 2500?  After 3 or 4 thousand feet, your power setting is already going to be at 25/2500, right?
  There are lots of other details that could be discussed regarding this question, but time and space restraints come into play on this sort of media, so I'll close.
  I hope this anwers your question.  Keep in mind, that any power management techinique you use should be in compliance with your Operator's Manual and other manufacturer's and FAA approved data.
  The conclusion drawn from all this is, that in a normally aspirated engine, relatively small amounts (less than 5" in my opinon) of "oversquare" power settings are not harmful, and an some cases can actually benefit the engine with cooler-running fuel/air mixtures and higher, more efficient, and better-sealing compressions within the cylinders, all while reducing engine heat-cycles and pilot workload.</HTML>

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Re: Power settings (RPM VS MP)

<HTML>(Please Forgive the typo's in the previous msg.  The dog and the cat got into a fight and I forgot to proof it.)  ;Þ</HTML>

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Re: Power settings (RPM VS MP)

<HTML>George

Thank you for all the effort you put in to writing that fantastic reply. I learned so much -- I bet many other could learn a thing or two by reading it.

Again, thanks

Archie</HTML>

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Re: Power settings (RPM VS MP)

<HTML>George,
I know that it took awhile to write, so I am thanking you for your input on the subject of rpm vs. mp. When I was learning to operate a 182 at a flight school, the practice was to bring the mp back to 22 and the rpm back to 2400 at about 500 ft above ground. I now have my own 182, and my present instructor has me leave the mp alone and bring the rpm back to 2450, once we are at least 500 agl. After a couple thousand feet into the climb the mp is moving down anyway. The airplane performs alot better with this method.
Thanks again for the info, that you so well explain.
Glenn Darr
182E</HTML>

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Re: Power settings (RPM VS MP)

<HTML>Well Done,George; Your response to (rpm vs. mp.) Was a work of art, and I might add a lot of research on your part.I'm one of those old school pilots of years gone by,c.p.t. airforce,airlines, and general aviation.I still use the old method, mp back then rpm to meto power after takeoff.I am going to try your procedure on my 0470-L and make note of any improvement on climbeout. I do a lot of flying for skydivers,hence a lot of climbing to altitude(don't ask "why would anyone jump out of a perfectly good airplane"?)You hav'nt seen my airplane!My Wife read your response,and said,"they aught to call this the.GEORGE WEB SIGHT.She's got a point there. regards
willy Owens  P.S.disregard any typos If I was smart, I wouldn't have spent most of my life flying airplanes.</HTML>

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Re: Power settings (RPM VS MP)

For Willy Owens: You wrote "If I was smart, I wouldn't have spent most of my life flying airplanes". Alloew me to disagree with your comment. Making it back on the ground safely every time and staying alive proves you are smarter than you think.

JP

Jean Paul Vincent
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Re: Power settings (RPM VS MP)

This is a great discussion and I am interested in operating my 182N in this manner.  I assume all the discussion on the oversquare power settings refer to cruise as well as climb power settings.  After reading I reviewed my POH.  The cruise charts or climb data do not indicate  oversquare settings.  Does anyone's POH indicate the oversquare settings? If they do not, do you feel comfotable operating oversquare?

I have always wanted to ask this question on oversquare as well as operating lean of peak.

Also, I wanted to recognize everyone in this discussion especially George.  It was nice that someone can feel free to ask such a question and receive such a great and informative response!

Brian Taylor
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Re: Power settings (RPM VS MP)

George:

Superbly done.

In addition:

1) The old "increase RPMs then MP" and "reduce MP before RPMs" came from the radials in which that was very important.  Reducing RPMS after takeoff could quickly make these engines detonate.  It does not apply to our GA engines.  Today, for us, it's an OWT.

2) There is no such thing as METO power in our engines.  That was only in radials.  Our TCM and Lyc. engines are rated at max power to TBO.  Reducing RPM and/or MP after takeoff is a LOT harder on the engine than leaving T.O. power in there.  It moves the thetaPP closer to TDC, for the mechanically inclined.

3) The easiest you can be on your engine during takeoff and climb is to leave the  throttle wide open (WOT) and the rpms at max (unless there is a time restriction in the POH on takeoff RPMs).  The stress on the engine is actually lower.

4) You should lean to a Target EGT in the climb (or at a high DA airport).  For a free PowerPoint presentation on this, see the Advanced Pilot Seminars website. The TECH page.

5) there is NO relationship between MP in INCHES and RPM in 100s.  Oversquare concerns are someone's Old Wives Tale.  HOWEVER, when ROP, be sure to adhere to the POH ranges on MP/RPM combinations.  When LOP those do not apply.  I can easily and safely run my TNIO-550 at 31"/2000 all day long-----as long as it's LOP.

Again, George, nicely done.  Compliments.

Walter Atkinson

Walter Atkinson
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Re: Power settings (RPM VS MP)

Hello Walter!  Glad to see guys like you and George are still out there looking out for us. I always enjoy your straight logical discussions with minimal opinions and OWTs.  thanks!  (By the way, after the big discussion on engine break-in versus oil types, I went with your advice and achieved quick ring seating.  Now have over 150 hrs on the cylinders and have not had to add a single quart between changes.  I am currently using about one third quart each 25 hr change! ) (Lyc 0320E2D)

Stevan J Vanwestenburg
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Re: Power settings (RPM VS MP)

Excellent discussion.

I quite enjoy going up and just doing touch and go circuits. Sounds like that's not so good for the engine.

What power and rpm settings are recommended?

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Re: Power settings (RPM VS MP)

Stevan:

Glad that's working well for you.  Thanks for the report.


Jamie:

Once the engine is broken in, there is no reason not to enjoy going out to do touch and goes.

Walter

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