As William Kershner writes in " Practice Area: Aviation Myths and Insight," on page 66, the danger of operating an aircraft engine oversquare is one of aviation's greatest myths. Combine the oversquare discussion with a debate on the downwind turn and flying on the step and you can wile away an entire evening — or two. A recent and heated hangar-flying session involving the operation of normally aspirated engines with constant-speed propellers reminded me of just how many pilots harbor the misinformation about operating oversquare. Like Kershner, every pilot has his own opinion on the subject. Here's mine.
According to legend, it is harmful to operate a non-turbocharged engine with power settings that result in manifold pressure (in inches) exceeding rpm (in hundreds), an example of which is 24 inches and 2,200 rpm. Such a power setting is colloquially referred to as being oversquare. A square power setting is attained when manifold pressure and rpm are equal (23 inches and 2,300 rpm, for example), and an undersquare power setting exists when rpm exceeds manifold pressure (such as 22 inches and 2,400 rpm).
Loosely speaking, manifold pressure can be regarded as the pressure that develops in a cylinder during its power stroke. In a sense, this pressure is relieved by rpm. The faster the engine turns, the more frequent are the exhaust strokes that allow spent gases to escape. It is natural to conclude, therefore, that as engine rpm increases, pressure within each cylinder is more quickly relieved and results in less strain on the engine.
This might have been significant during an era when aircraft engines were no more reliable than a politician's promise, but it does not apply to more modern powerplants. The myth is destroyed, for example, during every low-elevation takeoff. At such a time, manifold pressure is close to 30 inches and engine rpm (in hundreds) is substantially less than 3,000. Furthermore, it usually is permissible to operate these engines indefinitely at such oversquare power settings.
Also, if an airplane equipped with a fixed-pitch propeller were to be equipped with a manifold-pressure gauge, you would see that manifold pressure exceeds rpm by an even larger margin. This occurs because the rpm of a fixed-pitch propeller is substantially less than its redline during takeoff and climb. It is only during cruise and while diving that the fixed-pitch propeller can achieve redline rpm (because of reduced propeller loading at high airspeed). If the myth were correct, operating such an engine at full throttle would be much more harmful than when equipped with a constant-speed propeller.
From my perspective, it seems that pilots tend to operate their engines at square (or undersquare) because it is such an easy power setting to remember and can be applied without harm to virtually all normally aspirated engines. In other words, it is a lazy way to operate an engine. If these pilots were to consult the power charts provided in the pilot's operating handbook (or, better yet, in the applicable engine operating handbook), they would discover that operating an engine oversquare is not only permissible (within limits), but more efficient and economical.
During World War II, the pilots of U.S. military aircraft were having difficulty flying long-range missions and returning to base with a safe fuel reserve. Charles A. Lindbergh demonstrated that fuel economy could be improved by operating the engines of these aircraft at higher manifold pressure and reduced rpm, a discovery that increased range and operational safety. For general aviation pilots, this can translate into saving money — lots of it.
For example, the operating handbook of a 235-horsepower Lycoming O-540-B engine shows that there are several permissible combinations of manifold pressure and rpm that can be used to operate the engine at 65-percent power. The pilot is allowed to use extremes of 21 inches of manifold pressure and 2,575 rpm (an undersquare setting), 25.2 inches and 1,875 rpm (an oversquare setting), or any of several combinations in between. The most important reason for selecting the power setting with the lowest rpm and highest manifold pressure is fuel economy. When operating this engine at 21 inches and 2,575 rpm at sea level, fuel consumption is 13.8 gph. But when using 25.2 inches and 1,875 rpm, fuel flow drops to 12.1 gph. Fuel flow is reduced by 13 percent, a savings of 1.7 gallons during every hour of operation at the oversquare power setting (without affecting power output). A bonus of selecting such a low rpm is a quieter cockpit environment.
The main reason for this improved fuel economy is that reducing rpm reduces internal engine friction; at high rpm, more fuel is consumed in simply overcoming the increased friction associated with high rpm. This can pay long-term dividends, too. Reducing engine friction can extend the time between overhauls.
According to representatives of both Textron Lycoming and Teledyne Continental, a pilot can usually and safely operate his naturally aspirated engine when using a power setting such that manifold pressure exceeds rpm by "four." For example, he could feel comfortable using 24 inches and 2,000 rpm. To this must be added the caveat that a pilot should always consult and abide by the approved power charts for his engine.
Yes, a pilot may operate his engine using a square power setting, but he should be aware that it can be more costly to abide by this myth than to operate his engine more efficiently (oversquare).
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