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Thread: Over Square?

  1. #1
    Eddie Foy's Avatar
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    Over Square?

    My engine builder says that I should always run MP at least one higher than RPM.

    I.E. 24"MP 2300 RPM.

    What say you all?
    "Put out my hand and touched the face of God!"

  2. #2
    behindpropellers's Avatar
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    Quote Originally Posted by Eddie Foy View Post
    My engine builder says that I should always run MP at least one higher than RPM.

    I.E. 24"MP 2300 RPM.

    What say you all?

    So when you are at 10k and you can only get 21" MP you run 2000 rpm?

    I run our 520 at 2300rpm and 22".

    Tim

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    When I was flying the A36 with an IO550, it was always full throttle and bring the prop back to 2500 as I recall. In the Howard it was 1800 rpm and 26” up to about 6500’, then the MP would go down with altitude for cruise. Climb was 2000 RPM and 30” or what ever you could get based on altitude. Climb was “auto rich” cruise was”auto lean”, loved that automatic mixture control on the 985!


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    A Continental rep told me the green arc is for normal operations and it didn't matter where the respective needles were. On the other hand, my old flight instructor taught me to feel the vibration of the engine and prop and let the airplane tell me where it was happy. I usually fly 24/2400-2450 as a result. With the old 470 and 2 blade I liked 23-2350. Listen to your airplane.
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  5. #5

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    IO 520 with a 88” Mac, once level = 24 squared untill the manifold pressure drops but the prop stays at 2400. I’ve played with it and 2400 or a tad above is the fastest/smoothest and most consistent cylinder temps.
    Remember, These are the Good old Days!
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  6. #6
    G44's Avatar
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    Husky with MT prop, I frequently run 1,900rpm and 23 inches, incredible fuel burn and miles per gallon.
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    I would suggest a read of Mike Busch's book "Engines" on this very subject. Cliff note version, pull the prop back and get every BTU you can out of every revolution like G44 does.
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  8. #8
    SJ's Avatar
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    Quote Originally Posted by Eddie Foy View Post
    My engine builder says that I should always run MP at least one higher than RPM.

    I.E. 24"MP 2300 RPM.

    What say you all?
    This is not going to work well over 5000msl.

    I have always heard that the old oversquare prohibition came from radial engines and was never uplicable to HO designs, however I still hear people propagating misinformation about running over square often times it comes from instructors..

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    charlie husky's Avatar
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    Click image for larger version. 

Name:	AB483310-342F-4455-9C4C-2D5FC9578FBE.jpeg 
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ID:	42361A1-B, 80”hartzell, 1900rpm, 18inches, 5.6-5.8gph @ 80mph. 21inches 7gph @90mph. Or in that vicinity.
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  10. #10

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    There was an excellent article written by Glen Alsworth a couple of months ago in the "Transponder", the newsletter from the Alaska Airmen's Association. Cliff notes of that one... they had much better engine life after they quit following the "over square" old wives tale.

    The Transponder, Jan-Mar edition 2019, page 11.

  11. #11
    mvivion's Avatar
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    Quote Originally Posted by SJ View Post
    This is not going to work well over 5000msl.

    I have always heard that the old oversquare prohibition came from radial engines and was never uplicable to HO designs, however I still hear people propagating misinformation about running over square often times it comes from instructors..

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    Steve,

    i agree reed that the over square bugaboo is a wives tale.

    But don’t blame it on radials. P & W 985s run oversquare lots....

    MTV
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  12. #12
    SJ's Avatar
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    Mike, now I am curious how the wives tale got started!

    sj
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    hotrod180's Avatar
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    Quote Originally Posted by Eddie Foy View Post
    My engine builder says that I should always run MP at least one higher than RPM.
    I.E. 24"MP 2300 RPM. What say you all?
    Is this for a fresh engine, or all the time?
    The Cessna POH (except the early ones) have power charts showing (I guess) recommended settings.
    I like 22 squared in my 470-powered C180, about 65% power-- rumbles along smoothly and efficicently.
    (easy to remember also)
    Running a little over-square doesn't bother me, but FWIW the 1957 C180 owners manual doesn't show any oversquare settings.

