Running your engine “Over-square”?

Builders:

There is a long standing piece of “hangar wisdom” That says you should never fly a plane “Over-square” . This condition is defined has having a higher manifold pressure in inches of Hg than you have RPM in  hundreds. Example: 25″ map and 2400 rpm is said to be “over-square”, where as the reverse, 24″ map at 2500 rpm is said to be “under-square”. This rule is brought up primarily to warn pilots about putting the engine in a condition where it might be prone to detonation.

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Like many things said in hangar stories, there is some element of original truth in this, but it doesn’t apply to all engines nor conditions. In the end it was supposed to be a little memory device for those unwilling to understand the larger principle.

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The origins of the statement are of attributed to being a good way to run Lycomings, or more particularly Lycomings with constant speed props. Over time, the “over-square” rule became something that people tried to apply to any engine in any situation. But a basic look at a trip around the pattern shows that the rule doesn’t work, not even in Lycomings; If you are near sea level and begin your take off roll at wide open throttle, your map will be darn near the outside air pressure, very close to 29.92″ on a standard day. Since no direct drive Lycoming with a fixed pitch prop turns 2990 rpm static, every take off is “Over square.” Even Constant speed equipped planes have redlines between 2700 and 2800 rpm, and thus would still be “over-square on take off.

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So what protecting these over-square planes from detonation on takeoff? Something very simple. Aircraft carbs, by design, run rich at wide open throttle. In private pilot school many people were told this was for “Fuel Cooling” the air-cooled engines, but that isn’t what is going on. The mixture running rich at wide open throttle effectively increases the comparative octane of the fuel. When you look at Octane ratings of old fuels like 80/87 or 100/130 or 115/145, these dual ratings reflect the comparative detonation resistance both lean and rich.  regardless of it’s name, any fuel will have more detonation resistance when the mixture is rich, and aircraft carbs set properly do this for you.

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Lycomings and continentals are configured this way, and if you have a properly set Stromberg or Marvel carb on your Corvair, it will do it also. However, you have to remember that it is always preferable to run the carb wide open when it is heavily loaded, ie, “Over-square”. Never let anyone talk you into backing off the throttle slightly on climb out, it is a very poor practice.

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The way to make sure your carb is set correctly is to tie the tail of the plane to something solid (not just chocks) and run the engine up to full static rpm. Give the engine a few seconds to stabilize, and then just barely pull the mixture out slightly. If it is set correctly, the rpm will increase as you lean it out, because you are going from an anti-detonation air/fuel mixture of say 10:1 toward best power at 12:1.  The power goes up, so the rpm will climb slightly. This is a good condition. If your carb doesn’t do this, it isn’t set rich enough. You can also watch this on your EGT.

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Where the warning comes into play is avoiding running an engine over-square in cruise settings where the throttle is partial open, and there is no additional richness to suppress detonation. Many times people refer this condition as “Lugging” the engine.  With Corvairs, I consider it very important that the engine turn at least 2,700 rpm static with the prop pitch set for flight. This way, on every take off and climb out, The engine will be operating close to “square” which minimizes the chance of detonation, and additionally the engine has substantially better seat of the pants detectable performance with just 100 more rpm static.

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Some builders, particularly Pietenpol builders like bigger wood props that often yield low static rpms in the 2400-2550 rpm range. I feel that this is an undesirable condition because such a prop tends to “lug” the engine, not just restraining it’s performance, but it is prone to cruising in a condition of low rpm and higher manifold pressure, but without the carb being in the wide open throttle position. Many guys feel that flying around with a larger wood prop at lower rpm is easier on the engine, but I can make a pretty good case that just the reverse is true, especially if the builder ever runs car gas.

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More Rpm will not hurt the Corvair, keep in mind that it did more than 5,500 rpm in the car and was designed to cruise in the car over 3,000 rpm. Your aircraft engine is far better built than any stock Corvair Car engine from the factory, so more rpm will not hurt it, but loading it at lower rpm just might.

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-ww.

 

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Above, a new 2,850 running  on the test stand in our front yard. During the cam break in period, (20-30 minutes) We run the engine between 1800 and 2200 rpm. The throttle is only part of the way open to do this. During the later part of the break in runs we run the engine as high as 3,200 rpm. At that condition the MA3-spa carb on the run stand is wide open and the O2 sensors and the egt’s indicate the engine is in the anti-detonation rich zone. Read more at this link:  New 2850cc / 110hp Corvair in photos.

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Center, above Joe Foss (USMC-CMH) stands with Charles Lindbergh in the south Pacific in WWII. Lindbergh was a factory rep for Vought, and one of the things he taught pilots was how to get extremely good range by running their engines massively “Over-square”, (very low rpm, high blower and high prop pitch) It worked, but the training included elements of making sure the air/fuel was very rich. Foss went on to be the Governor of South Dakota, Commissioner of the AFL, host of “The American Sportsman” and president of NRA.