Engine Power Settings

Reduced engine wear and fuel savings can be achieved by lowering RPM and increasing manifold air pressure (MAP) to maintain horsepower and thrust. Modern engines have approved power ranges for which this practice is allowed. These ranges are in the POH but can also be found in the engine manufacturer's handbook.

Charles Lindbergh visited the South Pacific in World War II as a civilian representative of an aviation manufacturer. He helped pilots lift higher bomb loads and fly greater distances. He was successful on both counts, achieving greater range mostly through over square operation of the engines.

For example, the P-38 was regarded as having an operational range of about 400 miles; when he left the pilots regarded it as having a 950 mile range, plus combat time over the target and a fuel reserve.

However, the P-38 pilots took a lot of convincing that running their engines over square would not cause them to fail early, thereby dropping them in the vastness of the Pacific with little change of survival.

We are going to discuss why running over square is not harmful to the engine, you can think of it as switching to a higher gear as is done in a car. Obviously, this is for engines equipped with a controllable propeller.

Running over square

Over square means having the MAP higher than RPM divided by 100. Thus a MAP of 25" and a RPM of 2300 is called over square. In case of turbocharged engines they are almost always run over square, for example: The Lockheed Super Constellation runs on a MAP of 51" at take-off with 2900 engine RPM and cruised with 44" and a much lower engine (2600) RPM.

MAP & RPM

A turbo normalized engine has a lower MAP, usually 30-32 inch, which is kept constant until a certain altitude. RPM is set at 2700 at take-off and in the range of 2200-2500 RPM during cruise. Even in this situation you are running over square. An example is a C-172 with an O-320 and a fixed pitch propeller: RPM during initial take-off is around 21-2200 (it will rise when airspeed increases) and MAP is 29-30 inch, also over square.

This means that any type of aircraft with a fixed pitch prop and a static RPM around 2200 with full throttle (max MAP) runs over square. When taking off the MAP will be 29-30 inch and the RPM around 2200. On each and every take off that engine runs in a good over square condition but actually no harm is done.

Running your engine with lower RPM than MAP has a number of advantages:

For example, in one particular aircraft (like the Piper Turbo Arrow IV PA-28RT-201T with a TSIO-360FB engine), the recommended combinations for 65% power at sea level and higher are:

As indicated above you can use a low RPM and a high MAP or a high RPM and a low MAP to achieve exactly the same power output from the engine. The low RPM / high MAP combination result in a more efficient cylinder charging and better combustion plus less friction. The high MAP also acts as a cushion in the cylinders, reducing engine stress. MAP is usually measured in inches of mercury ("Hg) rather than hecto pascals (hPa). Standard sea level barometric pressure is 29.92 "Hg or 1013.25 hPa.

But to enjoy all the advantages mentioned above you will need to select the lowest RPM and highest MAP combination for the power required and lean the mixture when in cruise.

Take off

On take-off do the same: 30" MAP and 2700 RPM and after passing 500' AGL reduce RPM to 2500 but leave the throttle wide open, this will reduce engine noise. Do not worry about the MAP this will reduce too as air pressure drops with 1" per 1000 ft, thus engine power reduces as well. This will not work with turbocharged engines.

Engine efficiency

Gasoline aviation engines have their best volumetric efficiency running at low RPM and high MAP, preferably at their full throttle height for their rated power. This will result in the best specific fuel consumption increasing range and endurance (and lowering your fuel bill too).

So, over square is not bad at all. Just remain within the engine limits set by the manufacturer and you will enjoy a greater range with lower fuel consumption and hourly cost to run the aircraft.

More background information

For readers wanting more information on this subject we refer them to the next series of articles:

The above articles are a must read for anyone with an engine and controllable propeller interested in keeping the engine in good shape for years (and flying hours) to come.