    Click image for larger version. 

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    Last edited by hotrod180; 04-03-2019 at 10:27 AM.
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  14. #14
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    If you look at the engine operator manuals for most Lycoming and Continental engines you will see several "over square" power settings listed. It's not uncommon at all to run engines over square, and perfectly within the parameters specified by the engine manufacturers. (Obviously, a normally aspirated engine won't be able to develop over square power settings at higher altitudes, so full throttle and desired rpm is what works up there.)

    The "rule of thumb" about never having mp higher than rpm is a "safety" fall-back for when there are no published power settings available. If you keep mp equal or lower than rpm, you're pretty safe as far as not damaging the normally aspirated engine. But when proper documents are available, you'll find that many over square power settings are just fine.
    Joe

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    PerryB's Avatar
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    Nothing wrong with over square with a typical N/A aircraft engine, radials notwithstanding.
    During WW2, Charles Lindbergh used this method (in the extreme) to teach our P38 pilots how to maximize their range. I guess everyone put up quite a fuss thinking the engines would be quickly destroyed. After a few demonstration missions (which the powers did NOT want Lindbergh flying) with no ill effects, his methods became SOP.
    Last edited by PerryB; 04-03-2019 at 07:58 PM.
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  16. #16
    skywagon8a's Avatar
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    The thrust of this thread seems to be about cruise power settings. Glen Alsworth's article is about the initial power reduction after take off. He doesn't answer the question of why the EGT rises when the throttle is pulled back first. Fuel systems are set up to provide enriched mixtures at full power for cooling purposes. So when the throttle is pulled back first the fuel mixture is automatically leaned, thus causing an increase in EGT. When you reduce the rpm first the mixture remains unchanged because you have not moved the fuel control (throttle).
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  17. #17
    skywagon8a's Avatar
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    The link to Glen Alsworth's article has been removed, which is a shame since it was well written and does express important information relative to the over square topic. The more we all understand about our engines the better off we will be.
    N1PA
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  18. #18

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    As an additional data point about Engine Manufacturers recommendations......Lycoming publishes an Operating Manual for there engines that debunk old wives tales about over square.

    For my Lycoming IO-360-B series engine with 8.5:1 compression ratio, a general rule that will not put an operator outside of Lycoming's Chart is "Don't run more than 5 inches over square." Not all of the lycoming charts are this generous, but a cursory look showed 4 inches over square covered everything. Highest Compression ratio I saw was 8.7:1.

    Also Read Mike Bush. Fly often, fly Lean of Peak, Over Square when below 65% power, and engine overhaul decisions based on engine condition is a very good thing and will lower your cost to fly. If that sentence makes you mad, don't read Mike Bush.

    BTW, takeoffs are usually over square where I live (700' agl). Fixed Pitch prop guys seldom even know when they are over square.
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  19. #19
    charlie husky's Avatar
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  20. #20
    skywagon8a's Avatar
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    Quote Originally Posted by charlie husky View Post
    "the P–38 was considered to have an operational range of just 400 miles, Lindbergh taught the P–38 pilots how to stretch the range to 950 miles by operating them oversquare and leaning them brutally."
    Mike Bush overlooks the fact that a P-38 has turbo-charged engines and always were run "over square". I too have heard this story about Lindbergh but no one has said exactly what it was that he did to get these results. More than doubling the range is a considerable improvement. I have a P-38 manual. I'll have to see if it says anything.
    N1PA
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    charlie husky's Avatar
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  22. #22
    PerryB's Avatar
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    The other thing I read about extending the ranges during wartime was they'd run the engines lean, right at the edge of misfire. The logic was that below peak was below peak no matter how you got there. Might as well use excess air instead of excess fuel. Apparently it worked, look at what we're doing now. I don't know if Lindbergh was involved in that development or not. Amazing what we can figure out with a gun to our heads, literally.
    After Monday and Tuesday, even the calendar says WTF !

