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My flight instructor made a comment when talking about complex airplanes that "never over-square an engine" is a good rule of thumb but secondary to POH instructions. The POH, (I presume) is the best-informed source of how the engine was intended to operate.

He then went on to give an example: Supposedly, since the Doolittle Raid had to launch their B-25B Mitchels 200 miles further out to sea, they ran their engines over square which changed their fuel consumption such that they were still able to pull of the mission.

So here are my questions:

  1. Did the pilots during the Doolittle raid run their engines over square?
  2. By what mechanism would running over square change fuel consumption that dramatically? Is that unique to the B-25B's engines?
  3. If there is such a big fuel consumption benefit, why is the "never over-square an engine" rule so prevalent? (Seems like this competes with operating the engine at "best economy"?)

Image of B-25B launching off the USS Hornet:enter image description here

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    $\begingroup$ Did you read the Aviation.SE answer you linked? "Over Square" is a myth, a well-believed one at that, but a myth nonetheless. $\endgroup$
    – Ron Beyer
    Mar 5, 2019 at 19:19
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    $\begingroup$ "Never over square" is a handy rule of thumb because if you stay below square, you don't need to read the POH or remember what exactly is allowed for each plane. If you do, though, over square can be just fine. $\endgroup$
    – StephenS
    Mar 5, 2019 at 19:19
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    $\begingroup$ @RonBeyer - yeah, I originally had the question written more negatively, but opted to maintain civility... :) $\endgroup$
    – nodapic
    Mar 5, 2019 at 19:30

2 Answers 2

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According to this transcript of the actual mission report from one of the pilots, they did indeed run over square

I flew the last 500 miles into Tokyo at altitudes ranging from 1000 feet to 4000 feet in order to fly in the thin overcast and clouds to avoid detection. I used 28 inches of manifold pressure and about 1370 RPM and indicated between 150 and 165 MPH.

And for the latter part of the mission

I made the trip from Tokyo to China at about 500 feet altitude and 1300 RPM and started at 29 inches of mercury and gradually reduced to 25 inches as my gas load reduced. I indicated about 160 to 165 MPH.

Question 2: According to this copy of the manual (page 155) the engine is almost always run over square in cruise.

enter image description here

As for question 3 you can find a discussion on operating over square in this question and ill avoid answering it here to keep it form being a dupe.

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The whole "oversquare" thing is a myth based on peoples' perceptions, not reality. They are just numbers. As long as detonation isn't happening, you can run at any MP/RPM combination you want. When you shove the throttle to wide open from an engine that is idling, you are "oversquare" to the blazes for a few seconds aren't you, and by rights the engine you detonate itself to pieces right there, but it doesn't.

For best fuel economy, you want RPM as low as possible, while still running smoothly, with temperatures in limits, and producing the required horsepower, to minimize friction and heat losses in the engine.

This favours low RPM/High MP settings and also extends engine life. Bottom line is, the slower the piston rings are travelling up and down the cylinder bore per mile travelled, the better.

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