  23. #23

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    Here's one of Alsworth's articles talking about powering back after takeoff to reduce noise and reduce wear and tear on the cylinders. I don't know if this is the same article that was published in the transponder or not. This is from the Merrill Field Newsletter.
    Attached Files Attached Files
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  24. #24
    skywagon8a's Avatar
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    Quote Originally Posted by Ed Ruebling View Post
    Here's one of Alsworth's articles talking about powering back after takeoff to reduce noise and reduce wear and tear on the cylinders. I don't know if this is the same article that was published in the transponder or not. This is from the Merrill Field Newsletter.
    That's it, same one thanks.
    N1PA

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    Quote Originally Posted by SJ View Post
    Mike, now I am curious how the wives tale got started!

    sj
    Not by wives...

  26. #26
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    http://mechanicsupport.blogspot.com/...l-tsio520.html

    From John Schwaner's book, Aircraft Repair - The missing manual.

    I've been running most Continentals recently at 23"/22. (Mind you I've only done the occassional private flying in the last few years)

    Reading the above, I wonder whether I've been doing it wrong.

    Running 24/23 however gets you further away from max range, or more correctly on a non hired aircraft, min cost.

    This whole engine operation thing, power settings, EGT on an aircraft without an engine digital monitor, or even with, has me scratching my head.

    I think I'll go back to what we did 30 years ago before the internet. 23/23 and 50 rich of peak. My overhauled O-470R should be ready to go back in the C180 in a month.

    Dr Google doesn't seem to have Continental Service Bulletin SS107-5. Can someone post a copy?

  27. #27
    skywagon8a's Avatar
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    Quote Originally Posted by skywagon8a View Post
    "the P–38 was considered to have an operational range of just 400 miles, Lindbergh taught the P–38 pilots how to stretch the range to 950 miles by operating them oversquare and leaning them brutally."
    Mike Bush overlooks the fact that a P-38 has turbo-charged engines and always were run "over square". I too have heard this story about Lindbergh but no one has said exactly what it was that he did to get these results. More than doubling the range is a considerable improvement. I have a P-38 manual. I'll have to see if it says anything.
    In reviewing the manual it is unclear to me how Lindbergh was able to maintain formation with the other P-38s and still produce the fuel economy which has been claimed. As you will note, in order to increase the range by the claimed amount the cruising speed is also reduced by 90 mph.
    The Flight operation instruction chart has numerous power settings, altitudes, speeds and ranges.
    With 250 gallons of internal fuel available the range increase is available when the power is changed as in just this one example at 15,000 feet:
    2400 rpm, 37" mp, mixture auto-rich, 273 mph IAS, f/f 172 gph, range 430 miles.
    1600 rpm, 24" mp, mixture auto-lean, 183 mph IAS, f/f 55 gph, range 950 miles.

    As you can see the major difference is the large reduction of power along with the mixture changing from auto-rich to auto-lean which produced a considerable reduction of airspeed.

    I suspect that perhaps Lindbergh was using auto-lean mixture while the other pilots were using auto-rich.

    The only cruise instruction in the manual is in the interpretation of the power charts. The manual does mention using auto-rich for various phases of flight but neglects to mention when to use auto-lean. I'm suspecting that the lack of emphasis on the use of auto-lean could lead a pilot to just leave it in auto-rich. We pay attention to this because we pay for our own fuel.

    The P-38 has pressure injection carburetors which have an automatic mixture control. The mixture control has three positions: Auto-rich, Auto-lean and Idle-cutoff. The mixture is controlled by an aneroid operated valve which automatically compensates for the altitude pressure differences.
    N1PA
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  28. #28
    mvivion's Avatar
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    And, note that ALL the power settings noted in Pete’s note are oversquare.

    MTV

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    If you're going to muddy the water with supercharged engine MP you may as well add geared engine RPM to add to the confusion.
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  30. #30
    hotrod180's Avatar
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    Don't forget the nitrous.
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  31. #31
    mvivion's Avatar
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    Quote Originally Posted by stewartb View Post
    If you're going to muddy the water with supercharged engine MP you may as well add geared engine RPM to add to the confusion.
    Stewart,

    My point wasn't to "muddy the waters", it was related to radial engines, many if not most of which are supercharged.

    I've run O-360s over square regularly ever since and it works great.

    MTV

  32. #32
    WindOnHisNose's Avatar
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    Mr. Busch, from the Cessna Pilots Assn, strongly encourages running over squared. He feels that having that pressure is important, and suggests that when we descend we lower the rpm to maintain over squared. His reasoning seems sound.

    Randy
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  33. #33
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    Not only were the P38's able to extend their range by a wide margin but the Japanese were able to extend the range of their A6M Zeros using similar methods. I recall that after one raid on OZ the allies were sure the Japanese had a new base somewhere closer and spent time looking for it.....I also recall the Japanese developed special spark plugs for such operations. Their cruise speed was just over 100 Kts as I recall, it took a long time to get there but they went a long ways.
    RPM was so low you could count the prop blades going by....

    Jack

  34. #34
    hotrod180's Avatar
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    Quote Originally Posted by WindOnHisNose View Post
    ... He feels that having that pressure is important, and suggests that when we descend we lower the rpm to maintain over squared. ...
    When I descend in my 180, the MP goes up if I leave the throttle alone.
    Maybe you're thinking lower the rpm as the MP goes down as you climb?
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    I run a GO-480 in Piaggio 149D. Here’s something wierd - best cruise is 2 750 at 21” (I think, the book is metric) - but I tried taking it down to 2400 and the oil consumption goes up - suddenly it starts throwing oil out the exhaust! Thump it for aerobatics (full throttle and 3000 rpm) and it uses very little oil and the belly stays clean. I understand this engine is not getting over square just closer to it than recommended, but could the oil issue be the reason it is not recommended? No doubt I am displaying serious ignorance here but - having read this thread - I tried an oversquare setting in my Cherokee 235 this last weekend and found an oily belly from the O-540 which usually runs clean


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    Increasing oil consumption running running over square is due to increase in BMEP (brake mean effective pressure) While theoretical, when you develop the same horsepower at lower prop RPM, the duration of the combustion stroke is longer creating more time for high pressure to act on your rings. Oil expelling from the breather pipe indicates your rings are worn or ring end gaps are too large or incorrectly installed. You are not developing higher pressure so much as you are giving the pressure more time to act on the rings. At higher rpms there is less time to act and the oil control ring wipes up excess more quickly. On a high time engine or one with worn rings you may see more blow by into the case pushing oil out the breather. Given that most aviation engine oil sumps are over serviced for most missions "breather topping" is not uncommon and can be aggravated by more blow by. Simply put there is nothing free. Run over square and you get better range as more heat value of the fuel is directed to the propeller and not out the exhaust pipe and heating your engine heads. The flip side of that is your rings are under combustion stroke pressure longer and your oil control ring does not "swing the mop" as fast.
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  37. #37

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    Quote Originally Posted by GeeBee View Post
    Increasing oil consumption running running over square is due to increase in BMEP (brake mean effective pressure) While theoretical, when you develop the same horsepower at lower prop RPM, the duration of the combustion stroke is longer creating more time for high pressure to act on your rings. Oil expelling from the breather pipe indicates your rings are worn or ring end gaps are too large or incorrectly installed. You are not developing higher pressure so much as you are giving the pressure more time to act on the rings. At higher rpms there is less time to act and the oil control ring wipes up excess more quickly. On a high time engine or one with worn rings you may see more blow by into the case pushing oil out the breather. Given that most aviation engine oil sumps are over serviced for most missions "breather topping" is not uncommon and can be aggravated by more blow by. Simply put there is nothing free. Run over square and you get better range as more heat value of the fuel is directed to the propeller and not out the exhaust pipe and heating your engine heads. The flip side of that is your rings are under combustion stroke pressure longer and your oil control ring does not "swing the mop" as fast.
    Thanks very much for that easy to understand explanation - it makes sense. As both of these are low time engines (less than 300 hours TSMOH) its worrying, but it does make sense!
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